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Development and Demonstration of a Methodology to Evaluate High Burnup Fuel Susceptibility to Pulverization under a Loss of Coolant Transient

by Nathan A Capps, Ryan T Sweet, Brian D Wirth, Andrew T Nelson, Kurt A Terrani
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Nuclear Engineering and Design
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For economic reasons, the United States nuclear industry is renewing efforts to build a technical basis to extend rod average burnup limits above the current regulatory burnup limit of 62 GWd/MTU. The primary driver is to increase Pressurized Water Reactor cycle lengths to 24-months, reducing the number of fresh fuel assemblies and reduce core design constraints, thereby increasing core energy utilization efficiencies. However, fuel pellet fragmentation and pulverization, termed high burnup fuel fragmentation, has been observed in high burnup (>90 GWd/MTU) Halden loss-of-coolant-accidents integral test. The issue gained attention when fuel fragmentation and pulverization was also observed closer to the current United States regulatory limit during the U.S. Nuclear Regulatory Commission (NRC) sponsored out-of-core integral test at Studsvik Nuclear in early 2011. This led to NRC concerns with potential changes to fuel and core designs relative to fuel pellet pulverization. In a letter to the NRC Commission, the staff specifically identified a need to “…define the boundary of safe operation for key fuel design and operating parameters, the staff is challenged to evaluate the acceptability of future fuel design advancements and fuel utilization changes.” As such, it can be concluded that high burnup fuel fragmentation and potential dispersal into reactor coolant system introduces additional complications in LWR fuel safety evaluations. However, it is not clear how much fuel will be susceptible to high burnup fuel fragmentation, nor has there been a methodology developed to evaluate fuel susceptibility to high burnup fuel fragmentation. To that end, this paper proposes an analysis methodology to assess fuel susceptibility to high burnup fuel fragmentation during loss-of-coolant accident scenarios. The work presented here uses the BISON fuel performance code to evaluate a representative pressurized water reactor fuel rod exposed to a rod average burnup of 75 GWd/MTU. Sensitivity studies investigated the impact of the peak cladding temperature, transient fission gas released, and pre-transient fission gas release on cladding ballooning and burst timing. Subsequently, a methodology to assess fuel susceptibility to high burnup fuel fragmentation will be developed based on experimental data published in the open literature. The methodology will then be demonstrated by calculating the mass of fuel susceptibility to high burnup fuel fragmentation. The BISON results conclude that increasing peak cladding temperature: 1) drastically decreased time to failure, and also 2) decreased balloon size. Additionally, the effect of pre-transient and transient fission gas release impacted cladding balloon size and burst timing. Lastly, fuel susceptibility to high burnup fuel fragmentation significantly decreased as a function of peak cladding temperature.