Maximizing Radioisotope Output Through Beryllium Reflector Geometry Optimization in Small-Scale Medical Reactors
Abstract
Mohammad Yaghoub Abdollahzadeh Jamalabadi
Maintaining a reliable worldwide supply of medical radioisotopes requires ongoing advances in both compact reactor architecture and production efficiency. This investigation develops a comprehensive computational methodology to identify the ideal beryllium reflector thickness for small nuclear reactors used in radioisotope manufacturing. The theoretical foundation draws on multi-group neutron diffusion theory, extended to a ten-group energy framework with improved thermal resolution. Key physical phenomena are captured through temperature-dependent cross-sections incorporating Doppler broadening, thermal up-scatter effects governed by detailed-balance principles, and a two-stage Pareto multi-objective optimization scheme. The analysis simultaneously targets five radioisotopes of major clinical importance — Mo-99, Lu-177, I-131, Y-90, and Tc-99m — with production rates scaled to anticipated clinical demand profiles. The modeled system consists of a cylindrical plutonium core (5.0 cm radius, 10.0 cm height) enclosed by a beryllium reflector whose thickness is varied across a 0–30 cm range. Parametric simulations show that a reflector thickness of roughly 12–15 cm yields the best overall performance, improving isotope output by 35–40% compared to an unreflected configuration while keeping the effective neutron multiplication factor (k_eff) safely within the 1.05–1.10 window. Of the six nuclear data libraries evaluated, JENDL-3.2 achieved the closest alignment with experimental data from the Pakistan Research Reactor-2, with calculated-to-measured discrepancies held within ±5% — considerably better than the 8–11% deviations observed for other libraries. An independent OpenMC Monte Carlo model was constructed to cross-validate the diffusion theory predictions, and the satisfactory agreement obtained confirms that the multi-group framework is suitable for this class of design optimization. The findings offer practical, evidence-based guidance for enhancing isotope supply resilience in both greenfield reactor projects and retrofit scenarios at existing facilities.
