DAVINCI-SP tests for debris bed formation and spreading in a pool with center conical structure under two-phase condition

In case of severe accidents in light water reactors (LWRs), the coolability of relocated corium from the reactor vessel is a significant safety issue. The failure in cooling and stabilizing the molten core in the containment vessel can threaten the integrity of the containment boundary. In a flooded cavity, a porous corium debris bed is ex-pected to develop on the bottom of the cavity pool due to the melt jet breakup and fragmentation. The geometric configuration of the porous debris bed is important for accurate coolability assessment under accident condi-tions. Earlier, Kim et al. (2016a, 2016b) presented an analytical model to estimate the spreading of the debris bed in terms of two-phase flow and the debris injection parameters. Here we are testing the effect of conical bottom surface in passively dispersing the debris particle to obtain a levelled bed formation. For this, the previous model for debris bed development is modified to simulate the development of debris bed over conical bottom surface. The existing 'The Debris Bed Research Apparatus for Validation of the Bubble-Induced Natural Convection Effect Issue (DAVINCI) -Spreading (SP) experimental facility was modified with a center conical structure. Five ki-lograms of stainless steel simulant debris were injected stepwise from the top of the water level, while air bubbles simulating the two-phase vapor flow were injected from the bottom of the particle catcher plate. The test results from the current study to describe the spreading of the debris bed in terms of two-phase flow, debris injection parameters, and conical bottom surface geometrical parameters are discussed. Subsequently, the model for characteristics length and side slope angle of debris bed were modified using the empirical parameters from the tests. Comparison between the tests results and modified model prediction are presented. Lastly, the effectiveness of the conical bottom surface on particle dispersion is discussed.

Automated summary

The coolability of relocated corium from the reactor vessel is a significant safety issue in severe accidents in light water reactors (LWRs). Failure in cooling and stabilizing the molten core can threaten the integrity of the containment boundary. A porous corium debris bed is expected to develop on the bottom of the flooded cavity, and the geometric configuration of the bed is important for accurate coolability assessment.