Effect of packing fraction variations on the multiplication factor in pebble-bed nuclear reactors

Abstract

Abstract The effect of fuel element packing density on the effective multiplication factor, keff, in a typical pebble-bed reactor (PBR) is studied using MCNP and WIMS computer codes. It is observed that at normal operating conditions keff increases with packing density. Increase in packing density of 1% results in approximately 0.0015 increase in keff. A negative temperature effect is observed at all feasible packing densities. The effect of secondary coolant (water) ingress in the core at accidental conditions is studied at various packing densities. The effect of effective water density on keff depends on packing fraction, fuel enrichment and fuel-to-graphite ratio. It is strongly positive in highly enriched fuel at low water density regardless of packing fraction. For fuel enrichment 8% and normal packing density, a maximum positive effect of approximately +0.10 on keff is observed at a water density of ∼0.1 g/cm3. The positive effect is reduced at low enrichments. Above a water density of 0.5 g/cm3, the effect on keff becomes negative regardless of enrichment and packing density.