Development of a Source Terms Migration Model for CDA Bubble

Abstract

The probability of a core disruptive accident (CDA) is considered to be extremely low for sodium-cooled fast reactors (SFRs) because of their high inherent safety design. However, from a defense-in-depth perspective, it is still worthwhile to study CDA sequences for further assurance of safety and reliability of SFRs. During a CDA, a large CDA bubble rapidly expands inside the sodium pool, rises from the core, and cover the gas region, providing a potential migration path for radioactive materials. Source terms released provisionally to the cover gas region on the order of a few hundred milliseconds are called instantaneous source terms. In this study, we propose an instantaneous source term migration model that provides a simplified evaluation of the amount of source terms absorbed by coolant sodium during the ascent of the CDA bubble. In this model, a particle motion model within the CDA bubble based on the basic momentum equation is used to calculate the amount of source terms escaped from the bubble boundary. In addition, a model analogous to the aerosol scavenging by precipitation is used to assess the amount of source terms absorbed by entrained sodium droplets that occur during rapid expansion of the CDA bubble. The model is further validated by a past source term migration experiment in which a high-pressure, large bubble expanded and rose in a sodium pool. Good agreement with the measured retention factor of a source term demonstrates the reliability of the developed model.