The Massachusetts Institute of Technology (MIT) Reactor II (MITR-II) is a 5-MW research reactor presently fueled with highly enriched uranium (HEU) in uranium-aluminum plate-type elements. A low-enriched uranium (LEU)-fueled core has been designed using 20% enriched monolithic uranium-molybdenum fuel that maintains high experimental neutron flux and increases flexibility in meeting the needs of experiments. The configuration of the new plate fuel elements was selected using a full-core MCNP model, with which different in-core materials were evaluated to optimize the neutron fluxes, reactivity, and experimental neutron spectrum. In-core materials were chosen to meet experimental flux level and spectrum needs. Of the designs evaluated, the most promising consisted of half-width fuel elements with nine U-7Mo LEU fuel plates.

Results from the MCNP/ORIGEN linkage code MCODE depletion calculations showed that the refueling interval of the chosen LEU core would be twice as long as the HEU core at the same power level. Thermal-hydraulic analysis using the MULtiCHannel analysis code II (MULCH-II) indicated that the peak channel will remain below the onset of nucleate boiling under normal and loss-of-flow conditions. A thermal-hydraulic evaluation of the limiting channel using point kinetics showed that the LEU core could withstand a step reactivity insertion of 3.92 $, increasing by 60% the allowable reactivity for an in-core experiment. Finally, preliminary analyses show that it may be feasible to use the proposed design to double the core power, if the fuel cycle length is to be kept at its present length.