Deep Burn Fuels for Advanced Small Modular Reactors

New fuel forms are needed to enable gas-cooled fast fission reactors to operate using a once-through deep burn fuel cycle. These small, modular reactors will have a core life of 30 years and will burn the plutonium-239 that is created in the core as well as other transuranics. Deep burn rapidly and effectively reduces the inventory of transuranics from spent fuel without the need for repeated recycles, destroys any weapons-usable materials contained in spent fuel, and precludes the possible weapons-related use of the residuals, thereby minimizing proliferation risk. Such reactors are only fueled immediately before startup and limit the generated radioactive waste to fission products only. To realize this technology, a vented high temperature fuel form is required. Ultramet and Sandia National Laboratories previously developed a high temperature tricarbide foam fuel that is ideal for gas-cooled reactor applications. An important attribute of the foam fuel is the presence of an interconnected hollow- ligament structure, which can be exploited in the current work for fission product removal and venting. By venting the fission products with subsequent removal, the foam fuel can sustain a long lifetime, allowing any generated plutonium-239 to be burned along with the low-enriched uranium fuel. Conversion of thorium-232 also becomes a possibility in a blanketed core design. Oak Ridge National Laboratory (ORNL) will perform analysis of cooling and purge flows and thermomechanical analysis including volumetric nuclear heating, and investigate fission product production and transport from the ligaments into the purge channels. The effectiveness of the purge channels and the required thickness of the diffusion barrier coatings on the outside of the ligaments will be determined, and the optimal hollow-ligament foam will be predicted in terms of primary cooling and fission purge flow characteristics. Ultramet will fabricate development specimens, using surrogate fissile foam, based on the optimal foam/closeout layer structure defined through ORNL’s design and analysis work. Helium gas flow tests will be performed to establish flow characteristics and confirm a robust, gas- tight seal between the foam and closeout layer, which will prevent mixing of purge gas streams with coolant streams. The results will be compared with those predicted through modeling. Development of a high temperature vented fuel form is required to enable deep burn reactor designs that reduce proliferation risk and minimize transuranic waste. The vented foam fuel is ideal for small modular reactors that fully utilize all the energy content of low-enriched uranium fuels, require no refueling or spent fuel storage, and may consume existing spent fuel stockpiles.