Sustainable Design from Waste Up: Irradiated Graphite Disposal Assessments to Inform Reactor Design and Operation

As Gen IV graphite-moderated reactor technologies advance to demonstration and deployment, the question must be answered of how, where, and when to dispose of the irradiated graphite waste produced from the operation of these facilities. This work presents an integrated assessment involving the entire graphite lifecycle in nuclear power production: impurity measurement of graphite grades, reactor simulations for source-term characterization, and repository simulations for subsurface radionuclide transport from a hypothetical near-surface low-level waste repository. This integration enables the feedback from the disposal assessment to evaluate the engineering design parameters. We demonstrate our approach based on a hypothetical repository with hydrogeological parameters representative of the arid western region in the US. Our results show that mobile carbon-14 is the largest contributor to radioactivity and offsite groundwater concentrations, and that crediting a graphite waste form reduces the concentrations significantly, since it limits the radionuclide release. The sensitivity analysis then shows the dominant impact of release rates from the waste forms compared to sorption and other transport parameters. In addition, reduced-order models are developed separately for the reactor model and repository model to combine two models in a modular structure, and to explore the acceptable nitrogen impurity and operational period for satisfying the repository criteria. The framework developed in this study enables us to quantify the risks associated with the disposal of major radioactive waste streams from advanced reactors, and to consider such characteristics in the reactor engineering designs.