Empirical modeling of ex-vessel debris bed formation in a pre-flooded reactor cavity

To secure the containment integrity of a nuclear power plant (NPP) in severe accident situation of ex-vessel corium release into the reactor cavity, a pre-flooding strategy of a reactor cavity has been adopted in various NPPs, including Korean PWRs. Molten corium released into the pre-flooded water pool is fragmentized and forms a particulate debris bed at the bottom of the cavity. To prevent the containment failure, it is important to properly remove the heat from the debris bed, which can be accomplished by water ingression into the bed. The water ingression characteristics are highly dependent on the debris bed shape. In this study, several main pa-rameters affecting the debris bed shape were determined with respect to phenomena of free-fall & sedimentation, particle dispersion by steam spike and convection flow, and self-leveling. Then empirical model was developed and validated using the results of DEFCON (Debris bed Formation and COolability experimeNt) experiments. The model's error was within +/- 10% when only free-fall & sedimentation were present. When steam-spike or con-vection was added to free-fall & sedimentation condition, the error increased to within +/- 20%. When all three were simultaneously present (free-fall & sedimentation, steam-spike, and convection), the model predicted most of the experimental results within +/- 20% error, except for several cases where convection had a significant impact. The effect of self-leveling itself was within +/- 10%. Considering all phenomena related to particulate debris bed formation, most cases had an error of +/- 20%. In addition, the debris bed shape formed by poly-disperse particles was predicted appropriately by the mode using the mass mean diameter of the particle size distribution, with an error range of +/- 20%. Consequently, it is expected that the proposed model of the debris bed shape can be utilized in an integrated severe accident analysis code for the evaluation on the ex-vessel debris bed coolability in the pre-flooded reactor cavity.

Automated summary

To maintain the containment integrity of a nuclear power plant during severe accidents, a pre-flooding strategy is adopted to prevent the failure of the reactor cavity. The shape of the debris bed formed by molten corium released into the pre-flooded water pool plays a crucial role in removing heat from the bed and ensuring coolability. Various phenomena, such as free-fall & sedimentation, particle dispersion by steam spike and convection flow, and self-leveling, affect the debris bed shape.