Numerical analysis of a large two phase fuel bubble evolution under energetic CDA in a medium-sized pool type SFR

The two-phase fuel bubble behavior inside the liquid sodium pool under energetic core disruptive accident (CDA) is an essential consideration in the safety analysis of sodium fast reactors (SFR). The fuel bubble expansion and the resultant reactor vessel pressurization during energetic CDA drive the enhanced actinide transport from the damaged core to the cover gas region and the liquid sodium release to the reactor containment building (RCB). A numerical model is developed to evaluate the fuel bubble behavior by considering the sodium entrainment and the fuel-sodium heat transfer in a medium-sized pool type SFR. The model is validated with relevant benchmark experimental results. Under energetic CDA conditions, the fuel bubble mass typically varies between 1000 and 3000 kg, and the initial fuel bubble temperature ranges from 4200 to 4700 K. The parametric analysis with these input values shows that the sodium entrainment at the bubble-pool interface and the fuel bubble behavior are more sensitive to the initial fuel bubble temperature or superheat than the fuel bubble mass. The time period for the fuel bubble's first expansion-compression cycle range between 600 and 800 ms. The equilibrium or quasi-static pressure under adiabatic condition (i.e. without sodium entrainment and fuel-sodium heat transfer) for the 1000 kg-4200 K case is 1.57 × 10⁵ Pa. Results also show that the sodium vapor partial pressure dominates the total fuel bubble pressure in the 1000 kg-4200 K case when the sodium entrainment and the fuel-sodium heat transfer effects are included. As a result, the quasi-static pressure in the reactor vessel increases to 3.0 × 10⁵ Pa. The fuel bubble-sodium pool interface velocity history which dictates the actinide upward displacement from the core region is evaluated as a function of initial fuel bubble temperature and mass. The maximum fuel bubble-sodium pool interface velocity for the input values considered is 35 ms⁻¹. A mechanistic evaluation of the reactor vessel pressure history after the completion of bubble expansion shows that the radiation heat transfer and the fuel vapor condensation reduce the reactor vessel pressure from the quasi-static to the ambient pressure within 10 s. The evaluated velocity and pressure histories would serve as inputs for the mechanistic estimation of in-vessel and in-containment accident source terms due to the fuel bubble expansion under energetic CDA scenario in SFR.