Multiscale modeling simulations of point defect production in neutron-irradiated iron

A computer simulation and an experimental method were applied to determine a point defect concentration in reactor pressure vessel (RPV) steels irradiated by neutrons. The steel samples had the basic composition of SA508-3 type steels of which the RPV for commercial nuclear power plants are made. The samples were neutron irradiated at 250degC at a fluence of 1.69 x 1019 n/cm2 (En>1.0 MeV). For estimating the concentration of the point defects in the steels, we used computer simulation methods, including a molecular dynamics (MD) computation and a point defect kinetics model calculation. In order to obtain the primary damage parameters, MD simulations of the displacement cascades in Fe were performed. Then, we calculated the point defect concentrations by using the defect kinetics model and the MD calculation results. In parallel with the computation, we undertook a measurement of the positron annihilation lifetimes (PAL) of the irradiated samples. The PAL test results showed that a certain amount of single vacancies were present in the irradiated steels. The vacancy concentration from the PAL measurement was found to be 7.3 x 10-7 /atom, while that from the computational approach was 1.8 x 10-6 /atom. In spite of some uncertainties about the kinetic and materials parameters, a fair agreement between the two methods was obtained. (author)