Void swelling of Japanese candidate martensitic steels under FFTF/MOTA irradiation

Microstructural observations of six Japanese candidate 7-9% Cr reduced activation martensitic steels were carried out after heavy neutron irradiation in order to investigate the void swelling behavior of each steel. Neutron irradiations were performed in the FFTF/MOTA up to 67 dpa at temperatures between 638 and 873 K. Transmission electron microscope observations revealed that voids were formed in all the steels irradiated to 67 dpa at 703 K, and the highest void swelling was observed in JLM-1 which was added with 30 wt.ppm of boron (0.74%), and the minimum void swelling was observed in F82H steel (0.12%). The 9% Cr martensitic steels showed the peak of void swelling at temperatures around 700 K, where void swelling gradually increased with increasing irradiation fluence to 30 dpa and increased rapidly above it. It is considered that the incubation period of void swelling of 9% Cr martensitic steels (JLM series) is about 30 dpa. JLM-1 showed the highest void swelling rate (0.045%/dpa at most). The addition of 30 wt.ppm of boron enhanced void swelling, while it was suppressed by the addition of 100 wt.ppm Ti in the 9% Cr martensitic steel. The JLF-3 steel (7.03% Cr) and F82H (7.65% Cr) showed less void swelling than JLF-1 (9.04% Cr). The alloying effects on the swelling behavior of the steels were interpreted in terms of the difference in the precipitation morphology of carbides. (orig.)