A Fast Neutron Fluence Reduction Strategy for Pressure Vessel Lifetime Extension

Abstract

The feasibility of nuclear power plant lifetime extension was examined by the evaluation of fast neutron (>1 MeV) fluence at the reactor pressure vessel (RPV). A fluence reduction option, additional shields installation in outer core structures, was applied to the Kori Unit 1 reactor, and its fluence reduction effect was carefully analyzed. Full-scope explicit modeling of Monte Carlo simulation with MCNP4A code was employed in the fast neutron fluence calculations. An optimized design option was found from various choices in geometry and material for shield structure. It is expected that the magnitude of fast neutron fluence would be reduced by 39% at the circumferential weld of the RPV. On the basis of the criterion of pressurized thermal shock requirement of the RPV, it is predicted to obtain the additional life of 4.4 effective full power years. It was also investigated that the nuclear characteristics and thermal hydraulic factors at the internal core were negligibly influenced by the installation of additional shield structure.