High-Temperature Embrittlement of AISI Type 316 Austenitic Stainless Steels after Irradiation

Abstract

The results presented below are related to potential cladding materials for fast reactor fuel elements, that is, austenitic stainless steels and precipitation-hardened nickel alloys. These materials were irradiated to a dose of 3 × 1021 nvt (E > 1 MeV) in different thermal reactors and to a dose of 1.6 × 1022 nvt (total dose) in a fast reactor. The irradiation temperature was about 700 C in thermal reactors and about 500 to 600 C in a fast reactor. After irradiation, tension tests and uniaxial stress rupture tests were performed at temperatures in the 600 to 800 C range. For all alloys, high-temperature tests after irradiation have shown a more or less pronounced embrittlement, not recoverable by any post-irradiation annealing. This embrittlement is associated with intergranular wedge-type cracks initiating at triple points. For a given alloy, at a given temperature, the lower the rate of deformation, the more pronounced the embrittlement. For Type 316 austenitic steels, we observed that the radiation induced hardening was low and disappeared at a maximum temperature of 800 C. The embrittlement was associated with a decrease in the nonuniform part of the elongation in tension tests and with a decrease in the elongation and duration in the tertiary stage of stress-rupture tests. Two types of factors were studied: 1. Those concerning the structural parameters: grain size, precipitation, prestraining, and chemical composition. 2. These concerning the actual conditions of irradiation.