No abstract

Many components used in the pressure vessel of French pressurized water reactors are made of austenitic stainless steels : 316L for screws, 304L for core barrel and baffle assembly, etc. Among these components, the most irradiated ones undergo an average damage rate of about 1 dpa/an at temperatures which may reach about 400°C. Such irradiations may have very detrimental effects on the in-service behaviour of austenitic steels. In order to forecast these effects, Electricité de France has defined important programmes of studies. In the framework of these programmes, we have carried out a review concerning the effects of irradiations at relatively low temperatures (i. e. 500°C) on cold-worked 316 stainless steel. We were particularly interested in the evolution of microstructure as well as tensile (with or without notch), creep, fatigue (with or without creep) and fracture toughness properties. In some cases, we have also analysed the influence of cold-work level, irradiation temperature and test temperature. For some mechanical properties, the data was sufficient to qualitatively predict the properties of cold worked 316 stainless steels after long — term irradiations under PWR conditions.

We try to develop nondestructive methods to monitor the effects of neutron irradiation in ferritic alloys. In this paper we report results obtained with a method based on thermoelectric power (TEP). This method is already used to monitor the thermal aging of duplex stainless steels. It is shown that the measurement of the thermoelectric power allows the monitoring of the effects of i) electron irradiations on Fe-Cu model alloys, and ii) neutron irradiations on industrial pressure vessel steels. The recovery of these irradiation effects during thermal annealing can also be followed with the proposed method. On the basis of the results obtained on Fe-Cu alloys, we propose a law to determine the limit of copper solubility in α iron at temperatures below 600°C.

The evolution of the microstructure of the French Chooz A pressure vessel steel (characterized by a low copper content (<0.1 at.% Cu)) has been studied during annealing at 450°C. The irradiated material (fluence: 0.5 to 16×1023 n.m-2, flux : 2×1015 n.m-2.s-1, temperature : 275°C) shows a quick recovery of hardness after 20 h of aging. The intensity of the SANS scattered signal as well, as the thermoelectric power one, decreases with annealing time. In addition, 3D images, at the atomic scale, given by the Tomographic Atom Probe show that during the first hours of annealing, the neutron-induced Cu-Ni-Mn-Si clusters formed during the in-service irradiation are “dissolved” while copper precipitates are formed. Their low number density is in agreement with the full recovery of the hardness. These results are of primary importance for prediction of recovery of an irradiation embrittled steel and also for prediction of its behavior during reirradiation. This work also shows that the nondestructive technique, thermoelectric power, may be used to “follow” the microstructural evolution of materials during annealing treatments.