Analysis of the Reactivity Effects Exercises of the Neutronics Benchmark of the CEFR Start-Up Tests

The “Neutronics Benchmark of the CEFR Start-Up Tests” is an IAEA coordinated research project based on the simulation of the CEFR start-up tests. The main goals of the project are to improve the participant capabilities in SFR analysis and to perform an international validation of codes for Sodium Fast Reactor simulation. NINE-UNIPI work together on the creation of the Serpent 2 model and on the simulation of all the start-up tests proposed in the benchmark. In this work the three experiments related to the reactivity measurements are discussed. The geometry model is briefly described and the simulation set-up is presented. In particular, the geometry has been modeled considering the thermal expansion at the experimental temperatures. The nuclear data libraries used are the ENDF/B-VIII.0, pre-processed at the experimental temperatures and provided to the benchmark participants from SCK-CEN. The obtained results show a good agreement with the experimental data, except for the assembly-swap reactivity effect, which shows a small shift for all the considered cases. The results presented in this work could contribute to the validation of Serpent 2 for SFR criticality calculations.