Abstract: As interest in High-Temperature Gas Reactors (HTGRs), particularly Pebble Bed Reactors (PBRs), grows, there is a need for safe and effective storage solutions for their spent fuel. This senior design project proposes a dry cask storage system tailored to PBR spent fuel pebbles, emphasizing safety, regulatory compliance, and economic feasibility. Our cask will consist of three layers: an outer shell for impact protection, shielding, and an inner cask to hold the fuel. The spent fuel pebbles will be packed in a random configuration with a helium back-fill. Key design challenges — criticality control, graphite oxidation, radiation shielding, and structural stability — are addressed through a Dual-Purpose Cask (DPC) design that will be used for both transport and storage. The dry cask storage unit meets all applicable NRC regulations and follows waste acceptance criteria (WAC) established to site storage casks within the geological repository, Yucca Mountain. In addition to our no-insert design, potential inserts were evaluated for additional stability, heat transfer ability, and neutron absorption. Materials such as borated horizontal spacers and molten metal alloys were assessed, but it was determined that the inserts are not necessary. The inner canister, shielding canister, and outer canister are made of XM-19 stainless steel, DUCRETE, and
304L stainless steel, respectively. OpenMC and LAMMPS were employed to assess neutron behavior, thermal performance, and pebble dynamics. It was found that our cask design remains subcritical (keff <0.95), including under severe accident conditions, and structurally stable. Each cask will hold 17,333 PBR spent pebbles and cost approximately $288,000. Through the development of our design, we determined that a higher fidelity of modeling is required for real-life implementation, but the simple and conservative approach we used was effective for the preliminary design. This design supports the advancement of next generation nuclear waste management systems. |