High precision neutronic calculations for transient simulations for FRM II

A new high density fuel element consisting of an alloy of uranium-molybdenum (UMo) is being developed for the conversion of FRM IIs compact core from high enriched uranium towards lower enriched uranium. Required slight changes of the core geometry demand both neutronical and thermo-hydraulical re-calculations. This is achieved by coupled calculations of the deterministic neutronic TORT-TD code and the thermo-hydraulic system code ATHLET for transient calculations. Starting from the well proven full MCNP6 model of FRM II, a substitutional MCNP6 model was developed and validated. Based on the MCNP6 vertical stack model, a geometrically equivalent Serpent 2 model was created. With the validated Serpent 2 model the multi group cross sections needed for TORT-TD can be calculated. In the next step, the vertical stack model was translated into an r-φ-z geometry for TORT-TD. It is shown, that the developed system consisting of data processing tools and new models leads to matching results between Monte Carlo and deterministic codes. With that system of models and post-processing tools, high precision neutronic calculations can easily be embedded in transient calculations for FRM II.