Investigation of Point-Contact Strategies for CFD Simulations of Pebble Bed Reactor Cores

This study numerically investigated the effects of various point-contact strategies on the flow and heat transfer behavior within a simple cubic structured bed. Techniques in computational fluid dynamics (CFD) modeling of pebble beds to avoid point-contact singularities include decreasing the pebble diameter, increasing the pebble diameter, capping pebbles near the point-contact, and extending a bridge between pebbles near the point-contact. These four techniques were investigated on a simple cubic structure bed of 100 explicitly modeled pebbles using a 4 × 5 × 5 structured array. Pebbles were modeled as perfect spheres, and each had a central fuel spherical region and a surrounding fuel-free spherical shell. Two cases of reactor operating conditions were considered. Case 1 conditions were specified at nomimal full power and flow. Case 2 consider the pressurized loss of forced cooling (PLOFC) with reactor trip event. Numerical simulations of heat transfer and fluid flow within the simple cubic structured bed were performed using the Reynolds-averaged Navier-Stokes (RANS) approach with the realizable k-ε turbulence model and two-layer all y⁺ wall treatment. Quantities of interests obtained from the calculations were compared amongst all point-contact strategies. For Case 1, the hydraulic behavior was sensitive to the contact strategy, with a 30% difference in the predicted pressure drop and a 4% difference for the volume-averaged velocity magnitude. The thermal behavior within the pebbles and fluid were not sensitive. For Case 2, large differences between the contact strategies were observed in the predicted pebble temperature when only considering thermal conduction. The differences in the maximum pebble temperature for various strategies were 23% in Case 2a (conduction) and reduced to 2% in Case 2b (conduction and radiation). In Case 2c (conduction, radiation, and 7% convective flow), the thermal differences reduced further to 0.3% across the various point-contact strategies.