Simulation of severe accidents in sodium-cooled fast reactors using ASTRA code with a molten clad motion model

Accidents in sodium cooled fast reactors such as Unprotected Loss of Flow (ULOFA), and Total Instantaneous Blockage (TIB) involve coolant boiling and molten material motion inside the voided channel. Accurate modeling of molten clad dynamics is required to calculate transient power evolutions, fuel motion modeling, and transition and disassembly phase calculations. In the present study, a one dimensional molten clad motion model is developed. The model is validated with analytical calculations and benchmark experiment data. Integrating the clad motion model, TIB and ULOFA scenarios in a medium sized SFR is analyzed using the ASTRA code. The TIB analysis revealed that fast voiding resulted in rapid power rise and fuel melting. The coolant channel was blocked by the refrozen clad at the lower axial blanket. The parametric study showed that the heat generation rate affects the time of occurrence of key events such as sodium boiling, clad melting, fuel melting. The ULOFA analysis showed a gradual introduction of steel relocation feedback and a complete channel blockage at the top of the fissile region at the time of fuel melting. The parametric study showed that the heat generation rate is the most influential parameter that affects the time of occurrence of key events.