Development of a CASMO-4/SIMULATE-3/MCNPX calculation scheme for PWR fast neutron fluence analysis and validation against RPV scraping test data

A methodology is presented for the accurate assessment of the fast neutron fluence at the reactor pressure vessel (RPV) of a pressurised water reactor (PWR). The basis is the transfer of results from deterministic CASMO-4/SIMULATE-3 core-follow calculations (power distribution, fuel compositions) into a 3D volumetric (pin-by-pin, axially distributed) fixed neutron source, for ex-core neutron transport simulations using the Monte Carlo code MCNPX. The modelling is supported by sensitivity and optimisation studies, using precise MCNPX calculations and detailed reactor condition specifications. As validation of the new methodology, an analysis has been performed to compare calculation results with values based on experimental data related to the RPV of a PWR. The reference fluence estimates are based on a previously performed activity analysis of RPV scraping samples, taken after 10 cycles of operation of the nuclear power plant. The agreement between computed results, i.e., fluence values obtained using the currently developed CASMO-4/SIMULATE-3/MCNPX calculation scheme, and the estimates based on experimental data, has been found to be very satisfactory. The calculation results lie within ±5% of the reference values, i.e., well within the declared experimental uncertainty of ±10%. This agreement is considerably better than that reported for the results of earlier calculations, based on use of the deterministic code BOXER.