UC Berkeley

Breed and Burn (B&B) reactors are a special class of fast reactor that promises enormous sustainability and proliferation-resistance benefits compared to current commercial nuclear power systems. Utilizing a very tight neutron economy in the fast spectrum, B&B reactors are able to sustain criticality in the equilibrium state without either fuel enrichment or reprocessing. As with many advanced reactor systems, however, B&B reactors currently suffer from a number of uncertainties and infeasibilities hampering their practical deployment. This dissertation aims to better understand and propose solutions to some of these primary uncertainties. Utilizing state-of-the-art computational tools and techniques, the properties of a prototypical B&B core are investigated and quantified in high fidelity, enabling deeper understanding of the physical phenomenon driving their unique characteristics. Subsequently, three of the most pressing current uncertainties in B&B cores are investigated in detail: (1) reactor orificing, (2) transient performance and inherent safety, and (3) the tradeoff between material integrity and resource utilization.

To understand the feasibility of adequately cooling fuel assemblies over their in-core lifetimes, new mathematical optimization techniques are developed and applied to devise optimal orificing strategies subject to a variety of important operational constraints, the results of which demonstrate how certain methods of orificing are better suited to B&B operation than others. To understand B&B transient performance, the first open-literature transient models are developed and analyzed for a wide variety of hypothetical accident scenarios, finding challenges associated with the unfavorable reactivity feedback mechanisms inherent to B&B core designs. In order to improve transient performance, a novel passive safety system is incorporated into the core and its behavior simulated using a newly-coupled code system. Finally, the topic of material damage is explored by first using newly-proposed methods for uncertainty quantification on the material damage levels. Once the uncertainties have been understood, the study moves to exploring the tradeoff between material damage and resource utilization when low-enriched uranium is used as a feed fuel. In order to improve this tradeoff, new two-tiered reactor systems are designed and analyzed which enable significantly higher resource utilization than current light water reactor systems without requiring fuel reprocessing or physical/chemical separations. Each of these explorations allows for some of the uncertainties around B&B reactors to be clarified, and this thesis therefore enables improved viability of implementing the breed-and-burn mode of operation as part of future nuclear power systems.