Estimation of retention factor of cesium in sodium pool under fuel pin failure scenario in SFR

Radioactive source term in argon (Ar) cover gas region of the primary vessel due to cesium (Cs) leaked from the failed fuel pins into the primary coolant of Sodium cooled Fast Reactor (SFR) depends on its thermodynamic and kinetic behavior with the coolant sodium. Evaluation of this source term requires detailed knowledge on the distribution of Cs between large volume of the liquid sodium, and the inert Ar cover gas. Solute–solvent combination like liquid Cs and sodium, with relative volatility greater than unity, is an important system to be analyzed in the context of SFR safety. Distribution of Cs between Ar cover gas and liquid sodium pool is complicated by the imposition of temperature difference across the cover gas region and its resultant enrichment of the more volatile solute. An analytical model has been developed to obtain the geometry dependent Retention Factor (RF) of Cs in the sodium pool as a function of the height of cover gas, initial mass inventory of Cs, the temperature difference across the cover gas region (between the sodium pool surface and top roof bottom plate) for an infinite dilute solution of Cs in the sodium pool. The model predicted results are validated with available experimental results in the literature and found that they are fairly in good agreement. In the infinite dilute solution (IDS) regime sodium pool is having the retention capacity to keep the Cs from being released into the argon cover gas region.