Microstructural characterizations of austenitic stainless steels representative of PWR internals irradiated with ions from low to high doses and comparison to cluster dynamic simulations
Résumé
The French nuclear industry is looking into the extension of the operation time of pressurized water reactors (PWR) up to 60 years. The lower parts of the internals are composed of Solution Annealed (SA) 304 austenitic stainless steel plates and Cold Worked (CW) 316 stainless steel bolts. Due to their high exposition to irradiation it is expected to reach doses as high as 120 dpa after 60 years, at temperatures in the range of 300 to 370°C.
The microstructures under ion irradiations of two 304 austenitic stainless steels grades that mainly differ in carbon content are investigated by Transmission Electron Microscopy (TEM). The evolution of radiation-induced defects as cavities, Frank loops and precipitates with the dose are studied as well as the influence of carbon.
Ion irradiations from 5 to 100 dpa have been carried out at JANNuS-Saclay facility on using iron ions (10 MeV). To counterbalance the flux effect, irradiation temperature was set to 450°C.As observed in PWR internals, Frank loop density and size reach saturation at about 5 dpa. This validates the chosen shift of temperature. Major precipitation is observed in both 304 at high doses, however precipitates appears earlier and in higher concentration in the higher carbon content steel. Voids are present, and are relatively small in size and in low density considering the dose. As the dose increases the mean size and void density increase as well, with exception at high doses in 304L where a bimodal size distribution is observed. Carbon and precipitation seem to delay germination of cavities. In-situ irradiation up to 1.5 dpa were also performed at JANNuS-Orsay facility. It allows to investigate the evolution and creation of defects in the early moment of irradiation. Apparition of black dots and evolution of the dislocation network (Frank loops and perfect dislocations) are visible at the very beginning of irradiation. Finally, the evolution of the microstructure under irradiation with time (doses) and space (along the damage profile) is successfully modelled using Cluster Dynamic (CD) code CRESCENDO. Calculations along the profile damage show that surface and injected interstitials have an effect on the microstructure and confirm experimental observations. The effect of the surface is observable on cavities and Frank loops, on a large depth. Also, injected interstitials have an effect on both cavities and Frank loops. Microstructures as a function of the depth are explained based on the coupling between experiments and CD calculations