Numerical prediction of the effect of irradiation on the Charpy Upper Shelf Energy of Reactor Pressure Vessel steels

Abstract

Neutron irradiation affects the mechanical properties of Reactor Pressure Vessel (RPV) steels used in Light Water Reactors (LWR). Fracture properties are particularly monitored based on surveillance programs, mostly through Charpy impact tests. The evolutions of Ductile Brittle Transition Temperature (DBTT) and Upper Shelf Energy (USE) with irradiation are evaluated and used for structural integrity assessment depending on regulation procedures. Semi-empirical correlations have been proposed to predict DBTT and USE shifts as a function of material chemical composition and fast neutron fluence, based on extended experimental database. In this study, numerical simulations of Charpy impact tests are performed in order to predict quantitatively the evolution of USE with irradiation. In a first part, the mechanical properties of an A508 Cl3 RPV steel are presented, including tensile tests on flat and notched samples and Charpy impact tests, for various irradiation levels and test temperatures. USE values are found to be within the bounds of correlation curves. In a second part, the experimental data are used to calibrate constitutive equations describing the material behavior up to high strain levels. Charpy impact test finite element simulations are then detailed, including ductile fracture modelling. In a third part, the ductile fracture model is calibrated based on the experimental data, and numerical simulations are used to predict the evolution of USE with irradiation. The decrease of USE with irradiation is recovered, and the numerical results are found to be in quantitative agreement with the correlation curves. Additional simulations confirm the agreement for different reference unirradiated USE and chemical composition, as well as using a simplified ductile fracture model. These results indicate that numerical simulations of Charpy impact test relying on calibrated constitutive equations can be used to predict USE and required experimental characterization are finally discussed.