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EXECUTIVE SUMMARYThis report contains the results of reactor fuel element design for conversion of the Massachusetts Institute of Technology Reactor (MITR) from the use of highly enriched uranium (HEU) fuel to the use of low enriched uranium (LEU) fuel. The analyses were performed by staff members of the Global Threat Reduction Initiative (GTRI) Reactor Conversion Program at the Argonne National Laboratory (ANL) and the Massachusetts Institute of Technology Nuclear Reactor Laboratory. The core conversion to LEU is being performed with financial support of the U. S. government.The goal of this work was to design an MITR LEU fuel element that could safely replace the current MITR HEU fuel element and maintain mission performance while requiring minimal, if any, changes to the reactor system. As a means to accomplish this, neutronic and steady-state thermal hydraulic performance of the MITR was analyzed with various LEU fuel element designs. The evaluation included the impact of assumed manufacturing tolerances and other uncertainties in reactor parameters.Documents that were reviewed as bases for the design and safety evaluations were the MITR design drawings and historic analyses of the facility. All of the information and data needed to construct the reactor models and perform the analyses were provided by MITR. The current HEU fuel element has 15 plates that are 0.080 inch thick with 0.010 inch deep grooves along the length of the plate. These grooves serve as fins to increase heat transfer area to the coolant. The HEU aluminide fuel contains uranium with a 235 U enrichment of 93 wt%, and is 0.030 inch thick in each plate. The Al-6061 aluminum cladding at the base of the grooves on the HEU plates is 0.015 inch thick.Prior LEU element design analyses with high-density monolithic alloy fuel have obtained equivalent performance and fuel cycle with an 18-plate element with 0.020 inch thick fuel and 0.010 inch cladding thickness (at the base of the fins). Whereas this prior MITR LEU design was based upon cladding thickness of 0.010 inch, recent manufacturing development experience has led to a re-evaluation of the minimum cladding thickness for reliable fabrication. These core and element design activities were undertaken to determine if additional cladding thickness could be incorporated into an MITR LEU element design. Since increased cladding thickness would displace water and degrade core reactivity, removal of the fins was also explored as a goal of the work. Removal of the fins would not only increase water to metal ratio in the core, but would also improve the economics of an MITR LEU element by eliminating this fabrication step, which is unique to MITR fuel design among U.S. high performance research reactors which refuel with HEU. In order to compensate for the loss of heat transfer area, an increased core coolant flow rate has been considered, and distinct fuel thicknesses were introduced in the outer plates of each element to limit heat flux peaking.The proposed LEU fuel element designs have the same over...

Multi-physics calculations may involve coupling continuous-energy Monte Carlo neutronics codes to CFD codes that provide many thousands or even millions of region temperatures. The traditional Monte Carlo approach – using precalculated Doppler broadened nuclear cross-sections – is not feasible for these large multiphysics problems. Instead, an On-the-Fly (OTF) Doppler broadening methodology is required, whereby neutron cross-sections are broadened during the Monte Carlo transport. To this end, we have developed a methodology for MCNP to provide OTF broadening based on cell temperatures during neutron tracking. The method enables the use of many thousands or more temperatures in MCNP Monte Carlo calculations for multiphysics applications, significantly advancing the state-of-the-art by permitting the solution of problems that were not previously possible with continuous-energy Monte Carlo codes. A production library with an extended set of isotopes has been developed for use with MCNP6. Calculations of test problems with MCNP6 and the new library demonstrate the accuracy and effectiveness of the OTF approach.

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