Understanding neutron radiography post result extended reading xi a by charlie Chong

Understanding Neutron Radiography Extended Reading XI 2nd September 2016 Reading Post Result (passed)

Charlie Chong/ Fion Zhang

Neutron Source

Charlie Chong/ Fion Zhang

Photomultiplier tubes lining the walls of the Daya Bay neutrino detector. The tubes are designed to amplify and record the faint flashes of light that signify an antineutrino interaction. This experiment aims to measure the final unknown mixing angle that describes how neutrinos oscillate â&#x20AC;&#x201D; another chapter in Brookhaven National Laboratory's long history of neutrino research over the last several decades.

Charlie Chong/ Fion Zhang

https://www.bnl.gov/newsroom/news.php?a=24055

Neutron Source

Charlie Chong/ Fion Zhang

The Great Rationalizer on Neutron

Charlie Chong/ Fion Zhang

The Magical Book of Neutron Radiography

Charlie Chong/ Fion Zhang

数字签名者：Fion Zhang DN：cn=Fion Zhang, o=Technical, ou=Academic, email=fion_zhang@ qq.com, c=CN 日期：2016.09.06 12:37:11 +08'00' Charlie Chong/ Fion Zhang

ASNT Certification Guide NDT Level III / PdM Level III NR - Neutron Radiographic Testing Length: 4 hours Questions: 135 1. Principles/Theory • Nature of penetrating radiation • Interaction between penetrating radiation and matter • Neutron radiography imaging • Radiometry 2. Equipment/Materials • Sources of neutrons • Radiation detectors • Non-imaging devices

Charlie Chong/ Fion Zhang

3. Techniques/Calibrations

• Electron emission radiography

• Blocking and filtering

• Micro-radiography

• Multifilm technique

• Laminography (tomography)

• Enlargement and projection

• Control of diffraction effects

• Stereoradiography

• Panoramic exposures

• Triangulation methods

• Gaging

• Autoradiography

• Real time imaging

• Flash Radiography

• Image analysis techniques

• In-motion radiography • Fluoroscopy

Charlie Chong/ Fion Zhang

4. Interpretation/Evaluation • Image-object relationships • Material considerations • Codes, standards, and specifications 5. Procedures • Imaging considerations • Film processing • Viewing of radiographs • Judging radiographic quality 6. Safety and Health • Exposure hazards • Methods of controlling radiation exposure • Operation and emergency procedures Reference Catalog Number NDT Handbook, Third Edition: Volume 4, Radiographic Testing 144 ASM Handbook Vol. 17, NDE and QC 105 Charlie Chong/ Fion Zhang

Fion Zhang at Norway 2nd September 2016

Charlie Chong/ Fion Zhang

SME- Subject Matter Expert http://cn.bing.com/videos/search?q=Walter+Lewin&FORM=HDRSC3 https://www.youtube.com/channel/UCiEHVhv0SBMpP75JbzJShqw

Charlie Chong/ Fion Zhang

Gamma- Radiography TABLE 1. Characteristics of three isotope sources commonly used for radiography. Source

TÂ˝

Energy

HVL Pb

HVL Fe

Specific Activity

Dose rate*

Co60

5.3 year

1.17, 1.33 MeV

12.5mm

22.1mm

50 Cig-1

1.37011

Cs137

30 years

0.66 MeV

6.4mm

17.2mm

25 Cig-1

0.38184

Ir192

75 days

0.14 ~ 1.2 MeV (Aver. 0.34 MeV)

4.8mm

350 Cig-1

0.59163

Th232

Dose rate* Rem/hr at one meter per curie

Charlie Chong/ Fion Zhang

0.068376

八千里路云和月

Charlie Chong/ Fion Zhang

闭门练功

Charlie Chong/ Fion Zhang

http://greekhouseoffonts.com/

Charlie Chong/ Fion Zhang

Application potential of cold neutron radiography in plant science research d a e R

Charlie Chong/ Fion Zhang

e r o

Summary Though comprehensive knowledge of water status and water flow are important prerequisites for plant in many aspects of modern plant science truly non-destructive methods for the in-situ study of water transport are rare. Advanced imaging methods such as Magnetic Resonance Imaging (MRI) or Cold Neutron Radiography (CNR) may be applied to fill this gap. In CNR strong interaction of cold neutrons with hydrogen provides a high contrast even for small amounts of water. The combination of CNR with the low contrast tracer D2O (?) allows the direct visualisation of water flow and the calculation of water flow rates in plants with a high resolution at the tissue level. Here, we give a general introduction into this method, describe their latest developments, report about studies applying neutron radiography in plant science and provide most recent results of our experiments in this field. Note: Cold Neutron (0~0.03MeV)

Charlie Chong/ Fion Zhang

Z number Charlie Chong/ Fion Zhang

Neutron Absorption Cross Section

Charlie Chong/ Fion Zhang

Neutron Absorption Cross Section (at MeV?)

Charlie Chong/ Fion Zhang

http://periodictable.com/Properties/A/NeutronCrossSection.bt.html

Neutron Absorption Cross Section

Charlie Chong/ Fion Zhang

Neutron Absorption Cross Section

Charlie Chong/ Fion Zhang

Neutron Absorption Cross Section

Charlie Chong/ Fion Zhang

Neutron Absorption Cross Section

Charlie Chong/ Fion Zhang

Neutron Absorption Cross Section

Charlie Chong/ Fion Zhang

http://physics.stackexchange.com/questions/106532/energy-dependent-cross-sections-for-neutrons

Introduction Water in plants is one of the most important factors for life. As the major solvent as well as an important substrate it guaranties the well functioning of the metabolic mechanisms of the plants, such as photosynthesis â&#x20AC;&#x201C; the basic process for live on earth.

Charlie Chong/ Fion Zhang

https://www.bnl.gov/chemistry/AP/images/Home_01_HR.jpg

Water availability, water distribution and water flow also regulate various plant physiological phenomena (VON WILLERT et al., 1995; LÖSCH, 2001). In future years water may become a limiting factor in agriculture, horticulture or silviculture 造林学 production in many countries. Breeding of plants with improved drought tolerance may help to partially overcome this challenge (ARAUS et al., 2002). Conventional construction of plants with improved water uptake and transport performance such as grafting of high yield shoots on water effective roots may also be a solution (PROIETTI et al., 2008). Hence, for both breeding and grafting comprehensive knowledge about plant water relations and especially on water uptake and water flow phenomena is essential. However, non-destructive methods for the in-situ study of water transport are quite limited. Even modern heat balance method (VON WILLERT et al., 1995; LÖSCH, 2001) can’t be applied without at least partially affecting the water transport pathways.

Charlie Chong/ Fion Zhang

Hence, non-invasive and fully nondestructive methods such as Magnetic Resonance Imaging (MRI; KUCHENBROD et al., 1996) or, less well-known, Cold Neutron Radiography (CNR, MATSUSHIMA et al., 2005a, b, 2007) seem to be appropriate methods to study water status and water transport in intact plants. Neutron radiography is an effective imaging method, where the strong interaction of thermal or cold neutrons with hydrogen provides a high contrast even for small amounts of water. On the other hand, the neutron beam has a large penetration depth in metals (Al, Fe, Cu, Pb etc.). Therefore, neutron radiography has been mainly used for the visualizations of water and/or organic components in mechanical devices (TRABOLD et al., 2006). The full-scale entry of neutron radiography into biological field started in the early 1990th. Because soil is relatively transparent to neutron beam, neutron radiography became a good tool for non-destructive observation of plant roots (NAKANISHI et al., 1991; 1992).

Charlie Chong/ Fion Zhang

This imaging technique was applied to investigate the exchange of minerals between soil and plant roots (SAITO et al., 1997; OSWALD et al., 2005; MANNES et al., 2006a). The use of the CT technique allowed a 3 Dimensional investigation of the water distribution around soybean roots (NAKANISHI et al., 2005; KIM et al., 2006). In postharvest technology, successful application of neutron imaging for the observation of changes in the internal structure of corn kernels during storage has been reported (CLEVELAND et al., 2006). In Japanese cedar seedlings, YAMADA et al. (2005) detected fungal induced tissue discoloration and tissue drying by neutron imaging. In this experiment, differences in the size of water deficient tissue parts resulting from damage by the impact of various fungi were identified. Neutron imaging was also applied for internal inspection and moisture mapping of tree trunks and timber (MANNES et al., 2006b). were also described.

Charlie Chong/ Fion Zhang

In order to study the characteristics of building materials, water absorption process of timber was investigated (LEHMANN et al., 2005). New developments of neutron radiography using low energy neutrons and/or monochromatic neutron beams increase the potential of the method for plant science. The new developments provide better spatial resolution and a higher image contrast. In this report recent studies of the application of novel neutron radiography in plant science are introduced. Our recent attempts in this field were also described.

Charlie Chong/ Fion Zhang

Neutron radiography Neutron radiography visualizes the attenuation of neutrons through a medium. The probability of the neutronâ&#x20AC;&#x2122;s interaction with a nucleus depends on the structure and the stability of the core. Hence, the attenuation of neutrons in a medium is random. Interestingly, certain light elements such as hydrogen and B, Be, Li, N, etc. absorb and/or scatter neutrons rather well. On the other hand, neutrons penetrate very heavy elements such as lead, titanium and others rather easily. Elements having adjacent atomic numbers can have a widely differing absorption of neutrons. Neutron attenuation efficiency can vary even between different isotopes of the same element.

Charlie Chong/ Fion Zhang

The probability of neutron scattering and/or absorption by a matter is given by the so-called cross sections. When a material is irradiated by a neutron beam, the number of transmitted neutrons depends on the total cross section which is a sum of the scattering and the absorption cross-sections for the given material. Equation 1 explains the attenuation of neutron intensity, I, from the initial intensity I0 to I by penetrating a material with the thickness x and the density Ď

The beam detected by a two-dimensional imaging device results in an image that can be utilized to analyze the macroscopic structure of the samples interior because the mass attenuation coefficient, Îźm, of a respective material depends on its density and element composition. This means that this image of the penetrating neutrons provides a reflexion of the amount and the arrangement of certain elements in the sample. Charlie Chong/ Fion Zhang

Hydrogen is one of the elements, which have a large mass attenuation coefficient and, hence, produce clear images. The thus obtained good contrast allows for high sensitivity to small amounts of hydrogen in complex systems. Hydrogen also forms the major constituent of living plants. It is incorporated in water, sugars, fibers and lipid molecules. By far the most ubiquitous molecule in living plant material is water. Hence, changes in the amount and the distribution of plant water are usually much more pronounced and can occur much faster than changes in other molecules. Of course, some other elements with large total cross sections such as boron and lithium are also found in plants. However, their concentrations are normally too small to be observed by neutron radiography. Therefore, the effect of these elements is certainly negligible. This all make neutron radiography a valuable tool to investigate the variation of water content and distribution in plant material. Nevertheless, the are some disadvantages of this method. This includes the relatively high cost and potentially radiation safety problems. Radiation safety problems are, however, rare and where they exist they are usually easily handled by shielding (BASTĂ&#x153;RK et al., 2005a, b).

Charlie Chong/ Fion Zhang

Neutron energy and imaging According to their energy neutrons are roughly classified as fast (high energy), (slowâ&#x2020;&#x2019;) thermal, cold and very cold (low energy) neutrons. This difference in their energy influences the characteristics of neutron transmission. Because the probability for interaction with the sample is reciprocal to the neutron energy, the total cross section also depends on the energy of the neutrons. By using liquid deuterium or hydrogen in the neutron source the energy of thermal neutrons can be largely reduced (moderation) . This reduction in neutrons energy remarkably increases the total cross section and, hence, decreases transmission. Thus, low energy neutron beams provide high contrast neutron imaging. This means that the dynamic range in the observation of changes of plant water is larger with the cold neutron imaging than that of the conventional method. This property should be suitable to investigate plants with thin stems, leaves and/or petals, for example small seedlings, blooming flowers, small bean pods etc. Such plant samples have spatially complex structures and the volume in the space would be relatively small. Charlie Chong/ Fion Zhang

Magnetic resonance imaging, an alternative to CNR, has a high spatial resolution (KUCHENBROD et al., 1996). It can provide various information about the status and the distribution of water (KĂ&#x2013;CKENBERGER, 2001; GARNCZARSKA et al., 2007). MRI has also been applied to observe microscopic plant samples. For example, MANZ et al. (2005) studied the regulation of water uptake during germination of tobacco seeds by in vivo 1Huclear magnetic resonance with a spatial resolution of 30 Îźm. Magnetic resonance imaging has been successfully employd to study long distance xylem flow and hydraulics in intact lants (e.g. JOHNSON et al., 1987; KUCHENBROD et al., 1996; PEUKE et al., 2001; SCHEENEN et al., 2007).

Charlie Chong/ Fion Zhang

However, MRI is not suitable for small leafy plants, because both its spatial and time resolution are influenced by the ratio of the sample volume to the overall detectable space in the coil of the NMR detector. If this ratio is too small, i.e. the sample is small compared to the volume within the detector coil, the spatial and time resolution would be lower because of a low signal to noise relation (KĂ&#x2013;CKENBERGER, 2001). This limitation is much less pronounced in CNR imaging (MATSUSHIMA et al., 2005a). In case of CNR the resolution primarily depends on the sample thickness and not on the total sample volume to detectable space ratio. If the total thickness of the sample is smaller than the upper attenuation limit of the neutron beam, i.e. the neutrons are not fully absorbed or scattered by the sample material, the neutron radiography is able todetect water distribution with very high sensitivity.

Charlie Chong/ Fion Zhang

As mentioned above, in CNR, the image contrast is affected by the energy of neutron radiation. Fig. 1 shows contrast differences in ivy leaves CNR imagescreated by two different low energy neutron beams. The wavelength of neutron radiations used for the images in Fig. 1A and B were about 0.3 (3 Å) and 6.0 nm (60 Å), respectively. It is obvious that the contrast of the image obtained with longer wavelength radiation and hence lower energy is higher than at shorter wavelength and higher energy. In Fig. 1B the leaf veins are clearer visible than those of Fig. 1A because the low energy of the neutron beam (6.0 nm) resulted in a higher attenuation coefficient for H2O, which, in turn, contributed to the increased contrast of the neutron image (KARDJILOV et al., 2003). In case of the image shown in Fig. 1B, water thickness could be estimated from the image at a resolution of 50 μm in 95% of confidence interval (MATSUSHIMA et al., 2005a).

Charlie Chong/ Fion Zhang

Fig. 1: Neutron images of ivy leaves taken by different radiation wavelength. A: approximately 0.3 nm (CONRAD, BER-II), B: approx. 6.0 nm (VCN port, PF2, Institut Laue-Langevin, ILL, France) (adapted from KAWABATA et al., 2005; MATSUSHIMA et al., 2005c).

Charlie Chong/ Fion Zhang

Cold neutron radiography imaging system During recent years, several experimental setups for low energy neutron imaging have been established consistently. Well known devices are:  CNRF (“Cold Neutron beam for the Radiography Facility“ at JRR-3M, Japan Atomic Energy Res. Inst., Ibaraki, Japan),  ICON (“Imaging with COld Neutrons“ at Swiss Spallation Neutron Source, SINQ, Paul Scherrer Institut, Villigen, Switzerland),  CONRAD (“COld Neutron RADiography“ at BER II, Hahn- eitner Institut, Berlin, Germany), and  ANTARES („Advanced Neutron Tomography And Radiography Experimental System“ at FRM II, Technische Universität München, München, Germany). The wavelengths of the neutron radiation applied in these imaging facilities reflects that of a cold neutron beam of approximately 0.3-0.4 nm (3-4 Å). Hence, low energy neutron radiography performed at the facilities mentioned above will be referred to as cold neutron radiography (CNR) hereinafter.

Charlie Chong/ Fion Zhang

CNFR of Japan Atomic Energy Res. Inst., Ibaraki, Japan

Charlie Chong/ Fion Zhang

CNFR of Japan Atomic Energy Res. Inst., Ibaraki, Japan

Charlie Chong/ Fion Zhang

Swiss Spallation Neutron Source, SINQ, Paul Scherrer Institut, Villigen, Switzerland

Charlie Chong/ Fion Zhang

Swiss Spallation Neutron Source, SINQ, Paul Scherrer Institut, Villigen, Switzerland

Charlie Chong/ Fion Zhang

Swiss Spallation Neutron Source, SINQ, Paul Scherrer Institut, Villigen, Switzerland

As an example of a low energy neutron devices, CONRAD, at the HahnMeitner Institut (HMI) will be introduced in more detail. The experimental setup is placed at the end of a curved Ni- oated neutron guide at the experimental reactor BER II. The peak wavelength of the beam spectrum is about 0.31 nm (HILGER et al., 2006). There are two different experimental positions available. The neutron beam at Position I has a high neutron flux and a low spatial resolution. In contrast, Position II is adapted for high spatial resolution by optimization of the pinhole geometry. Users can chose the appropriate position in dependence of the specific purpose of their investigations.

Charlie Chong/ Fion Zhang

The beam size of Position II is 10cm×10cm and the spatial resolution is about 200 μm. In the irradiation room there is plenty of working space for the sample setup on the right hand of beam propagation direction. More details about the experimental device are given by KARDJILOV et al. (2005) and HILGER et al. (2006). Spatial resolution has been further improved by reducing scintillator thickness and employed low energy neutron beam (KÜHNE and LEHMANN, 2006; FREI and LEHMANN, 2006; KÜHNE et al., 2006). Images appearing on the scintillator are deflected by a mirror into the 50 mm focus Nikon camera lens and is recorded by an Andor DW436N-BV CCD camera with 2048×2048 pixels, each 13.5×13.5 μm2 large. This detector system can provide a spatial resolution of up to 25 μm. Fig. 3 shows the root system of a tomato seedling obtained with the spatial resolution about 70 μm by the thin scintillator system. Fine roots of the seedling are clearly visible. This high resolution imaging system can be a powerful tool to investigate microscopic plant structures. However, to observe plant cells, a spatial resolution of less than 5 μm will be necessary. Images at the cellular level would clearly broaden the application of neutron imaging, thus, further technical developments are urgently required. Charlie Chong/ Fion Zhang

Fig. 2: Irradiation room of position II, CONRAD.

Charlie Chong/ Fion Zhang

Fig. 3: Water distribution in the roots and the lower stem of a tomato seedling. The image was obtained with a spatial resolution of approx. 70 Îźm.

Charlie Chong/ Fion Zhang

Applications in plant science As mentioned above, application of neutron radiography in plant science has been primarily focused on water mapping in wood (LEHMANN et al., 2005; YAMADA et al., 2005; MANNES et al., 2006a, b), in corn (CLEVELAND et al., 2006), around and in living root systems (NAKANISHI et al., 1991; 1992; 2005; OSWALD et al., 2005; KIM et al., 2006) but also in leaves and stems of ornamentals (MATSUSHIMA et al., 2005a, b; 2007). MATSUSHIMA et al. (2005b) investigated the effects of dehydration by vacuum cooling on the water content and the water distribution in chrysanthemum leaves by low energy neutron imaging at the CN-3 neutron port of the Research Reactor Institute, Kyoto University. It could be shown that during vacuum cooling overall water content declined by only approximately 5% of the initial fresh mass.

Charlie Chong/ Fion Zhang

However, differential water mapping before and after vacuum cooling application indicated that critical dehydration had occurred only at wound cuts of the leaf sample. Furthermore, the process dynamics of plant leaf water losses was also studied in detail using lower energy neutron beam (MATSUSHIMA et al., 2005a). However, only further improvement of cold neutron radiographic facilities will open new perspectives for various additional usage of this method in plant science research. In the following, we present two novel applications using the advantages of CNR.

Charlie Chong/ Fion Zhang

Study of water flow by D2O tracer In case of neutron or x-ray radiography, any growth or dehydration induced dynamic variation in structure and/or density of plant materials causes changes of the contrast in transmission images. However, steady state flows in the biological samples such as water transport in plants that do not affect distribution or content of H2O can not be directly detected by radiographic techniques. Therefore, contrast agents such as Iodine for x-ray radiography need to be applied as tracers to actually visualize the flow of H2O with CNR. In case of plant water transport deuterium oxide (D2O) is suitable for this purpose. The physical and chemical properties of D2O are very similar to those of H2O (WIKIPEDIA, 2008). Hence, heavy water has been yet used in many investigations on its metabolic effects in fungi, animals and plants (ALEXANDROV et al., 1965; PITTENDRIGH et al., 1973; SIEGEL and GALUN, 1978; IGNATOV and LITVIN, 1998).

Charlie Chong/ Fion Zhang

Note: 2 H 1 0.00052 Ď&#x192;(barns) neutron absorption deuterium, in D2O, good collisional energy transfer with modest absorption 1

0.332 Ď&#x192;(barns) neutron absorption hydrogen, in H2O, optimum collisional energy transfer, medium absorption http://www.its.caltech.edu/~chem2/NuclearEnergySlides%204-28-09.pdf

Note2: Target type dependence The neutron cross section is defined for a given type of target particle. For example, the capture cross section of hydrogen-2 (referred to as deuterium) is much smaller than that of common hydrogen-1.[1] This is the reason why some reactors use heavy water (in which most of the hydrogen is deuterium) instead of ordinary light water as moderator: fewer neutrons are lost by capture inside the medium, hence enabling the use of natural uranium instead of enriched uranium. This is the principle of a CANDU reactor. https://en.wikipedia.org/wiki/Neutron_cross_section

Charlie Chong/ Fion Zhang

In plants it has also been applied at low concentrations as a tracer to study various water exchange and water transport processes (ILVONEN et al., 2001; SEKIYA and YANO, 2004; ICHMASA et al., 2005). Concerning CNR, the mass attenuation coefficient of D2O is smaller than that of normal water, which allows neutron to better penetrate D2O than normal water resulting in a large variation of image contrast. Hence, we attempted to observe steady state water flow with CNR using D2O as a tracer. To our best knowledge, no use of a contrast agent for plant research using neutron radiography has been documented before.

Charlie Chong/ Fion Zhang

Hydrogen & Deuterium Neutron Cross Section (Total)

Charlie Chong/ Fion Zhang

http://www.technology.matthey.com/article/60/2/132-144/

Ionized Deuterium

Charlie Chong/ Fion Zhang

https://en.wikipedia.org/wiki/Deuterium

For the experiments conducted at CONRAD at HMI tomato seedlings (Solanum lycopersicum L. cv. Harzfeuer) were grown from seeds on sand in a climate cabinet (VB 1014, Vötsch Industrietechnik GmbH, Balingen, Germany). Climatic conditions were set to Tday/night = 22/ 15°C and RHday/night = 40/70%. Plants received photosynthetic active photon fluence rates of approximately 220 μmol m-2 s-1 at a 14 h day period and were daily watered with tap water. After they had developed 2 or 3 true leaves plants were used for the experiments. Each seedling was transplanted into a quartz glass tube filled with a soda glass beads medium (Fig. 4 left). During the experiment the tomato seedlings were irradiated with a halogen lamp (Osram, Hamburg, Germany).

Charlie Chong/ Fion Zhang

Fig. 4: Experimental setting for the application of D2O tracer during CNR measurements. Left: Sample in a glass tube filled with soda glass beads medium. During the experiment the tomato seedling is irradiated with a halogen lamp with a Perspex water flow heat shield. Right: D2O injection system.

Charlie Chong/ Fion Zhang

The plants were protected by a Perspex water flow heat shield. Fig. 4 (right) shows the experimental set up of the investigation on the D2O-tracer application. With a custom-made PC-controlled injection system D2O and H2O could be automatically supplied to the samples in the quartz glass tube from bottles as requested. During the experiments, CNR images were taken every 15 seconds with an exposure time of 10 seconds and read out time of the detector of 5 seconds. Water flow from root system to stem was clearly visualised by CNR through the positive contrast created by the D2O tracer (Fig. 5). This clearly indicates that heavy water is a very suitable tool to comprehensively and non-destructively investigate the temporal and spatial dynamics of water movement at the plant organ level.

Charlie Chong/ Fion Zhang

Fig. 5: An example of water flow into a tomato seedling stem indicated by the level of the D2O tracer at different times after exchange of H2O for heavy water. The dark area corresponds to the amount of heavy water in the stem.

Charlie Chong/ Fion Zhang

Furthermore, the observation of the D2O level in the stem at different times after the exchange process enables the calculation of the velocity of water uptake and water flow in the seedling. In this measurements, flow rates of 2.6 cm h-1 could be estimated. These flow rates, corresponding to 0.01 mm s-1, were much lower than those found (0.2 to 0.4 mm s-1) for adult ricinus plants by MRI (PEUKE et al., 2001). At the respective developmental stage of the tomato seedling, xylem water flow may be slower than in mature plants due to the still developing vascular bundle system. Furthermore, the low VPD of less than 7 kPa MPa-1 prevailing during the entire measurement may have also reduced transpiration of leaves. Anyway, such a low flow velocity in small stems is difficult to measure with existing techniques such as heat balance systems (VON WILLERT et al., 1995).

Charlie Chong/ Fion Zhang

Because the chemical structure and the physical properties of D2O is similar to H2O this tracer easily passes the casparian strip that selectively restricted the intake of chemical compounds. This is a big advantage of the D2O tracer method. Boron, for example, can also be a valuable contrast agent for neutron radiography due to its high attenuation of neutron. Further more, boron is an essential nutrient to higher plants (BROWN et al., 2001). However, even at high external boron supply, plants do not take up enough boron to create a contrast in radiographic images. It is expected that boron, like other chemical compounds used as CNR tracers, would be partially excluded at the different transmembrane transport processes occurring during primary uptake by epidermal, cortical or endodermal cells and at the casparian strip, or during xylem loading in the root system (BASSIL et al., 2004). Therefore, tracers such as boron must be either injected by syringe, drip-infused or taken up after removal of the root system. Furthermore, the higher concentrations boron is highly toxic and it is known to concentrate in particular tissues of plant; and it is obvious that boron solution doesnâ&#x20AC;&#x2122;t behave as normal water. Hence, this obviously indicates that D2O is the preferential tracer for nondestructive CNR water flow studies in plants. Charlie Chong/ Fion Zhang

Furthermore, combining the D2O tracer technique with the neutron computed tomography (NCT) imaging system it is possible to construct 3-dimensional maps of the distribution of D2O or H2O, respectively. Fig. 6A shows the D2O distribution in a single vertical tomographic slice of the upper stem (peduncle) and flower bud of a rose. Using several horizontal NCT slices (Fig. 6A) taken at different but close locations it is possible to construct a D2O replacement map. With this approach the intensity of the water movement within a vascular bundle and between the vascular bundle and the parenchyma cells could be traced (Fig. 6B).

Charlie Chong/ Fion Zhang

According to the given scale, the grey scale in Fig. 6B reflect the amount of D2O replaced in the different tissues. In this flow activity image the highly efficient vascular bundles are highlighted. The comparison of the slice of the D2O map and a light microscope image of the peduncle cross section of a rose of the same cultivar (Fig. 6C) further indicates that the replacement of D2O from the vascular bundles to the pith was more intensive than to the other tissues. For investigations of mechanisms and dynamics of short and long-distance water flow in plants it is very desirable to observe 3 Dimensional water flow characteristics at a high spatial resolution. For this purpose, a rapidly scanning neutron CT with improved resolution is necessary, hence further development of this technique is required.

Charlie Chong/ Fion Zhang

Fig. 6: D2O map on a slice image in a neutron CT of rose peduncle. A: Vertical sliced NCT image. The lines indicates the NCT slice surface of those image used to construct the D2O replacement map given in B. B: D2O replacement map. C: Optical microscopic image of a rose peduncle (Rosa hybrid cv. Akito).

Charlie Chong/ Fion Zhang

Combination with other imaging analysis The obvious drawbacks of MRI, compared to neutron radiography, are the lack of space and the high magnetic field around the sample which prevents the use of additional electronic devices. In case of CNR such devices can be protected from neutron and/or gamma ray by shielding, if necessary at all. This advantage of CNR was used in order to combine neutron imaging with an other method that monitors the photosynthetic activity of plants. Pulse modulation chlorophyll fluorescence analysis imaging (CF imaging) is a tool that provides deep insight into photosynthetic efficiency and integrity of plants (NEDBAL et al., 2000; HERPPICH, 2002). Especially the parameter Fv/ Fm, an indicator of the potential photosynthetic efficiency, is highly related to plant stress responses (VON WILLERT et al., 1995). By the combination of neutron radiography and CF imaging, it is possible to parallely study the effects of environmental stresses on both water status and photosynthetic activity of the plant sample.

Charlie Chong/ Fion Zhang

Chlorophyll Fluorescence Analysis Imaging (CF Imaging)

Charlie Chong/ Fion Zhang

http://qubitphenomics.com/chlorophyll-fluorescence-imaging/

With these techniques, the effects of toxic auto-exhaust, simulate by 2 ppm SO2 in air, on the physiological efficiency of street trees was evaluated. Hibiscus, which is a popular street tree in Okinawa, Japan, was used for experiments. Young rooted cuttings were enclosed in atemperature and humidity controlled aluminium cuvette fitted with a quartz glass window to be able to take the CF images. With their roots the plants were placed in glass tubes filled with glass beads. These tubes were connected to an automatic, PC-operated exchange system for H2O and D2O which could alternately supply each liquid. Before the start of the experiments, i.e. before the exposure of the plant to the simulated auto-exhaust gas an initial CF image was taken. Then, the toxic SO2-in-air gas mixture was supplied for one hour while CF images were taken every 20 min. Afterwards, the cuvette was again flushed with normal air and plants were maintained under this condition for another hour with CF images taken regularly. During the entire experimentCNR images were recorded every 15 seconds with an irradiation time of 10 seconds. H2O and the liquid tracer D2O were alternately exchanged every 30 min.

Charlie Chong/ Fion Zhang

The CF imaging system was installed in front of the neutron radiography facility CONRAD (Fig. 7). It was placed vertically to the neutron beam line to avoid direct irradiation. The sample was located in the neutron beam in orderto take neutron images simultaneously. Therefore, the sample cuvette was rotated by an automatic rotation table to face it to the fluorescence imaging camera. Fig. 8 shows the variation of the maximum photochemical efficiency, Fv/Fm, of hibiscus leaves as analysed from the CF images taken during the course of the experiment. In this figure, Fv/Fm, which is a sensitive indicator of both the efficiency and integrity of plant photosynthesis, was presented in a false colour scale ranging from a minimum at 0.3 (dark) to a maximum at 0.8 (light). After the supply of the simulated auto-exhaust gas the average maximum photochemical efficiency of the exposed leaves, within few minutes, dropped by more than 30% from the initial mean of 0.64 to less than 0.40.

Charlie Chong/ Fion Zhang

Fig. 7: CF imaging system installed in front of the CNR facility, CONRAD.

Charlie Chong/ Fion Zhang

Fig. 8: CF images of hibiscus samples during the course of the experiment. A: Before supply of the SO2 gas. B: 65 minutes after supplying 2 ppm of SO2. C: 120 minutes after supply of normal air. Note that the low activity of the upper small leaf on the right hand side was due to a partial shading of the saturation light by the cuvette ventilator (c.f. Fig. 9).

Charlie Chong/ Fion Zhang

Reduction of photochemical competence seems to be equal all over the entire leaf (Fig. 8), i.e. no clear-cut gradient developed during stress. On the other hand, these effects were fully reversible and Fv/Fm slowly recovered to its initial level after approximately 2 h in normal air (data not shown). Recovery seemed to be most rapid close to the major veins, probably also indicating a dilution effect of the cell sap, acidified by dissolved SO2 (SCHMIDT et al., 1990). Our results clearly indicate that 2 ppm of SO2 rapidly and seriously affect primary metabolism of hibiscus plants thus substantiating earlier findings that fumigation with SO2 reversibly inhibited Calvin cycle activity and may even cause damage at the photosystem II level (SHIMAZAKI et al., 1984; SCHMIDT et al., 1990). Exposure to SO2 also rapidly affected plant water uptake and water status in the hibiscus stem as can be seen from the neutron images obtained simultaneously (Fig. 9) during the different treatments. The time interval between the four images was about 30 minutes.

Charlie Chong/ Fion Zhang

The D2O tracer successfully both quantitatively and qualitatively indicated water flow and distribution in the stem of the plant sample. When the atmosphere in the cuvette was again changed from the simulated auto exhaust gas to normal air, the amount of tracer and, hence, the rate of water uptake increased. Therefore, it can be concluded that hibiscus trees may sensitively reduce stomatal conductance and transpiration in response to SO2 stress as also found in peanut and tomato (KONDO and SUGAHARA, 1978). In contrast, stomata of radish, perilla, spinach (KONDO and SUGAHARA, 1978) or poplar (VAN HOVE et al., 1991) have been shown to respond much less extensive to this toxic gas.

Charlie Chong/ Fion Zhang

Hence, the presented results show that simultaneous CNR and CF imaging successfully visualizes the effects of air polluting gases like SO2 on photosynthetic activity and water uptake, water movement and water distribution in plants at least in small samples. The combination of these two methods can greatly contribute to increase our understanding of the complex and interactive effects of toxic auto exhaust on the different aspects and levels of the metabolism of plants. Consequently, this is a very helpful approach to efficiently screen for street tree species and varieties with a high tolerance against auto exhaust, having the potential to increase CO2 absorption capacity in cities (YANG et al., 2005; NORWAK et al., 2006).

Charlie Chong/ Fion Zhang

Fig. 9: Water uptake into the hibiscus stem during the different treatments as indicated by the D2O tracer. The images were normalized to the initial state for each treatment step. A: 30 minutes after supply of SO2. B: 60 minutes after supply SO2 gas. C: 30 minutes after supply of air. D: 60 minutes after supplying air. The dark area corresponds to the amount of water in the stem.

Charlie Chong/ Fion Zhang

Conclusions CNR is suitable to investigate water distribution in small and/or thin plant materials, which determines a broad field of applications of thismethod in various field of applied plant science. CNR combined with D2O as a tracer directly visualizes water uptake, water flow and water distribution in seedlings and small plants. Following a further increase in spatial and temporal resolution of cold neutron radiography, water flow in microscopic pathways of plant can be monitored. CNR can be easily and effectively combined with other advanced nondestructive and non-invasive imaging techniques such as CF, thermographic or hyperspectral imaging. Using the combination of CNR and CF imaging, we successfully investigate the complex effects of simulated auto-exhaust gas on the physiological performance of Hibiscus cuttings. The result clearly demonstrated the integrated impact of SO2 on both the photosynthetic activity and on transpiration. Non-destructive imaging methods for plants are important tools to investigate plants. However, there is no imaging method suitable for all purpose. Cold neutron radiography is very effective to examine thin and spared plant materials. On the other hand, MRI or X-ray imaging can be a powerful tool for study thick compact plant materials. Charlie Chong/ Fion Zhang

Materials for Nuclear Power Generation

Charlie Chong/ Fion Zhang

http://www.doitpoms.ac.uk/tlplib/nuclear_materials/index.php

Contents Main pages 1. Aims 2. Before you start 3. Introduction to Nuclear Processes 4. Introduction to Nuclear Power Generation 5. Cross-Sections 6. Mechanisms of Radiation Damage 1 7. Mechanisms of Radiation Damage 2 8. Effects of Radiation Damage 9. Fuel and Cladding 10.Moderators 11.Summary 12.Questions 13.Going further Additional pages Approximated Equation

Charlie Chong/ Fion Zhang

http://www.doitpoms.ac.uk/tlplib/nuclear_materials/printall.php

1. Aims On completion of this tutorial you should be able to:  understand the basic physics behind nuclear fission;  describe the common features of nuclear reactors;  understand the various neutron cross-sections;  explain the mechanisms of radiation damage, and its consequences, particularly for structural steels;  understand the material problems associated with extreme conditions, in particular large radiation fluxes;  explain the materials selection for the components at the heart of a nuclear reactor:  moderators;  control rods;  cladding.

Charlie Chong/ Fion Zhang

Before You Start Readers should be familiar with the concept of a crystal lattice, dislocations, and diffusion. A familiarity with the basics of mechanical behaviour and corrosion of materials would also be useful. Readers should be familiar with standard nuclear terminology: the definitions of isotope and nuclide, the composition of nuclei, the definitions of atomic number and mass number. A note on units: throughout this TLP the unit used for energy is the electron volt , the energy associated with one electronic charge of (1.619 × 10-19 Coulomb) subjected to a potential difference of 1 V, i.e. 1 eV ≈ 1.619 × 10-19 J

Charlie Chong/ Fion Zhang

Introduction to Nuclear Processes Each nucleus, consisting of protons and neutrons (collectively known as nucleons), has an associated binding energy. A graph of binding energy per nucleon is shown in the graph below. The total binding energy of a nucleus is the energy released when a nucleus is assembled from individual nucleons; the greater the energy release, the lower the potential energy of the nucleus, so higher binding energy in the graph represents greater stability. When one nucleus is converted to another or others of higher binding energy, whether that be through a natural radioactive process or through an artificially induced process, the difference in the total binding energies of the nuclei is released as (1) kinetic energy of the particles produced and (2) gamma rays. This (these?) energy can be harnessed through traditional methods, e.g. by heating water to generate steam to drive a turbine, and so electricity can be produced.

Charlie Chong/ Fion Zhang

A graph of the binding energy per nucleon, in MeV, for common nuclides.

Charlie Chong/ Fion Zhang

A graph of the binding energy per nucleon, in MeV, for common nuclides.

higher binding energy in the graph represents greater stability

Charlie Chong/ Fion Zhang

Origins of Binding Energy The measured binding energies of the nuclides can be fitted reasonably well by Weizsäcker’s formula (see below). The formula is derived by treating the nucleus as analogous to a liquid drop, with surface energy and volume energy terms leading to the two dominant contributions: a term proportional to A, the atomic mass and to the volume of the nucleus, and a term proportional to -A2/3 due to the surface energy. These two terms compete, much in the same way they do in other processes (e.g. nucleation), facilitating a qualitative understanding of why nuclei split up or join together under certain conditions. Keywords: ∝A ∝ -A2/3

Charlie Chong/ Fion Zhang

Fusion Energy is given off when a nucleus becomes more stable, i.e. approaches the maximum on the graph above. Moving from lighter nuclei towards this maximum requires two nuclei to combine and form a heavier one (fusion), whereas moving from heavier nuclei towards this maximum requires the nucleus to split apart (fission). The energy release per mass of nuclide is much higher for fusion than for fission. Fusion has many other attractive attributes as a basis for power generation, but since nuclei are positively charged, sufficient energy most be put into the system to overcome the repulsion between nuclei so that a fusion process can occur. This Coulomb barrier can also be expressed as an ignition temperature. The technical challenges are many, and nothing close to a commercially viable reactor currently exists. Fusion for power generation is still a prominent research topic, and experimental reactors are in the process of being built, such as ITER (International Thermonuclear Experimental Reactor), which is planned to be completed by 2018. Since nuclear fusion is not yet a practical power source, this TLP will instead focus on nuclear fission as means to generate heat and electricity. Charlie Chong/ Fion Zhang

split apart

combine and form a heavier one

Charlie Chong/ Fion Zhang

Fission Nuclear fission, as previously mentioned, involves splitting a heavier nucleus into two lighter nuclei. Fission can be induced if a nucleus absorbs a neutron of sufficient energy. If a nucleus undergoes fission regardless of the incident neutron energy, the nucleus is referred to as fissile; otherwise, if there is a threshold energy then the nucleus is referred to as fissionable. Keywords: â&#x2013; If a nucleus undergoes fission regardless of the incident neutron energy, the nucleus is referred to as fissile; â&#x2013; if there is a threshold energy then the nucleus is referred to as fissionable. Examples of fissile nuclides include 233U, 235U and 239Pu. The nuclide most commonly used in nuclear reactors is 235U.

Charlie Chong/ Fion Zhang

A neutron will not necessarily induce fission if it passes through the nucleus. For example, fast neutrons are less likely to induce fission in 235U than thermal neutrons (i.e. neutrons with kinetic energy of the order of kT) (?) . Qualitatively, this makes sense since the faster a neutron is travelling the less time it spends inside the nucleus and so the less opportunity it has to induce fission within the nucleus. The actual reasons for this are complicated, and this topic is explored further on the “Cross Sections” page. Fissionable nuclides, such as 238U and 239Pu, are also used in so-called “fast” reactors, where the neutrons are travelling fast enough (commonly around 10% the speed of light, or 1 MeV) to overcome the activation energy required to make fissionable nuclides decay. Note: 239Pu is both fissile & fissionable.

Charlie Chong/ Fion Zhang

Video illustrating nuclear fission

Charlie Chong/ Fion Zhang

As can be seen in the movie, the parent nucleus decays into two fission fragments of unequal mass with a combined kinetic energy of about 169 MeV and several neutrons with a kinetic energy of about 2 MeV each (for 235U, the average number of neutrons produced is 2.4, but can be as high as 5). These neutrons are highly energetic, with 7~8 orders of magnitude more energy than thermalized neutrons. A gamma ray of about 7 MeV is also released. The neutrons could induce further fission events in other nuclei and thus cause a chain reaction, but in practice they are too fast and must first be slowed down inside the reactor.

Charlie Chong/ Fion Zhang

Graph showing the distribution of fission fragment mass numbers for three nuclides, U-233, U-235 and Pu-239. The fragments formed tend to be of unequal masses, with each fragment showing a Gaussian distribution about a particular lower or higher mass. [Graph is under a CC[BY][NC][SA] licence and was created from source data at http://www-nds.iaea.org/sgnucdat/c1.htm] The nuclides produced by fission are usually of unequal mass, as shown in the graph below. The x-axis of the graph is by atomic mass, not atomic number. Many fission fragments are highly unstable, and decay by giving off beta radiation: this involves a neutron changing into a proton within the nucleus, leaving the overall number of nucleons (and hence the mass of the nucleus) the same.

Charlie Chong/ Fion Zhang

Unequal Fragments of Fissions

Charlie Chong/ Fion Zhang

Introduction to Nuclear Power Generation There are two main types of nuclear reactor, characterized by the speed of the neutrons which induce fission: â&#x2013; Thermal reactors. These are the predominant kind, using slower neutrons to induce fission, the basic fissile nuclide being U-235. â&#x2013; Fast breeder reactors. In these less-common reactors, the fast neutrons are used directly to create (breed) fissile nuclides from fissionable nuclides; most commonly Pu-239 is bred from U-238. Pu-239 is also used in nuclear weapons.

Charlie Chong/ Fion Zhang

Fast Breeder Reactor- Introduction & History The Integral Fast Reactor (IFR), also known as Advanced Liquid-Metal Reactor, is a fast neutron reactor with a very efficient and clean nuclear fuel cycle. It was once a very active research project of the U.S. Department of Energy, and even a prototype (called Experimental Breeder Reactor-II) was build in Argonne National Laboratory. However, the project was hampered in 1994 after a series of controversies involving safety issues and nuclear nonproliferation policies. [1] More recently, in 2002, the Integral Fast Reactor was elected the best nuclear reactor design in a Department of Energy report, even though there are no IFR in commercial use today. [2] Here, we analyze how a fast reactor works, focusing on the qualitative comparison with the more common thermal reactors, and assess the safety issues which are at the core of their unsuccessful spreading.

Charlie Chong/ Fion Zhang

http://large.stanford.edu/courses/2013/ph241/waisberg1/

The Integral Fast Reactor

Charlie Chong/ Fion Zhang

http://large.stanford.edu/courses/2013/ph241/waisberg1/

What Are Fast Neutrons? They are free neutrons with a kinetic energy on the order of 1 MeV, which corresponds to a speed of around 13,815 km/s, according to the expression for the relativistic kinetic energy: EKE = (Îł - 1) m0c2, Where: Îł is the Lorentz factor and m0 the rest mass. These neutrons have intermediary speed between thermal neutrons (which are in thermal equilibrium with the background) and higher-energy neutrons in particle accelerators and cosmic rays. Fast neutrons can be produced either by nuclear fusion or fission (although these processes produce neutrons following a Maxwell distribution with mode energy usually below 1 MeV, so that most of the neutrons produced do not qualify as "fast".

Charlie Chong/ Fion Zhang

http://large.stanford.edu/courses/2013/ph241/waisberg1/

EBR-II

Charlie Chong/ Fion Zhang

http://large.stanford.edu/courses/2013/ph241/waisberg1/

EBR-II

Charlie Chong/ Fion Zhang

http://large.stanford.edu/courses/2013/ph241/waisberg1/

EBR-II

Charlie Chong/ Fion Zhang

http://large.stanford.edu/courses/2013/ph241/waisberg1/

EBR-II

Charlie Chong/ Fion Zhang

http://large.stanford.edu/courses/2013/ph241/waisberg1/

EBR-I

Charlie Chong/ Fion Zhang

http://large.stanford.edu/courses/2013/ph241/waisberg1/

EBR-I

Charlie Chong/ Fion Zhang

http://large.stanford.edu/courses/2013/ph241/waisberg1/

Overview of Functionality Almost all thermal reactors currently in use are thermal reactors, which means that they make use of a moderator (regular water in light-water reactors, heavy water in heavy-water reactors, graphite in gas-cooled reactors etc) to slow down fast neutrons into thermal neutrons. This is necessary because the cross section of fissible nuclei (e.g. U-235, Pu239, Pu-241) is inversely proportional to the bombarding neutron energy (roughly, the fissible atom vibrates proportionally to its absolute temperature, and thermal neutrons are ideal for their fission since they are at the same temperature as the surrounding material). On the other hand, the cross section for fertile nuclei (a fertile isotope is one that is not fissionable by thermal neutrons, but can be converted to fissile material if the neutrons have high enough kinetic energy; e.g. U-238 to Pu239) is directly proportional to the neutron energy.

Charlie Chong/ Fion Zhang

http://large.stanford.edu/courses/2013/ph241/waisberg1/

Unfortunately for thermal reactors, natural uranium contains approximately 99.28% of fertile U-238 isotope and only 0.71% of fissile U-235. As a result, making sure that neutrons are thermalized maximizes the probability that a given neutron will be captured by fissile material, allowing thermal reactors to use low-enriched (or even natural, in the case of heavy-water reactors) uranium. [3]

Charlie Chong/ Fion Zhang

http://large.stanford.edu/courses/2013/ph241/waisberg1/

Fast reactors dispense the use of a moderator and make use of fast neutrons to sustain the nuclear fission chain reaction. The irony is that U-238 will, at low energies, absorb more neutrons than it consumes (due to the phenomenon of resonance absorption at neutron energies from 1 eV-10 keV). Therefore, U-238 cannot be made "critical", and fast neutron reactors need higher ratio of fissile-to-fertile material (i.e. higher enrichment) to ignite the nuclear reaction. Despite this drawback, once the chain reaction achieves criticality, fast fission reactions have a larger neutron output, and fast neutrons are more likely than thermal neutrons to cause fission once they are absorbed. This makes fast reactors capable of achieving a so- called "breeding" condition, in which they generate more fissible material than they consume. [3,4]

Charlie Chong/ Fion Zhang

http://large.stanford.edu/courses/2013/ph241/waisberg1/

Breeding - the Key to Efficiency and Cleanness Basically, fast reactors can achieve "breeding" condition by transmuting fertile U-238 into fissible Pu-239 with fast neutrons. This is more sufficient to replace the consumed U-235 i.e. fast reactors can produce more fissible material than they consume. Once the reaction has achieved critically (which, again, requires relatively high enriched starting fuel), the breeder reactor can be continuously re-fueled with natural uranium. This makes fast reactors extremely fuel-efficient: they are capable of extracting nearly all the energy in the fuel (in contrast, thermal reactors are typically able to extract less than 10% of the energy in the enriched uranium). [4,5] More than that, fast reactors are able to fissile the actinides resulting from Pu239 fission (which are all fissible with fast neutrons) into non-radiative or radiative elements with short half-live. These actinides are the core component of unrecyclable nuclear waste in thermal reactors.

Charlie Chong/ Fion Zhang

http://large.stanford.edu/courses/2013/ph241/waisberg1/

In thermal reactors, even though around 1/3 of the output energy actually comes from Pu-239 (originated from fertile U-238) consumption, this is not sufficient to stop the build-up of reactor-grade plutonium (even mass number plutonium isotopes which are not fissible) and of high concentrations of nasty actinides in the reactor. [6] Therefore, fast reactors are much cleaner than conventional nuclear reactors in terms of not generating hazardous nuclear waste. Specifically to the IFR, pyroelectric (voltage generation with heat) separation is used to remove the transuranic (elements with higher atomic number Z than uranium) and concentrate them; they are then used to re-fuel the reactor.

Charlie Chong/ Fion Zhang

http://large.stanford.edu/courses/2013/ph241/waisberg1/

Safety - Achilles' Heel? Controversies that led to the hampering of the IFR project were largely centered on safety considerations. [1] One worry about fast neutron reactors is the lack of moderators, which in thermal reactors serves as negative feedback in case the reaction rate raises too much (basically, increasing reaction leads to increasing temperatures, which leads to moderator boiling, which leads to less thermalized neutrons, which leads to smaller reaction rate). Therefore, in fast reactors it is necessary to find a substitute capable of providing negative feedback. [7] This can actually be achieved by reactor design, since the fuel and cladding expansion at higher temperatures, which allows more neutrons to escape the core, can stop the chain reaction and provide negative feedback.

Charlie Chong/ Fion Zhang

http://large.stanford.edu/courses/2013/ph241/waisberg1/

Another concern about the safety of fast reactors comes from the coolant, which carries the thermal energy generated by fission to the water to be boiled to move the steam turbines. Thermal reactors can use the moderator (usually water) as a coolant, but fast reactors must use coolants other than water. Typically, fast reactors use liquid metals as coolants (namely, sodium and lead). For one thing, using a liquid metal coolant has the advantage of being able to maintain the core of the reactor at lower pressures (in fact, at ambient pressures), since they have boiling points much higher than water (883 Â°C for Na, 1749 Â°C for Pb). [3] In water reactors, the pressure has to be maintained extremely high to keep water liquid even at high temperatures.

Charlie Chong/ Fion Zhang

http://large.stanford.edu/courses/2013/ph241/waisberg1/

However, using liquid metal coolants has important drawbacks. Integral fast reactor usually uses liquid sodium as coolant, which is highly reactive: it ignites when in contact with air and explodes when in contact with water. This problem has already caused reactor accidents in the past (e.g. Monju Nuclear Power Plant in Japan was forced to shutdown in 1995 after sodium leakage) and has contributed to the hampering of fast reactors. [1] It is possible to overcome this problem and ensure safety by engineered safety mechanisms (e.g. a coolant loop between reactor and steam turbines). Using lead as the coolant is still not a very practical alternative, since lead's very high melting point and vapor pressure makes it very difficult to refuel and service a nuclear reactor. [7]

Charlie Chong/ Fion Zhang

http://large.stanford.edu/courses/2013/ph241/waisberg1/

IFR

Charlie Chong/ Fion Zhang

Conclusion The Integral Fast Reactor is an example of a great reactor design that was abandoned in the past due to safety considerations. Today, however, cost rather than safety may be the current most significant factor for the drawback of the IFR. The costs of pyroelectric separation and secondary coolant loops as compared to uranium enrichment makes fast reactors more expensive than conventional thermal reactors. [5] Just the same, as natural uranium reserves are depleted and nuclear waste becomes a crucial environmental concern , IFR's superior efficiency and cleanness could cause them to resurge and fulfill their promise. ÂŠ Idel Waisberg. The author grants permission to copy, distribute and display this work in unaltered form, with attribution to the author, for noncommercial purposes only. All other rights, including commercial rights, are reserved to the author.

Charlie Chong/ Fion Zhang

http://large.stanford.edu/courses/2013/ph241/waisberg1/

Why Were Breeder Reactors Developed? The main fuel that is used in almost all nuclear reactors is uranium. It has a total of 6 isotopes from uranium-233 (U-233) to uranium-238 (U-238). All these isotopes are unstable, meaning, they will undergo radioactive decay over time and change their form. Typically, their decay-rate ranges from 70 years to 4.5 billion years. Because of such long decay times, uranium is considered to be mildly radioactive. Of the 6 isotopes of uranium, two are of importance in nuclear energy generation - U-235 and U-238. U-235 has been traditionally used in nuclear reactors, because unlike U-238, it is fissile in nature, and is therefore capable of sustaining a fission chain reaction. For years it has powered many nuclear reactors across the globe. However, of the total naturally occurring deposits of uranium in the world, U-235 constitutes only about 0.72%, and because of its increased usage in recent years, it has begun depleting fast.

Charlie Chong/ Fion Zhang

http://www.buzzle.com/articles/working-principle-of-a-breeder-reactor.html

On the other hand, U-238 constitutes almost 99.28% of the total uranium deposits. But the problem in using it is that it is non-fissile. Nuclear scientists realized that, if somehow U-238 could be used, it would be able to power reactors for hundreds of years. So they started looking for the means of making its use possible, until finally they found an answer in the form of breeder reactors. What follows here is the principle and working of breeder reactors. Charlie Chong/ Fion Zhang

http://www.buzzle.com/articles/working-principle-of-a-breeder-reactor.html

How Does a Breeder Reactor Work Nuclear scientists, upon experimentation, discovered that though U-238 isn't fissile, it is fertile. In atomic science, a fertile material is one which, though isn't fissionable by thermal neutrons, can be converted into one by being bombarded by neutrons (at a threshold energy?) , which subsequently leads to the transformation of its nucleus. This fact forms the basis of the working of a breeder reactor. When a neutron strikes a U-238 atom, it gets captured by its nucleus. This additional neutron, increases the atomic mass by a factor of one, and thus, U238 changes to the isotope U-239. The half-life period of U-239, that is the time taken by half the radioactive atoms in a sample to undergo decay, is about 23 minutes, after which it decays and changes form to neptunium-239, while releasing energy in the order of 1.29MeV. Nu-239, after another 2 - 3 days, further undergoes beta decay, finally forming into plutonium-239. The following diagram represents this process.

Charlie Chong/ Fion Zhang

http://www.buzzle.com/articles/working-principle-of-a-breeder-reactor.html

Uranium to Plutonium Conversion Process

Charlie Chong/ Fion Zhang

http://www.buzzle.com/articles/working-principle-of-a-breeder-reactor.html

In breeder reactors, the core is made up of plutonium Pu-239. It is encased by a layer of non fissionable uranium-238. Plutonium being fissile, undergoes spontaneous fission and releases neutrons. These neutrons are projected towards the surrounding layer of U-238. The uranium-238 atoms in the layer, capture these neutrons and undergo two beta decays, which change the structure of their nuclei, converting them to fissile plutonium-239. The newly formed Pu-239 atoms, again ejects more neutrons via fission. This process continues on until all the U-238 is converted to Pu-239. Once that is done, the reactor is refueled, and it can carry on working by producing more nuclear reactions. It is interesting to note that though originally only an x amount of fissile Pu-239 was added to the reactor, in the end, via the phenomenon of nuclei transformation, the reactor was able to 'breed' Pu-235 (Pu-239?) in multiples of that amount. The following is a graphical representation of this process. This process of Pu-239 generation produces a tremendous amount of heat. This heat is absorbed by different coolants running through the reactors, and is transported to heat exchangers. It is this heat which is collected by the heat exchangers, that is used to convert water to steam and drive the large turbines of electricity generators. Charlie Chong/ Fion Zhang

http://www.buzzle.com/articles/working-principle-of-a-breeder-reactor.html

Charlie Chong/ Fion Zhang

http://www.buzzle.com/articles/working-principle-of-a-breeder-reactor.html

Types of Breeder Reactors A breeder reactor is simply one which can use existent fissile material to convert non-fissionable matter into fissionable matter. As such, many different types of breeder reactors have surfaced over the years. However, the following are two of the most significant ones from among them. â&#x2013; Liquid Metal Fast Breeder Reactor (LMFBR) It is considered to be one of the most promising types of breeder reactors. In it, U-238 is converted to PU-239 through bombardment of fast neutrons, as described in the section above. The newly formed PU-239 atoms again eject neutrons, converting more U-238 atoms to P-239. This leads to a selfsustaining chain reaction. The heat that is released continuously during this process is absorbed by a liquid metal (sodium) coolant and transported further to be used in electricity generation.

Charlie Chong/ Fion Zhang

http://www.buzzle.com/articles/working-principle-of-a-breeder-reactor.html

â&#x2013; Thermal Breeder Reactor A thermal breeder reactors use thorium instead of uranium as its main fuel. In it, thorium is converted to uranium-233, which is fissionable. For this conversion to take place, thorium atoms have to be bombarded with neutrons that have been slowed down or thermalized using neutron moderators. Hence, the reactor is named thermal breeder reactor. The U-233 that is produced undergoes spontaneous fission, which starts a chain reaction producing a lot of energy in the form of heat. This energy is collected by water that gets turned into steam, which is used for the generation of electricity. It is estimated that the thorium deposit is three times more abundant than uranium deposit. Hence, it may one day serve as an alternative to uranium.

Charlie Chong/ Fion Zhang

http://www.buzzle.com/articles/working-principle-of-a-breeder-reactor.html

Drawbacks of Breeder Reactors 1) It is estimated that the cost of construction of a breeder reactor is twice that of conventional nuclear reactors. This was one of the main reasons cited for the cancellation of the Clinch river breeder reactor project. 2) Liquid sodium, which is used as a coolant in LMFBR, is very volatile when exposed to air or water. It reacts violently with both of these and produces hydrogen gas which is highly flammable. This can lead to a large-scale catastrophe in case of accidents. 3) Plutonium, which is generated in breeder reactors, is highly toxic and known to cause lung cancer in human beings. Also, its half life period is very long (24,100 years). Thus, its disposal is a serious problem. 4) Plutonium can also be easily used to make nuclear bombs. Hence, it poses a threat if it were to fall in the wrong hands.

Charlie Chong/ Fion Zhang

http://www.buzzle.com/articles/working-principle-of-a-breeder-reactor.html

Pu-239 Bomb

Charlie Chong/ Fion Zhang

The magnesium cases for the worldâ&#x20AC;&#x2122;s first three plutonium cores. Left: Herb Lehr at Trinity base camp with the Gadget core. Center: Luis Alvarez at Tinian with the Fat Man core. Right: The third coreâ&#x20AC;&#x2122;s case at Los Alamos, 1946.

Charlie Chong/ Fion Zhang

http://blog.nuclearsecrecy.com/tag/plutonium/

he magnesium box used for transporting the plutonium core to the Trinity site. Via Los Alamos.

Charlie Chong/ Fion Zhang

http://blog.nuclearsecrecy.com/tag/plutonium/

A mockup of the third coreâ&#x20AC;&#x2122;s experimental setup, August 21, 1945.

Charlie Chong/ Fion Zhang

http://blog.nuclearsecrecy.com/tag/plutonium/

Little Boy from Hansenâ&#x20AC;&#x2122;s 1988 U.S. Nuclear Weapons.

Charlie Chong/ Fion Zhang

Little Boy from Hansenâ&#x20AC;&#x2122;s 1988 U.S. Nuclear Weapons.

Charlie Chong/ Fion Zhang

Introduction to Nuclear Power Generation There are two main types of nuclear reactor, characterized by the speed of the neutrons which induce fission: Thermal reactors. These are the predominant kind, using slower neutrons to induce fission, the basic fissile nuclide being U-235. Fast breeder reactors. In these less-common reactors, the fast neutrons are used directly to create (breed) fissile nuclides from fissionable nuclides; most commonly Pu-239 is bred from U-238. Pu-239 is also used in nuclear weapons.

Charlie Chong/ Fion Zhang

There are many varieties of nuclear reactor, but all have the following common elements: Fuel: The material that undergoes fission. This neednâ&#x20AC;&#x2122;t have the fissionable nuclides in the form of the element. The fuel is often in the form of a ceramic. Cladding: This encases the nuclear fuel, isolating it mechanically and chemically from its immediate environment. Moderator: Necessary in thermal reactors to slow down the neutrons produced by the fission process. Commonly, the moderator is in the form of a rod, but can be in liquid form or even be mixed with the fuel itself. Control: This can be used to absorb excess neutrons, or even shut down the reactor in an emergency. Most often, the control material is in the form of a rod.

Charlie Chong/ Fion Zhang

Core: The heart of the reactor, containing the fuel. The fuel is encased in cladding, and core must also accommodate the coolant and allow for more moderating rods or control rods to be added. Coolant: The coolant removes heat from the reactor core into a heat exchanger. Note that the coolant itself is not cool, just that it removes heat from the core. Reactor vessel: This contains the reactor core and the coolant. It often also acts as a reflector, reducing the loss of neutrons to the outside environment. Generator/turbine: The heat generated by the reactor core generates steam, used to drive a turbine, which can generate electricity.

Charlie Chong/ Fion Zhang

Nuclear Reactor Simulation

Charlie Chong/ Fion Zhang

The types of reactor are loosely grouped into generations describing the time period in which they were first used. Advances in technology have led to new designs. The current generation of reactors can be defined by the materials used for each of these components. They include: ■ Pressurized Water Reactors (PWR), the most common reactor type, ■ Boiling Water Reactors (BWR), ■ CANDU or Pressurized Heavy Water Reactor (PHWR). These all include water as a coolant in some form. There are also: ■ Gas Cooled Reactors (GCR) and ■ Advanced Gas Cooled Reactors (AGR), which use CO2 as coolant. Finally, there are also: ■ Liquid Metal Fast Breeder Reactors (LMFBR), which are cooled by a liquid metal (sodium or lead). There are also many other forms of reactors used for research purposes.

Charlie Chong/ Fion Zhang

The next generation, commonly referred to as Generation IV, in some cases are just incremental improvements on these designs, but in other cases are radically different designs aimed at increasing efficiencies and reducing risk. The latter may demand materials which can sustain exposure to much more extreme environments.

Charlie Chong/ Fion Zhang

What is a Cross-Section? A cross-section quantifies the probability that a particle passing through a material will interact with the material. For example, a neutron absorption cross-section would quantify the probability that a neutron is absorbed as it travels through a material. The following equation is a definition of the nuclear cross section σ

σ = C/N·δx·I For neutrons passing through a plate of thickness δx (m), C is the number of events occurring per unit area (m−2), N is the number of nuclei per unit volume, or nuclear number density (m−3), and I is the number of neutrons passing through a unit area (m−2). As the behaviour depends on neutron energy, the cross-section must be specified for neutrons of a given energy (i.e.monoenergetic). The N·δx term is often grouped together, since when multiplied by σ it is equal to C/I, a dimensionless quantity that is the probability of a neutron interacting, i.e. the ratio of the number of events occurring per unit area to the number of neutrons travelling through that same area. Charlie Chong/ Fion Zhang

http://www.doitpoms.ac.uk/tlplib/nuclear_materials/printall.php

Microscopic Cross Section of Neutron Probability of interaction of neutron with 1 nuclei in unit volume?

σ = C/N·δx·I I

N C

Charlie Chong/ Fion Zhang

Types of Cross-Section Several different cross-sections will be mentioned in this TLP. Standard notation is used below, where (a,b) means an atomic interaction in which a is absorbed and b is emitted. Elastic scattering (n,n): the cross-section of a neutron undergoing elastic scattering by a nucleus The total kinetic energy of the neutron and the nucleus is the conserved. Any energy that the neutron loses is due to the nucleus recoiling after the neutron is scattered. Inelastic scattering (n,n'): a neutron is briefly absorbed by a nucleus, leaving it in an excited state. The nucleus can later return to its ground state, losing its excess energy as a gamma ray. Radiative capture (n,Îł): a neutron is absorbed by a nucleus, which gives out a gamma ray as a result.

Charlie Chong/ Fion Zhang

Fission (n,f): neutron causes a nucleus to split into fragments and more neutrons. Alpha decay (n,α): neutron causes a nucleus to lose two protons and two neutrons in the form of a helium nucleus. This interaction is important when considering the transmutation of elements, and how radioactivity is induced in a material. Virtually any possible interaction has its own specific cross section; the ones above are just some of the most common. Other important interactions include (n,p) and (n,2n). (n,n) (n,n’) (n,γ) (n,f) (n,α) (n,p) (n,2n)

Charlie Chong/ Fion Zhang

Neutron Cross Section Area In nuclear and particle physics, the concept of a neutron cross section is used to express the likelihood of interaction between an incident neutron and a target nucleus. In conjunction with the neutron flux, it enables the calculation of the reaction rate, for example to derive the thermal power of a nuclear power plant. The standard unit for measuring the cross section is the barn, which is equal to 10â&#x2C6;&#x2019;28 m2 or 10â&#x2C6;&#x2019;24 cm2. The larger neutron cross section, the more likely a neutron will react with the nucleus.

Charlie Chong/ Fion Zhang

An isotope (or nuclide) can be classified according to its neutron cross section and how it reacts to an incident neutron.  Radionuclides that tend to absorb a neutron and either decay or keep the neutron in its nucleus are neutron absorbers and will have a capture cross section for that reaction.  Isotopes that fission, are fissile fuels and have a corresponding fission cross section.  The remaining isotopes will simply scatter the neutron, and have a scatter cross section. Some isotopes, like uranium-238, have nonzero cross sections of all three. Isotopes with a large scatter cross section and have a low mass are good neutron moderators (see chart below). Nuclides which have a large absorption cross section are neutron poisons if they are neither fissile nor undergo decay. A poison that is purposely inserted into a nuclear reactor for controlling its reactivity in the long term and improve its shutdown margin is called a burnable poison.

Charlie Chong/ Fion Zhang

Cross-Section and Neutron Energy

Charlie Chong/ Fion Zhang

Graph showing neutron cross-section against neutron energy. [Adapted from graph by Napy1Kenobi CC[BY][SA], source data unknown] As the log-log graph above shows, cross-sections vary with neutron energy. Since most neutrons are in the thermal range (about 0.025 eV, or about 4 × 10−21 J), cross-sections are often quoted for this neutron energy. Even though cross-sections do vary with energy, nuclides still have characteristically "high" or "low" cross sections. For example, as the graph shows, 235U (n,γ) has a higher cross section than 233U (n,γ) over almost all energy ranges. The peaks in the graph are due to resonance effects. The reasons for these are beyond the scope of this TLP.

Charlie Chong/ Fion Zhang

The Macroscopic Cross-Section So far we have examined the microscopic cross-section. When talking about actual materials, the macroscopic cross-section is more commonly used. Each element present in a material has its own macroscopic cross-section (m−1) defined by the following equation, where N is the nuclear number density as used earlier (m−3). Σi=Niσi And for the material as a whole, its macroscopic cross-section is therefore: Σ=N1σ1+N2σ2+⋅⋅⋅+Niσi+⋅⋅⋅ The macroscopic cross-section is the probability that a neutron will undergo a reaction per unit path length travelled in the material.

Charlie Chong/ Fion Zhang

The probability that a neutron travels a distance x without interacting therefore is: exp(â&#x2C6;&#x2019;ÎŁx) And the neutron mean free path, i.e. the average distance a neutron travels before interacting, can be found by integrating over this quantity as follows:

Charlie Chong/ Fion Zhang

Interactive Graph of Macroscopic Cross Section (μ) Try out the graph below to see what effect mass (weight?) , density and microscopic cross-section have on the macroscopic cross-section. The nuclear number density is calculated by simply working out the number of nuclei present in the material given the molar mass and its density. This method makes the approximation that all the mass is present as nuclei, which is true to a reasonable degree of accuracy (electrons also have mass, but are only about 1/2000 the mass of a single nucleon and so do not contribute significantly). N = Ṅ·ρ/A N= Nuclei density, Ṅ = Avogadro's number, ρ = density, A=atomic weight

μ = (Ṅ·ρ/A) σ

where μ is the linear attenuation coefficient (cm-1 ) (Macroscopic cross section) ; p is the material density (gm/cm3); Ṅ is Avogadro's number (6.023 X 1023 atoms/gram-molecular weight) ; σ is the total cross section in barns (cm2 ) ; and A is the gram atomic weight of material.

Charlie Chong/ Fion Zhang

The graph is editable: double-click on a cell to edit the numbers given. The arrows along the xaxis show the mean free path of the neutron through the material.

Charlie Chong/ Fion Zhang

Cross-Section and Neutron Energy

Charlie Chong/ Fion Zhang

Mechanisms of Radiation Damage 1 Most of the radiation damage in a reactor is from the neutron flux being produced in the core. Other forms of radiation, such as gamma radiation, are very weakly interacting and donâ&#x20AC;&#x2122;t produce much effect. The principles in this section can in theory apply to any material, but the key materials are steels (e.g. a cold-worked 316 stainless steel).

Charlie Chong/ Fion Zhang

NEUTRON CROSS SECTIONS Cross Sections The microscopic cross section (σ) is a property of a given nuclide; σ is the probability per nucleus that a neutron in the beam will interact with the nucleus; this probability is expressed in terms of an equivalent area that the neutron "sees." The macroscopic cross section (Σ) takes into account the number of those nuclides present

[cm −1 ]

∑=Nσ

(1)

The mean free path is mfp = λ = 1/Σ. The microscopic cross section is measured in units of barns (b): 1 barn equals 10–24 cm2 = 10–28 m2. Cross Section Hierarchy Total Absorption Capture

Scattering

Fission

σ t = σ s + σ a = σ s + (σ c + σ f )

where σ c ≈ σ γ

(2)

Σ t = Σ s + Σ a = Σ s + (Σ c + Σ f ) For mixtures of isotopes and elements, the Σ's add. For example

Σ aH 2O = Σ aH + Σ Oa = N H σ aH + N O σ aO

(3)

= 2 N H 2O σ aH + N H 2O σ aO = N H 2O (2 σ aH + σ aO ) 1/v Law For very low neutron energies, many absorption cross sections are 1/v due to the fact the nuclear force between the target nucleus and the neutron has a longer time to interact 1 v

σa ∝ ∝

1 E

∝

(4)

Energy dependence of cross sections ⋅ σs is independent of thermal energy (and temperature) ⋅ σa (σf and σc) is energy dependent

σ a (E) v E0 T = 0 = = 0 σ a0 v( E ) E T

EEE460-Handout

(5)

K.E. Holbert

Transmutation – (n, α) – Production of Helium As seen in the previous section, there are several ways in which neutrons can interact with nuclei, including absorption of the neutron by the nucleus, making the nucleus unstable so that it decays, releasing an alpha particle in the process. Alpha particles consist of two protons and two neutrons, i.e. a 4He (42He2+) nucleus. Since they are 2+ positively charged, they are very highly ionizing, and will they quickly pick up electrons from the surrounding lattice and become elemental helium. In stainless steels, the (n, α) interaction does not occur often with iron itself, but is mostly as a result of the nickel content of the alloy, as the graph of its cross section below shows. The presence of helium in the metal causes embrittlement and can act as a nucleation point for voids, which can lead to swelling (cracking) . Additionally, the neutron flux can induce further radiation. This occurs when a neutron transmutes an element into a radioactive one. This is undesirable, because it creates more low-level radioactive waste to contain when the reactor is eventually decommissioned. Charlie Chong/ Fion Zhang

Activation cross sections for (n, p) reactions on nickel

Charlie Chong/ Fion Zhang

Frenkel Defects 弗仑克尔缺陷 There are many proposed mechanisms of radiation damage, but on a fundamental level a single neutron scattering event can be considered. If a neutron of sufficient energy scatters off a nucleus, the nucleus itself is displaced. The atom associated with the nucleus finds itself embedded into the structure elsewhere in a high-energy, interstitial site. It is termed a selfinterstitial as the matrix and interstitial atoms are in principle the same. The site the atom previously occupied is now empty: it is a vacancy. In this way, self interstitial-vacancy pairs are formed, and these are called Frenkel defects.

Charlie Chong/ Fion Zhang

Threshold Energy At lower energies, the neutron collision causes the nucleus to vibrate, but the nucleus is not displaced. The excess energy is dissipated through the lattice as heat. The threshold energy to form a Frenkel defect depends on the nuclei present and the structure of the material (e.g. the phase of iron). It is typically in the range 10~50 eV (2~8 Ă&#x2014; 10â&#x2C6;&#x2019;18 J). Note that when the neutron scatters off a nucleus, not all of its energy is transferred. This means that the minimum kinetic energy of the neutron is be larger than this threshold value, typically by a factor of 2~3. This threshold energy is commonly given the symbol Ed. It is the energy required to overcome the potential barrier to move from one lattice site to another, and is approximately twice Es, the energy of sublimation, since twice as many bonds are broken to move an atom within a lattice as removing it from its surface, plus a contribution of 4~5 Ec, where Ec is the energy loss by electron stopping (required to allow the lattice to relax after the atom has been displaced).

Charlie Chong/ Fion Zhang

Displacement Spikes Neutron scattering events are not isolated. On average, each displaced atom might then go on to displace further atoms, and likewise the neutron that caused the first displacement might go on to displace further atoms. This means that there is a local cascade of displacements, known as a displacement spike, within which there is a large amount of disorder in the structure. This is illustrated with a simulation, below:

Charlie Chong/ Fion Zhang

The Kinchin and Pease Model A neutron scattering from an atom imparts an energy Ep to it. This primary knock-on atom (PKA) with energy Ep then displaces other atoms, ultimately giving a displacement cascade if Ep is high enough. The number of atoms displaced by the PKA is difficult to calculate, but a simple model (attributed to Kinchin and Pease) can capture much of the basic physics. The assumptions are: the cascade is a sequence of two-body elastic hard-sphere collisions; a minimum energy transfer Ed is required for displacement; the maximum neutron energy available for transfer is the cut-off energy Ec, set by loss to the electrons (electron stopping); the atoms are randomly distributed, so that channelling and other effects of crystal structure are ignored.

Charlie Chong/ Fion Zhang

The average number of atoms displaced by a PKA of energy Ep is: 0 1 Ep/2Ed Ec/2Ed

Charlie Chong/ Fion Zhang

for Ep< Ed for Ed < Ep< 2Ed for 2Ed < Ep<Ec for Ep â&#x2030;¥Ec

Formation of Dislocation Loops Both the interstitial atoms and vacancies can diffuse through the lattice, but the interstitial atoms are more mobile. Both interstitials and vacancies are eventually removed from the lattice (when they reach sinks such as dislocations or grain boundaries). However, they are also always being generated by the neutron radiation. Thus steady-state populations of interstitials and vacancies are formed. There is a tendency for interstitial atoms and vacancies respectively to aggregate together into discs. This is again illustrated through an animation, below:

Charlie Chong/ Fion Zhang

Formation of Dislocation Loops

Charlie Chong/ Fion Zhang

When there is a sufficient supersaturation of vacancies, the disc of vacancies grows and the gap between the planes on either side collapses to form a continuous lattice with a dislocation loop. Since the Burgers vector is normal to the plane loop, it is an edge dislocation and grows/shrinks by climb and moves by glide along a prism; it is termed a prismatic loop.

Charlie Chong/ Fion Zhang

Dislocation Loop

Charlie Chong/ Fion Zhang

http://www.tf.uni-kiel.de/matwis/amat/def_en/kap_6/illustr/i6_3_6.html

Dislocation Loop

Charlie Chong/ Fion Zhang

http://www.tf.uni-kiel.de/matwis/amat/def_en/kap_6/illustr/i6_3_6.html

Dislocation Loop

Charlie Chong/ Fion Zhang

http://www.tf.uni-kiel.de/matwis/amat/def_en/kap_6/illustr/i6_3_6.html

Dislocation Loop

Charlie Chong/ Fion Zhang

Dislocation Loop

Charlie Chong/ Fion Zhang

Dislocation Loop

Charlie Chong/ Fion Zhang

Dislocation Loop

Charlie Chong/ Fion Zhang

http://www.globalsino.com/EM/page3454.html

Dislocation Loop Defect-clusters (dislocation loops) behavior by electron microscope irradiation.

Charlie Chong/ Fion Zhang

http://nsec.jaea.go.jp/fme/en/group5/group6_i2-2.htm

Type of Dislocations a) Interstitial impurity atom, b) Edge dislocation, c) Self interstitial atom, d) Vacancy, e) Precipitate of impurity atoms, f) Vacancy type dislocation loop, g) Interstitial type dislocation loop, h) Substitutional impurity atom

Charlie Chong/ Fion Zhang

http://www.tf.uni-kiel.de/matwis/amat/def_en/overview_main.html

Dislocation Loop Insert007

Charlie Chong/ Fion Zhang

Nucleation and Growth of Voids Vacancy dislocation loops should reduce the volume of the material whilst interstitial dislocation loops should increase it, as seen in the animation above. And, in general, we expect compensating vacancy and interstitial effects to leave the material with approximately the same volume. However, irradiated materials are in fact observed to swell. To explain this, we consider what happens when vacancy loops join together. In practice, when the loops join they form three dimensional cavities a few nm in diameter. These voids contribute no net change in volume to the material, and so this just leaves the interstitial loops, which do lead to swelling in the material. In the absence of any driving force, it would seem unlikely that enough voids would form for any appreciable effect to be observed on the material. This is where the transmutation of nickel becomes important, since the helium atoms produced are very small and are thus extremely mobile as interstitial atoms in the lattice. They quickly form bubbles, and these helium bubbles can act as nucleation points for void formation. Charlie Chong/ Fion Zhang

Effects of Radiation Damage The previously discussed changes in microstructure due to radiation damage affect the macroscopic, mechanical properties of the material. These effects happen for a variety of reasons, but are generally less noticeable at higher temperatures as the damage caused by radiation is constantly being annealed out: at higher temperatures vacancy and interstitial mobility are increased so they are removed from the lattice faster. The following table gives an overview of the effects observed.

Charlie Chong/ Fion Zhang

A stress-strain curve for a stainless steel irradiated or not.

Charlie Chong/ Fion Zhang

Fuel and Cladding Choice of Fuel There are several important factors when choosing a nuclear fuel:  The fuel itself must be easily fissionable, preferably fissile.  The fuel must release sufficient quantities of neutrons per neutron captured to be able to sustain a fission chain reaction. Too many neutrons produced and a runaway, supercritical, reaction would occur which would be disastrous in the case of a nuclear reactor. The ratio of neutrons produced to neutrons absorbed can, however, be adjusted through use of control rods and moderators.  The fuel must have a sufficiently long half-life. Fissile materials, by their very nature due to their instability, are radioactive. Radioactive materials decay exponentially, and this decay is quantified by their half-life, the time it takes for half of the radioactive nuclei present to decay into a more stable form. Nuclear fuels must therefore have a sufficiently long half-life, otherwise the nuclei would decay into a useless form before fission could be induced in a controlled manner.

Charlie Chong/ Fion Zhang

http://www.doitpoms.ac.uk/tlplib/nuclear_materials/printall.php

ď Ž Economic factors are also important. The fuels must be abundant and readily available. Uranium is the only naturally abundant fissile material and exists in an ore, called uraninite (also known as pitchblende), which is primarily uranium (IV) oxide, mined primarily in Canada, Australia and Kazakhstan. It has an isotopic composition of 99.3% of the fissionable but not fissile 238U and just 0.7% of the fissile 235U. This means that it must first be enriched, a difficult and expensive process which raises the proportion of 235U to 238U. ď Ž No plutonium occurs naturally, except in trace amounts as a result of the natural decay of uranium. It is instead made as a by-product in nuclear reactors, and must first be extracted from use nuclear fuels before it can be used.

Charlie Chong/ Fion Zhang

http://www.doitpoms.ac.uk/tlplib/nuclear_materials/printall.php

ď Ž Political concerns are important; heavily enriched uranium and plutonium can be used for atomic weaponry and so are not favoured. This is why there is current interested in the thorium cycle, which produces 233U. Though this can in theory be used in atomic weaponry, it is always contaminated with 232 U, which is highly dangerous because of the amount of gamma radiation it emits and making it very difficult to handle. Before the 233U could be used as a weapon, the 232U would have to be removed, which is again very difficult. This inherent proliferation resistance, and thoriumâ&#x20AC;&#x2122;s natural abundance (3~4 x as abundant as uranium), has increased interest in it in recent years

Charlie Chong/ Fion Zhang

http://www.doitpoms.ac.uk/tlplib/nuclear_materials/printall.php

ď Ž The common fission fragments formed are also important, both in the short-term due to the effects they have on structural materials in the reactor and also in the long-term since some fragments have very long half-lives and so will present problems as nuclear waste since it will need to be stored for much longer periods of time. It should be noted, however, that the longer the half-life of the fission product the less dangerous it is to people. This is a somewhat counter-intuitive point that is often missed, since a longer half-life means that less of the material decays and hence gives off dangerous radiation in a given period.

Charlie Chong/ Fion Zhang

http://www.doitpoms.ac.uk/tlplib/nuclear_materials/printall.php

Form of Fuel Metallic uranium is not favoured as a fuel since it is dimensionally unstable under irradiation, flammable, can readily corrode in oxygen-containing atmospheres, and can produce uranium dust which has a low-temperature flash-point and which can cause serious health problems if inhaled. There is some interest in using metallic U alloys as fuel when a particularly high density of fissile or fissionable nuclides is required. As an alternative, ceramic forms can be used, including UO2, U3O8, UC, U2C3, UN, U3Si and USi. The most common of these is uranium dioxide, which has the calcium fluorite structure shown in the image below.

Charlie Chong/ Fion Zhang

http://www.doitpoms.ac.uk/tlplib/nuclear_materials/printall.php

Uranium Fuel Cell

Charlie Chong/ Fion Zhang

http://geoinfo.nmt.edu/resources/uranium/power.html

Uranium Fuel Cell

Charlie Chong/ Fion Zhang

http://lobby.la.psu.edu/066_Nuclear_Repository/Agency_Activities/DOE/DOE_Spent_Nuclear_Fuel.htm

Uranium Fuel Cell

Charlie Chong/ Fion Zhang

http://www.world-nuclear.org/information-library/nuclear-fuel-cycle/conversion-enrichment-and-fabrication/fuel-fabrication.aspx

Uranium Fuel Cell Fuel bundles employed in Canadian reactors are made of a metal extremely resistant to corrosion and heat called Zircaloy in which uranium ceramic pellets are inserted.

Charlie Chong/ Fion Zhang

http://nuclearsafety.gc.ca/cnsconline/fuel/eng/index.cfm

Choice of Cladding The nuclear fuel cannot be allowed to make direct contact with the coolant inside the reactor vessel, due to the potential for radioactivity to be released into the environment. Instead, cladding has to be used to surround the fuel. Key design criteria are that the cladding should:  be transparent to neutrons, so that it doesn’t absorb neutrons that could be used to induce further fission.  have a high thermal conductivity, and not have a high thermal expansion coefficient. Key problems include:  hydrogen embrittlement due to (n, p) reactions inside cladding.  swelling due to release of fission product gases. Common choices for cladding material are stainless steel (in FBRs) Zircaloy (in PWRs) and, in the past, Magnox.

Charlie Chong/ Fion Zhang

http://www.doitpoms.ac.uk/tlplib/nuclear_materials/printall.php

Magnox is an alloy—mainly of magnesium with small amounts of aluminium and other metals—used in cladding unenriched uranium metal fuel with a non-oxidising covering to contain fission products in nuclear reactors. Magnox is short for Magnesium non-oxidising. This material has the advantage of a low neutron capture cross section, but has two major disadvantages:  It limits the maximum temperature (to about 360 Celsius), and hence the thermal efficiency, of the plant.  It reacts with water, preventing long-term storage of spent fuel under water in spent fuel pools. The magnox alloy Al80 has a composition of 0.8% aluminium and 0.004% beryllium.

Charlie Chong/ Fion Zhang

http://www.doitpoms.ac.uk/tlplib/nuclear_materials/printall.php

CaFl2

Charlie Chong/ Fion Zhang

http://www.doitpoms.ac.uk/tlplib/nuclear_materials/printall.php

Moderators A moderator is designed to slow down fast neutrons such that they are more easily absorbed by fissile nuclei. There are two main factors in choosing a moderator: 1. The moderator must not absorb neutrons itself. This means it should have a relatively low neutron absorption cross-section. 2. The moderator should efficiently slow down the neutrons. Modelling neutron-nuclei collisions as a classical elastic collision, in much the same way as gas molecules are modelled, gives the result that the closer the nucleusâ&#x20AC;&#x2122; mass is to that of the neutron, the more energy will be transferred in the collision. This means that lighter elements are favoured.

Charlie Chong/ Fion Zhang

http://www.doitpoms.ac.uk/tlplib/nuclear_materials/printall.php

The following equation shows the fractional energy lost per collision, Îž, on average for a neutron colliding with a nuclide of mass A. E0 is the initial energy of the neutron, and Es is the energy after scattering has occurred.

Charlie Chong/ Fion Zhang

http://www.doitpoms.ac.uk/tlplib/nuclear_materials/printall.php

It is beyond the scope of this TLP to derive this equation, but the basic physics is straightforward. In elastic collisions kinetic energy and momentum are conserved and the energy lost by the neutron can be calculated for any given angle of contact. In three dimensions it is necessary to integrate over all possible angles to obtain an average. The equation is well approximated by:

Charlie Chong/ Fion Zhang

http://www.doitpoms.ac.uk/tlplib/nuclear_materials/printall.php

This is good enough for most purposes. Since this is a classical derivation applied to a quantum situation, there is probably more error due to the original assumptions than this mathematical approximation. Try out the interactive Flash movie below to see this effect in action. The movie obeys the same physics used to derive the above equations, except in a two-dimensional rather than three-dimensional case. The simulation is meant to show energy lost per collision, and does not give an accurate impression of how often these collisions occur: interatomic distances have been greatly reduced for illustrative purposes. In practice it is the scattering cross-section which determines the rate of neutron collisions.

Charlie Chong/ Fion Zhang

http://www.doitpoms.ac.uk/tlplib/nuclear_materials/printall.php

Insert010-Choice of Moderator

Charlie Chong/ Fion Zhang

http://www.doitpoms.ac.uk/tlplib/nuclear_materials/printall.php

Finally, the above analysis can be modified with respect to the neutron crosssections, by considering the ratio ξ (Σs / Σa). This weights ξ with the absorption and scattering cross-sections. The higher this ratio, the more appropriate the material is as a moderator.

Charlie Chong/ Fion Zhang

http://www.doitpoms.ac.uk/tlplib/nuclear_materials/printall.php

Graphite Historically, graphite has been a very popular neutron moderator, and is used in the majority of British reactors. However, the graphite used has to be highly pure to be effective. Graphite can be manufactured artificially using boron electrodes, and even a small amount of contamination from these electrodes can make the graphite unsuitable as a moderator since boron is a highly effective neutron absorber, and so it â&#x20AC;&#x153;poisonsâ&#x20AC;? the graphite by increasing the overall absorption cross section, ÎŁa. It also has unique problems: it stores energy in metastable local defects when it is irradiated, particularly at lower temperatures. This so-called Wigner energy can be released suddenly when the graphite spontaneously returns to its stable phase, and this sudden rise in temperature is not desirable since it can cause further structural damage within the reactor. This means that graphite has to be annealed to remove the excess energy in its lattice in a controlled manner. The following Flash movie shows three-dimensional models of the graphite lattice and demonstrates the origins of this metastable phase within the graphite lattice.

Charlie Chong/ Fion Zhang

http://www.doitpoms.ac.uk/tlplib/nuclear_materials/printall.php

Insert011

Charlie Chong/ Fion Zhang

http://www.doitpoms.ac.uk/tlplib/nuclear_materials/printall.php

Other common choices: Light Water Hydrogen is a good candidate for a neutron moderator because its mass is almost identical to that of the incident neutron, and so a single collision will reduce the speed of the neutron substantially. However, hydrogen also has a relatively high neutron absorption cross-section due to its tendency to form deuterium, and so light water is only suitable for enriched fuels which allow for a higher proportion of fast neutrons. Heavy Water Heavy water has similar benefits to light water, but because its water molecules already have deuterium atoms it has a low absorption cross section. Additionally, because of the high energy of the fast neutrons, an additional neutron might be knocked out of the deuterium atom when a collision occurs, thus increasing the number of neutrons present. The main disadvantage of heavy water as a moderator is its high price.

Charlie Chong/ Fion Zhang

http://www.doitpoms.ac.uk/tlplib/nuclear_materials/printall.php

Beryllium Beryllium-9 is favoured, because in addition to being a light element, on collision with a fast neutron, it can react as follows: 9Be + n â&#x2020;&#x2019; 8Be + 2n The main problems with beryllium are its brittleness as a metallic phase and its toxicity, which make it less favoured as a moderator than the other materials mentioned here. Lithium Fluoride Lithium fluoride is commonly used in molten salt reactors. It is mixed with the molten metal and the fuel, and so its structural properties as a solid are not important.

Charlie Chong/ Fion Zhang

http://www.doitpoms.ac.uk/tlplib/nuclear_materials/printall.php

Summary In this TLP, the process of nuclear fission has been described, thus explaining the common choices for nuclear fuel used commercially. Materials selection for the major components of a nuclear reactor have also been explored, including:  Moderators, and how they work best when they consist of light nuclides with relatively low absorption cross-sections.  Control rods, which require high absorption cross-sections, and how the same nuclides found in control rods, e.g. boron, can act as poisons significantly reducing the efficiency of a reactor if found elsewhere, such as in moderators.  Cladding, which experiences much stronger radiation fluxes and extremes of temperature than any other structural material in the reactor, and so must be able to withstand these conditions.

Charlie Chong/ Fion Zhang

http://www.doitpoms.ac.uk/tlplib/nuclear_materials/printall.php

Concepts such as neutron cross-section and neutron flux have been explained, and this allowed mechanisms of radiation damage inside structural steels, and the consequences of this, to be discussed. Radiation materials science is a mature field, but there any many challenges for materials to permit more efficient operation, improve safety and reliability and reduce costs. As this TLP has shown, the basic mechanisms of damage caused by low levels of radiation are now well understood, but the much higher levels of radiation such as those that will be experienced in the new experimental fusion reactor, ITER, have yet to be satisfactorily contained. This TLP has given only an introduction to some of the important phenomena. To learn more, consult the Going Further section.

Charlie Chong/ Fion Zhang

http://www.doitpoms.ac.uk/tlplib/nuclear_materials/printall.php

Charlie Chong/ Fion Zhang

http://www.doitpoms.ac.uk/tlplib/nuclear_materials/printall.php

Quick questions You should be able to answer these questions without too much difficulty after studying this TLP. If not, then you should go through it again! Q1. Check which elements are fissionable but not fissile: A U-233 B U-235 C U-238 D Pu-239 E Th-232 Q2. Which of the following are NOT suitable moderating materials? A Deuterium (A=2) B Helium (A=4) C Beryllium-9 (A=9) D Boron (A=11) E Graphite (A=12)fIron (A=56)

Charlie Chong/ Fion Zhang

http://www.doitpoms.ac.uk/tlplib/nuclear_materials/printall.php

Q3. Which of the following would NOT be classified as “Absorption" cross sections? A (n, n) B (n, n') C (n, γ) D (n, f) E (n, α) F (n, p) Q4. Which of the following discourages void formation? A More interstitial atoms. B Fewer interstitial atoms. C More vacancies. D Fewer vacancies. E More transmutation. F Less transmutation. Standard answer for Q4 is “E”

Charlie Chong/ Fion Zhang

http://www.doitpoms.ac.uk/tlplib/nuclear_materials/printall.php

Q5. Which of the following material properties have lower values after irradiation? A Yield strength B Thermal conductivity C Electrical conductivity D Tensile strength E DuctilityfDensitygCreep rate

Charlie Chong/ Fion Zhang

http://www.doitpoms.ac.uk/tlplib/nuclear_materials/printall.php

Deeper questions The following questions require some thought and reaching the answer may require you to think beyond the contents of this TLP. Zirconium minerals are often found with small amounts of hafnium present due to their chemically similar nature. Zirconium is also used as a primary component of Zircaloy, a cladding material designed to be almost transparent to neutrons. By comparing how the mean free path of a thermal neutron in pure zirconium differs from that of zirconium with 0.01% hafnium impurities, comment on the consequences of hafnium impurities in Zircaloy. Data: Zr: A = 91.22, ρ = 6.52 g cm−1, σc = 0.18 barns; Hf: A = 178.49, ρ = 13.31 g cm−1, σc = 105 barns

Charlie Chong/ Fion Zhang

http://www.doitpoms.ac.uk/tlplib/nuclear_materials/printall.php

Neutron scattering IOP Institute of Physics

Charlie Chong/ Fion Zhang

Neutron scattering is routinely used in modern science to understand material properties on the atomic scale. Originally developed as a tool for physics, the method has led to advances in many areas of science, from clean energy and the environment, pharmaceuticals and healthcare, through to nanotechnology,materials engineering, fundamental physics and IT.

Charlie Chong/ Fion Zhang

What is neutron scattering? The goal of modern materials science is to understand the factors that determine the properties of matter on the atomic scale, and then to use this knowledge to optimise those properties or to develop new materials and functionality. This process regularly involves the discovery of fascinating new physics, which itself may lead to previously unthought-of capabilities. Almost all of the major changes in our society, from the dramatic growth of computing and the internet to the steady increase in average life span, have their origin in our understanding and exploitation of the physics and chemistry of materials. To investigate atomic-scale structure and dynamics, scientists use a variety of tools and techniques, often based on the scattering of beams of particles. An â&#x20AC;&#x153;idealâ&#x20AC;? probe might be one that has a wavelength similar to the spacing between atoms, in order to study structure with atomic resolution, and an energy similar to that of atoms in materials in order to study their dynamics.

Charlie Chong/ Fion Zhang

It would have no charge, to avoid strong scattering by charges on the electrons or the nucleus and allow deep penetration into materials. It would be scattered to a similar extent by both light and heavy atoms and have a suitable magnetic moment so that we can also easily study magnetism. The scattering cross-section would be precisely measurable on an absolute scale, to facilitate comparison with theory and computer modelling. This particle exists – it is the neutron. Unfortunately, it is difficult to produce high-intensity beams of neutrons – which are normally only found strongly bound to protons in the nuclei of atoms. This can be done by fission in a nuclear reactor, where the release of neutrons is the fundamental process that produces heat. A research reactor, such as that at the Institut Laue-Langevin (ILL) in Grenoble, France, is optimised to produce bright beams of neutrons. Another way to produce intense neutron beams is using an accelerator-based source, such as the ISIS facility near Oxford in the UK, where a highenergy beam of protons releases neutrons from tungsten nuclei in a process known as spallation. Both research reactors and “spallation sources” are large and expensive facilities, so there are relatively few in the world. The UK is fortunate in having access to the world’s best in each class – ILL and ISIS. Charlie Chong/ Fion Zhang

Institut Laue-Langevin

Charlie Chong/ Fion Zhang

The science Neutron scattering provides information that is highly complementary to that from other microscopic scattering techniques, such as those using photons (from visible light to synchrotron X-rays) or electrons (microscopy and diffraction), as well as to standard laboratory measurements. In modern materials science, it is normally the case that a variety of techniques are required to tackle any particular problem. The ILL neutron beams are continuous, whereas those at ISIS are produced in short bright pulses 50 times a second, allowing different optimisation. Each facility operates some 30 separate experimental stations (â&#x20AC;&#x153;instrumentsâ&#x20AC;?), which are individually tailored for a particular type of measurement and range of scientific applications.

Charlie Chong/ Fion Zhang

Many neutron scientists use instruments at both ILL and ISIS and there is a large amount of knowledge sharing between the facilities, with advances in techniques and instrumentation benefiting both laboratories. This knowledge sharing extends across the international community, with smaller neutron sources providing input to the building of instruments and various national governments providing funding and expertise to particular projects. The network of European sources is complemented by international facilities at the Japanese Proton Accelerator Research Complex, and Oak Ridge National Laboratory, US. Neutron scattering in the UK started at the Dido and Pluto reactors at the Harwell Laboratory in the 1960s. The ILL, jointly owned by France, Germany and the UK, has 10 additional scientific member countries and began operation in the early 1970s. It has had several modernisation programmes, developing new neutron infrastructure and introducing new instrument concepts. The ILLâ&#x20AC;&#x2122;s Millennium Programme continues to refresh its neutron guide infrastructure and instrument suite, increasing its effective performance overall by a factor of 20 since the year 2000.

Charlie Chong/ Fion Zhang

Japanese Proton Accelerator Research Complex

Charlie Chong/ Fion Zhang

Japanese Proton Accelerator Research Complex

Charlie Chong/ Fion Zhang

Japanese Proton Accelerator Research Complex

Charlie Chong/ Fion Zhang

ISIS is located at the Rutherford Appleton Laboratory, and is owned by the Science and Technology Facilities Council. It began operations in 1984 and has attracted scientific partnerships from across the globe. In 2009 it expanded its scientific capability and capacity for experiments by adding a second target station with additional instruments designed to study soft matter, advanced materials and bioscience. The second phase of this project aims to further increase the range of capabilities in soft matter, life sciences, neutron imaging and microchip irradiation testing. Every year more than 1000 experiments are completed at ILL and ISIS, covering a wide range of research ranging from clean energy and the environment, pharmaceuticals and healthcare, through to nanotechnology, materials engineering, fundamental physics and IT. These facilities serve an international research community of more than 4000 scientists and each produces more than 500 research publications annually. A future European Spallation Source, to start operation in the 2020s, is currently being planned by several European countries. The site for construction has been chosen as Lund in Sweden.

Charlie Chong/ Fion Zhang

Applications Neutron scattering is used in many different scientific fields. Neutrons can be used to study the dynamics of chemical reactions at interfaces for chemical and biochemical engineering, in food science, drug synthesis and healthcare. Neutrons can probe deep into solid objects such as turbineblades, gas pipelines and welds to give microscopic insight into the strains and stresses that affect the operational lifetimes of crucial engineering components. Neutron studies of nanoparticles, low-dimensional systems and magnetism are used for the development of next-generation computer and IT technology, data storage, sensors and superconducting materials. Neutron scattering is a delicate and non-destructive measurement technique, making it ideal for use in heritage science.

Charlie Chong/ Fion Zhang

■ Understanding magnetism The neutron is capable of seeing both the nuclei of atoms and at the same time the magnetic interactions of their electrons. Neutron scattering has made seminal contributions to our understanding of magnetism – from the early demonstration of anti-ferromagnetism in simple systems through to the complex magnetic structures found in hard magnets or the synthetic multilayer structures used for data-storage applications. ■ Investigating polymers Neutrons have been used to investigate polymers since the early 1970s. Originally, neutron research unveiled the structure and formation of polymersto understand how they assembled and bonded. Now neutron science is studying the dynamics of thin polymer films, further increasing their range of applications into areas such as anti-reflective coatings and timerelease medications. The significant difference in the neutron scattering cross- section between hydrogen and deuterium allows selective “labelling” of chemically specific parts of complex molecular systems, giving a unique insight; this powerful technique is used for almost all soft-matter studies.

Charlie Chong/ Fion Zhang

â&#x2013; Revealing invisible worlds Neutron diffraction has been used to reveal the molecular structure of both crystalline and disordered materials since the early days of the discipline. Powerful computational modelling applied to neutron data allows accurate structures of pharmaceutical compounds to be derived, material structures in fuel-cell and battery electrodes to be optimised, and the orientation and packing of molecules in liquids and glasses to be understood. When materials bend, break or disintegrate it is their atomic structure that changes. Neutrons are used in a wide range of engineering applications to test the strength and suitability of materials under certain conditions, from studying the performance under strain of materials in aeroplane wings or train wheels to safely extending the operating life of nuclear power stations.

Charlie Chong/ Fion Zhang

â&#x2013; Biophysics: neutrons and the body A real understanding of the essential processes of life requires knowledge of how proteins and other macromolecules perform their roles. Techniques first used to investigate physics-based problems at ILL and ISIS have now been harnessed to identify water organisation in proteins and other biological systems. This is giving new insight into the way drugs and medicines move through the body and how they can be controlled and delivered to the specific area of concern. Neutron science continues to break new ground in investigating how drug-delivering polymers can move through membranes, how antibodies are structured and how active parts of medicines interact with lipids and proteins.

Charlie Chong/ Fion Zhang

â&#x2013; Unlocking the potential of hydrogen Hydrogen has the largest scattering interaction with neutrons of all the elements in the periodic table. Early experiments showing how hydrogen diffuses in simple metals have been built on to provide data supporting the development of materials for fuel cells and hydrogen storage. Hydrogen has been identified as a fuel with great potential for providing clean energy for transport, but its use is constrained by our inability to store it in a dense enough form suitable for vehicles. Neutron studies, currently being undertaken, will facilitate the understanding and development of materials that can store hydrogen safely and efficiently.

Charlie Chong/ Fion Zhang

â&#x2013; Unveiling our heritage The delicate, sensitive and deeply penetrating nature of neutron beams enables heritage scientists to determine unique information from historic objects, museum artefacts or geological fossils with no risk to their value or integrity. Adapting techniques from crystallography and engineering, analysis of crystal structures in ceramic or pottery fragments can determine the period and region of manufacture and reveal ancient trade routes, while texture analysis of metal objects can identify manufacturing techniques and forgeries.

Charlie Chong/ Fion Zhang

Impacts Around half of all experiments at both ILL and ISIS have direct connections with industry, often through partnerships with university groups. Neutron scattering can be used to address theglobal challenges facing society, and to make developments that have immediate or long-term economic impact. Such applied research is built on a foundation of fundamental investigations and techniques developed over the last 30 years, so it is crucial to continue such basic work to underpin the theories and technologies of tomorrow.

Charlie Chong/ Fion Zhang

Neutron experiments have provided definitive data to the chemical industry, which has enabled process optimisation and the saving of millions of pounds in energy costs and improvedthe environment by reducing waste effluent. Materials testing data have given aerospace companies confidence in new alloy compositions and manufacturing techniques. Health-based research has obtained key datasets required in preparation for clinical trials or to understand why certain drug treatments can be more successful than others. Neutron facilities have unique requirements for advanced components and equipment that can challenge suppliers to innovate and develop new technologies. Partnerships with large research centres can support small businesses by giving them the security and confidence to embrace new areas of activity. UK businesses have benefited from work with neutron-scattering centres through technology development and knowledge transfer, which has led to substantial overseas exports.

Charlie Chong/ Fion Zhang

Future developments Neutron-scattering developments have not, historically, been driven byadvances in source capability. Indeed, reactor-source brightness has increased by less than an order of magnitude since the 1950s. But instrumentation â&#x20AC;&#x201C; large position-sensitive detectors, focusing optics, innovative exploitation of neutron polarisation â&#x20AC;&#x201C; has produced enormous improvements in capability. The more recent development of acceleratorriven pulsed neutron sources has further stimulated advances in instrument design, which have now been implemented at continuous sources. This has fuelled an expansion of the field, originally focused on condensed matter physics, into materials science, soft matter and biomolecular systems, engineering, earth sciences, archaeology and the arts.

Charlie Chong/ Fion Zhang

The simplicity of the neutron interaction, and the fact that it can be measured on an absolute scale with high accuracy, gives an easy and direct link totheory and computer modelling. In future it will be the norm for neutron experiments to be coupled with advanced computation. The high scattering cross-section for light atoms means that neutrons are well suited to study many of the important topics in modern energy research â&#x20AC;&#x201C; such as hydrogen storage, fuel cells and lithium-ion batteries. The performance of all of these materials and devices intrinsically depends on the motion of atoms (dynamics) and the structural changes this causes; neutrons are able to measure both aspects. They are also important in studying the tailored self-assembly processes that will be needed to improve the efficiency of organic photovoltaics. It is therefore clear that the use of neutron scattering in energy research, already significant, will continue to grow, with an increasing proportion of in situ measurements in real operating devices.

Charlie Chong/ Fion Zhang

Fundamental studies of magnetism will remain a core topic of neutron scattering. For example, our understanding of correlated electrons, with its links to important practical applications such as magnetoresistance and superconductivity, is still in its infancy. The optimisation of devices increasingly makes use of thin films and nano-structuring, putting increased demands on instrumentation to make accurate measurements on ever smaller samples. The unique information that can be obtained by isotopic labelling in soft matter and biology and health studies provides a powerful incentive to use these techniques, but the sample volumes required are relatively large and remain a hindrance to greater exploitation. A future drive will be to further develop neutron optics, possibly in combination with more powerful sources, to reduce sample volumes. The use of neutron reflectivity, where sample volumes are intrinsically small, to study, for example, biological membranes, is increasing rapidly. There is also an increasing demand to study kinetics, for example, processes and processing, which further drives technique development.

Charlie Chong/ Fion Zhang

Neutrons can also be used at a more macroscopic scale â&#x20AC;&#x201C; for example in radiographic and tomographic imaging. Neutronscan easily penetrate large objects, giving a picture that is effectively opposite (and hence complementary) to that provided by X-rays. Future development will include the use of neutron energy selective imaging, which allows distinction between different elements or even different crystallographic phases and textures â&#x20AC;&#x201C; which can be extremely important in determining, for example, the strength of engineering materials. This can be combined with neutron diffraction to provide a three-dimensional map of residual stress, which in turn can be related back to the materialsâ&#x20AC;&#x2122; properties. Potential applications range from improved gears for higher-power wind turbines to the advanced technology needed for Formula 1 motor racing.

Charlie Chong/ Fion Zhang

Key facts and figures ■ Neutron sources are a powerful creative hub for science and technology,bringing together a range of research disciplines from physics to earth science, medicine and engineering to solve current research problems and generate new research projects. ■ There are around 15 neutron sources operating worldwide with significant capabilities for materials science research. ■ The UK has the largest community of neutron-scattering expertise in the world, closely followed by Germany and France. In total, Europe has more than 4000 neutron users. ■ More than 1000 experiments are completed each year at ILL and ISIS, and each facility produces more than 500 research publications annually.

Charlie Chong/ Fion Zhang

â&#x2013; The US and Japan have recently built billion-dollar neutron sources to help develop their neutron-scattering expertise and user communities, and to gain parity with Europe. â&#x2013; Industry use of neutron sources for productdevelopment, process optimisation and quality control can generate significant financial savings, export opportunities and reduce environmental impact. â&#x2013; Neutron sources support a wide range of basic and applied research. Basic research today is rapidly moved to underpin the technologies of tomorrow.

Charlie Chong/ Fion Zhang

Timeline ■ 1911 Ernest Rutherford develops the atomic model in which the nucleus carries most of the mass of the atom but occupies a very small part of its volume. ■ 1923 Prince Louis-Victor P R de Broglie proposes that particles with mass may also show wave-like properties, now referred to as the de Broglie wavelength of a moving particle. ■ 1932 James Chadwick discovers the neutron at the University of Cambridge. He receives the Nobel Prize in Physics in 1935 for discovering this missing part of the atom. ■ 1938 Enrico Fermi receives the Nobel Prize in Physics for his work investigating the atomic scattering and absorption cross-sections of slow and thermal neutrons.

Charlie Chong/ Fion Zhang

■ 1946 Ernest O Wollan and Clifford G Shull, using the Graphite Reactor at Oak Ridge National Laboratory, US, establish the basic principles of the neutron diffraction technique. They prove the existence of antiferromagnetism, as predicted by Louis Néel who won the Nobel Prize in Physics in 1970. ■ 1955 The first measurements of phonons from a prototype triple-axis spectrometer built by Bertram N Brockhouse confirm the quantum theory of solids. ■ 1956 The Dido research reactor comes online at the Harwell Laboratory. It is the first reactor in the UK devoted to materials research and is instrumental in developing the use of neutron beams by university researchers. ■ 1972 ZING-P and ZING-P’ pulsed spallation neutron source concepts are demonstrated by Jack Carpenter at Argonne National Laboratory, US.

Charlie Chong/ Fion Zhang

■ 1972 The Institut Laue-Langevin (ILL) in Grenoble, France, one of the most intense thermal neutron sources in the world, comes into operation. It pioneers the use of neutron optics (guides) to substantially increase the experimental capacity of a neutron source and operation as a “user facility”. ■ 1974 Small-angle neutron scattering shows that polymer chains in the liquid state have a random coil conformation, as predicted by Paul J Flory who wins the Nobel Prize in Chemistry for his fundamental achievements in understanding macromolecules. ■ 1984 The ISIS pulsed spallation neutron source opens at the Rutherford Appleton Laboratory, UK. It is the first major neutron user facility based on a high-energy proton accelerator. ■ 1987 J Georg Bednorz and K Alexander Müller receive the Nobel Prize in Physics for the discovery of high-temperature superconductors. Later, neutron spectroscopy shows that magnetic interactions are crucial to understanding the mechanism of this phenomenon.

Charlie Chong/ Fion Zhang

■ 1991 Pierre-Gilles de Gennes receives the Nobel Prize in Physics for his work on liquid crystals and polymers. Neutron spin-echo spectroscopy was used to validate his models of polymer reptation dynamics. ■ 1994 Clifford G Shull and Bertram N Brockhouse receive the Nobel Prize in Physics for pioneering the development of neutron-scattering techniques that can show “where atoms are” and “what atoms do”. ■ 2009 Next-generation accelerator-based pulsed neutron sources come online in the UK (ISIS Target Station 2), Japan (J-PARC) and the US (SNS). ■ 2010 Lund, Sweden, is chosen as the site for the construction of the European Spallation Source. Construction is planned to be completed around 2018–19. The ESS will provide neutron beams up to 30 times brighter than present day neutron sources.

Charlie Chong/ Fion Zhang

More Reading

Charlie Chong/ Fion Zhang

TOKAMAK Fusion Reactor

Charlie Chong/ Fion Zhang

http://science.howstuffworks.com/fusion-reactor.htm

Fusion reactors have been getting a lot of press recently because they offer some major advantages over other power sources. They will use abundant sources of fuel, they will not leak radiation above normal background levels and they will produce less radioactive waste than current fission reactors. Nobody has put the technology into practice yet, but working reactors aren't actually that far off. Fusion reactors are now in experimental stages at several laboratories in the United States and around the world. A consortium from the United States, Russia, Europe and Japan has proposed to build a fusion reactor called the International Thermonuclear Experimental Reactor (ITER) in Cadarache, France, to demonstrate the feasibility of using sustained fusion reactions for making electricity. In this article, we'll learn about nuclear fusion and see how the ITER reactor will work.

Charlie Chong/ Fion Zhang

ITER fusion reactor plant at Cadarache, France

Charlie Chong/ Fion Zhang

Physics of Nuclear Fusion: Reactions Current nuclear reactors use nuclear fission to generate power. In nuclear fission, you get energy from splitting one atom into two atoms. In a conventional nuclear reactor, high-energy neutrons split heavy atoms of uranium, yielding large amounts of energy, radiation and radioactive wastes that last for long periods of time (see How Nuclear Power Works). In nuclear fusion, you get energy when two atoms join together to form one. In a fusion reactor, hydrogen atoms come together to form helium atoms, neutrons and vast amounts of energy. It's the same type of reaction that powers hydrogen bombs and the sun. This would be a cleaner, safer, more efficient and more abundant source of power than nuclear fission.

Charlie Chong/ Fion Zhang

The Sun

Charlie Chong/ Fion Zhang

There are several types of fusion reactions. Most involve the isotopes of hydrogen called deuterium and tritium: ď Ž Proton-proton chain - This sequence is the predominant fusion reaction scheme used by star such as the sun. Two pairs of protons form to make two deuterium atoms. Each deuterium atom combines with a proton to form a helium-3 atom. Two helium-3 atoms combine to form beryllium-6, which is unstable. Beryllium-6 decays into two helium-4 atoms. These reactions produce high energy particles (protons, electrons, neutrinos, positrons) and radiation (light, gamma rays) ď Ž Deuterium-deuterium reactions - Two deuterium atoms combine to form a helium-3 atom and a neutron.

Charlie Chong/ Fion Zhang

The protonâ&#x20AC;&#x201C;proton chain reaction dominates in stars the size of the Sun or smaller.

Charlie Chong/ Fion Zhang

The protonâ&#x20AC;&#x201C;proton chain reaction dominates in stars the size of the Sun or smaller.

Charlie Chong/ Fion Zhang

â&#x2013; Deuterium-tritium reactions - One atom of deuterium and one atom of tritium combine to form a helium-4 atom and a neutron. Most of the energy released is in the form of the high-energy neutron. Conceptually, harnessing nuclear fusion in a reactor is a no-brainer. But it has been extremely difficult for scientists to come up with a controllable, non-destructive way of doing it. To understand why, we need to look at the necessary conditions for nuclear fusion.

Charlie Chong/ Fion Zhang

Isotopes Isotopes are atoms of the same element that have the same number of protons and electrons but a different number of neutrons. Some common isotopes in fusion are:  Protium is a hydrogen isotope with one proton and no neutrons. It is the most common form of hydrogen and the most common element in the universe.  Deuterium is a hydrogen isotope with one proton and one neutron. It is not radioactive and can be extracted from seawater.  Tritium is a hydrogen isotope with one proton and two neutrons. It is radioactive, with a half-life of about 10 years. Tritium does not occur naturally but can be made by bombarding lithium with neutrons.  Helium-3 is a helium isotope with two protons and one neutron.  Helium-4 is the most common, naturally occurring form of helium, with two protons and two neutrons.

Charlie Chong/ Fion Zhang

Conditions for Nuclear Fusion When hydrogen atoms fuse, the nuclei must come together. However, the protons in each nucleus will tend to repel each other because they have the same charge (positive). If you've ever tried to place two magnets together and felt them push apart from each other, you've experienced this principle firsthand. To achieve fusion, you need to create special conditions to overcome this tendency. Here are the conditions that make fusion possible:

Charlie Chong/ Fion Zhang

http://imgarcade.com/1/deuterium-and-tritium/

High temperature - The high temperature gives the hydrogen atoms enough energy to overcome the electrical repulsion between the protons. Fusion requires temperatures about 100 million Kelvin (approximately six times hotter than the sun's core). At these temperatures, hydrogen is a plasma, not a gas. Plasma is a highenergy state of matter in which all the electrons are stripped from atoms and move freely about. The sun achieves these temperatures by its large mass and the force of gravity compressing this mass in the core. We must use energy from microwaves, lasers and ion particles to achieve these temperatures.

Charlie Chong/ Fion Zhang

High pressure - Pressure squeezes the hydrogen atoms together. They must be within 1x10-15meters of each other to fuse. The sun uses its mass and the force of gravity to squeeze hydrogen atoms together in its core. We must squeeze hydrogen atoms together by using intense magnetic fields, powerful lasers or ion beams. W-ith current technology, we can only achieve the temperatures and pressures necessary to make deuterium-tritium fusion possible. Deuteriumdeuterium fusion requires higher temperatures that may be possible in the future. Ultimately, deuterium-deuterium fusion will be better because it is easier to extract deuterium from seawater than to make tritium from lithium. Also, deuterium is not radioactive, and deuterium-deuterium reactions will yield more energy.

Charlie Chong/ Fion Zhang

What is Plasma If you boost a gas to extremely high temperatures, you get plasma. The energy begins to break apart the gas molecules, and the atoms begin to split. Normal atoms are made up of protons and neutrons in the nucleus (see How Atoms Work), surrounded by a cloud of electrons. In plasma, the electrons separate from the nucleus. Once the energy of heat releases the electrons from the atom, the electrons begin to move around quickly. The electrons are negatively charged, and they leave behind their positively charged nuclei. These positively charged nuclei are known as ions. When the fast-moving electrons collide with other electrons and ions, they release vast amounts of energy. This energy is what gives plasma its unique status and unbelievable cutting power.

Charlie Chong/ Fion Zhang

http://home.howstuffworks.com/plasma-cutter3.htm

Commonplace Plasma Almost 99 percent of all matter in the universe is plasma. It's not common on Earth because of its extremely high temperatures; but somewhere like the sun, it's the norm. On Earth, you find it in lightning, among other places. Plasma cutters are not the only devices to harness the power of plasma. Neon signs, fluorescent lighting and plasma displays, just to name a few, all rely on it to get the job done. These devices use "cool" plasma. Though cool plasma cannot be used to cut metals, it has tons of other useful applications. Check out How Fluorescent Lamps Work to learn more.

Charlie Chong/ Fion Zhang

http://home.howstuffworks.com/plasma-cutter3.htm

Fusion Reactors: Magnetic Confinement There are two ways to achieve the temperatures and pressures necessary for hydrogen fusion to take place: ď Ž Magnetic confinement uses magnetic and electric fields to heat and squeeze the hydrogen plasma. The ITER project in France is using this method. ď Ž Inertial confinement uses laser beams or ion beams to squeeze and heat the hydrogen plasma. Scientists are studying this experimental approach at the National Ignition Facility of Lawrence Livermore Laboratory in the United States.

Charlie Chong/ Fion Zhang

National Ignition Facility of Lawrence Livermore Laboratory

Charlie Chong/ Fion Zhang

National Ignition Facility of Lawrence Livermore Laboratory

Charlie Chong/ Fion Zhang

National Ignition Facility of Lawrence Livermore Laboratory

Charlie Chong/ Fion Zhang

National Ignition Facility of Lawrence Livermore Laboratory

Charlie Chong/ Fion Zhang

Microwaves, electricity and neutral particle beams from accelerators heat a stream of hydrogen gas. This heating turns the gas into plasma. This plasma gets squeezed by super-conducting magnets, thereby allowing fusion to occur. The most efficient shape for the magnetically confined plasma is a donut shape (toroid). A reactor of this shape is called a tokamak. The ITER tokamak will be a selfcontained reactor whose parts are in various cassettes. These cassettes can be easily inserted and removed without having to tear down the entire reactor for maintenance. The tokamak will have a plasma toroid with a 2-meter inner radius and a 6.2-meter outer radius. Let's take a closer look at the ITER fusion reactor to see how magnetic confinement works. Tokamak "Tokamak" is a Russian acronym for "toroidal chamber with axial magnetic field."

Charlie Chong/ Fion Zhang

Toroidal Plasma

Charlie Chong/ Fion Zhang

Let's look at magnetic confinement first. Here's how it would work:

Charlie Chong/ Fion Zhang

Magnetic Confinement: The ITER Example The main parts of the ITER tokamak reactor are:  Vacuum vessel - holds the plasma and keeps the reaction chamber in a vacuum  Neutral beam injector (ion cyclotron system) - injects particle beams from the accelerator into the plasma to help heat the plasma to critical temperature  Magnetic field coils (poloidal, toroidal) - super-conducting magnets that confine, shape and contain the plasma using magnetic fields  Transformers/Central solenoid - supply electricity to the magnetic field coils  Cooling equipment (crostat, cryopump) - cool the magnets  Blanket modules - made of lithium; absorb heat and high-energy neutrons from the fusion reaction  Divertors - exhaust the helium products of the fusion reaction

Charlie Chong/ Fion Zhang

■ ωσμ∙Ωπ∆ ∇ º≠δ≤>ηθφФρ|β≠Ɛ∠ ʋ λ α ρτ√ ≠≥ѵФε ≠≥ѵФdsssa

Charlie Chong/ Fion Zhang

The Tokamak

Charlie Chong/ Fion Zhang

http://www.iter.org/mach

The Tokamak

Charlie Chong/ Fion Zhang

http://www.iter.org/mach

The Tokamak

Charlie Chong/ Fion Zhang

http://www.iter.org/mach

The Tokamak

Charlie Chong/ Fion Zhang

http://www.iter.org/mach

The Tokamak

Charlie Chong/ Fion Zhang

http://www.iter.org/mach

The Tokamak

Charlie Chong/ Fion Zhang

http://www.iter.org/mach

The Tokamak

Charlie Chong/ Fion Zhang

http://www.iter.org/mach

The Tokamak

Charlie Chong/ Fion Zhang

http://www.iter.org/mach

The Tokamak

Charlie Chong/ Fion Zhang

http://www.iter.org/mach

The Tokamak

Charlie Chong/ Fion Zhang

http://www.iter.org/mach

The Tokamak

Charlie Chong/ Fion Zhang

http://www.iter.org/mach

The Tokamak

Charlie Chong/ Fion Zhang

http://www.iter.org/mach

The Tokamak

Charlie Chong/ Fion Zhang

http://www.iter.org/mach

Here's how the process will work: 1. The fusion reactor will heat a stream of deuterium and tritium fuel to form high-temperature plasma. It will squeeze the plasma so that fusion can take place. The power needed to start the fusion reaction will be about 70 megawatts, but the power yield from the reaction will be about 500 megawatts. The fusion reaction will last from 300 to 500 seconds. (Eventually, there will be a sustained fusion reaction.) 2. The lithium blankets outside the plasma reaction chamber will absorb highenergy neutrons from the fusion reaction to make more tritium fuel. The blankets will also get heated by the neutrons. 6 Li + 1 n â&#x2020;&#x2019; 4 He + 3 H + 4.8 MeV 3 0 2 1 3. The heat will be transferred by a water-cooling loop to a heat exchanger to make steam. 4. The steam will drive electrical turbines to produce electricity. 5. The steam will be condensed back into water to absorb more heat from the reactor in the heat exchanger. Initially, the ITER tokamak will test the feasibility of a sustained fusion reactor and eventually will become a test fusion power plant. Charlie Chong/ Fion Zhang

Charlie Chong/ Fion Zhang

Lithium Ore

Charlie Chong/ Fion Zhang

Lithium Ore

Charlie Chong/ Fion Zhang

Fusion Reactors: Inertial Confinement The National Ignition Facility (NIF) at Lawrence Livermore Laboratory is exp-erimenting with using laser beams to induce fusion. In the NIF device, 192 laser beams will focus on single point in a 10-meter-diameter target chamber called a hohlraum. A hohlraum is "a cavity whose walls are in radiative equilibrium with the radiant energy within the cavity" (Science & Engineering Encyclopaedia). At the focal point inside the target chamber, there will be a pea-sized pellet of deuterium-tritium encased in a small, plastic cylinder. The power from the lasers (1.8 million joules) will heat the cylinder and generate X-rays. The heat and radiation will convert the pellet into plasma and compress it until fusion occurs. The fusion reaction will be short-lived, about one-millionth of a second, but will yield 50 to 100 times more energy than is needed to initiate the fusion reaction. A reactor of this type would have multiple targets that would be ignited in succession to generate sustained heat production. Scientists estimate that each target can be made for as little as $0.25, making the fusion power plant cost efficient.

Charlie Chong/ Fion Zhang

The laser used for inertial confinement process

Charlie Chong/ Fion Zhang

Inertial Confinement Process

Charlie Chong/ Fion Zhang

Inertial Confinement Process

Charlie Chong/ Fion Zhang

Like the magnetic-confinement fusion reactor, the heat from inertial-confinement fusion will be passed to a heat exchanger to make steam for producing electricity.

Charlie Chong/ Fion Zhang

Applications of Fusion The main application for fusion is in making electricity. Nuclear fusion can provide a safe, clean energy source for future generations with several advantages over current fission reactors: Abundant fuel supply - Deuterium can be readily extracted from seawater, and excess tritium can be made in the fusion reactor itself from lithium, which is readily available in the Earth's crust. Uranium for fission is rare, and it must be mined and then enriched for use in reactors. Safe - The amounts of fuel used for fusion are small compared to fission reactors. This is so that uncontrolled releases of energy do not occur. Most fusion reactors make less radiation than the natural background radiation we live with in our daily lives. Clean - No combustion occurs in nuclear power (fission or fusion), so there is no air pollution.

Charlie Chong/ Fion Zhang

Less nuclear waste - Fusion reactors will not produce high-level nuclear wastes like their fission counterparts, so disposal will be less of a problem. In addition, the wastes will not be of weapons-grade nuclear materials as is the case in fission reactors. NASA is currently looking into developing small-scale fusion reactors for powering- deep-space rockets. Fusion propulsion would boast an unlimited fuel supply (hydrogen), would be more efficient and would ultimately lead to faster rockets.

Charlie Chong/ Fion Zhang

Cold Fusion In 1989, researchers in the United States and Great Britain claimed to have made a fusion reactor at room temperature without confining hightemperature plasmas. They made an electrode of palladium, placed it in a thermos of heavy water (deuterium oxide) and passed an electrical current through the water. They claimed that the palladium catalyzed fusion by allowing deuterium atoms to get close enough for fusion to occur. However, several scientists in many countries failed to get the same result. But in April 2005, cold fusion got a major boost. Scientists at UCLA initiated fusion using a pyroelectric crystal. They put the crystal into a small container filled with hydrogen, warmed the crystal to produce an electric field and inserted a metal wire into the container to focus the charge. The focused electric field powerfully repelled the positively charged hydrogen nuclei, and in the rush away from the wire, the nuclei smashed into each other with enough force to fuse. The reaction took place at room temperature. See Coming in out of the cold: Cold fusion, for real (csmonitor.com) to learn more.

Charlie Chong/ Fion Zhang

Lots More Information Related Articles • How Atoms Work • How Atom Smashers Work • How Electricity Works • How Hydropower Plants Work • How Nuclear Bombs Work • How Nuclear Power Works • How Nuclear Radiation Works • How Power Grids Work • How Solar Cells Work• Nuclear Power Quiz More Great Links • Inertial Fusion Energy: A Tutorial on the Technology and Economics • National Ignition Facility Project: How NIF Works • Princeton Plasma Physics Laboratory • Project ITER • World Nuclear Association: Nuclear Fusion Power

Charlie Chong/ Fion Zhang

http://science.howstuffworks.com/fusion-reactor7.htm

Swiss Plasma Center to harness the sun's energy September 22nd, 2015

Charlie Chong/ Fion Zhang

At EPFL, the Center for Research in Plasma Physics (CRPP) has become the Swiss Plasma Center (SPC), and for good reason: the Center is upgrading its facilities and expanding its scope of activities. These improvements strengthen the role the Lausanne-based tokamak will play as one of three research facilities selected by the EUROfusion consortium to develop nuclear fusion as part of the international project known as ITER. Once mastered, nuclear fusion will be able to produce enough energy â&#x20AC;&#x201C; clean, reliable energy â&#x20AC;&#x201C; to meet the needs of mankind for centuries to come. Unlike fission, fusion does not create radioactive waste with a long lifespan, and it is based on abundant materials that are easier to extract than uranium.

Charlie Chong/ Fion Zhang

Numerous international research projects are under way, and one of the most crucial challenges they face is plasma confinement. This refers to confining a gas that is heated to more than a hundred million degrees â&#x20AC;&#x201C; considerably hotter than the core of the sun â&#x20AC;&#x201C; so that the component hydrogen atoms will fuse and release huge amounts of energy. But these extreme temperatures must not damage the reactor, which means the plasma must be kept away from the walls. This is done using a magnetic field that is contained inside a ring-shaped chamber called a tokamak. "Tokamak" is a Russian acronym for "toroidal chamber with axial magnetic field."

Charlie Chong/ Fion Zhang

One-of-a-kind research facility The Variable Configuration Tokamak, which was built in 1992 at the Swiss Federal Institute of Technology in Lausanne (EPFL, Switzerland), has always been on the leading edge among research facilities in this field. The TCV tokamak, as it is known, is operated by the Center for Research in Plasma Physics (CRPP) and is unique because â&#x20AC;&#x201C; as its name indicates â&#x20AC;&#x201C; it can produce plasma in various shapes. This feature allows scientists to determine the most appropriate configuration for use in an energy-producing reactor. And it was thanks to this feature that in late 2013 the TCV tokamak was selected by the EUROfusion consortium as one of three national facilities on the European continent to be used to help design the international power plant ITER, currently being built in the south of France, and develop its successor, DEMO, a prototype commercial reactor.

Charlie Chong/ Fion Zhang

The Lausanne-based lab recently received 10 million francs from the Swiss government to upgrade certain aspects of its facility. Thanks to these funds, the Center will soon be equipped to carry out new experiments on the TCV tokamak, particularly in relation to extracting energy and particles from the plasma. New mechanisms for heating the plasma with microwaves and with the injection of neutral particles may also be installed. At the same time, the Center is expanding its sector for lower density and lower temperature plasmas in order to explore new applications for plasma, such as in the medical field, the food industry and astrophysics. These improvements will encourage many Swiss and European researchers to visit Lausanne and conduct new experiments.

Charlie Chong/ Fion Zhang

The "Swiss Plasma Center", a new international reference Alongside these developments, the Lausanne-based lab is changing its name. It is now the Swiss Plasma Center that will impress its credentials on Switzerland, Europe and the rest of the world as a leading institution in this field. The renamed Center was officially inaugurated today in Lausanne. Attendees included Bernard Bigot, Director-General of ITER, along with officials from the EUROfusion consortium, who emphasized the importance of the research being carried out in Switzerland in support of the objective of the reactor being built in Cadarache. The reactor, using nuclear fusion, aims to generate ten times more power than was injected into it.

Charlie Chong/ Fion Zhang

Swiss Federal Institute of Technology in Lausanne â&#x20AC;&#x201C; Rolex Learning Center

Charlie Chong/ Fion Zhang

EPFL â&#x20AC;&#x201C; Rolex Learning Center

Charlie Chong/ Fion Zhang

How Tokamak Works

http://www.fusionforenergy.europa.eu/understandingfusion/technology.aspx

Charlie Chong/ Fion Zhang

http://www.fusionforenergy.europa.eu//understandingfusion/Technology/tokamok.swf

Wendelstein 7-x stellarator

Charlie Chong/ Fion Zhang

History of Tokamak

http://www.iter.org/news/videos/36

Charlie Chong/ Fion Zhang

Technology In order to produce a self-sustaining fusion reaction, the tritium and deuterium plasma must be heated to over 100 million Â°C â&#x20AC;&#x201C; this requires powerful heating devices and minimal thermal loss. To sustain such a temperature the hot plasma must be kept away from the walls of the reactor. However, because the plasma is an electrically-charged gas it can be held or contained by magnetic fields. This allows the plasma to be held, controlled and even heated by a complex cage of magnets, whilst enabling the neutrons to escape as they have no electric charge.

Charlie Chong/ Fion Zhang

In a tokamak the plasma is held in a doughnut shaped vessel. Using special coils, a magnetic field is generated, which causes the plasma particles to run around in spirals, without touching the wall of the chamber. The Magnetic Fields Experimental arrangement for controlled nuclear fusion. In a Tokamak, two superimposed magnetic fields enclose the plasma: (1) this is the toroidal field generated by external coils on the one hand and (2) the field of a flow in the plasma on the other hand. In the combined field, the field lines run helicoidally around the torus centre.

Charlie Chong/ Fion Zhang

https://www.euronuclear.org/info/encyclopedia/t/tokamak.htm

In this way, the necessary twisting of the field lines and the structure of the magnetic areas are achieved. Apart from the toroidal field generated by the external field coils and the field generated by the flow in the plasma, the Tokamak requires a third vertical field (poloidal field), fixing the position of the flow in the plasma container.

Charlie Chong/ Fion Zhang

Magnetic Fields

Charlie Chong/ Fion Zhang

https://www.laetusinpraesens.org/iter/iter8.php

The flow in the plasma is mainly used to generate the enclosing magnetic field. In addition, it provides effective initial heating of the plasma. The flow in the plasma is normally induced by a transformer coil. Owing to the transformer, the Tokamak does not work continuously, but in pulse mode. Since, however, a power plant should not be operated in pulse mode for technical reasons, methods are examined to generate a continuous flow - for example by high-frequency waves. The fusion research plant JET is built according to the Tokamak principle. The fusion reactor ITER is also planned according to this principle.

Charlie Chong/ Fion Zhang

‘Toroidal magnetic confinement fusion’ is the advanced technology that is the main approach for European fusion research and is at the heart of the ITER experiment. The reactions take place in a vessel that isolates the plasma from its surroundings it has a torus or ‘doughnut shape’ – essentially a continuous tube. The confining magnetic fields (toroidal and poloidal fields ) are generated by electromagnets located around the reactor chamber and by an electrical current flowing in the plasma itself. This current is partly induced by a solenoid at the centre of the torus which acts as the primary winding of a transformer. The resulting magnetic field keeps the plasma particles and their energy away from the reactor wall.

Charlie Chong/ Fion Zhang

C:\Users\Administrator\Desktop\Fusion For Energy - Understanding Fusion - Technology.htm

To achieve net fusion power output in a deuterium tritium reactor, three conditions must be fulfilled: ■ a very high temperature greater than 100 million °C; ■ a plasma particle density of at least 1022 particles per cubic metre; and ■ an energy confinement time for the reactor of the order of 1 second. In order to control the plasma we need to understand fully its properties. For example: how it conducts heat, how particles are lost from the plasma, its stability, and how unwanted particles (impurities) can be prevented from remaining in the plasma. One of the major challenges in fusion research has been to maintain plasma temperature. Impurities cool the plasma and ways must be found to extract them. Plasma is heated by the electrical current induced by the transformer arrangement, but additional heating is needed to reach the high temperatures required. This includes the injection of beams of highly energetic fusion fuel particles (deuterium and or tritium) which, on collision with plasma particles, give up their energy to them, and radio-frequency heating where high-power radio waves are absorbed by the plasma particles. Charlie Chong/ Fion Zhang

Europe has a large track record in fusion. Europeâ&#x20AC;&#x2122;s JET (Joint European Torus) located at Culham (UK) is the worldâ&#x20AC;&#x2122;s largest fusion facility and the only one currently capable of working with a Deuterium-Tritium fuel mixture. JET has reached all its originally planned objectives and in some cases surpassed them. In 1997 it achieved a world record fusion power production of 16 MW and a Q = 0.65. Europe has also been building on the knowledge accumulated through the Tore Supra tokamak in France , the first large tokamak to use superconducting magnets; the ASDEX device in Germany with ITER-shaped plasmas; the reversed pinch device RFX in Italy and the stellarators TJ-II in Spain and W7-X in Germany.

Charlie Chong/ Fion Zhang

View of the plasma inside the tokamak MAST in the United Kingdom. (Source: UKAEA-Culham)

Charlie Chong/ Fion Zhang

Deuterium-Tritium Reaction D(T,n)He 2 D(3 T,1 n)4 He 1 1 0 2

Charlie Chong/ Fion Zhang

http://video.elementy.ru/smith/smith-eng.mp4

Charlie Chong/ Fion Zhang

http://elementy.ru/nauchno-populyarnaya_biblioteka/430851/The_Path_to_Fusion_Power?context=20451

Charlie Chong/ Fion Zhang

https://m.forocoches.com/foro/showthread.php?t=4452871

Charlie Chong/ Fion Zhang

https://m.forocoches.com/foro/showthread.php?t=4452871

Charlie Chong/ Fion Zhang

https://m.forocoches.com/foro/showthread.php?t=4452871

Charlie Chong/ Fion Zhang

https://m.forocoches.com/foro/showthread.php?t=4452871

Charlie Chong/ Fion Zhang

https://m.forocoches.com/foro/showthread.php?t=4452871

Charlie Chong/ Fion Zhang

Charlie Chong/ Fion Zhang http://horizon-magazine.eu/sites/default/files/pictures/HO-ITER-TK-cooling_1.jpg

Wendelstein 7-X Fusion Reactor

Charlie Chong/ Fion Zhang

https://en.wikipedia.org/wiki/Wendelstein_7-X

Wendelstein 7-X The Wendelstein 7-X (W7-X) reactor is an experimental stellarator (nuclear fusion reactor) built in Greifswald, Germany, by the Max Planck Institute of Plasma Physics (IPP), and completed in October 2015.[2] It is a further development of Wendelstein 7-AS. The purpose of Wendelstein 7-X is to evaluate the main components of a future fusion reactor built using stellarator technology, even if Wendelstein 7-X itself is not an economical fusion power plant. The Wendelstein 7-X reactor is the largest fusion device created using the stellarator concept which was the brainchild of physicist Lyman Spitzer. It is planned to operate with up to 30 minutes of continuous plasma discharge, demonstrating an essential feature of a future power plant: continuous operation. The name of the project, referring to the mountain Wendelstein in Bavaria, was decided at the end of the 1950s, referencing the preceding project from Princeton University under the name Project Matterhorn. The research facility is an independent partner project with the University of Greifswald. Charlie Chong/ Fion Zhang

Mountain Wendelstein

Charlie Chong/ Fion Zhang

University of Greifswald

Charlie Chong/ Fion Zhang

Design and main components The Wendelstein 7-X device is based on a five field-period Helias configuration. It is mainly a toroid, consisting of 50 non-planar and 20 planar superconducting magnetic coils, 3.5 m high, which induce a magnetic field that prevents the plasma from colliding with the reactor walls. The 50 nonplanar coils are used for adjusting the magnetic field. It aims for a plasma density of 3Ă&#x2014;1020 particles/cubic metre, and a plasma temperature of 60~130 million K. The main components are the magnetic coils, cryostat, plasma vessel, divertor and heating systems. The coils (NbTi in aluminium) are arranged around a heat insulating cladding with a diameter of 16 meters, called the cryostat. A cooling device produces enough liquid helium to cool down the magnets and their enclosure (about 425 metric tons of 'cold mass') to superconductivity temperature (4 K). The coils will carry 12.8 kA current and create a field of up to 3 Tesla.

Charlie Chong/ Fion Zhang

The plasma vessel, built of 20 parts, is on the inside, adjusted to the complex shape of the magnetic field. It has 254 ports (holes) for plasma heating and observation diagnostics. The whole plant is built of five almost identical modules, which were assembled in the experiment hall. The heating system includes 10 megawatts of microwaves for Electron Cyclotron Resonance Heating (ECRH), for up to 10 seconds, and can deliver 1 megawatt for 50 seconds during operational phase 1 (OP-1).[7] For operational phase 2 (OP-2), after completion of the full armor/water-cooling, up to 8 megawatts of neutral beam injection will also be available for 10 seconds,[8] while the microwave system will be extended to true steady state (30 minutes). An Ion Cyclotron Resonance Heating (ICRH) system will become available for physics operation in OP1.2.[9] A system of sensors and probes based on a variety of complementary technologies will measure key properties of the plasma, including the profiles of the electron density and of the electron and ion temperature, as well as the profiles of important plasma impurities and of the radial electric field resulting from electron and ion particle transport. Charlie Chong/ Fion Zhang

Wendelstein 7-X

Charlie Chong/ Fion Zhang

Wendelstein 7-X

Charlie Chong/ Fion Zhang

https://upload.wikimedia.org/wikipedia/commons/8/8f/Wendelstein7-X_Torushall-2011.jpg

The History The German funding arrangement for the project was negotiated in 1994, establishing the Greifswald Branch Institute of the IPP in the north-eastern corner of the recently integrated East Germany. Its new building was completed in 2000. Construction of the stellarator was originally expected to reach completion in 2006. Assembly began in April 2005. Problems with the coils took about 3 years to fix.[4] The schedule slipped into late 2015.[4][11][12] A three-lab American consortium (Princeton, Oak Ridge, and Los Alamos) became a partner in the project, paying $7.5 million USD of the eventual total cost of 1.06 billion Euros.[13] In 2012, Princeton University and the Max Planck Society announced a new joint research center in plasma physics,[14] to include research on W7-X. The end of the construction phase was officially marked by an inauguration ceremony on 20 May 2014.[15] After a period of vessel leak-checking, beginning in the summer of 2014, the cryostat was evacuated, and magnet testing was completed in July 2015.[5] Charlie Chong/ Fion Zhang

The reactor successfully produced helium plasma (with temperatures of about 1Ă&#x2014;106 K) for about 0.1 s on December 10, 2015. For this initial test with about 1 mg of helium gas injected into the evacuated plasma vessel, microwave heating was applied for a short 1.3 MW pulse.[16] More than 300 discharges with helium were done in December and January with gradually increasing temperatures finally reaching six million degrees, to clean the vacuum vessel walls and test the plasma diagnostic systems. Then on February 3, 2016, operational phase 1 (OP-1) began, with production of the first hydrogen plasma to initiate the science program. A 2 MW microwave pulse resulted in a plasma temperature of 80Ă&#x2014;106 K, with a lifetime of Âź second, fulfilling all expectations. Such tests were planned to continue for about a month, followed by a scheduled shut-down to open the vacuum vessel and install protective carbon tiles lining the vessel, and a "diverter" for removing impurities from the plasma. Then the science program will continue while gradually increasing the power and duration of the discharges

Charlie Chong/ Fion Zhang

Schema of the coil system (blue) and plasma (yellow). A magnetic field line is highlighted in green on the yellow plasma surface.

Charlie Chong/ Fion Zhang

Superconducting feed lines being attached to the superconducting planar coils.

Charlie Chong/ Fion Zhang

https://upload.wikimedia.org/wikipedia/commons/6/61/Stellarator_Wendelstein_7-X_Planar-Spulen_Vermessung.jpg

Construction as of May 2012. Visible are the torus, offset in the test cell, and the large overhead crane. Note the workers for scale.

Charlie Chong/ Fion Zhang

https://upload.wikimedia.org/wikipedia/commons/8/87/W7-X_Stellarator_Experiment.jpg

Wide-angle view inside the W7-X stellarator (under construction), showing the stainless cover plates and the water-cooled copper backing plates (which will eventually be covered by graphite tiles) that are being installed as armor to protect against plasma/wall interactions.

Charlie Chong/ Fion Zhang

History[edit] The German funding arrangement for the project was negotiated in 1994, establishing the Greifswald Branch Institute of the IPP in the north-eastern corner of the recently integrated East Germany. Its new building was completed in 2000. Construction of the stellarator was originally expected to reach completion in 2006. Assembly began in April 2005. Problems with the coils took about 3 years to fix.[4] The schedule slipped into late 2015.[4][11][12] A three-lab American consortium (Princeton, Oak Ridge, and Los Alamos) became a partner in the project, paying $7.5 million USD of the eventual total cost of 1.06 billion Euros.[13] In 2012, Princeton University and the Max Planck Society announced a new joint research center in plasma physics,[14] to include research on W7-X.

Charlie Chong/ Fion Zhang

The end of the construction phase was officially marked by an inauguration ceremony on 20 May 2014.[15] After a period of vessel leak-checking, beginning in the summer of 2014, the cryostat was evacuated, and magnet testing was completed in July 2015.[5] The reactor successfully produced helium plasma (with temperatures of about 1Ă&#x2014;106 K) for about 0.1 s on December 10, 2015. For this initial test with about 1 mg of helium gas injected into the evacuated plasma vessel, microwave heating was applied for a short 1.3 MW pulse.[16]

Charlie Chong/ Fion Zhang

More than 300 discharges with helium were done in December and January with gradually increasing temperatures finally reaching six million degrees, to clean the vacuum vessel walls and test the plasma diagnostic systems. Then on February 3, 2016, operational phase 1 (OP-1) began, with production of the first hydrogen plasma to initiate the science program. A 2 MW microwave pulse resulted in a plasma temperature of 80Ă&#x2014;106 K, with a lifetime of Âź second, fulfilling all expectations. Such tests were planned to continue for about a month, followed by a scheduled shut-down to open the vacuum vessel and install protective carbon tiles lining the vessel, and a "diverter" for removing impurities from the plasma. Then the science program will continue while gradually increasing the power and duration of the discharges.

Charlie Chong/ Fion Zhang

Timeline Year

Event

1994

Project initiated

2005

Assembly began

2014

Inaugurated

2015

Successful helium plasma test at 1Ă&#x2014;106 K for ~0.1 s

2016

Hydrogen plasma at 80Ă&#x2014;106 K for 0.25 s according to expectations

Charlie Chong/ Fion Zhang

东方超环 - 世界首个全超导托卡马克“东方超环（EAST）”2010年度实验将于12月24日

圆满结束，目前已获得1兆安等离子体电流、100秒1500万度偏滤器长脉冲等离子体、大于30倍能量约束时间高约束模式等离子体、3兆瓦离子回旋加热等多项重要实验成果，大大推进了“东方超环”实现其总体科学目标的进程；实验中广泛开展的国际合作，使“东方超环”已成为国际上最重要的高参数长脉冲等离子体物理实验平台。

Charlie Chong/ Fion Zhang

http://www.ipp.cas.cn/xwdt/ttxw/201207/t20120710_96336.html

“合肥超环”进行末次放电实验我国首个托卡马克装置“光荣退役”（安徽日报， 2012年10月24日，头版）

http://tieba.baidu.com/p/2223121270 Charlie Chong/ Fion Zhang

http://www.hf.cas.cn/lmjx/cmsj/201210/t20121024_3666056.html

Charlie Chong/ Fion Zhang

Read More http://www.nuclear-power.net/neutron-cross-section/

Charlie Chong/ Fion Zhang

Good Luck! Charlie Chong/ Fion Zhang