Fluence rate effects on irradiation embrittlement of model alloys

Abstract

The Reactor Pressure Vessel (RPV) material of Nuclear Power Plants (NPP) is exposed to neutron irradiation during its operation. Such exposure generally induces degradation of the mechanical and physical properties of the materials: e.g. an increase of the ductile to brittle transition temperature (DBTT) and a decrease of the upper shelf impact energy. At a given irradiation temperature, dose and neutron spectrum, the sensitivity of materials to neutron irradiation depends on their chemical composition. In particular, elements like phosphorus, P, copper, Cu, and nickel, Ni play a key role in RPV steels. The effect of fluence rate on irradiation embrittlement of RPV materials is also a key issue for the correct interpretation of accelerated data and surveillance data in view of reactor pressure vessel life assessment of nuclear reactors. Much effort was done in the last decades to tackle such issues and quite contradictory results have been obtained. Model alloys can successfully be used to study embrittlement mechanisms and the effect of fluence rate. A parametric study of the response to neutron irradiation of 32 different model alloys with systematic variation of elements (Ni from 0.004 to ∼2 wt%, P from 0.001 to 0.039 wt%, Cu from 0.005 to ∼1 wt%) was completed by some members of the European Network AMES. The irradiation of the 32 model alloys took place in the LYRA rig at the High Flux Reactor (HFR) of the Joint Research Centre, Petten, The Netherlands. Some model alloys were also irradiated in commercial reactors, namely in Rovno Nuclear Power Plant (NPP), Ukraine, and Kola NPP in Russia. Data available on these model alloys are presented and analysed in this paper, proving to be very important for the study of fluence rate effect.