Examination of the conversion of the U.S. submarine fleet from highly enriched uranium to low enriched uranium

Abstract

The nuclear reactors used by the U.S. Navy for submarine propulsion are currently fueled by highly enriched uranium (HEU), but HEU brings administrative and political challenges. This issue has been studied by the Navy before, and promising new fuel technology developments, combined with new motivations, warrant a fresh look at low enriched uranium (LEU). Evidence suggests that the mission space of the submarine force might be changing in such a way to relax previous constraints that made LEU cores more challenging. Estimates show that high density fuels can produce LEU core sizes that are comparable to HEU core sizes. One of the difficult constraints will be meeting the reactivity requirements to overcome xenon poisoning throughout the lifetime of the core, especially at higher burnups, where LEU cores exhibit different reactivity behavior. A new LEU design should not prove prohibitive from a thermal hydraulic perspective, unless increases in core volume or fuel-to-moderator ratio require a significant increase in pumping power. LEU designs that require increases in weight or size of the reactor compartment are not outright infeasible, and alterations to the reactor compartment might provide a necessary recovery of weight and volume. The concept of LEU redesigns resulting in unattractively large new cores that fail to meet performance standards is a thing of the past. A major investigation of advanced naval reactor technology and LEU designs should be made, and a thorough analysis should be conducted soon.