High-Temperature Gas-Cooled Microreactors in Continuous Recycle Nuclear Fuel Cycles for Reducing Nuclear Waste and Environmental Impact

This work adapts both prismatic and pebble-bed micro high-temperature gas-cooled reactor (HTGR) point designs for Pu/TRU driver fuel under a continuous recycle fuel cycle. The adapted prismatic and pebble-bed micro-HTGR point designs are optimized to maximize fuel discharge burnup to reduce waste disposed, fuel cycle costs, and environmental impact. Optimization was performed for inert matrix fuel (IMF) concepts using different composite moderators in prismatic and pebble-bed designs. The prismatic and pebble-bed designs use (TRU) oxide tristructural-isotropic (TRISO) fuel. Moderator material includes beryllium (Be), beryllium oxide (BeO), yttrium hydride (YH_x=1.9), or zirconium hydride (ZrH_x=1.6) entrained in a MgO host matrix. For each prismatic IMF concept, optimization studies were performed to maximize discharge burnup by varying the TRISO packing fraction and assembly lattice pitch. For each pebble-bed IMF concept, optimization studies were performed by varying the TRISO packing fraction, the pebble fueled radius, the ratio of fueled to unfueled pebbles, and the active reactor core radius. Energy-normalized metrics for the mass of spent nuclear fuel and high-level waste (SNF&HLW) disposed, volume of low-level waste (LLW) disposed, metric tons of natural uranium (MTNU) used, area of land used, volume of water used, mass of carbon dioxide (CO₂) produced, and activity of SNF&HLW at 100 and 100,000 years are calculated for the continuous recycle prismatic and pebble-bed IMF concepts. These metric results are compared to those of a graphite reference design, once-through prismatic and pebble-bed IMF concepts, a light water reactor (LWR), and a small modular reactor (SMR). All continuous recycle IMF concepts outperformed the graphite reference case for all evaluated metrics. Additionally, all continuous recycle IMF concepts outperformed their once-through counterparts, the LWR, and SMR for nearly all evaluated metrics. For the mass of SNF&HLW disposed, the continuous recycle designs saw a 92.7% reduction. For the volume of LLW disposed, the continuous recycle designs saw between 29.7% to 31.1% reduction. Overall, the continuous recycle IMF concepts significantly reduced waste disposed, fuel cycle cost, and environmental impact compared to conventional once-through designs.