Effect of Downgraded Moderator Purity on Lattice Reactivity and Coolant Void Reactivity for a LEU Lattice in the Zed-2 Reactor

Abstract

ZED-2 criticality measurements have been compiled using a lattice comprising Low Enriched Uranium (LEU) fuel assemblies. The data obtained are to determine the effect of down-graded moderator purity on lattice reactivity and Coolant Void Reactivity (CVR) for the test lattice. The data span moderator purities ranging from 99.882 to 98.519 weight % D₂O. Since the fuel assemblies comprise LEU, the measurements did not require changing the lattice configuration over the range of purities studied. These data therefore provide unique applicability to address the reactor physics phenomenon Moderator-Purity-Change-Induced Reactivity and its impact on Coolant Void Reactivity (CVR).

An analysis was performed using the Monte Carlo code MCNP5 (v1.40) employing nuclear data derived from the ENDF/B-VII.0 evaluation. Reactivity coefficients are determined from the experimental data and compared to code predictions. The comparison indicates that there are suspected errors in the nuclear-data library regarding the cross sections for hydrogen in heavy water.