A new uncertainty arising in reactor pressure vessel fluence calculation

To identify and estimate a discrepancy of the reactor pressure vessel (RPV) fast neutron fluence between heterogeneous and homogeneous assembly models, Kori Unit 1 cycle 1 core was modeled on three-dimensional representations by MCNP4A. While all fuel rods were represented exactly in the heterogeneous model, the assemblies of the homogeneous model consisted of uniform compositions which were homogenized by volume fractions of components i.e. fuel, cladding, and water. In the conventional RPV fluence calculations of the deterministic methods, the homogenized assemblies have been used to model on the reactor core region. Because an uncertainty associated with the assembly modeling has not been revealed, it was identified and estimated by full-scope Monte Carlo simulations in this study. The MCNP version 4A was consistently used in all procedures such calculations as criticality, source terms, and RPV fluence. The heterogeneous model converges to a k value of 1.00039±0.00084 and the relative assembly power distributions were nearly the same as that of the Kori Unit 1 FSAR. In the MCNP criticality calculation, pin-by-pin source terms were also calculated to use in RPV fluence calculations. Each calculation of the heterogeneous and the homogeneous models were performed by the MCNP. The results showed that the RPV fast neutron fluence of the homogeneous model were higher than that of the heterogeneous model. A new uncertainty associated with the assembly modeling was found to be 10 ∼ 20%, which is too high to be ignored. It is concluded that the homogeneous model of the assembly region should be one cause of the uncertainty in the RPV fluence calculations