Evaluating Cladding Creep during Dry Storage and Repository Emplacement
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Evaluating Cladding Creep during Dry Storage and Repository Emplacement Eric Siegmann1 and Pierre Macheret2 1 Duke Engineering & Services, 2 Bechtel SAIC Company, LLC, (both) 1180 Town Center Drive, Las Vegas, NV 89144. ABSTRACT Creep strain has been identified as the dominant failure mode for commercial spent nuclear fuel cladding during dry storage, including the vacuum drying phase. It could also be important during the early period of repository closure. A statistical analysis of creep failure during these three phases was performed. Statistical analysis is an important tool for predicting fuel behavior and the distributions can be modified for specific applications. A burnup distribution (rod average = 44 MWd/kgU, range = 2 to 75 MWd/kgU) was assumed and a distribution of rod properties, including stress was developed. The Murty creep correlation was selected after comparing six different correlations with results from five different experiments. It was then modified to better predict irradiated cladding creep data. Creep failure criteria is a Complementary Cumulative Distribution Function (CCDF) based on 52 failure tests. The fuel rods are exposed to three consecutive temperature histories that are typical of what is to be expected: 24 hours of vacuum drying, 20 years dry storage, and 1000 years of repository thermal history. Each phase has a peak temperature, treated as an independent variable, and temperature history taken from the literature. Uncertainties in the temperatures and strain rate are included. The radial temperature distribution across the waste package is also modeled. For the first phase, vacuum drying, rod failures start to occur at about 550°C and exceed 1% failure at 600oC. With a peak vacuum drying temperature of 430oC, rods begin to fail during dry storage when the peak temperature reaches 400oC and approached a 1% failure level at 450oC. With representative peak temperatures of 430oC for drying and 350oC for dry storage, rod failures start to occur during repository closure at a peak cladding temperature of 390oC. They reached 1% at about 430oC. In the current repository design, the cladding temperatures are below 210oC and rod failures from creep are not expected INTRODUCTION Creep is the slow ballooning and plastic flow of the metal cladding due to the fuel rod internal pressure under sufficiently high temperatures and over long time periods (days and years). During reactor operation, the creep may be inward from the high reactor pressure but during dry storage and emplacement, it would tend to be outward because of the higher internal pressure. The internal rod pressure is caused by fission gasses and an initial helium fill gas introduced during manufacturing. Creep has been identified as the primary degradation mechanism during dry storage [1] and it is also possible during the initial thermal cycle of repository closure. The method used to predict creep failures was to develop a statistical distribution of fuel rod properties and then analyzing 12,000 rods to evaluate the probabilit
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