Draft evaluation of the frequency for gas sampling for the high burnup confirmatory data project
This report provides a technically based gas sampling frequency strategy for the High Burnup (HBU) Confirmatory Data Project. The evaluation of: 1) the types and magnitudes of gases that could be present in the project cask and, 2) the degradation mechanisms that could change gas compositions culminates in an adaptive gas sampling frequency strategy. This adaptive strategy is compared against the sampling frequency that has been developed based on operational considerations. Gas sampling will
... ovide information on the presence of residual water (and byproducts associated with its reactions and decomposition) and breach of cladding, which could inform the decision of when to open the project cask. Initial Condition After drying (using the vacuum drying process as opposed the forced helium dehydration process), sealing, leak testing, and backfilling the cask with high purity helium, there may be two types of gases within the cask cavity: 1) fission gases (mainly 85 Kr) released from breached fuel rods, and 2) residual water and its decomposition products formed by corrosion or radiolysis (e.g., H 2 and O 2 ). The initial conditions relevant to gas sampling frequency will be established after the cask has been backfilled with helium while the cask is inside the North Anna decontamination building. Gas Types, Magnitudes, and Detection Limits The minimum detectible concentration of 85 Kr without using high sensitivity measurement methods is less than one pCi/cm 3 . At this detection limit, even if as little as 1% of 85 Kr in a single pellet were to be released, it would be detectible (assuming the 85 Kr gas is uniformly distributed within the cask). Therefore, potential 85 Kr concentrations (initial and subsequent due to potential cladding failures) will be detectable with standard measurement methods. The detectible concentration of water without using high sensitivity measurement methods is about one part per million (ppm). If the NRC drying guidelines are followed, the total unbound water within the cask should be below 0.25 volume percent which is about 11 parts per thousand, an amount easily detected in a gas sample.