Experimental study on coolability of mixed particle size debris bed under top flooding

During severe accident in reactor, a debris bed is formed after the core melt interacts with the coolant, this is the key stage to realize the melt retention in vessel and terminate the accident process. Studying the drying characteristics of such stacked particle beds can expand the safety margin of the In-Vessel Retention (IVR) strategy. In this paper, based on the debris bed's characteristics of prototype fuel coolant interaction (FCI) experiment, a onedimensional cylindrical debris bed with irregular and multi-sized sand particles was constructed. At the uniformly stacked case, the typical characteristics were observed. Two vapor phase concentration areas appeared in the debris bed before and after drying, bubble blocking area was formed in the middle of the debris bed before drying, then a dry zone appeared at the bottom of the debris bed due to lack of liquid, the dry area extended first radially and then axially. Meanwhile, the influence of system pressure is significant, dryout heat flux (DHF) under 1.0 MPa is almost twice than normal pressure. At the sand particles layered case, it was found that DHF increased by 40%similar to 50% compared with uniformly stacked condition, the bubble blocked area and dry area coincided, and occurred in the middle of the debris bed. Finally, a DHF model based on Reed pressure drop model is established for one-dimensional uniform sand debris bed, and the calculated values are in good agreement with the experimental values.

Automated summary

During severe accidents, a debris bed is formed after core meltdown and coolant interaction, which is crucial for melt retention in vessel and accident termination. Studying the drying characteristics of such particle beds can enhance the safety margin of the In-Vessel Retention (IVR) strategy.