Chapter 5
5.4 Fusion/Fission Power Generation Materials
Johsei Nagakawa
IMEL (Innovative Materials Engineering Laboratory), National Institute for Materials Science
1. Introduction
Nuclear fission of heavy elements like uranium and nuclear fusion of light elements like hydrogen are the two nuclear reactions applicable for the generation of electricity. Thermal energy is produced by these nuclear reactions and utilized to operate a steam turbine for the electrical generator. Nuclear power generation by fission reaction of uranium is already commercialized and now supplies more than 30% of electricity in Japan. Fusion energy, on the other hand, has been regarded as an inexhaustible energy resource that could save the world, and International Thermonuclear Experimental Reactor (ITER) is now being constructed as a big step toward its realization. One of the most essential problems for the fusion/fission power materials is the radiation damage which is a degradation caused by high energy particles like neutrons produced in the nuclear reactions. In this article the radiation damage is briefly explained and the status of research and development of fusion/fission power materials will be described.
2. Radiation Damage
Fig. 5.4.1 shows how the crystal lattice atoms are displaced by incident neutrons. Such atomic displacements produce point defects (interstitial atom and atomic vacancy). Occasionally the displaced atom itself can force other atoms out of their lattice points and this phenomenon is called cascade damage. Most of the point defects would annihilate through mutual recombination or absorption by grain boundaries or dislocations. Portion of point defects, however, agglomerate and continuously grow under irradiation. In general vacancies form three-dimensional void (cavity), resulting in a swelling of materials, and interstitial atoms form two-dimensional dislocation loops, mainly responsi-
ble for radiation hardening and conversely an enhanced creep deformation under irradiation. Moreover, point defect reacts with solute atoms in the material so that its migration causes a locally high concentration of solutes, which is called irradiationinduced segregation and affects various properties like corrosion.
Reaction cross-sections for (n, α) and (n, p) increase steeply as neutron energy is higher than about 5 Mev, and resultant nuclear transmutations will produced large amounts of transmuted elements and gases (helium and hydrogen), which is called transmutation damage. Most of the transmuted elements are radioactive and would be a burden to the environment. The produced gaseous atoms may possibly induce embrittlement and degrade the mechanical properties considerably.
3. Fusion Reactor Materials
Figure 5.4.2 schematically shows ITER. ITER is a Tokamak type fusion device, which magnetically confines the fusion plasma of deuterium (D) and tritium (T), and its major mission is to prove a long-lasting burn of D-T fusion plasma. Since neutrons with energy as high as 14 MeV are produced in the D-T fusion plasma, the first -wall/blanket components, which protect a vacuum vessel and superconducting magnets, will be exposed to an extremely severe environment. Thus, these components consist of modules and will be replaced during the ITER operation (20 years) 1). Beryllium will be applied to the plasma-facing portions and copper alloys dispersion-strengthened with alumina will be used for the thermal sinks. Outer portions of the blanket will be made of nitrogen-added low-carbon steel (316LN-ITER grade)
Fig. 5.4.1 Atomic displacement by neutrons. |
Fig. 5.4.2 Schematic figure of ITER. |
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Materials Outlook for Energy and Environment
which shows good high-temperature strength.
In DEMO (the next device after ITER, and the first to generate electricity) fusion plasma will be more intense and the firstwall/blanket materials will be exposed to a much harder damage at higher temperatures. Therefore, materials with excellent swelling resistance and thermal conductivity are required for the firstwall/blanket components. In addition low-activation materials composed of specific elements of which radio-activity would decay swiftly after the nuclear transmutation are essential for the sake of environments. All the above consideration leads to lowactivation ferritic steels as the most promising structure material 2), and the synergetic effects of irradiation and externally applied loads (stresses and environments) have been vigorously investigated. Also, vanadium alloys and SiC-composites are expected to be low-activation fusion materials with more excellent hightemperature strength.
4. Fission Reactor Materials
Electric power generation by light water moderated reactors
(LWRs) has already been established. Figure 5.4.3 shows the basic components of a pressurized water reactor (PWR). Since the energy of neutrons produced by the fission reaction is about 1 MeV at the most, transmutation damages are not so serious in LWRs. Major materials used are zirconium alloys, which show low neutron absorption and good corrosion resistance, for the fuel cladding, nickel alloys for the steam generator, and SUS 304 and SUS 316L for the in-core structure components 3). Irradiationinduced stress corrosion cracking (IASCC) induced by the deterioration of corrosion resistance through irradiation-induced solute segregation has been worried about as the reactor aging has been proceeding. For the suppression of IASCC, SUS 316L with lowered phosphorous content and austenitic stainless steels with high chromium content have been considered.
The fast breeder reactor (FBR) has been developed as the next-step fission nuclear reactor following LWR 4). FBR shows utilization efficiency of uranium of about 60%, much higher than that of about 0.5% in LWRs, capable of using the uranium resources far more efficiently. In the demonstration FBR Monju, SUS 304 is used for the reactor vessel and the pinging from the viewpoint of compatibility with liquid sodium coolant, and SUS 321 or 2 1/4Cr-Mo steel is applied to the components in contact with boiling water such as the steam generator. In the development of next-step demonstration FBRs, SUS 316LN steels modified with trace elements such as Ti or Nb and improved 9Cr-1Mo steels have been considered for the reactor vessel and for the steam generator, respectively. Efforts are now being made to gather and expand the database for these advanced FBR materials.
Fig. 5.4.3 Basic components of PWR.
References
1)M. Mori et al., J. Atomic Energy Soc. of Japan, 44 (2002) 16 (in Japanese).
2)R.L. Klueh et.al.: J. Nucl. Mater. 307-311 (2002) 455.
3)J.T. Adrian Roberts: Structural Materials in Nuclear System, Olenum Press, New York and London (1981).
4)Engineering Committee for Fast Breeder Reactors, Status and Prospects of Fast Breeder Reactors, Japan Nuclear Energy Soc., (1987) (in Japanese).
For more information on fusion/nuclear power generation materials,
1)Fusion reactor Materials, Ed. By N. Igata, Baihu-kan, Tokyo 1081) (in Japanese).
2)Nuclear materials – Lecture Series Modern Metallurgy Materials Issue 8, Ed. By Japan Inst. of Metals, Maruzen, Tokyo, (1989) (in Japanese).
Chapter 5
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Chapter 5. Materials for High-Efficiency Major Power Plants