Investigation of pellet-clad interaction during load-follow operation in a pressurized water reactor using VERA-CS
This thesis is a comprehensive study on the effect load-follow operation has on fuel failure risk in a PWR. PWR1, a commercial reactor that performed load-follow power maneuvers in cycle 21, was modeled using VERA-CS. The effects of a jump-in approach, starting depletion in cycle 17 as compared to cycle 1, were shown to be negligible after two cycles of depletion. Both low power physics tests and critical boron comparisons show excellent agreement between the VERA-CS predictions and the plant-measured data after cycle 19. In addition to cycle depletion, five load-follow power maneuvers in the first 24 days of cycle 21 were simulated using hourly depletion steps. Predictions showed an overestimation of the axial offset behavior during the load-follow operation but the overall reactivity of the core was in good agreement. The linear heat rate and coolant temperature calculated by VERA-CS were used as boundary conditions for BISON fuel performance calculations. Using the maximum hoop stress as the indicator of fuel failure risk, the different load-follow power maneuvers were compared to the initial start-up ramp. The maximum clad hoop stress occurred during the start-up ramp and was estimated to be 94 MPa, the subsequent maneuvers showed similar magnitudes regardless of the change in power. No correlation was observed between the characteristics of the load-follow power maneuver and the resulting maximum clad hoop stress. Neither the magnitude of the decrease in power, hold period, nor intermediate power steps showed a variation in the maximum clad hoop stress observed after a return to full power this early in the cycle. To reduce the number of fuel performance calculations required to determine the safety of a power maneuver, a screening process was developed based on a weighted change in conditioned power. By selecting the top 1000 fuel rods showing a positive weighted change in linear heat rate, the limiting fuel pin for the maneuver is included, with most of the selected pins showing elevated hoop stresses.
Pellet-Clad Material Interaction, Load-follow operation, VERA-CS, BISON
University of Illinois - Urbana-Champaign
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University of Illinois at Urbana-Champaign