Development of fabrication technology for low activation vanadium alloys as fusion blanket structural materials

Vanadium alloys, candidate fusion reactor blanket materials, consist of intrinsic low activation elements for 14 MeV fusion neutrons (e.g. vanadium, chromium and titanium). From 500 to 1000 K, the tensile strength of vanadium alloys is independent of temperature, enabling the fusion blanket to be operated at least 100 K higher than a blanket made from ferritic steel. However, most of the data have come from laboratory- scale fabricated vanadium alloy, so a feasibility study on large scale production of vanadium alloy products is required. In the present study, high-purity vanadium alloy products (e.g. plates, wires and tubes) were fabricated from reference high-purity V-4Cr-4Ti ingots designated as NIFS-HEAT, with the use of technologies applicable to industrial scale fabrication. A critical issue for NIFS-HEAT large-scale melting was to reduce the levels of interstitial impurities (e.g. C, N and O) that are known to deteriorate mechanical properties before and after neutron irradiation. Impurity behaviour during working and annealing processes and its effect on mechanical properties were investigated. Mechanical properties of the products were significantly improved by the control of Ti-C, N and O precipitation induced during the processes. Good mechanical properties comparable to laboratory- scale alloys were obtained at above 96% in cold working degree.