Study on Heavy Neutron Irradiation Effects of Refractory-Metal-Base, High-Z and High-Heat-Flux Materials for Fusion Power Reactors

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Neutron irradiation effects of refractory metals, molybdenum (Mo), tungsten (W) and their alloys were studied in the program. The aim is to propose the availability of high-Z materials in the condition of high thermal load and heavy neutron irradiation as in fusion power reactors. In power reactors, high-heat flux materials must be survived after neutron exposure at high temperatures. In these materials, it is important to control the microstructure during irradiation in order to suppress the neutron-irradiation embrittlement.In this study, Mo, TZM,Mo-Re and W-Re alloys were used. These materials are added with titanium and zirconium (for TZM alloy) and rhenium to improve the weldability and ductility after recrystallization. Material properties of these alloys after neutron heavy irradiation up to high fluence have not been studied well. The influence of irradiation conditions and materials parameters on the mechanical properties was studied.The specimens were irradiated in the Fast Flux Test Facility (FFTF) at temperatures from 360C to 800C and fluence up to 34 dpa. With these specimen, the observations of microstructure, tensile tests and bend tests were carried out. It was found that heat treatments before irradiation affect the ductility change caused by neutron irradiation. Especially, stress-relieved treatment is found to be effective to suppress irradiation-induced embrittlement. Re addition is effective for suppression of recrystallization during irradiation at high temperatures over 800C.But excessive Re content enhances irradiation-induced precipitation, which results in conspicuous irradiation hardening and embrittlement.As used in irradiation environment at high temperatures, such as in power fusion reactors, the alloys of Mo and W must be required with optimum Re content, as much as to suppress the growth of grain boundary and retard irradiation-induced precipitation.