MCNP neutron streaming investigations from the reactor core to regions outside the reactor pressure vessel for a Swiss PWR

The Swiss National Cooperative for the Disposal of Radioactive Waste, partly in collaboration with ETH Zurich and the Swiss NPP utilities, has developed MCNP NPP models for neutron activation analysis and NPP decommissioning studies. These models exhibit mainly rotational symmetry and are bound to the reactor pit/drywell area. In this paper, the PWR Goesgen model is chosen as a reference model for further investigation and extended to incorporate detailed reactor pressure vessel support structures, cooling pipes/penetrations and farther ex-reactor pressure vessel areas such as the adjacent steam generator or cooling pump compartment as well as the layout of the basin area above the reactor pressure vessel. The impact of these azimuthal asymmetries on the ex-reactor pressure vessel neutron distribution is quantified and the variance reduction methodology used is outlined. Finally, a description of the in-situ foil irradiation campaign initiated in collaboration with the Swiss NPP Goesgen for the purpose of future model validation is presented. (author)