TEMPERATURE EFFECT OF Xe ION IRRADIATION TO 316 AUSTENITIC STAINLESS STEEL

Abstract

Molten Salt Reactors (MSR) is one of the six most promising Generation IV fission reactors. In the ultimate goals, MSR should run at temperatures over 1000 K, and its neutron irradiation damage doses could reach 100 dpa or more for the core components. Hence, the evaluation of irradiation damage under high temperature for structural materials is of particular importance for ensuring the safe operation of MSR. It is generally accepted the structural materials used for MSR should be Ni-based alloys, especially the Hastelloy N alloy. Recently, the 316 austenitic stainless steel (316SS) was considered as a candidate structural material for MSR. In this study, bulk and TEM specimens of 316SS have been characterized by nanoindentation and TEM to determine the change of microhardness and microstructural evolution after 7 MeV Xe26+ and 1 MeV Xe20+ iron irradiation, respectively. The irradiation experiments were carried out at room temperature (about 22 degrees C) and 600 degrees C, and the ion fluences correspond to calculated peak damge dose of 0.62 and 3.7 dpa. The nanoindentation results for bulk specimens showed the irradiation induced hardening of 316SS irradiated at room temperature, and the hardenability increases with increasing ion damage dose. However, in the case of the irradiation at 600 degrees C, the hardness of 316SS keep the same level with that of the unirradiated specimen. The recovery of irradiation induced hardening occurred at 600 degrees C compared with the room temperature irradiation. The TEM results showed that the presence of high number density of nanoscale dislocation loops, with the diameter of 3 similar to 8 nm, in 316SS irradiated at room temperature. The number density of these dislocation loops increase with the increase of ion damage dose. As far as the irradiated 316SS under 600 degrees C, several solute clusters were observed with the size range from 4 to 12 nm, which a little larger than the dislocation loops. It should be noted that the number density is far smaller than that of the dislocation loops in former case. The radiation induced defects (dislocation loops, solute clusters) were believed to be responsible for the hardening in 316SS. The temperature effect of Xe ion irradiation to 316SS was discussed using the Orowan mechanism. The stronger diffusion of point defects caused by ion implantation at 600 degrees C was considered to be the main reason for the recovery of irradiation induced hardening, in which the vacancies and interstitials have greater probability for recombination and then disappear, resulting in the exiguous nucleation sites for the formation of solute clusters.