Sodium boiling is a critical transient phenomenon prior to severe accidents in sodium-cooled fast reactors. Accurate prediction of the two-phase state and thermodynamic parameters of the coolant during sodium boiling is essential for sodium-cooled fast reactors accident analysis.

This study aims to develop a core thermal-hydraulic model for the sodium-cooled fast reactor integrated analysis program ISAA-Na (Integrated Severe Accident Analysis code) on the basis of the multi-bubble liquid slug assumption.

In the multi-bubble slug model developed for ISAA-Na, bubbles were classified as small bubbles (not exceeding the minimum grid size) and corresponding large bubbles, with the liquid between them forming a slug. Separate sets of three conservation equations (mass, momentum, energy) were established and solved for both the bubbles (according to their type) and the liquid slug. Based on this model setup, ISAA-Na was used to model and calculate the loss-of-flow experiment in the high burnup irradiation core of CABRI-BI1 (C?ur à Haut B?rnup Irradiation). Finally, simulation results of various analysis codes were compared with that of experimental results.

Comparison results show that ISAA-Na provides more accurate predictions of coolant temperature and pressure prior to boiling compared to other codes such as SAS4A, ASTEC-Na, and SIMMER. Specifically, in the fission zone, ISAA-Na predicts a maximum coolant temperature difference of approximately 15 K during steady state and about 20 K prior to transient boiling, which align well with experimental observations. However, after the onset of boiling, overestimations of bubble growth rate and two-phase interface movement are observed due to the lack of fuel melting and cladding failure models.

Overall performance of ISAA-Na demonstrated in this study indicates that the application of the multi-bubble liquid slug boiling model in ISAA-Na for the analysis of the CABRI-BI1 experiment is deemed reasonable and reliable.