Abstract
Molybdenum-99 (99Mo) has been in short supply in the recent decades. The most common production
mechanism, irradiation in a reactor fueled by highly enriched uranium (HEU), raises
proliferation concerns. Novel efforts are being pursued to meet the demand for 99Mo without
HEU irradiation, relying on the processing of low-enriched uranium or natural uranium. One
such concept is to use an electron linear accelerator to generate bremsstrahlung that, through a
photonuclear reaction, can help multiply neutrons in a subcritical assembly. The goal of this work
was to neutronically optimize the subcritical assembly for keff and 99Mo production. This optimization
is unique for The design was optimized in terms of reflector type, pitch, and LEU and
NU mass. Results of the paper show the effects of fuel mass, pitch, and reflector material on keff
and the NU fission fraction. keff was found to be mostly driven by LEU mass, and the NU fission
fraction was dependent on several parameters, primarily pitch and mass.
