A Comparison of Gadolinia and Boron for Burnable Poison Applications in Pressurized Water Reactors

Extended cycle lengths and fuel burnups are receiving increased attention. Frequently, the attendant fuel management strategies in pressurized water reactors (PWRs) require burnable poison shims to control power distribution and to maintain a negative moderator coefficient. High energy (∼450 effective full-power days) fuel cycles utilizing both out-in and low-leakage assembly placement schemes provide some insight to the relative merits of UO₂-Gd₂O₃ (gadolinia)- versus boron-bearing shims for PWR applications. Relative to using boron-bearing burnable shims in PWRs, gadolinia has important potential advantages and disadvantages. With proper application, the advantages point to a reduction in fuel cycle costs and increased fuel management flexibility. However, for proper application, the more complex gadolinia neutronics and thermal-mechanical design characteristics must be modeled accurately.