Neutron-Use Optimization with Virtual Experiments to Facilitate Research-Reactor Conversion to Low-Enriched Fuel

For almost three decades, there have been signicant international eorts to convert research reactors from weapon-usable highly enriched fuel to a fuel based on uranium enriched to less than 20% in the isotope U-235 (LEU). The most challenging type of reactors to convert are high- ux research reactors, which, along with some spallation sources, are the most important neutron sources for modern neutron scattering experiments. Advanced Monte-Carlo computer codes are now available that make it possible to track neutrons from the reactor core, through a beam tube, to the instrument and detector. These \\virtual experiments\" allow optimizing the performance of a research facility as a whole instead of the reactor alone. This may open additional reactors for conversion and could signicantly accelerate the \\global cleanout\" of civilian HEU. This article brie y reviews performance gains obtained for high- ux reactors during previous facility-upgrades. The Monte-Carlo code VITESS is used to compare results for typical neutron scattering experiments using obsolete versus state-of-the-art technologies. The analysis shows that performance gains due to instrument upgrades or neutron guide renewals dwarf potential neutron ux losses due to conversion to low-enriched fuel.