Cladding Evaluation in the Yucca Mountain Repository Performance Assessment
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to begin to react with the moisture or air and leads to the cladding unzipping phase. In the unzipping phase, the cladding is torn open by the formation of secondary mineral phases on the fuel, and the radionuclides are available for release. The various components of the model are discussed below. Cladding Condition as Received The initial cladding condition analysis describes the condition of the commercial nuclear fuel as it is expected to be received at the YMP site. This analysis generates the initial boundary condition for the subsequent analysis of degradation of the cladding in the repository. It also evaluates the fraction of fuel rods that are perforated before emplacement and are immediately available for cladding unzipping when the WP fails. Earlier studies of cladding initial conditions have been performed 2,4,8. The TSPA-VA used a single value for these initiating conditions but statistical distributions have since been developed. The cladding degradation model is based on the Westinghouse 17 x 17 rod fuel design. This design represents over 30% of the PWR fuel discharged to date and also has the thinnest Zircaloy cladding. It is assumed that the BWR cladding degrades in a similar manner. This is conservative since BWR cladding is thicker and is discharged with lower bumups and stresses. In addition, most BWR assemblies are enclosed in flow channels (sheet metal boxes) which offer additional protection. Starting with a distribution of PWR fuel bumups that are anticipated for storage at YMP, this model develops distributions for various cladding properties. Table 1 summarizes these distributions and includes the mean and upper 5% values. Table 1 Model Results of Expected Fuel Stream into YMP
Property Burnup Internal Pressure Oxide Thickness Peak Hydride Content Crack Size Stress (27°C) Stress Intensity Factor, K,
Mean Value 44.1 MWd/kgU 4.8 MPa 54 pim 358 ppm 19 grm 38.4 MPa 0.47 MPa-mu*'
Upper 5% Value 63.3 MWd/kgU 7.3 MPa 112 4m 738 ppm 57 gtm 61.8 MPa 1.08 MPa-mU5
A distribution for the fraction of cladding within a WP that failed as a result of reactor operation was developed from the fraction of rods failed as a function of calendar years by assuming that the fuel assemblies are loaded into WPs in their order of discharge from the reactor. This loading sequence tends to place fuel with high failure rates (BWR fuel in 1970, also 1973-1976, and PWR fuel in 1972, 1983, and 1989) into consecutive WPs and produces larger variations in rod failure fractions than would be expected with thermal blending. A factor of four uncertainty was applied to represent the uncertainty in rod failure data to address incipient failures of the surrounding four rods in the square array assembly. The creep failure analysis included rod failure from dry storage and transportation using temperature profiles starting at 350°C. This analysis shows that a small fraction of the fuel with high stresses would fail if exposed to design basis storage and shipping temperatures. Table 2 gives the calculated percentage of rods tha
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