Monte Carlo analysis of the CROCUS benchmark on kinetics parameters calculation

The experimental configurations, the construction data and the material compositions of the CROCUS reactor, a water-moderated zero-power core operated at the Ecole Polytechnique Fédérale de Lausanne (EPFL), Switzerland, have been accurately determined and compiled into an OECD/NEA international benchmark for reactivities and kinetics parameters calculations. As such, the CROCUS reactor configurations provide an outstanding testbed for validating Monte Carlo codes in the context of reactor physics. In this work, we have performed extensive code-code comparisons between Monte carlo code Tripoli-4® developed at CEA and the enhanced version of MCNP5 developed at CRIEPI based on MCNP5.1.30 on the whole set of CROCUS configurations available from OECD/NEA. First, we have verified the coherence of the respective independent Monte Carlo models and of the simulation options by computing reactivities from multiplication factors (the so-called direct reactivities). Then, we have calculated the kinetics parameters by resorting to the Iterated Fission Probability (IFP) method and deduced the reactivities from the inhour equation (the so-called dynamic reactivities). An excellent agreement has been found between the codes. Finally, we have carried out a detailed analysis of the simulation results and we have carefully assessed the impact of nuclear data libraries on the computed quantities, with special emphasis on possible discrepancies between direct and dynamic reactivities.