Takeshi Miyazawa

Moderation of Negative Oxygen Effects by Small Yttrium Addition to Low Activation Vanadium Alloys

Abstract In order to improve irradiation embrittlement of vanadium alloys for fusion reactors, yttrium (Y) has been added reducing the interstitial oxygen impurity. However Y addition can also degrade high-temperature strength, because Y could scavenge oxygen in solid solution, which is a strong hardening agent in vanadium alloys. In this study, the effect of Y addition and oxygen level on the mechanical properties was investigated from the view points of both the high-temperature strength and low temperature ductility. Y addition was suggested to moderate the hardening and embrittlement induced by oxygen impurity sustaining the high-temperature strength within an acceptable level.

Impact Properties of Low Activation Vanadium Alloy After Low-Temperature Neutron Irradiation at 450°C and Below

Abstract A reference low-activation vanadium alloy NIFS-HEAT-2 was neutron-irradiated at 450 °C and below, in order to estimate the resistance to low temperature irradiation. DBTT of NIFS-HEAT-2 was -85 °C after irradiation up to 8.5 dpa at 450 °C in Na atmosphere, while DBTT was below -196 °C for 3.7 dpa at 430 °C in Li atmosphere. On the other hand, DBTT was lower than about -90 °C for the irradiation up to 0.1˜1 dpa at 60, 290 and 400 °C. The DBTT shift was increased with increasing hardness after neutron irradiation for limited irradiation conditions. The mechanisms of DBTT shift and irradiation hardening at low temperature was discussed.

Impact property of low-activation vanadium alloy NIFS-HEAT-2 after electron beam welding and neutron irradiation

Abstract In order to investigate the effect of post-weld heat treatment (PWHT) and post-irradiation annealing (PIA), electron-beam-weld specimens of the reference low activation vanadium alloy, NIFS-HEAT-2, were neutron-irradiated to a fluence of 7.62 × 1023 neutron m−2 (E > 1 MeV) at 563 K in Belgian Reactor-2. In the present experiments, unexpected oxidation of the surface of the samples occurred during the neutron irradiation, and significantly degraded impact properties of the weld metal, while the degradation was not significant for the base metal. The removal of the oxidized layer by electro-polishing improved the impact properties of the weld metal. Although complete removal of the oxidized layer could not be confirmed, it is revealed that impact absorbed energy of the weld metal with post-weld heat treatment at 1073 K was comparable to that of the base metal after the post-irradiation polishing. In other words, irradiation embrittlement of the weld metal was successfully suppressed by the PWHT. PIA at 773 K and above was effective to recover the irradiation hardening and irradiation embrittlement. Mechanisms of the irradiation hardening, irradiation embrittlement and its recovery were discussed.