Toward Development of a Fuel Specification for Uranium Mononitride Particle Fuel Kernels

Interest in coated particle fuel architectures based upon uranium mononitride (UN) has expanded in recent years. Compared with uranium carbide / uranium oxide (UCO) kernel chemistries familiar to tristructrural isotropic (TRISO) advanced gas reactor qualification activities, the primary advantage offered by UN kernels is a superior uranium density. Research focused on UN kernel fabrication has advanced to overcome early challenges in densification and porosity control, and the open literature now includes reactor irradiation experience and post-irradiation examination activities. These outcomes have demonstrated basic viability of UN TRISO fuel forms. However, fabrication activities performed to date have largely centered on laboratory scale efforts focused on sub-kilogram scales as needed to support separate effects studies and proof-of-concept irradiations, and available irradiation data remains limited. The next step needed to support qualification of UN TRISO or other UN-based coated particle fuel is research to inform a fuel specification. The properties and performance of a nuclear fuel are affected by numerous chemical, microstructural, and geometric factors; a fuel specification prescribes acceptable ranges for these parameters. Validation that key parameters have been controlled within stated limits provides confidence that future material fabricated to the same specification will possess the same properties and offer the same performance. This applies not only to intrinsic material properties (e.g. thermal conductivity, melting point) but also critical aspects of fuel performance (e.g. fission gas release, dimensional changes). The challenges in development of a fuel specification are dictated not only by the fabrication process used, but also by developing sufficient understanding of how various chemical, microstructural, or other physical properties of the fuel material correlate to performance. Historic TRISO fuel specifications have been informed based upon reactor performance, but more often driven by constraints of fabrication processes and available characterization methods. Rapidly expanding applications for particle fuels will place a greater emphasis on understanding impacts to performance rather than defaulting to what can be measured and controlled. Narrow tolerances placed on parameters that have minimal impact on fuel performance may cause economics of a fabrication process to suffer needlessly. Conversely, poor process control or excessively broad tolerances in critical parameters may result in greater uncertainty in irradiation performance, causing more conservatisms in reactor design or operation. The present paper will summarize the critical differences in UN kernel chemistry from UCO kernels with an emphasis on sol gel fabrication followed by carbothermic reduction-nitridation routes used to produce UN. The nominal UN single-phase microstructure presents a simpler unirradiated template than the uranium carbide / uranium oxide composite of UCO kernels. However, the impact of processing variables on this initial microstructure and chemistry of UN is less understood. Available methods for characterization of factors that must be captured in a fuel specification for UN will be outlined and contrasted to those used for UCO at the laboratory scale and when scaled to production levels. A major uncertainty in development of a fuel specification for UN coated particle fuels is how residual carbon and oxygen impurities resulting from initial carbon loading and the conversion profile impact fission product thermochemistry. The carbon and oxygen contents prescribed in modern UCO kernel specifications were determined based upon a comprehensive understanding of kernel thermochemistry evolution during service and other performance factors lacking or absent from UN literature. Ongoing irradiation tests and post irradiation examination campaigns underway will be discussed, and additional irradiation needs proposed. Finally, modern accelerated burnup irradiation capabilities within the context of an accelerated fuel qualification paradigm will be discussed when used for the objective of informing a fuel specification for UN coated particle fuels.