Fig. 9-1 Temperature-neutron fluence diagram for nuclear materials

This picture, 15KB

Fig. 9-2 Organization and Activities of the Department of Materials Science and Engineering

Nuclear materials are subject to irradiation with neutrons generated by nuclear fission reactions in the core region of fission reactors, such as the light water reactor (LWR), the high temperature gas-cooled reactor (HTGR) and the fast breeder reactor (FBR). Neutron irradiation significantly changes the material properties by the displacement of lattice atoms and the generation of helium and hydrogen by nuclear transmutation. The radiation damage, including embrittlement or loss-of-ductility and dimensional change caused by swelling, creep and irradiation growth, limits the life-time of the nuclear materials. The endurance limit of the material is naturally dependent upon the temperature of the environment. Moreover, the rate of chemical reactions with the coolant, or corrosion of the structural material is also enhanced by the neutron irradiation as well as by an increase in coolant temperature. The temperature-neutron fluence diagram for nuclear materials used in fission reactors is presented in Fig. 9-1, together with the materials expected to be used in fusion reactors. Since the life-time of materials in a nuclear environment changes significantly with neutron energy, neutron fluence is represented by a unit of dpa (displacement per atom) in the figure. The blanket structure material in a fusion power reactor is required to have a life-time of about 400 dpa, which is roughly twice the endurance limit of fuel assembly materials currently used in a liquid metal-cooled fast breeder reactor. In addition, the energies of neutrons produced in a fusion reactor are much higher than those in fission reactors. It is, therefore, essential for the long-term fusion program to develop longer-lived structural material for use in a higher-energy neutron environment than those for fission reactors. Furthermore, tritium must be produced to fuel fusion power reactors; significant quantities of tritium are not naturally available. The tritium breeding material required has never been used in fission reactors, and so the integrity of the material and tritium recovery should be examined with fission reactors before installing a test module in a fusion experimental reactor (FER). It is natural from the economic standpoint that structural materials should be continuously developed also for use in fission reactors at higher temperature and at higher neutron fluence. Research and development of fusion reactor material is promoted by using materials testing reactors and fast experimental reactors, together with the ion-irradiation facility and high-voltage electron microscope; no fusion experimental reactor is available at present. Fundamental studies of radiation damage of reactor materials are also carried out to evaluate the behavior of materials in a fusion reactor environment with a data base acquired from the irradiation experiments using fission reactors and other facilities. In the circumstances, an intense high-energy neutron source that provides neutrons similar to those produced by nuclear fusion reactions is planned to be built in the near-future to establish correlations of radiation damage in a fusion reactor with that in a fission reactor and other irradiation facilities. Radiation-induced modifications of the atomic arrangements in crystalline material are, on the other hand, applied to the syntheses of novel functional materials as well as to the improvement of material properties. The radiation effect and other fundamental studies on material properties are carried out with non-metallic materials including high-temperature oxide superconductors. It is useful for the fundamental studies to utilize the matter/wave characteristic of neutrons slowed down in a research reactor. In the neutron scattering experiments, crystalline and magnetic structures are investigated for understanding specific physical properties in the oxide and magnetic functional materials.

Select a topic in left column