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In this work, we have studied the effect of helium and its injection mode on the microstructure of Type 316 and titanium-modified Type 316 stainless steels, both in their solution-annealed and cold-worked states. Irradiations have been conducted in a dual-beam accelerator to doses up to 150 dpa in a wide range of temperatures, from 550 to 750°C. Different injection modes have been investigated: cold preinjection, hot preinjection, and dual beam. The results show that the effect of helium on swelling is largely dependent on the injection mode and also on the chemical composition of the alloy considered. In Type 316 steel, the presence of helium decreases swelling, the major effect being observed when helium is preinjected at room temperature. In titanium-modified steels, if a high density of tiny helium bubbles are present in all the specimens containing helium, void swelling only occurs for dual-beam irradiations. In conclusions, this work shows that the effect of helium on void swelling depends on its injection mode, but also that the sense of this effect is clearly different when comparing the behavior of a low and a high swelling alloy.

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This paper deals with the irradiation behavior of three ferritic steels, namely F17 (17Cr), EM12 (9Cr-2MoNbV) and EM10 (9Cr-lMo). These alloys were irradiated up to 100 dpa in Phénix as samples or wrapper tubes. The immersion density measurements confirm their high swelling resistance, but the tensile and impact tests reveal great differences in mechanical properties. The ductile-brittle transition temperature (DBTT) of F17 is strongly increased, and compared to an aging treatment, the irradiation amplifies and shifts the embrittlement towards lower temperatures. In contrast to F17, the mechanical properties of EM10 are unaffected by irradiation, while EM12 has an intermediate behavior. The transmission electron microscopy (TEM) examinations show that all the small density changes come from irradiation-induced voids and that the embrittlement of F17 results from α′ phase formation enhanced by irradiation. In conclusion, EM10 is by far the most attractive candidate for wrapper applications. Its fully martensitic structure provides an improved swelling resistance and its chemical composition should inhibit the microstructural instabilities that are responsible for the embrittlement of F17 and EM12.

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