Preferential Radionuclide Release Due to Alpha Decay: Effects on Repository Performance
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Preferential Radionuclide Release Due to Alpha Decay: Effects on Repository Performance David A. Pickett1 and William M. Murphy2 1 Center for Nuclear Waste Regulatory Analyses (CNWRA), Southwest Research Institute®, San Antonio, TX 78228-0210, USA 2 Department of Geological and Environmental Sciences, California State University, Chico, CA 95929-0205, USA ABSTRACT We model preferential release of 237Np, 234U, 230Th, 226Ra, and 210Pb from disposed commercial spent nuclear fuel as a result of alpha recoil damage, using the U.S. Nuclear Regulatory Commission (NRC) Total-system Performance Assessment (TPA) model for the potential repository at Yucca Mountain. Time-dependent augmentation of the ingrown component is simulated by increasing the initial parent inventory; we have used a factor of five increase, based on natural system observations. For 237Np, the magnitude of preferential release is subject to solubility limits. Stochastic TPA runs show a significant effect on modeled dose of preferential 237Np release, but low impact from the other four radionuclides. The mechanism could be ineffective if 237Np is incorporated into secondary phases. While our results are exploratory in nature, this approach to modeling decay-related enhancement of release can be applied in other nuclear waste disposal settings. INTRODUCTION Performance assessment models for high-level nuclear waste disposal typically assume congruent release of radionuclides from nuclear waste forms, except for prompt release fractions [1,2]. Considerations of natural uranium-series isotope systematics [3,4] and spent nuclear fuel characteristics [5] show that alpha decay-related, time-dependent preferential radionuclide release can occur from radioactive waste forms. Because current models do not account for this type of release from spent fuel, we have tested the potential effect on repository performance using a total-system performance assessment model of the potential Yucca Mountain repository. Our simulation of preferential release is based on natural system observations. Decay-series isotopes of radioelements are known to fractionate in water-rock systems, with the best known example the common observation of 234U/238U activity ratios of greater than one in surface and groundwaters [6-8]. Ratios greater than 30 have been reported, and values as high as ten are common, but the majority of reported values are between one and five [6]. Uranium isotope fractionation is evident in waters at Yucca Mountain, with measured 234U/238U up to 8.5 [9], and the ratio measured in unsaturated zone waters from the Nopal I natural analogue ranges up to 5.1 [10]. Preferential 234U enrichment in waters is a cumulative reflection of its alpha decay ancestry, owing to processes such as direct recoil and lattice damage [7,8]. We previously combined these natural system observations with considerations of spent fuel radionuclide inventories, decay/ingrowth behaviors, and dose to identify radionuclides for which alpha decay-related preferential release is most likel
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