Credible Criticality Safety Margin in the 30B Package with LEU+ UF6 and Hypothetical Water Ingress

The commercial nuclear industry is pursuing advancements in fuel and reactor design that increase the uranium enrichment above 5 wt. % 235U. These advancements will necessitate the ability to transport bulk quantities of UF6 at increased enrichments. Currently, the 30B cylinder is the primary container used by the industry for UF6 storage and transportation and has a long history of successful shipments. This container can transport up to 2,277 kg of UF6 at a maximum enrichment of 5 wt. % 235U. Previous evaluations have assessed the potential impact of criticality safety for 30B transport at higher enrichments but assumed moderator intrusion would not require evaluation. Although current regulations allow for the exception of moderator intrusion for UF6 packages through the design and quality control of the package content, this exception is limited to enrichments up to 5 wt. % 235U. Thus, an investigation of moderator intrusion into a 30B cylinder should be performed. Moderator intrusion into a 30B cylinder is a unique condition for criticality safety evaluation in transportation because of the violent chemical reactions that occur between UF6 and H2O. The resulting intrusion is strongly dependent on the breach size, breach location, breach interface solid/ullage of the UF6 content, the UF6 distribution (which is temperature dependent), and temperature/pressure conditions which are dynamic in accident conditions. Additionally, the complexity of the HF-UO2F2-H2O interface that occurs during the event can influence the potential solubility of uranium in the system, as well as influence the amount of UF6 reaction with H2O. With the operating experience from Orano Federal Services LLC, the UF6 chemical expertise from Oak Ridge National Laboratory (ORNL), and the criticality safety expertise from ORNL, this paper evaluates the neutronic conditions (i.e., keff) that apply the understood chemistry and experimental conditions that occur during moderator intrusion of a breached 30B cylinder under postulated accidents. This report examines the historical evaluations of UF6 transport and expands these evaluations for the enrichments expected for nuclear industry advancement. These simulations primarily feature a homogeneous mixture of UF6 and H2O as an infinite media system and as a sphere with water reflection given an impurity limit of 0.5 wt. % UF6. This report demonstrates that this H/U limit is valid up to 8 wt. % enrichment for both H2O and HF as the moderating mixture. The water-reflected homogeneous mixed spheres evaluated demonstrate the amount of safety margin applied by restricting the 30B cylinder impurity limit to 0.5 wt. %. Additionally, this report evaluates moderator ingress scenarios of a 30B cylinder. The 30B cylinder simulations vary the cylinder orientation, the mass of UF6 in the system, the mass of H2O in the system, and how much H2O has reacted with UF6. The moderator ingress of a 30B cylinder was evaluated for the water-reflected homogeneous mixed spheres to demonstrate the amount of safety margin applied by restricting the 30B cylinder impurity limit to 0.5 wt. %. These simulations are standard practice and are independent of the UF6-H2O reaction. This work also explored more complex simulations that layer the H2O over the UF6, which incorporates the amount of reacted UF6 as a separate layer and considers solubility limits of UO2F2 in H2O as it is produced. This approach is intended to simulate the postulated event of a large hairline crack occurring on a submerged cylinder. Finally, this work considers the potential of H2O mixing into the UF6 solid heterogeneously using a sponge-like model. The water ingress into the UF6 is treated as random size spheres, and the reaction products form into a layer over the spheres.