Effect of Ta addition on irradiation resistance properties of austenitic stainless steels for reactor pressure vessel internals

In order to elevate the resistance against irradiation-assisted stress corrosion cracking (IASCC) of reactor pressure vessel internals (RINs) in the environment of boiling water reactor (BWR), austenitic stainless steels (SSs) with tantalum (Ta) addition have been examined. The oversized element Ta is considered to reduce the concentration of free vacancy due to trapping effect. It is expected that radiation-induced grain boundary (GB) segregation of constituent element such as chromium (Cr) can be suppressed by Ta addition. In this study the irradiation tests were performed with Fe²⁺ ion in a dose range from 0.1 to 3 dpa on the specimens sampled from base materials and heat-affected zone (HAZ) of welding joint plates. Considering the Cr segregation might also occur during the heat history of welding process, we first confirmed and discussed the effect of Ta addition to SUS310S on irradiation resistance properties in the HAZ. The radiation-induced segregation (RIS) on random GBs in HAZ after the irradiation tests was evaluated by scanning transmission electron microscope (STEM) and X-ray energy dispersive spectroscopy (XEDS). The improvement of resistance against RIS by Ta addition was evaluated. Further the effect of Ta addition on radiation induced hardening was investigated on the base materials, but from the results of nanoindentation test the obvious relief of radiation-induced hardening was not confirmed. The corrosion properties after heavy ion irradiation were also discussed on the base materials in the conditions of electrochemical potentiokinetic reactivation (EPR) test. GB corrosion was observed on the ion-irradiated surface of commercial material of SUS310S. However, no GB corrosion was observed on the irradiated surface of SUS310S with 0.4 % Ta addition, indicating the effect of Ta addition on the improvement of resistance against IASCC.