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The purification and its influence on the property are important in developing low activation ferritic steels for fusion applications. In order to clarify the influence of purity on microstructure development in Fe-Cr alloys, the cavity formation caused by relatively high amount of helium was studied. High-purity Fe-9Cr binary alloys (99.99-99.995 wt% purity) and commercial-grade ones (99.9-99.95 wt% purity) were prepared. About 1000 appm helium associated with about 0.2 dpa was implanted into TEM disks at room temperature by using a Dynamitron accelerator with energy degrader. After post-irradiation annealing at 1123 K for 3.6 ks small cavities were observed in the high-purity alloys half in average size and 3-10 times higher in density compared with commercial-grade ones. The role of trace elements in cavity formation was discussed.

Phosphorus and/or titanium modified Fe-16Cr-17Ni austenitic alloys doped with ^B were irradiated in FFTF/MOTA at 684 K for the purpose of investigating the influence of helium generation on the swelling suppression effects by phosphorus and titanium. The increase in void density caused by ^B addition resulted in swelling increase in phosphorus-modified alloys. The close relation between the dislocation density and the void swelling implied the enhancement of dislocation density as a possible mechanism of the swelling suppression by phosphorus addition at the present temperature.