The effect of material homogenization on the calculated gamma-ray dose rate was studied for several arrangements of typical pressurized water reactor (PWR) spent fuel pins in an air medium using the Monte Carlo code MCNP. The models analyzed increased in geometric complexity, beginning with a single fuel pin; progressing to small lattices, i.e., 3 × 3, 5 × 5, and 7 × 7 fuel pins; and culminating with a full 17 × 17 pin PWR bundle analysis. The fuel pin dimensions and compositions were taken directly from a previous study, and efforts were made to parallel this study by specifying identical flux-to-dose functions and gamma-ray source spectra.

The analysis shows two competing components to the overall effect of material homogenization on the calculated dose rate. Homogenization of pin lattices tends to lower the effect of radiation channeling but increase the effect of source redistribution. Depending on the size of the lattice and the location of the detectors, the net effect of material homogenization on the dose rate can be insignificant, or it can range from a 6% decrease to a 35% increase relative to the detailed geometry model.