Converting research reactors from highly enriched uranium (HEU) fuel to more proliferation-resistant low-enriched fuel is critical for achieving the objective of ending the use of directly weapon-usable materials in the civilian nuclear fuel cycle. The most challenging type of reactors to convert are high-flux research reactors, which, along with upcoming strong spallation sources, are the most important neutron sources for sophisticated neutron scattering experiments. Advanced Monte-Carlo computer codes are now available that make it possible to track neutrons from the neutron source, through neutron guides, to the detector of a neutronic experimental setup, including realistic samples. These "virtual experiments" allow optimizing the performance of complete beamlines, where in many cases a large unused potential exists for increasing the neutron flux at the sample or detector position. The Monte-Carlo codes VITESS and McStas are used to compare results for typical neutron scattering setups using typical versus state-of-the-art technologies. The analysis shows that performance gains due to instrument upgrades or neutron guide renewals can dwarf potential neutron flux losses due to conversion to low-enriched fuel. Combined convert-and-upgrade strategies therefore offer unique opportunities for reactor operators and neutron scientists to significantly improve the overall performance of research facilities, and turn them into centers of excellence, while supporting the objective of phasing out the use of highly enriched uranium in the civilian nuclear fuel cycle as soon as possible.
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