Fast neutron fluence reduction strategy at the pressure vessel for a pressurized water reactor lifetime extension

The feasibility of nuclear power plant lifetime extension was examined by reducing the fast neutron fluence at the RPV and relieving irradiation embrittlements of materials, and thus ensuring enough structural integrity to last beyond the design lifetime. Two fluence reduction options, peripheral assembly replacements and additional shield installation in the outer core structures, were applied to the Kori Unit 1 reactor, and fluence reduction effect was carefully analyzed. For an accurate estimate of the neutron fluence at the RPV and a reasonable description of the modified peripheral assemblies, a full-scope explicit modeling of a Monte Carlo simulation was employed in ail calculations throughout this study. The Kori Unit 1 cycle 16 core was modeled on a three dimensional representation by using MCNP4B code, and the fluence distribution was estimated at inner wall beltline around the circumferential weld of the RPV. On the basis of fracture toughness requirements of the RPV, the two modified cases were predicted to have additional life of 7-10 EFPYs. Throughout the core nuclear characteristics analyses, it was confirmed that the critical peaking factors for safe reactor operation were satisfied with the design limits