Optimization studies for Seed-and-Blanket Unit (SBU) fuel assemblies in PWRs

The objective of recent studies as part of a joint NERI project between Brookhaven National Laboratory and Massachusetts Institute of Technology was to develop a seed and blanket unit (SBU) fuel assembly (FA) for thorium utilization in pressured water reactors (PWR). This heterogeneous design utilizes enriched uranium seed and mixed uranium/thorium dioxide blanket fuel pins in physically separable sub-assemblies, and could be used as a one-for-one replacement of more conventional UO₂ FA's in PWRs. The seeds are used for three 18-month cycles in a 3-batch fuel core management strategy, while the blankets are used for six 18-month cycles before being removed for long-term storage; the concepts considered here assume a once-through fuel cycle consistent with current U.S. policy. The SBU should have improved proliferation, radiation safety, and comparable economic characteristics in comparison to existing PWR FA's. Optimization studies using the BOXER, COBRA-EN, and ORIGEN codes have led to an improved SBU FA design for PWRs that achieves high burnup (80 GWd/t over 3 cycles for SBU) with reduced proliferation and toxicity characteristics, while satisfying thermal-hydraulic safety limits (minimum departure from nucleate boiling ratio greater than 1.3 for 118% over-power). The SBU is an attractive fuel for existing and next-generation PWR.