Michio Yamawaki

Corrosion behavior of Hastelloy-N alloys in molten salt fluoride in Ar gas or in air

The effects of air on the corrosion of Hastelloy-N alloys in molten salt coolant containing fission product elements were investigated to determine the safety of structural materials in high-temperature reactors cooled with fluoride salt. Corrosion tests of Hastelloy-N in the molten fluoride salt FLiNaK in an alumina crucible and a graphite crucible under argon gas or air were performed at 773–923 K for 100 h. The depth of corrosive attack, as well as the extent of chromium and molybdenum depletion, increased with increasing temperature. The extent of Hastelloy-N corrosion in molten salt under air was significantly greater than under argon gas. The effect of adding the impurity cesium iodide to molten salt containing nuclear waste fuel on the corrosion behavior was negligible.

Utilization Research and Development of Hydride Materials in Fast Reactors

Metal hydrides have high hydrogen atom density, which is equivalent to that of liquid water. An application of the hafnium hydride has been investigated as a neutron absorber in the Fast Breeder Reactors (FBRs). Fast neutrons are efficiently moderated by hydrogen in Hf hydrides and are absorbed by Hf. Since three isotopes of Hf have large cross sections, increase in the life of control rod is considered by Hf hydride. Results of design study of the core with Hf hydride control rods shows that the long lived hafnium hydride control rod is feasible in the large sodium-cooled FBR. Results of irradiation test conducted in BOR-60 has demonstrated the integrity of the capsules during irradiation. Na bonded capsule has an advantage in confinement effect of hydrogen compared with He bonded one. An application of hydride technique to transmutation target of MA was also discussed. MA hydride target is able to enhance the transmutation rate in FBR.

Depleted Uranium as Hydrogen Storage Material

Large amounts of depleted uranium kept as uranium fluoride or solid form after enrichment of natural uranium is sought to be utilized in the form of UNiAl intermetallic compound for hydrogen absorber. First principles calculation on UNiAl hydride has been performed in this study to predict the change of the crystal structure and the lattice constants with varying the hydrogen content. The results of the calculations have supported the experimental trends, suggesting that the present approach is promising in predicting the better hydrogen absorber based on depleted uranium.

Substantial Reduction of High Level Radioactive Waste by Effective Transmutation of Minor Actinides in Fast Reactors Using Innovative Targets

The problem of managing high-level long-lived radioactive wastes is one of the difficult issues associated with fission reactors. Long-term radiotoxicity is dominated primarily by minor actinides (MAs: Np, Am, Cm) and long-lived fission products (FPs: 99Tc, 129I, and so on). The potential radiotoxicity of an isotope is defined as the ratio of its radiotoxicity to the annual limit on intake (ALI) by ingestion. Calculations have shown that the potential radiotoxicity level of the waste would be reduced to that of natural uranium ore after 1,000 years provided that the MAs and long-lived FPs are removed from the waste and transmuted in reactors (Kondo & Takizuka, 1994). Extensive core design studies have been performed to assess the fast reactor (FR) capability for transmuting the MAs (Wakabayahi et al., 1995), (Kawashima et al., 1995). It was pointed out that while the MAs are to be charged to the core in FRs and an annual transmutation rate of more than 10% is possible, significant problems would be encountered in the core safety characteristics, such as the sodium void reactivity and the Doppler coefficient. Many concepts for transmutation of the MAs have been proposed using light water reactors (LWRs) (Takano et al., 1990), (Masumi et al., 1995) as well as FRs. In LWRs, the neutron fluxes are lower than those in FRs, but neutron spectra are so softer that the neutron cross sections of the MAs are larger. Thus, LWRs provide similar MA transmutation performance to FRs according to core analysis. However, more of the higher actinides are produced by MA recycling and the reactivity penalty for MA loading is larger than that in FRs.