Numeral Research on Thermal-Hydraulic Characteristics of Pebble Bed in WCCB Blanket Under In-Box LOCA

Abstract After the breakage of the coolant pipeline in Water Cooled ceramic Breeder blanket (WCCB), the coolant at high pressure inside the pipeline will be sprayed into the pebble bed environment, and the specific behavioral characteristics of this process are not clear. In order to investigate the flow characteristics of the coolant in the pebble bed after the rupture occurs, numerical simulation of this physical process is carried out based on the three-dimensional computational fluid dynamics (CFD) method. The flash phase transition process is described by the VOF model and the buckling flash model. The simulation results show that, after the In-box LOCA occurs, the coolant enters the proliferating spherical bed and rapidly vaporizes into water vapor by flash evaporation. Due to the geometrical characteristics of the blanket structure, the water vapor diffuses inside the pebble bed and then gradually diffuses to both sides and contacts the underside of the pebble bed; in addition to this, helium will flow out of the cladding along the helium pipeline during the In-box LOCA process; The In-box LOCA process can be initially divided into 2 stages. Stage 1: water vapor rapidly diffuses to contact the closer wall of the pebble bed at the beginning of the breach, which has a longer duration and a faster rate of pressure rise. Stage 2: water vapor diffuses mainly to the farther side of the wall, and because the gas flows out along the helium pipeline, this stage will last longer and the pressure rises slowly and is relatively stable.