Effects of irradiation on 316 stainless steel application to screws used in the CP0 reactors

The main conclusions concerning the mechanical behaviour of the Type 316 stainless steel (cold worked) after irradiation in the service temperature range of REP are the following: - The yield stress and the ultimate tensile stress increase as the fluence rises up to 2 1022 n.cm-2 (E > 0.1 MeV). For higher fluences, these characteristics remain constant. - The fatigue behaviour is lightly deteriorated by irradiation. However, the endurance limit does not seem to be affected. In service, the only notable structural evolution which can take place in the cold worked Type 316 stainless steel is the formation of dislocation loops and of microcavities. The presence of these latter may induce a swelling of the baffle assembly screws which should not exceed 0.3% at the end of life. The neutron flux should also induce an increase of Si, Ni, P contents and a decrease of the Cr and Fe in the vicinity of the grain boundaries. This literature survey has been used to carried out a first analyse of possible causes of the cracking of baffle assembly screws observed in some CP0 (Contrat Programme zero) reactors (Bugey, Fessenheim...). We have tried to determine if this damage had a mechanical origin. It is unlikely that the cracking of screws results from the application of an overstress (σ > UTS) or from an embrittlement due to helium. There are not enough available data to determine if creep or fatigue solicitations have induced this cracking. 38 refs., 16 figs., 8 tabs