Analysis of unprotected Loss of Flow accidents for SPX-1 using the SAS3D code system

The unprotected loss of flow accident (LOF) has been analysis for the SPX-1 reactor in a collaborative effort by the French CEA/DSN and the KfK. The objective was to analyze the core behaviour and the possible energy release of the primary excursion that is triggered by a coolant expulsion after sodium boiling and that includes further enhancements from possible reactivity effects after failure with fuel coolant interaction in unvoided core regions. This report informs about the core representation and the adjustments in SAS3D to simulate the pre-boiling phase with the SPX-1 specific slow flow coast down and its feedback effects. The calculated accident sequence after boiling onset is presented for a defined reference case together with a discussion of the key phenomena and the assumptions for modelling. Emphasis is placed on the simulation of the fission gas kinetics, which has a strong influence on the possible consequences. Sensitivities to the axial fuel expansion effect, to failure conditions and to the selection of the post-failure modelling are shown. Investigations with a fast flow coast down and the influence of a full pellet vs. hollow pellet are summarized to show up the possible influence of different design options in a SPX-type reactor core