Design of a mixed-spectrum long-lived reactor with improved proliferation resistance

Abstract

Long-lived fast reactors have been suggested as an effective way of spreading nuclear energy to new countries. These small reactors can be produced at centralized locations, shipped to area of need, then returned to the main hub at the end of their lifetime for decommissioning. Such ‘hub-spoke’ arrangements disincentivizes states front building sensitive front and back-end technology; however, critics argue they still pose a proliferation risk due to the large quantity of weapon-grade plutonium they produce during their operating lifetime. The dissertation attempts to address this issue by proposing a mixed-spectrum core configuration. A fast neutron zone can increase fissile material production, while a thermalized zone reduces plutonium quality. Moderating material (ZrH1.6) is inserted within peripheral assemblies, while the center of the core maintains a fast configuration. Assemblies are then shuffled to ensure all are exposed to the thermalized spectrum. This allows the new design to simultaneously improve proliferation resistance and reduce fast fluence damage, a limiting criteria for long-lived core designs. The objectives are achieved with minimal impact on overall performance. Core lifetime can be maintained at 25 years, without the need for any additional fuel. Inherent passive safety criteria can be met, and power peaking phenomena at the fast/thermal interface was deemed to be manageable. Different design variants that can alleviate power peaking or leverage the ability of thorium-cycle breeding in the epithermal regime, were also investigated. Mixed-spectrum cores pushes the boundaries of what deterministic codes are capable of modeling accuracy. The REBUS suite of codes is modified to provide a more accurate tool to explore the design space. MCNP6 is then used for detailed analysis and safety evaluation of optimal core configurations. The thesis demonstrates the viability of using a mixed-spectrum reactor design to improve proliferation resistance of long-lived cores. The main identified tradeoff was an increase in overall resource consumption, a slightly larger core size, and the reliance on shuffling midway through the core lifetime.