2017 | AFC ACCOMPLISHMENTS 150 3.2 AR COMPUTATIONAL ANALYSIS The present lack of a domestic fast neutron flux irradiation capability combined with continued development of fast reactor fuels in the U.S. motivated an innovative engineering solution to utilize a unique neutron flux tailoring capability in the Advanced Test Reactor (ATR) at the Idaho National Laboratory.To achieve the objectives of the fast reactor fuel irradiation tests, the incident neutron flux was hardened substantially by placing fueled irradiation capsules inside specially designed cadmium shrouds. Use of cadmium prevents thermal neutrons from reaching the fuels being tested and alleviates the plutonium self-shielding that would normally arise during irradiations of high density, highly enriched fuels in a thermal neutron spectrum. Modeling Restructuring in MOX Fuels Principal Investigator: Pavel Medvedev Collaborators: Steven Hayes, Samuel Bays, Stephen Novascone, Luca Capriotti Project Description: This work illustrates the profound effect this engineered solution has on the efficacy of the experiments. Based on the comparison of postirradiation measurements of the columnar grain region in fast reactor, mixed oxide fuels with fuel performance calculations, it is demonstrated that thermal conditions achieved in these cadmium-shrouded fuel experiments are substantially prototypic of a sodium fast reactor and are suitable for concept-screening tests supporting development of new fast reactor fuels. It is also shown that if the experiments were conducted in an unmodifiedATR neutron spectrum, gross plutonium self shielding would cause a strong depression of the fission power at the fuel centerline preventing fuel restructuring, a hallmark feature of Mixed Oxide (MOX) fuel behavior under fast reactor conditions. This is an important step in establishing the relevancy of fuel performance data generated for fast reactor fuels using ATR cadmium-shrouded experiments. Figure 1. Radial distribution of fuel thermal conductivity. Figure 2. Radial distribution of fuel stoichiometry. Figure 3. Radial distribution of the fission rate.