Prismatic and Pebble-Bed Micro-HTGRs in Continuous-Recycle Nuclear Fuel Cycles

This work adapts both prismatic and pebble-bed micro high-temperature gas-cooled reactor (HTGR) point designs for Pu/transuranic (TRU) driver fuel under a continuous-recycle fuel cycle. This article indicates the potential for HTGR microreactors to be deployed as burner reactors in continuous-recycle fuel cycles. The adapted prismatic and pebble-bed micro-HTGR point designs are optimized to maximize fuel discharge burnup to reduce nuclear waste and environmental impact. Optimization was performed for inert matrix fuel (IMF) concepts, each using different composite moderators in the prismatic and pebble-bed designs. The prismatic and pebble-bed designs use TRU oxide tristructural-isotropic (TRISO) fuel entrained in a magnesium oxide (MgO) ceramic compact. The moderator materials considered include beryllium, beryllium oxide, yttrium hydride, and zirconium hydride entrained in the same 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 also varied the TRISO packing fraction while further considering variables that included the pebble fueled radius, the ratio of fueled to unfueled pebbles, and the active reactor core radius. Energy-normalized metrics, such as the amount of spent nuclear fuel (SNF) and waste, activity, and environmental impacts, are reported for the continuous-recycle prismatic and pebble-bed IMF concepts. These metric results are compared to those of the graphite reference design, once-through prismatic and pebble-bed IMF concepts, a light water reactor (LWR), and a small modular reactor (SMR). All the continuous-recycle IMF concepts are shown to outperform the graphite reference case for all the evaluated metrics. Additionally, all the continuous-recycle IMF concepts outperformed their once-through microreactor counterparts, the LWR, and the SMR for most of the evaluated metrics. For the mass of SNF and high-level waste disposed, the continuous-recycle designs saw a 92.7% reduction with a concomitant reduction in the volume of low-level waste disposed between a 29.7% to 31.1% reduction. In addition to the levelized metrics, the levelized cost of fuel (LCOF) was calculated for the continuous-recycle prismatic and pebble-bed microreactor designs. The LCOF analysis was performed using cost drivers, uncertainties, and potential for cost reduction developed by employing deterministic modeling, sensitivity analyses, and Monte Carlo simulations. Significant reductions were seen in the LCOF, particularly in the pebble-bed concepts. Compared to the graphite reference, the prismatic designs saw between a 72.6% and 76.5% reduction and the pebble-bed designs saw about a 77.8% reduction in the LCOF. Overall, the continuous-recycle IMF concepts significantly reduced nuclear waste, environmental impact, and LCOF compared to conventional once-through reactor fuel cycles. It is important to note that the cases in this paper are representative of the maximum theoretically possible performance in these systems, and that significant research and development would be required to realize these potential benefits.