Studies of 2D reflector effects in cross section preparation for deep burn VHTRs

The prismatic VHTR neutronic simulation presents challenges due to thermalization of the neutrons in the graphite reflector which leads to a spectral change in the peripheral fuel blocks. Two calculation schemes were tested on a simple 2D core calculation: a single block path wherein a classical single block lattice calculation provides the homogenized cross-sections, and a supercell path where the homogenized cross-sections are generated using a lattice of a fuel block surrounded by some of its surroundings. In both paths, several group condensations were performed to assess the effect of increasing the number of groups in the core calculation from 2 to 295. Core and lattice calculations were validated with respect to MCNP. The study revealed that the supercells lead to improvement in the calculation of power shape over the single-block path. This improvement is rather pronounced with small numbers of energy groups. For larger numbers of energy groups, however, both solution methods appear to yield adequate accuracy and the improvement gained through supercells in these cases may not warrant the computational cost. Lattice depletion calculations also show that the presence of the reflector creates strong heterogeneities on isotopic densities after 1000 days of burning.