A New Fuel Management Plan for The Pennsylvania State University TRIGA Reactor Including Supporting Experiments and Calculations

The Pennsylvania State University Breazeale (TRIGA) Reactor (PSBR) has operated for 25 yr (440 MWd) using a mixed 12 wt% ZrH_x-U and 8.5 wt% ZrH_x-U fuel configuration (both enriched to 20 wt% ²³⁵U, and x, the ratio of H to Zr, is nominally 1.65). In this configuration, the most reactive 12 wt% ZrH_x-U fuel is always in the B-ring. The B-ring is the innermost hexagonal ring, incorporating 6 fuel elements, and the C-ring is the next outward ring, having 12 fuel elements. PSBR experience during pulsing and steady-state operation indicates that with these configurations the maximum fuel temperatures should be reduced in order to extend the useful life of the 12 wt% ZrH_x-U fuel. This is because during the past 10 yr, the fuel temperatures of the new fuel have been significantly higher than the original fuel. The instrumented fuel element (I-15) loaded into the core ~10 yr ago and the most recent batch of fresh 12 wt% ZrH_x-U fuel elements (six total, including I-16 and I-17) measured temperatures more than 100°C higher than any previous instrumented fuel element. Subsequent pulsing of I-15 increased its measured fuel temperature to where it began to approach the limiting safety system setting. Recent pulsing of I-16 and I-17 caused their steady-state fuel temperatures to decrease slightly, but they remain high. The new fuel management plan reduces these fuel temperatures by replacing the used 12 wt% ZrH_x-U fuel in the C-ring with fresh 12 wt% ZrH_x-U fuel. The 12 wt% ZrH_x-U fuel in the B-ring is replaced with 8.5 wt% ZrH_x-U fuel. Experiments have been performed to verify the predicted core parameters for the new plan. The lifetime of the new 12 wt% ZrH_x-U fuel should now be limited by its maximum allowed burnup, which has not occurred so far.