Assessment of shutdown margin requirements for high conversion BWR with Th-U233 fuel

A number of reactivity control system design options are explored in order to satisfy shutdown margin (SDM) requirements in a high conversion BWRs operating in Th-233U fuel cycle (Th-RBWR). The studied core has an axially heterogeneous fuel assembly structure with a single fissile zone located between two fertile blanket zones. The utilization of an originally suggested RBWR Y-shape control rod in Th-RBWR is shown to be insufficient for maintaining adequate SDM to balance the high negative reactivity feedbacks, while maintaining fuel breeding potential, core power rating, and minimum Critical Power Ratio (CPR). Implementation of alternative reactivity control materials, reducing axial leakage through non-uniform enrichment distribution, use of burnable poisons, reducing number of pins as well as increasing pin diameter are shown to be incapable of meeting the SDM requirements. Instead, an alternative assembly design based on Rod Cluster Control Assembly with absorber rods was investigated. This design matches the reference ABWR core power and has adequate shutdown margin. The study focused on the neutronic performance of the concept and was based on a single 3-dimensional fuel assembly model having reflective radial boundaries. The analyses were performed using BGCore code system, which consists of the MCNP code coupled with fuel depletion and thermo-hydraulic feedback modules.