Experimental Research on Formation of Large Mass Debris Bed at Severe Nuclear Accident

Abstract

In a severe accident, the molten core material enters the lower head with water or the reactor cavity is cooled and forms a debris bed. The types of the debris bed formed under different conditions are different, including loose granular, or agglomerated cake. Regardless of the types of debris bed, it is desirable to be able to be cooled to prevent further melting. Therefore, the debris bed formation experiment is a very worthy study, as it is a preliminary input condition for evaluating the cool ability characteristics, possibility of remelting, and cooling measures of the debris bed. A test facility was built to study the characteristics of formation of the debris bed in situation of PWR severe accident. The test facility could generate 100 kg molten ZrO2 as corium simulant with melting temperature of 2 700 ℃.The core test section included melting device and debris bed formation tank (water tank).The melting device melted the ZrO2 by electromagnetic induction, with the technology of cold crucible. The water tank structure was a stainless steel frame with segmented parts. There were sealed glass windows on both sides of the tank, so the progress of fuel-coolant interaction can be observed. Under these conditions, the experiments of formation of large mass debris bed were carried out. The main research includes different combinations of molten mass, jet diameter, pool depth and subcooling degree of water. So far, four experimental conditions were carried out. Based on these four conditions, the parameters of debris bed structure, debris shape, debris size distribution, and debris bed porosity were obtained. Under the conditions of this experimental device, the main observations and conclusions are obtained through analysis of experimental data: The height to diameter ratio of the debris bed formed in the natural state is less than 0-2, and the spread degree of the debris bed is relatively large. As the mass of molten material increases, the share of large sized debris in the formed debris bed increases. As the water subcooling decreases, the share of large sized debris increases. The average porosity of the formed debris bed is 0.345, and the porosity of the debris bed will increase as the diameter of the leakage port increases and the water subcooling decreases. Due to the sufficient cooling of the molten material in the water, there is no hightemperature impact on the bottom plate of the experimental water tank. These data will support the study of the coolability and remelting of the debris bed.

Keywords

• severe accident
• corium melt
• fuel-coolant interaction
• jet-breakup