One-dimensional coolability and concrete ablation analysis of volumetrically heat-generating debris bed

Highlights: • A one-dimensional dryout heat flux model is derived and validated with experiments. • A coolability map is provided for the debris bed expected to be formed in pre-flooded cavity. • Coolability and concrete ablation depth analysis of debris bed has been conducted. - Abstract: We present a preliminary study on the feasibility of the pre-flooded cavity strategy for ex-vessel cooling of APR1400 NPPs. Fragmented particles form a porous debris bed, so understanding the heat transfer mechanism in a porous medium and its critical heat flux is essential to assess the coolability of the debris bed. An one-dimensional dryout heat flux model of porous medium is derived from a water and vapor momentum equation for porous medium, two-phase permeability modifications and the correlation between capillary pressure and liquid saturation. The developed model is validated by comparing dryout heat flux data of DEBRIS, STXY, POMECO and COOLOCE tests. STXY BED, POMECO BED-5 and COOLOCE-8 BED are made to reflect the particle size distribution results of the melt fragmentation tests such as FARO, CCM, MIRA and DEFOR. We analyze the ex-vessel coolability by assuming that these three beds represent debris beds in actual accidents. In addition, one-dimensional coolability analysis is conducted for three cases of cylinder shape debris bed at containment pressures of 1–5 bar. Based on these, one-dimensional ablation concrete depth analysis for the three cases of debris shape and the three kinds of the debris beds of APR1400 NPPs is conducted. As a result, if fragmented particles are distributed evenly and the POMECO BED-5 or STXY BED like debris bed is formed in the cavity, the debris bed coolability is so good that ablation depth is negligible or less than the thickness of sacrificial concrete. Even if the COOLOCE-8 BED like debris bed is formed, the integrity of the containment is kept during ex-vessel cooling if the containment pressure is maintained 3 bar or more. However, if the solidified particles spread only in a particular area, the leak tightness of containment building could not be maintained in the ex-vessel cooling process. Because solidified particles lose their fluidity, so it is questionable that all of them are distributed uniformly in the cavity. Therefore, a further research is required on how fragmented particles are distributed in pre-flooded cavities to validate the pre-flooding strategy of APR1400 NPPs.