High-temperature creep properties of NIFS-HEAT-2 high-purity low-activation vanadium alloy

A low-activation vanadium alloy with the composition of V-4 mass% Cr-4 mass% Ti is an alternative to reduced activation ferritic/martensitic steels for blanket structural material in fusion reactors. The NIFS-HEAT-2 (NH2) is the world's highest-purity V-4Cr-4Ti alloy among over 30 kg scale heats. The processes to purify this heat have successfully enhanced workability, weldability and low-activation characteristics, however degradation of high-temperature strength was a concern due to possible purification softening. Therefore, the present study evaluated high-temperature creep properties of NH2 at around the operation temperature, 700 °C-800°C. The degradation of creep properties, such as shorter rupture time and enhanced creep rate, were observed for NH2, compared with a conventional US heat in the stress range of 150-200 MPa. On the other hand, it was revealed that these creep properties were comparable to those of the US heat in the lower applied stress of 100 MPa, though only short term rupture data are available at this moment. Since design stress for fusion blankets is expected below 100 MPa, purification up to the level of NH2 will induce no negative impact on the blanket service conditions.