Effect of Subchannel Void Fraction Distribution on Lattice Physics Parameters for Boiling Water Reactor Fuel Bundles

In boiling water reactor (BWR) cores, the radial void distribution in fuel bundles is thought to deviate from uniform distribution. The effect of heterogeneity in the subchannel void fraction distribution, caused by the presence of Gd-poisoned and cold surfaces as well as control blades on BWR lattice physics parameters, has been evaluated. The cross-sectional void distributions in each axial plane of a fuel bundle are calculated using the subchannel thermal hydraulics code COBRAG. The rod power distributions to be fed to COBRAG are calculated using either the Monte Carlo code MCNP4C or the fuel lattice code TGBLA. Iterative methods consisting of COBRAG and MCNP4C (or TGBLA) are established. A set of test cases was generated for a typical BWR 8 × 8 fuel bundle. The results of the coupled MCNP4C and COBRAG method reveal that both Gd rods and control blade increase in worth due to the void heterogeneity, showing a maximum decrease in K∞ of ∼0.7%Δk/K∞ and ∞1.5%Δk/K∞, respectively, at moderator density conditions equivalent to ∼40% void fraction. With the capability of the coupled TGBLA and COBRAG method to deplete fuel bundles, the acceleration of Gd depletion was evaluated. The impacts on the burnup characteristics of K∞ reached ±0.6%ΔK∞ at maximum.