Enhancement and validation of the NPP Mühleberg MCNP activation simulations for Swiss decommissioning planning

The Swiss National Cooperative for the Disposal of Radioactive Waste (NAGRA) regularly performs analysis of cost estimates associated with the NPP decommissioning. For this purpose, NAGRA has over the past ten years developed a NPP activation analysis methodology based on MCNP models of Swiss NPPs. The validation of these models is accomplished using measurements from oil activation campaigns, in which foil samples are activated at key locations inside the NPP for the duration of one cycle. The measurement campaigns have already been carried out at the Gösgen PWR (KKG) and the Mühleberg BWR (KKM). The first validation has already been successfully conducted for the KKG MCNP model. This thesis describes the efforts to validate the KKM MCNP model. This process included modifications, such as modeling of steam separators individually and improving the definition of jet pumps. Furthermore, the core definition was completely redefined, going from a 6-cell cylindrical model to a 940-cell model, shaped like the actual KKM core, which more accurately represented the void distribution. In order to benchmark the new model, the locations of samples during the two KKM foil activation campaigns were implemented into the model using the GSAM code. The interface between the MCNP model and GSAM was improved by creating a new energy group structure, optimized specifically for the activation of the three foil materials. Their activation was stimulated the state of the art hybrid VR code ADVANTG. The calculated results were then compared against the measured values for each foil material separately. The numerous improvements introduced in the 2014 model led to good agreement in many areas. The agreement is within the factor of two on the inner side of the bioshield, at the core height and above, and factor of three above the bioshield. Furthermore, distinct suggestion for improving the agreement in other areas was presented. This includes modeling of pipes extending from the RPV and going through the openings in the bioshield, as well as modeling of the control rods. Overall, the work described in this thesis represents a major step forward in KKM activation analysis accuracy. (author)