Investigation of the Influence of Water Injection Strategies and Spatial Heterogeneity on the Coolability of a Particulate Debris Bed in the Reactor Lower Head

摘要: The core melt interacts with the coolant and breaks into a particle bed. Effective cooling of the particle bed can achieve retention of the melt and terminate the accident process. Based on the fragment particle size distribution and porosity after the Reactor fuel and coolant interaction(FCI ) experiment, this paper constructed a mixed particle size sand and gravel fragment bed containing an internal heat source, and conducted experimental studies on the drying characteristics under different water injection cooling methods (top submerged pool, natural circulation driven bottom water injection, circumferential water inflow) and various uneven structures of debris bed(particle size stratification, top steam channel, cake-shaped block layer). It was found in the experiment that under the condition of the top submerged pool, bubble congestion areas and liquid-deficient dry areas appeared successively in the upper and middle parts of the debris bed; under the condition of bottom water injection driven by natural circulation, the liquid shortage state at the bottom of the debris bed was greatly improved, and the dry heat flux density (DHF) increased by more than 2.5 times, and the dry area was located in the middle and upper part of the debris bed; under the circumferential water inflow method, the dry heat flux density also increased by more than 2.5 times. The particle size stratification feature greatly eases the vapor-liquid counterflow in the lower half of the debris bed. Compared with the uniform particle bed, the dry area moves upward and appears in the middle. The steam channel at the top of the debris bed inhibits the heat dissipation of the debris bed. The steam channel with a larger pore size makes the vapor-liquid counterflow limit in the debris bed arrive earlier, and the drying heat flux density decreases. The cake-shaped block layer located in the middle and bottom of the debris bed increases the bubble resistance, increases the local heat release rate, and intensifies the bubble congestion state, causing drying to occur earlier than in a uniform debris bed. In the range of 0 to 1.0MPa, the increase in the cake-shaped block layer reduces the drying heat flux density under the corresponding system pressure by 10% to 18%. At the same time, compared with the uniform bed, the dry area appears at the bottom of the debris bed, and when cake-shaped blocks exist, it appears between two cake-shaped block layers.