THE INFLUENCE OF TWO-PHASE FLOW ON PORE CLOGGING BY FINE PARTICLE SETTLEMENT DURING EX-VESSEL DEBRIS BED FORMATION IN SEVERE ACCIDENT

摘要

In severe accident, the assurance of long term coolability of the corium melt is one of the key issues of the severe accident management (SAM) for the mitigation and termination of the accident progression. The Korean NPPs adopts the SAM strategy that water floods into cavity levels up to the upper part of the reactor vessel to provide the external reactor vessel cooling for the in-vessel retention of corium (IVR-ERVC). However, if the IVR-ERVC strategy fails and the corium relocation onto the cavity occurs, the long term coolability of the ex-vessel corium on the cavity floor should be assured. However, this scenario takes a little account of the effect of the two-phase flow from the corium particles and pre-settled debris bed on the cavity floor. Recently there begins research concerning the effect of two-phase flow on the bed formation, like the particle relocation by self-leveling phenomenon reporting that the cavity pool convection by the two phase flow from the debris bed largely distorts the settling trajectories of the debris by their own sizes. In the perspective, the present study aims to investigate the pore clogging by the fine debris particles in two-phase conditions at the DAVINCI facility, POSTECH, Korea. In the experiments, pre-defined debris particles with various diameters and size distributions are plunged into a water pool in where two-phase flow regime is simulated by the gas injected from the bottom of the test section. The configuration of the resultant debris bed is analyzed to identify the role of two-phase flow on the settlement of fine particles and clogging of debris bed pores. In the test, it was observed that the resultant bed shape and particle distribution was significantly affected by the two-phase bubble behavior during the particle settlement. It is also showed that in the two-phase test the fine particles are more settled in the outer region compared to the pre-settled debris bed region, illustrating the potential positive effect against the coolability degradation due to the fine particle pore clogging phenomena.