Cross section homogenization for transient calculations in a spatially heterogeneous geometry

The current norm in reactor accident simulation is to use homogenized cross sections that are computed using a fundamental mode flux. It is shown however, that using such cross sections in super prompt critical kinetic calculations can introduce large errors in the time dependent power. This work uses a previously developed homogenization technique (Dugan et al., 2016a), but applied to a spatially heterogeneous domain. The new homogenization method is shown to reduce the error in maximum power from ∼40% when using fundamental mode homogenized cross sections to ∼6% when using the new homogenization technique in the case studied. Focus is given to the time discretization used in the new homogenization technique; refinement of the temporal discretization is used to show that error reduction is possible until the point at which other sources of error dominate. Additionally, a study of the computational cost involved in this method is examined and improvements are suggested to reduce the cost incurred from using this new homogenization method.