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Analysis of Gas Samples Taken from the High Burnup Demonstration Cask

Bryan, Charles R.; Jarek, Russell L.; Flores, Christopher; Leonard, Elliott J.

The High Burn-Up Demonstration Project was recently initiated by the Department of Energy (DOE) to evaluate the effects of fuel drying and long term dry storage on high burn-up spent nuclear fuel. As part of the project, samples of the He backfill gas were collected 5 hours, 5 days, and 12 days after completion of drying. The samples provide information on the state of the fuel at closure, and on the environment within the cask. At Sandia National Laboratories, the samples were analyzed by gamma-ray spectroscopy to quantify fission product gases and by gas mass spectrometry to quantify bulk and trace gases; water content was measured via humidity probe. Gamma-ray spectroscopy results indicated no detectible ⁸⁵Kr, indicating no failed fuel rods were present after drying. Mass spectrometry indicated build-up of CO₂ to 930 ppmv over two weeks, attributed to oxidation of organic compounds (possibly vacuum grease or vacuum pump oil) within the cask. H₂, generated by either radiolysis or metal corrosion, also increased up to —500 ppmv. Water contents in the cask were higher than anticipated, increasing to —17,400 ppmv ±10% after 12 days. Measuring water content proved challenging, and possible improvements to the method for future analyses are proposed.