INFLUENCE OF THE NEUTRON SPECTRUM ON THE TENSILE PROPERTIES OF IRRADIATED AUSTENITIC STAINLESS STEELS, IN AIR AND IN PWR ENVIRONMENT

摘要

In order to address the issue of highly irradiated austenitic stainless steels of the core internals of PWRs, EDF has initiated a R&D program based upon the irradiation of test materials in several experimental reactors (fast breeder reactor and light water reactors) in a temperature range 300-400℃, up to the damage of the end of life (～ 80 dpa for 40 years service). However, irradiation conditions in fast breeder reactors are not those of a PWR: the neutron flux is higher, the helium and hydrogen production rates are lower, due to lower thermal neutron flux. Helium is responsible for high degradation of the ductility at irradiation temperatures above ～ 500℃. At temperatures below 400℃, helium is believed to have no influence on strengthening or loss of ductility of austenitic stainless steels. An experiment has been designed in order to verify that conclusion. To assess the influence of the neutron spectrum both on the tensile characteristics in air and in PWR environment, the Samara experiment is aiming at the irradiation of tensile specimens in the SM experimental light-water reactor of the Research Institute for Atomic Reactors (RIAR, Russia), at a temperature close to ~ 300℃. Specimens were irradiated in two positions: a position with a pure fast neutron spectrum, and a position with both fast and thermal neutron fluxes (in order to produce a high helium content). The tensile tests results obtained after irradiation in the frame of this experiment show that helium has no influence on strengthening or loss of ductility of austenitic stainless steels (CW 316 and SA 304L) at low temperature (300℃). This experiment also allows assessment of the coherence of the mechanical behavior of these materials in the different irradiation experiments. With regard to mechanical properties, the irradiation in fast breeder reactor seems very similar to irradiation in light water reactors at low temperature. In simulated PWR water, the ductility of the specimens (both CW 316 and SA 304L) is strongly reduced and their fracture surface partly brittle. The ductility seems to be lower and the fracture surface more intergranular for the specimens with "low helium", which is, irradiated with pure fast neutrons. Nevertheless, with regard to corrosion-mechanical properties, the irradiation with fast neutron spectrum seems comparable to irradiation with mixed neutron spectrum at low temperature.