【徐遐生院士談地球新能源-海報節錄英文版】 EARTH ENERGY: GIFTS FROM NATURE (The English Poster Version) Mitigating Climate Change Human burning of fossil fuels has increased the atmospheric concentration of carbon dioxide from 280 ppm before the industrial revolution to 395 ppm at the time of the writing of this article (Fig. 1). Overwhelming scientific consensus holds that this increase is the main cause of modern climate change. To avoid climate catastrophe, we need to transition away from burning fossil fuels. In this article, we introduce two alternative paths that are being developed at ASIAA.
Figure 1. The concentration of CO2 in the atmosphere in ppm as a function of time during the past ten thousand years (up to 2005). Data source: IPCC
Nuclear Breeder Reactors Conventional nuclear energy based on fissioning U-235 (enriched relative to natural uranium) in light water reactors (LWRs) is not a sustainable replacement for fossil fuels because there is only six years of energy use at the levels needed globally in 2050 if all that energy were to come from U-235 in high-grade uranium ore. In addition, LWRs are (unfairly) perceived to possess a nuclear waste problem, safety issues against the massive release of radioactivity into the environment, and security issues against weapons proliferation. U-238 is more than 100 times as abundant as U-235, and adding a neutron to U-238 makes U-239, which becomes Pu-239 after two beta decays to turn two neutrons into two protons. Pu-239 is fissile. Such a program of “breeding” to turn a fertile (U-238) into a fissile (Pu-239) raises the high-grade uranium ore use (if all power came from fission reactors) to 600 years. Uranium-bearing minerals are soluble in seawater, leading to Japanese proposals to use polymer filters to trawl for uranium from seawater. The supply of uranium in the oceans suffices to power a “plutonium economy” for hundreds of thousands of years. The potential for thorium breeder reactors is even better. Thorium has only one stable isotope, Th-232, which eliminates the need for expensive isotope separation. Moreover, while Th-232, an even-even nuclide with 90 protons and 142 neutrons, is only fertile, it can be made fissile by absorbing a neutron. This turns Th-232 into 1/10
Th-233, which, after two beta decays that convert two neutrons into two protons, produces U-233. An even-odd nuclide with 92 protons and 141 neutrons, U-233 is fissile. When U-233 has a slow neutron added to it (one with a spin opposite to the unpaired neutron that must be in U-233 because it has an odd number of neutrons), the increase in the energy of the large nucleus is enough to cause the resulting nucleus to vibrate violently into two uneven pieces, called fission products. Fission products from the breakup of a neutron-rich parent are too neutron rich to remain in such states without spitting out an additional 2 or 3 neutrons. When a U-233 nucleus absorbs a slow neutron and fissions, an average of 2.49 (fast) fission neutrons will be produced in the aftermath. Because this average output of neutrons per fission is greater than 2, apart from the 1 neutron needed to sustain the chain reaction, another is available to turn a neighboring Th-232 nucleus into Th-233, that then decays into a new fissile U-233. If the neutron economy is managed properly by building the reactor core out of materials that do not absorb fission neutrons parasitically while slowing them down to low speeds, the extra 0.49 neutrons on average per fission reaction can make more U-233 from Th-232 than we started out with. In principle, then, thorium breeder reactors could exponentially expand their numbers until we have enough to supply the total energy needs of the world. Thorium is 3 to 4 times as abundant as uranium in the crust of the Earth. What is a 600 year depletion time for high-grade uranium ore becomes something more like 2000 years for the depletion of high-grade thorium ores. As a chemical element, thorium behaves oppositely to uranium in one important respect: thorium minerals are not soluble in seawater. Thus, they are not found in the oceans of the Earth, but are ample in beach sand of a variety black in color called monazite. Lots of monazite exits on Taiwan beaches. If you think it is not enough, just go out in the ocean and get some more from the ocean bottom. Because thorium has no other commercial applications, no one has surveyed how much thorium might exist in the world as potential nuclear fuel. The reserves are likely to last millions of years, if not billions, if one were to go to lower grades of ore. Thus, thorium MSBRs are sustainable. Molten Salt Breeder Reactor Our more detailed discussion of MSBRs begins with the observation that it offers a solution to the nuclear waste problem that has accumulated from half a century of operating LWRs. Figure 2 schematically provides the solution. The high-level nuclear waste from the spent fuel rods of LWRs consists of three main components: ď Ź Unreacted U-235, mixed with U-238, ď Ź Pu-239 and higher actinides from collateral neutron irradiation of U-238, ď Ź Fission products from the splitting of fissile nuclei.
2/10
MSRs Can Rid LWR Waste &
Unreacted uranium can be safely separated from the Pu-239 and minor actinides by Safely Breed for U-233 the standard process of Chain reaction, breeding, and processing in liquid salt fluorination to produce a Pu in • LWR spent fuel Th-232 Blanket gas UF6 that rises out of a core Enough in Lehmi Pass for – U-238, U-235 molten salt system. Once turns 1,000 yr of USA energy use – Pu/actinides Th-232 separated, the large into – Fission prod’s amounts of U-238 mixed in U-233 Core • Th-232 with the U-235 (converted U-233 Blanket processing: from the UF6 form to more in core UF4 (liquid) + F2 (gas) gives stable oxide forms) makes 300 yr ! UF6 (gas) Yucca IFR or breeder both U-233 & U-232 this material unsuitable for Mtn TWR Ground bomb making, and it can 2/15/13 Frank H. Shu 9 either go to a geological Figure 2. Schematic diagram of how solving the nuclear waste problem of LWRs provides a method to start up MSBRs. repository (like Yucca ©ASIAA mountain or its replacement), or be given as fuel for proponents of reactor technology like the integral fast reactor (IFR) or traveling wave reactor (TWR). A process called “pyroprocessing” developed at the Idaho National Laboratory then safely separates the Pu-239 and minor actinides from the fission products. With a few unimportant exceptions, the fission products contain radioactive elements that have half-lives of order 30 years or shorter. Such material can be packed in dry casks and stored underground for 300 years, after which their radioactivity has dropped below background levels. The casks can be opened to retrieve rare substances that have great economic and medical value. The Pu-239 and minor actinides are chemically made into fluoride compounds, such as PuF3, and dissolved in eutectic NaF/BeF2 molten salt (our preferred choice of the carrier solvent salt). We pump enough of PuF3/NaF/BeF2 fuel salt into the core of a molten salt converter reactor (MSCR) to achieve a critical mass and to sustain a chain fission reaction. The excess neutrons above what is needed to sustain the chain reaction (against parasitic neutron captures by non-fissiles in the system) random walk their way out of the core to irradiate a blanket salt in a pool surrounding the reactor core that consists of ThF4 dissolved in molten eutectic NaF/BeF2. The thorium is entirely in the form Th-232, and neutron captures by Th-232 result, after two beta decays, in U-233. When the Pu-239 and minor actinides are consumed, we have solved the nuclear waste problem of LWRs. The solution for LWR waste has two side benefits: It has eliminated the “dirty bomb” risk from the existence of LWR plutonium.
3/10
 It offers a way to start up MSBRs when U-233 does not exist in nature. The manufactured U-233 in the blanket salt exists chemically as UF4 in the pool. To extract it, we continuously pump small amounts of the pool salt to a chamber where gaseous F2 bubbled through the molten salt combines with UF4 in solution to form a gas UF6 that bubbles out of the liquid. The UF6 then flows to another chamber where it attacks metallic Be to produce UF4 and BeF2. When we dissolve the 233UF4 in eutectic NaF/BeF2 molten salt and pump this fuel salt into the core of the reactor, the replacement fissile has turned a MSCR (converter reactor) into a MSBR (breeder reactor). Electrolysis of the BeF2 can recover the Be and F2 needed to process the next batch of 233UF4. The chemical processing is straightforward and can be carried out remotely without endangering the operators. The energy needed for the chemical processing is minuscule (~ 10-5) compared to the nuclear energy benefit.
Figure 3. One design possibility for a two-fluid MSBR (patent pending). Four molten-salt pumps in the foreground, fuel salt circulates into the vertical channels in the black-colored core. Reaching a compact configuration with moderator graphite all around it, the fuel salt sustains a chain reaction. Pumps in the background pull blanket salt through the core in horizontal channels that alternate with the vertical channels, but separated from them by walls of graphitic material. Heat from fission reactions in the vertical channels conducts across the graphite into the blanket salt in the horizontal channels. The blanket salt then flows into a secondary heat exchanger in the background outside the pool. The secondary heat exchanger transfers the heat from the radioactive blanket salt to a non-radioactive working salt (e.g, the NaAc/KAc used for supertorrefaction of biomass). After the secondary heat transfer, the cooled blanket salt flows to rejoin the pool at the top. The cooler blanket salt lying above the hotter blanket salt induces a convection patter that keeps the blanket salt well mixed. In the interim the cooled fuel salt flows out of the core into the foreground pumps, where any fission gases in the salt are flushed out of the system by helium gas flowing through the white pipes. The fuel salt then circulates back into the core via the red pipes to begin the process anew. Š ASIAA
Because the fuel salt in MSBRs circulates indefinitely until all fissiles are consumed, there are only fission products to deal with by underground storage for 300 years. Thus, MSBRs have no waste problem of their own without a good solution. What about security? Cannot U-233 be used to make bombs? No, when one has fast fission neutrons flying around, one cannot avoid reactions with one fast incoming 4/10
neutron and two outgoing neutrons. Such reactions create U-232 that accompanies the U-233. In its decay chain, U-232 is a powerful gamma emitter, and U-232 is almost impossible to separate from U-233. Even if martyrs were willing to make a bomb using unseparated U-233/U-232, the presence of the U-232 would make the bomb easily detectable by Geiger counters if one tried to smuggle it into a city, say, in a port container. The gamma rays would also interfere with the sensitive electronic control mechanisms that must be part of any weapons assemblage. No nation or terrorist organization would attempt to make a bomb this way, when much simpler alternatives are possible. Thus, MSBRS are secure. Figure 3 shows a possible design for a two-fluid molten-salt breeder-reactor of a type described schematically in Figure 2. To slow the fission neutrons from the fast speeds at which they emerge from the fission reactions without absorbing them, we build the reactor core entirely out of carbon-based materials (except for metallic nuts and bolts). Graphite is impervious to chemical attack by hot NaF/BeF2 as long as there is no water in the salt. Doubled for safety of containment, the walls of the pool are made of metal (Hastelloy N resistant to attack by the salt). The random walking neutrons in the pool will be mostly absorbed by Th-232 (in the form of ThF4 dissolved in molten NaF/BeF2 in the pool) before they can strike the walls of the pool and activate the metal to become nuisance low-level waste. Nuclear Accidents All nuclear reactors are designed to shut themselves off automatically in the case of an emergency. The MSBR is no different, it just has larger safety margins. No reactor accident has ever occurred because of a runaway chain reaction (with the exception of the Chernobyl reactor, which had a horrible flaw in its design that could never pass the nuclear regulatory review outside of the former Soviet Union). Most nuclear accidents occur after the reactor has shut down safely. They arise because of problems in dissipating the decay heat from the fission products. For reactors with fixed solid fuel elements, the possible problems are exemplified by Fukushima. An emergency arises (a tsunami of historical proportions strikes the station). The reactors shut down safely, but the fuel rods continue to put out decay heat that is a few percent of reactor full power. Something knocks out the cooling systems normally used to cool the fuel rods (the whole electrical grid goes down because of the earthquake and tsunami). Emergency equipment has to cool the fuel rods while they remain in the same cramped space of the operational configuration. The auxiliary power goes out (fuel for diesel generators swept away, batteries run down), and there is a loss of coolant fluid (because the water boils away). Now, the plants are in big trouble. Without active cooling of the fuel rods, the rods melt down. Steam interacts with the superhot fuel rods, generating hydrogen. The hydrogen escapes into the containment buildings and explodes. Not designed to be strong, the buildings blast apart. Containment is breached, and massive amounts of radioactivity escape into the environment. None of these events would have occurred in two-fluid molten-salt breeder-reactors of the design in Figure 3 because of the following safety features:
5/10
MSBRs do not use water, so they do not need to be located near large bodies of water, like rivers or ocean sides, where people like to live. They can survive earthquakes and cannot be overwhelmed by tsunamis. Molten salt reactors run themselves, without operator intervention needed; Neutron absorber elements buoyant in the blanket salt automatically descend into the core if the pool loses coolant (the blanket salt of the pool). If the fuel salt overheats for any reason, a drain plug melts that dumps the fuel salt into an air-cooled tank absent of moderators and of a geometry where reaching accidental criticality is impossible.
In MSBRs, if reactions run too fast, the fuel salt heats up. The molten fuel salt will then expand partially out of the core, and the reactions will slow. Conversely, if we need extra power, we pump on the blanket salt harder. This cools the fuel salt, causing it to contract into the core more, thereby making the reactions run faster. These principles are exactly how the Sun, having a gaseous core that expands when heated and contracts when cooled, regulates its thermonuclear fusion reactions in the core to balance what is lost in radiation from the surface. We no more have to worry about a molten salt reactor overheating or overcooling than we have to worry that the Sun tomorrow won’t be the same as it is today. The idea of a drain plug originated at Oak Ridge National Laboratory, who invented the concept of reactors with liquid fuel elements. With solid fuel elements, as we have seen in the example of Fukushima, if something goes wrong with the primary cooling system, the problem needs fixing with the equipment in the same place where something broke. With liquid fuel, we can move it to another place (the dump tank) where we have prepared a separate emergency cooling system. We choose the coolant to be air, because although we can lose water, and we can lose molten salt, it is almost impossible to lose air. To be able to use air to cool nuclear power equipment, however, the decay heat cannot be overwhelming. This is where online cleaning of the fission products (needed to maintain the breeding ratio above unity) makes its contribution to reactor safety – it allows even reactors with fairly large full-power operations to have relatively little decay heat when one has reactor shutdown in an emergency. To be supersafe, we should avoid building reactors that are too big (because the amount of decay heat scales with operational full power). Nevertheless, it is conceivable that with complete station blackout (as happened with Fukushima), the power needed even to run fans won’t be available. Suppose the fuel salt then melts through the air-cooled dump tank. For this contingency, we’ve added a steel salt catcher into which the molten salt will spread into a thin sheet, conducting its heat to inside the steel as it flows. The design is such that the salt freezes in less than 10 seconds to immobilize any fission products that the fuel salt might contain. Because solid salt has a very low vapor pressure, no radioactive gases will escape. One extra precaution must be taken: a containment dome that can prevent intrusion by jet airplanes that try to crash into the reactor. We have to design the dome so that in case the unthinkable happens, and the operators have to abandon the site, the reactor is walk-away safe. This means that decay heat cannot be trapped inside the dome, but 6/10
needs to be able to work its way out. A good design is exemplified by the Westinghouse AP 1000, which has a thin steel cap that traps gases inside but allows conduction of heat to the upper surface, which is cooled by convection in a protective concrete dome partially open to circulating outside air. Finally, MSBRs can be located in remote places where any accident would have a minimal impact on surrounding human populations. Thus, MSBRs are walk-away safe. Supertorrefaction of Biomass into Biofuel Torrefaction is generally recognized as the most efficient way of harnessing biomass energy (Fig. 4). The traditional method involves burning a fuel and letting the flue gas heat biomass in a partially enclosed environment that has a limited intake of oxygen in air. The process drives out volatile organic compounds (VOCs), including water vapor, leaving behind a blackened solid residue,
Figure 4. Torrefaction of woody plant material. Data source: Bergman et al. 2005).
charcoal. The VOCs are usually burned to supplement the fuel, which can be natural gas or a portion of the biomass or its torrefaction products. Supertorrefaction (patent pending) is an improved process conceived as part of a general program using molten salts to generate alternative energies by the first author and brought to maturity at
Figure 5. The Crankberry machine for tabletop supertorrefaction. ŠASIAA 7/10
Academia Sinica. Supertorref action uses molten salt as a medium to transfer heat to the biomass with which the salt is in direct contact. Immersion beneath the surface of the salt excludes Figure 6. Examples of charcoal making by supertorrefaction with molten acetate oxygen and salt (NaAc/KAc) from different biomass feedstocks. ŠASIAA air. In contrast with traditional torrefaction, where many hours are required for the completion of the charring process, supertorrefaction requires typically only ten minutes because the heat capacity of molten salt per unit volume is about 2000 times larger than that of flue gas if both heat-transfer fluids are at atmospheric pressure and a given temperature. The second author of this article designed a tabletop machine (“crankberry�, Fig. 5) which automates the process of supertorrefaction on a laboratory scale. Using the crankberry, the third author and his group have supertorrefied a wide variety of biomass feedstocks, with uniformly good results (Fig. 6). If the temperature of the salt is 300 oC, a product ecocoal results that is a clean-burning, carbon-neutral, replacement for natural coal; whereas if the temperature is 500 oC, the product biochar is a fine carbon-negative soil amendment (Fig. 7). We note that burying bichar is a carbon-negative activity, beneficial not only to the host country, but to the whole world. Because the VOCs driven from the biomass are recovered rather than burned, the economic return per unit weight of the biomass is higher than in traditional torrefaction. In particular, apart from water (which we recover and recycle for washing and recovering the salt in the finished biochar), acetic acid is the most abundant component of the VOC yield. We are able to generate acetone and Na2CO3/K2CO3 if we take NaAc/KAc above 460 oC. By reacting the Na2CO3/K2CO3 with acetic acid, which is a fast acid-base reaction, we are able to recover the NaAc/KAc that we decomposed (plus CO2 and H2O). Acetone is a high-value chemical, useful as an industrial solvent as well as a feedstock for general aviation fuel, so the technique not only creates a 8/10
high-throughput solid biofuel to compete with natural coal, but also a liquid feedstock to lessen the dependence on petroleum for one segment of the transportation industry. We also get uncondensed gases combustible as a replacement for natural gas. Supertorref action allows a greatly reduced size of the equipment needed to produce a given Figure 7. Scanning electron microscope (SEM) images of (left) ecocoal made throughput from leucaena supertorrefied at 300 oC for ten minutes, and (right) biochar made (tonne per from leucaena supertorrefied at 500 oC for eleven minutes. The bar at the bottom day) for the left of the left image ois 10 microns; of the right image, 20 microns. Supertorrefaction at 300 C drives out VOCs from ecocoal, but leaves many biomass microstructures within cell walls, whereas supertorrefaction at 500 oC decomposes processing, some acetate salt into carbonate salt and leaves behind only cell walls. Below the even when image we give the Brunauer-Emmett-Teller (BET) measure of porosity (area per the slight unit mass) in m2/g. ŠASIAA loss of the salt encased in the pores of the charcoal is taken into account. This reduction lowers considerably the initial investment of capital equipment. Indeed, it is possible to have supertorrefaction throughputs that generate attractive economic returns with batch-process equipment compact enough to be transportable by truck to remote batch supertorrefaction sites where the biomass is harvested. These capabilities make commercialization of supertorrefaction possible in startup environments that hold many barriers for traditional torrefaction technologies. For example, a bad situation exists in Western North America, where winters that are too mild, combined with drought-like conditions in the summers, are blamed for an outbreak of pine bark beetle disease in mountain forests stretching from Southern California to British Columbia. Hundreds of thousands of pine trees fall per day. We propose that the felled trees should be supertorrefied before they become ground tinder for wildfires, or rot and release greenhouse gases into the atmosphere, or have falling limbs that bring down power lines and cause expensive and dangerous outages. We would bury the resulting biochar in the same forests, not only sequestering for thousands of years the resulting carbon, but also encouraging new growth that would lock up more carbon. The forest crisis affects more than just North America. A survey published in Nature magazine in 2012 found that 70% of 226 forest species in 81 forests of the world are on the verge of dying from the stress placed on root systems when there is too little water in the soil. This existential threat deserves an adequate response. The Grand Challenge Climate change is the grand challenge of the twenty-first century. The fate of human 9/10
civilization may well depend on whether we rise in a rational and scientific manner to meet this challenge. The ultimate goal of our group is to marry the technologies of molten salt reactors and supertorrefaction. We can transfer the heat carried in the radioactive blanket salt (ThF4/NaF/BeF2) to a non-radioactive working salt (NaAc/KAc) via a secondary heat exchanger (an easy coupling depicted in the background of Fig. 3). We can then use nuclear heat to produce from biomass, at very high throughputs, biochar, acetone, and syngas cheaper and cleaner than the highly invasive processes of extracting coal by strip mining and mountain-top removal, petroleum by drilling in the ocean deeps, and natural gas from the hydraulic fracturing of shale rock. Baseload electric power can be generated from syngas; liquid transportation fuels can be made from acetone; and carbon-negative sequestration can be achieved with biochar. Coal, oil, and natural gas are valuable Earth resources, and they would not contribute to climate change if they were used to make durable goods, rather than burned. We do not need fossil-fuel companies to go out of business; we need them to go into a different business. Other researchers may have even better ideas for effecting a realistic transition from an economy based on fossil fuels. If so, they should get to work. Through nearly fourteen billion years of the evolution of the physical universe, nature has given us a bounty of Earth energy that can, in principle, replace fossil fuels. It is time for us to do our part. （ Authors/Frank Shu, Michael Cai, Fen-Tair Luo; Translator/Chun-Hui Yang; Reviewers/ Michael Cai）
天聞季報海報版與網路版由中央研究院天文及天文物理研究所製作， 以創用 CC 姓名標示-非商業性-禁止改作 3.0 台灣 授權條款釋出。 天聞季報網路版衍生自天聞季報海報版。超出此條款範圍外的授權，請與我們聯繫。 創用 CC 授權可於以下網站查閱諮詢 https://isp.moe.edu.tw/ccedu/service.php。
10/10