Dry-out characteristics of debris bed under different water injection conditions

A porous debris bed may form through the interaction between melting corium and coolant during a pressurized water reactor severe accident, which continues to emit large amounts of decay heat. The cooling of debris bed is a critical phenomenon and serves as a potential mitigation measure to halt the accident. In this paper, an experimental facility DBC was constructed to study the cooling properties and dry-out heat flux under different water injection conditions and system pressures. The debris bed consisted of stone particles ranging in diameter from 1～10 mm, with a porosity of 0.37. In the debris bed, a total of thirty-three electrical heating elements, twelve optical fibers equipped with 204 temperature sensors, and thirty-three thermocouples were embedded. Additionally, a computer program MIDEC was developed to investigate the vapor-liquid flow characteristics and dry-out mechanism. The experimental results indicated that the cooling of debris bed can be significantly improved by increasing system pressure. Furthermore, it was observed that the dry-out heat flux under bottom water injection conditions was considerably higher compared to that of top water reflooding. Under top reflooding conditions, the counter-current flow of vapor-liquid impeded the downward flow of coolant, resulting in insufficient coolant supply and leading to a dry-out phenomenon at the bottom region. However, under bottom injection conditions, both vapor and liquid flowed in an upward direction. As a result, the void fraction reached its maximum value at the top region of the debris bed. The distinct vapor-liquid flow characteristics under top and bottom injection conditions resulted in notable discrepancies in dry-out heat flux and dry-out positions. Based on the comparison between the calculated dry-out heat flux (DHF) and experimental results, it was observed that the Reed model accurately predicted the cooling of debris bed under top-flooding conditions when compared with measured data obtained from DBC experiments. However, for very small particle sizes, the Lipinski model would be more suitable and provided better predictions. •The experimental facility was constructed to simulate the cooling properties of corium debris bed.•The dry-out characteristics and dry-out heat flux (DHF) under top- and bottom-flooding conditions were obtained.•The gas-liquid two-phase flow phenomenon during dry-out process was analyzed by a new computer program.