Evaluation of the in- and ex-vessel neutron fluence distributions in a Siemens/KWU PWR

Neutron fluence is one of the key parameters required for calculating the extent of the neutron activation levels in a nuclear power plant (NPP). Therefore, it must be estimated with great accuracy. The neutron fluence is often evaluated using Monte Carlo codes, and its accuracy strongly depends on the level of detail in the reactor geometrical model and the representation of the neutron source. For decommissioning planning of NPPs in Germany, a 3D detailed Monte Carlo model of a Siemens/KWU pressurized water reactor (PWR), one of Germany's most common NPP types, was developed to calculate the neutron fluence distributions within the reactor core and its immediate surrounding area. This paper provides a detailed description of the reactor model, validates the accuracy of the model predictions by comparing the calculation results with metal foil-activation measurements, and determines the neutron fluence distribution in selected components and a fine rectangular mesh covering the entire geometry of the reactor model. The paper indicates that the developed Monte Carlo model can be used to obtain the neutron fluence with high accuracy for calculating the activation distribution of the reactor components.