Neutronic Performance and Thermal Hydraulic Analysis of the MIT Reactor Fission Converter Experimental Facility Using High-Density U-10Mo Low-Enriched Uranium Fuel Elements

Abstract

The MITR fission converter (FC) is a core-driven subcritical assembly at the MIT Nuclear Reactor Laboratory, located on the MIT campus in Cambridge, MA. The assembly is made of eleven partially-depleted MITR-II fuel elements in a separate cooling tank attached to the side of the core-tank graphite reflector. The FC serves to boost the thermal flux from the core and send a hardened neutron spectrum to an irradiation target, providing a fission energy flux spectrum without the need to put a sample inside the core tank. It was previously used for boron-neutron capture therapy clinical trials before its decommissioning in the 2010s. Recently, it has been modified from a medical beamline to a general-use engineering and materials testing facility. The new FC-based experimental facility has roughly one cubic meter of empty space downstream intended to contain large experiments, called the m³. This work is a safety and performance study aimed at quantifying the impact of modifying the facility’s geometry as part of the FC’s recommissioning, as well as the impact of changing its fuel from HEU to LEU fuel as part of the MITR LEU conversion project. Neutronics and thermal hydraulics analysis of the renovated facility have been performed using the codes MCNP5 and STAT7, respectively. This analysis quantified the FC’s k_eff, power distribution, multi-group neutron flux, and conditions which cause onset of nucleate boiling (ONB). It was determined that the FC assembly will remain subcritical (k _eff < 0.9) and low power (≤200 kW) under a wide range of performance conditions, including with both types of fuel and a variety of materials on the target-side of the FC tank. The HEU-fueled FC is expected to require no changes to the limiting safety system settings (LSSS) outlined in the original technical specifications document. The LEU fuel is expected to increase the FC performance, but as a tradeoff, will require minor changes to the LSSS setpoints to maintain margin to ONB under the most limiting thermal-hydraulic conditions. Additionally, this study evaluates the feasibility of using the FC for in-assembly fuel experiments, particularly as a pathway for testing the new LEU fuel elements at low power. This study indicated that this proposed FC configuration with one LEU and ten HEU elements is feasible and maintains wide safety margins.