Alternate Source Term Models for Yucca Mountain Performance Assessment Based on Natural Analog Data and Secondary Minera
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© 1999
Materials Research Society
maximum constant rate of oxidative alteration of uraninite [2,4]. Using this analysis, a sensitivity study is undertaken based on the premises that i) the oxidation rate of spent fuel in the proposed Yucca Mountain repository will be equal to or less than the maximum constant oxidation rate of uraninite at Pefia Blanca, and ii) this oxidation rate places an upper limit on the release rate of radionuclides from the repository engineered barrier system (EBS). Differences between the Pefla Blanca and proposed repository systems such as alteration products of engineered components of the proposed repository, differences between spent fuel and natural uraninite, and silicification and possible saturated hydrothermalism at Pefla Blanca detract from the precise relevance of the Pefia Blanca data. An additional caveat is that consistency among a few dates on related oxidized uranium minerals at Pefia Blanca [7,8] suggests that oxidation may have occurred during a relatively brief geologic event with greater rapidity than the maximum constant rate. Nevertheless, the Pefa Blanca data provide an opportunity for comparison of model results based on analogous geologic phenomena and a relevant geologic time scale. Spent nuclear fuel is chemically unstable in an oxidizing aqueous environment and will alter to secondary uranyl minerals. Incorporation of radioelements other than uranium in secondary alteration products (i.e., coprecipitation) has been predicted [91 and directly observed for Np [1O]. A realistic source term for the proposed Yucca Mountain repository could be constructed to reflect the properties of secondary uranyl minerals and their role in release of uranium and coprecipitated radioelements. Solubility limits for source term controlling phases [e.g., 11] are commonly invoked to control release rates. Uranyl mineral solubilities depend largely on the carbonate concentration and speciation in the aqueous phase, pH, and temperature. Thermodynamic data permit calculation of solubility relations for the hydrous uranyl mineral schoepite for a range of environmental conditions. An alternate source term model for Yucca Mountain performance assessment is based on the premises that i) releases of uranium are controlled by schoepite solubility, and ii) all other matrix radioelements are incorporated by coprecipitation in schoepite and released from schoepite together with uranium in proportion to their concentrations relative to uranium in spent fuel. The latter premise is unrealistic for species incompatible with secondary phase structure. However, species such as Np appear to be preferentially incorporated in secondary phases [10]. Nevertheless, the schoepite solubility model permits comparison of model results based on a conceptually realistic role of secondary phases in the Yucca Mountain environment. Comparisons of performance assessment results for the alternate source term models indicate that doses calculated using the natural analog model and the secondary schoepite model are sma
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