Activation Neutronics for a Swiss Pressurized Water Reactor

In Switzerland 40% of the electricity generation is produced by nuclear power. With all five reactors being already beyond their 30th year of operation, Nagra (National Cooperative for the Disposal of Radioactive Waste) in collaboration with the utilities periodically contributes to the Swiss Nuclear Power Plant (NPP) decommissioning cost studies. These studies are of relevance to the estimation of the financial input of the utilities to the Swiss decommissioning fund and the planning of decommissioning activities. During reactor operation, a fraction of the neutrons produced in the reactor core will escape the core boundaries and eventually interact with the surrounding matter. The most heavily irradiated components are located in the proximity of the reactor core [e.g., core baffle, core support plates, core barrel, and reactor pressure vessel (RPV)]. Neutrons will also stream in farther ex-RPV areas and activate components such as the reinforced concrete bioshield. Decommissioning costs are dependent, inter alia, on the radioactive waste volumes and on the corresponding isotopic inventories. Neutron-activated components are the main source of radioactivity within a NPP under immediate dismantling (i.e., spent fuel has been removed from the reactor). Reliable neutron transport and activation calculations are, therefore, essential for the estimation of radioactive waste volumes, the selection of an optimal dismantling strategy, the development of the radioactive waste packaging and logistics concept, and consequently for the estimation of the decommissioning costs. In this context, Nagra has developed a state-of-the-art NPP activation calculation sequence that enables the radiological characterization of the Swiss NPPs. This paper focuses on aspects relevant to the neutron transport calculations for a Swiss pressurized water reactor. More specifically, the MCNP5 modeling approach together with the use of the ADVANTG hybrid, variance-reduction acceleration code, is outlined. Furthermore, the validation of the neutron transport calculations with an in situ full-cycle foil activation campaign is presented.