Simulation of GR19 sodium boiling experiments with cathare 2 system code and Trio_U MC subchannel code

Abstract : The CEA R&D program on SFRs includes the study of reactor behavior during accidental transients possibly leading to sodium boiling, such as an unprotected loss of flow. To that end, two existing CEA thermal hydraulic codes are being extended to model two-phase sodium flows: CATHARE (system dynamics) and Trio_U MC (subchannel). In the case of the 6-equations CATHARE code, the applicability of its current closure laws to two-phase sodium flow must be evaluated; for the 3-equations MC code, new numerical methods are needed as well. This paper reports simulations with these codes of the GR19 sodium boiling experiments done at CEA Grenoble in the 80’s. Simulations of boiling test on this 19-pin out-of-pile mock-up were undertaken in order to provide a preliminary validation of the two codes and to guide future code improvements. The results presented here show that CATHARE can achieve good agreement with the experimental data: however, some closure laws (especially for heat transfer) will still have to be revised to better fit sodium boiling physics. Meanwhile, Trio_U MC is capable of predicting correctly the occurrence and extension of local boiling in boiling steady states: however, further improvements will be needed to correctly model unstable and transient boiling.