DEVELOPMENT OF A SIMPLIFIED ONE-DIMENSIONAL CDA BUBBLE MODEL FOR SOURCE TERM EVALUATION

<p>The probability of core disruptive accident (CDA) occurrence in sodium-cooled fast reactors (SFRs) is considered extremely low. However, for further verifying the safety and reliability of SFRs, the CDA sequence is still worth studying. In a case of SFR’s severe accident, such as unprotected loss of flow (ULOF), the CDA may be triggered, and then fuel and some fission products (or called source terms) may be released instantaneously from a CDA bubble through the potential leak paths on the vessel top slab or released with a delay from boiling sodium pool after vessel melt-through, widely known as instantaneous and delayed source terms. Therefore, reasonable prediction of CDA bubble behavior is necessary to investigate instantaneous source terms migration in the vessel pool. In this study, a simplified one-dimensional CDA bubble model that includes the formulation of thermal-hydrodynamic behaviors of the bubble mixture rising through the sodium pool toward the cover-gas region is proposed. The model includes mass transfer processes such as the condensation of gas mixture on liquid fuel/steel/sodium and the bubble interface. In this model, droplet entrainment phenomena at bubble interface are modeled based on Rayleigh-Taylor instability and Kelvin-Helmholtz instability, and the effect of non-condensable gas on condensation process is also considered. To validate the developed model, a past experiment on the expansion of high-pressure bubble in a stagnant liquid pool conducted by Purdue University in the late 1970’s using a 1/7-scale model of Clinch River Breeder Reactor was analyzed. The results showed generally good agreement with measured data and demonstrate that the developed model can reasonably represent the essential characteristics of dynamic behaviors of a high-pressure large-size bubble with heat and mass transfer at the bubble interface. This supports subsequent calculations to carry out the migration of transient source terms in the future.</p>