2009 NTT NEWS
IV Generation Nuclear Power Reactors and BlueSteel C/C C.H. Bosch NTT Group / NTT Aerospace 25‐Jan‐2009
Generation IV Nuclear Power Reactors and BlueSteel C/C
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By C.H. Bosch
What is Carbon/Carbon BlueSteel™ Reinforced Carbon-Carbon (carbon-carbon or C/C) is a composite material consisting of carbon fiber reinforcement in a matrix of graphite, often with a silicon carbide coating to prevent oxidation. It was developed for the nose cones of intercontinental ballistic missiles, and is most widely known as the material for the nose cone and leading edges of the Space Shuttle. The Brabham team pioneered its use in the brakes of Formula One racing cars in 1976, and more recently it has also appeared in the brakes of some high end supercars, such as the Bugatti Veyron. Carbon-carbon is well-suited to structural applications at high temperatures, or where thermal shock resistance and/or a low coefficient of thermal expansion is needed.
Production of Carbon/Carbon The material is made in three stages. First, material is laid up in its intended final shape, with carbon filament and/or cloth surrounded by an organic binder such as plastic or pitch. Often, coke or some other fine carbon aggregate is added to the binder mixture. Second, the lay-up is heated, so that pyrolysis transforms the binder to relatively pure carbon. The binder loses volume in the process, so that voids form; the addition of aggregate reduces this problem, but does not eliminate it. Third, the voids are gradually filled by forcing a carbon-forming gas such as acetylene through the material at a high temperature, over the course of several days. This long heat treatment process also allows the carbon to form into larger graphite crystals, and is the major reason for the material's high cost. C/C is a generally hard material that can be made highly resistant to thermal expansion, temperature gradients, and thermal cycling, depending on how the fiber scaffold is laid up and the quality/density of the matrix filler.
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BlueSteel™ C/C General Properties GROUP Satisfy Your Progress®
The most important class of properties of carbon-carbon composites is their thermal properties. BlueSteel™ C/C composites have very low thermal expansion coefficients, making them dimensionally stable at a wide range of temperatures, and they have high thermal conductivity. BlueSteel™ C/C composites retain mechanical properties even at temperatures (in nonoxidizing atmospheres) above 2000°C. | 3/8 They are also highly resistant to thermal shock, or fracture due to rapid and extreme changes in temperature. The material properties of a BlueSteel™ C/C composite vary depending on the fiber fraction, fiber type selected, textile weave type and similar factors, and the individual properties of the fibers and matrix material. For this reason NTT manufacturing phases are checked three times prior, during and after the C/C process. Fiber properties depend on precursor material, production process, degree of graphitization and orientation, etc. The tensioning step in fiber formation is critical in making a fiber (and therefore a composite) with any useful strength at all. Matrix precursor material and manufacturing method have a significant impact on composite strength. Sufficient and uniform densification is necessary for a strong composite. Generally, the elastic modulus is very high, from 15-20 GPa for composites made with a 3D fiber felt to 150-200 GPa for those made with unidirectional fiber sheet. Other properties include low-weight, high abrasion resistance, high electrical conductivity, low hygroscopicity, non-brittle failure, and resistance to biological rejection and chemical corrosion. Carbon-carbon composites are very workable, and can be formed into complex shapes.
What is Generation IV Generation IV is a multinational collaboration for the research, development, and construction next generation pilot nuclear power plant by 2015. Under Generation IV several options are being studied internationally: Very High Temperature Gas-Cooled Reactor Gas-Fast Reactor Molten Salt Reactor Super Critical Water Reactor Lead Fast Reactor Sodium Fast Reactor Generally speaking in a VHTR the application of carbon/carbon has produced since early tests very good result, as it is a composite combination with graphite-like features. NTT Group has developed a line of expertise in various power management industries, and under the Generation IV project it has included studies mostly for VHTR and MSR. The most interesting part of this application is relating to its relatively low costs compared with benefits of many different applications in the nuclear power plant.
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In the pictures below the carbon/carbon composite material behaviour under irradiation is well described.
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On the long range, as Composite allows “engineering” of properties such as dimensional change, the following pictures show the result on the composite material after 1 year exposure and different temperatures (500°C and 800°C respectively).
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Among many viable alternatives C/C composite are more mature and have clear advantages in cost, manufacturability and some thermomechanical properties (eg thermal conductivity.) Very interesting the comparative graphic, where the composite Carbon/Carbon, the BlueSteel™, gets the top values in the longest range.
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More than everything though we have found the widest interest in developing the MSR, Molten Salt Reactors.
What is a Molten Salt Reactor (MSR) Molten Salt Reactors (MSRs) are liquid-fueled reactors that can be used for production of electricity, actinide burning, production of hydrogen, and production of fissile fuels. Electricity production and waste burndown are envisioned as the primary missions for the MSR. Fissile, fertile, and fission isotopes are dissolved in a high-temperature molten fluoride salt with a very high boiling point (1,400°C) that is both the reactor fuel and the coolant. The near-atmospheric-pressure molten fuel salt flows through the reactor core. The traditional MSR designs have a graphite core that results in a thermal to epithermal neutron spectrum. Alternative designs are now being explored with no reactor internals and a fast neutron spectrum. In the core, fission occurs within the flowing fuel salt that is heated to ~700°C, which then flows into a primary heat exchanger where the heat is transferred to a secondary molten salt coolant. The fuel salt then flows back to the reactor core. The clean salt in the secondary heat transport system transfers the heat from the primary heat exchanger to a hightemperature Brayton cycle that converts the heat to electricity. The Brayton cycle (with or without a steam bottoming cycle) may use either nitrogen or helium as a working gas.
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The use of a liquid fuel, versus the solid fuels of the other Generation IV concepts, creates potentially unique capabilities that are not achievable with solid-fuel reactors, but it also implies a different set of technical challenges than other Generation IV concepts. The unique capabilities include: Destruction of long-lived radionuclides without the need to fabricate solid fuels A wider choice of fuel cycles (once through, waste burning, fissile fuel production [breeding]) without major changes in the reactor design Very low fissile fuel inventory relative to other reactor concepts (fissile inventory may be as low as a tenth of a solid-fuel fast reactor per kWe) that may create alternative safeguards strategies Full passive safety in very large reactors with associated economics of scale (under accident conditions, the fuel is drained to passively cooled, critically safe storage tanks) Limiting the radioactivity in the reactor core (accident source term) by on-line removal and solidification of the mobile fission products Limited excess reactivity requirements in the core due to on-line fuel
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The MSR was the high-temperature reactor developed to provide high-temperature heat for aircraft propulsion in the 1950s. It was then developed as a breeder reactor in the 1960s and early 1970s. Many of the technical challenges were a direct or indirect consequence of the limits of high-temperature technologies at that time. The Next Generation Nuclear Plant (NGNP) baseline concept is the modular, Very-High-Temperature Reactor (VHTR) using helium cooling. Because the NGNP is a high-temperature reactor, the | 7/8 development of the NGNP provides multiple key technologies for an Advanced Molten Salt Reactor (AMSR) such as Brayton power cycles (to replace earlier MSR steam cycles), compact heat exchangers (to replace tube-and-shell heat exchangers), and carbon-carbon composite materials (to replace some metallic components). The new technologies developed for the NGNP potentially imply major reductions in capital cost and reduce or eliminate about half of the technical challenges identified with MSRs.
Materials R&D, the BlueSteel™ for Nuclear applications The major goal of the materials R&D is to identify and qualify materials with properties appropriate for MSR operating conditions, including corrosion resistance, mechanical performance, and radiation performance. The primary materials of interest are the moderator (graphite) for thermal neutron spectrum MSRs and the reactor vessel/primary loop alloy (presently a Ni-based alloy). It is also necessary to develop corrosion control and coolant monitoring strategies for protecting the reactor vessel and primary piping alloys. The viability R&D will establish the primary candidate materials and control and monitoring strategies for further testing. In addition to the historical experimental experience with molten salts at very high temperatures (~900°C) obtained for the Aircraft Nuclear Propulsion Program, an extensive materials development effort supported engineering code qualification for the MSBR to operate at 705°C. This temperature limit was largely due to the coupling required for steam cycle operations and did not represent a fundamental limit. Thus, there temperature graphite as applications; NGNP.
is a natural base to build on to extend candidate materials for the higher objectives of the Generation IV Program. The MSR and NGNP both use a moderator and various carbon-carbon composites for multiple structural consequently, graphite and carbon-carbon research will be coupled to the
Also, because the NGNP is currently pursuing Ni-based super alloys for reactor components, development of Ni-based alloys for molten salts is coupled to the NGNP efforts. In parallel, there is ongoing European Community work on testing of advanced MSR alloys. NTT R&D Department supplies many components and variation of the BlueSteel CarbonCarbon materials in order to perform different tests under the GIF (Generation IV International Forum) schedule. 2009 and 2010 are the years of GIF coordination and assessment of integrated development and commercialization plan. Then a final development stage will be defined from 2011 to 2016 to reach the Generation IV Nuclear Power Reactor.
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Dr. Carl-Heinz Bosch, is the Head of the NTT Group’s Research & Development activities. In this role, he structures the Company development programs, to be on the edge leading the carbon fiber market. Dr. Bosch has over 15 years of experience in developing and managing carbon-related industrial technologies, with a specialty on carbon-carbon technologies and aerospace applications.
Contact: NTT Aerospace LONDON United Kingdom website: http://www.gontt.com e-mail: info@gontt.com