A major challenge for MITR is to convert to LEU while still meeting the performance requirements for experiments in the reactor. Meeting these re­quirements will entail optimizing the fuel design to maximize heat transfer

and neutron flux. In particular, the neutron flux optimization is focused on maintaining HEU-equivalent fast neutron fluxes in in-core materials experi­ments and thermal neutron fluxes in out-of-core experiments. To prepare for conversion, the existing neutronics and thermal/hydraulics models for the MITR core were improved in several ways.

Several major improvements were made to the neutronics codes. The primary change enabled more accurate burnup modeling and benchmark­ing. The improvements entailed an extensive review of the model’s core structure and dimensions as well as an update of the cross-section li­braries, homogenized volume fractions, and discrete structures. Two ini­tial HEU cores were modeled, and the results compared favorably with measurements.

The neutronics codes were improved in other ways as well. A graphi­cal user interface was added, as was the capability to model HEU, LEU, and mixed core geometries. In addition, it is now possible to model all fuel movements, including flipping, rotating, and fuel storage. The models are also now able to track and plot the mass of isotopes as well as the power distribution in the core.

The improved burnup modeling has shown good results. Twelve recent cores have been modeled; the results were in good agreement with measured beginning — and end-of-cycle control blade positions. There was also good agreement among different models.

In addition to the neutronics modeling, thermal/hydraulics models were also updated and modified. Specifically, the models were modified to include the fuel’s longitudinal fins for the steady-state and loss-of-flow analyses. Initial results have shown that the LEU design core has a higher margin to onset of nucleate boiling and a lower peak cladding temperature with loss of flow.

The results of the neutronic and thermal-hydraulic analyses have been used to design an LEU fuel for this reactor. The LEU fuel assembly will contain more plates and use a thinner fuel meat (0.51 mm for UMo LEU fuel versus 0.76 mm for HEU fuel) and cladding (0.28 mm for UMo LEU fuel versus 0.35 mm for HEU fuel). Fuel developers have informed MITR staff that fuel and cladding of this thickness is feasible to manufacture.

As was noted in Chapter 2, the reactor’s operational power will need to be increased from 6 MW to 7 MW to counter the expected loss of per­formance after conversion. This will result in an increase in the cycle length from 40-50 days for the HEU fuel to 60-70 days for the LEU fuel.