Transient coupled neutronics-thermalhydraulics study of ULOF accidents in sodium fast reactors using spatial kinetics. Comparison of the TFM Monte Carlo and S_N approaches

The purpose of this paper is to study and compare different neutronic approaches used to calculate ULOF transients in sodium cooled fast reactors. A first objective is to compare two codes used for spatial kinetics calculations. Two neutronic models are compared. The first one is based on a deterministic (discrete ordinate S_N) approach, using tabulated self-shielded cross sections, where the core reactivity and the power shape distribution are evaluated at each time step of the transient calculation. The second model relies on the Transient Fission Matrix (TFM) approach, condensing the response of a Monte Carlo neutronic code in time dependent Green functions characterizing the local transport in the reactor. This second approach allows a fast estimation of the reactivity and of the flux redistribution in the system during the transient with a precision closed to that of the Monte Carlo code. Both models have been coupled to the thermalhydraulics and applied on an ASTRID representative assembly. This application case is supposed to be sensitive to power redistributions. A second comparison between spatial kinetics and point kinetics calculations has been led to study this point. Finally we obtain a good agreement between spatial and point kinetics on the ULOF calculation, while some discrepancies are observed between the TFM and the S_N approaches on the power level stabilization, due to difference on the feedback estimation in both models. (author)