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Retrograde Solubilities of Source Term Phases

Published online by Cambridge University Press:  03 September 2012

William M. Murphy
William M. Murphy*
Affiliation:
Center for Nuclear Waste Regulatory Analyses, Southwest Research Institute, 6220 Culebra Road, San Antonio, TX 78228–0510 USA
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Abstract

Natural analog, experimental, and thermodynamic studies indicate that theproperties of secondary uranyl minerals are likely to control the sourceterm for U and other radioéléments incorporated in these phases in theproposed Yucca Mountain repository. Thermodynamic calculations using datafrom the 1992 NEA data base indicate an increase in the equilibrium constantfor schoepite dissolution from 103.1 to 104.8 withdecreasing temperature from 100° to 25°C, i.e., retrograde solubility.Enthalpies for mineral transformation reactions that consume protons andrelease cations are typically negative, suggesting that solubilities ofother uranyl phases such as uranophane increase more than that of schoepitewith decreasing temperature. Solubilities of mineral phases associated withspent nuclear fuel will be initially relatively low under the elevatedrepository temperature regime. As the temperature of the repositorydecreases due to radioactive decay and heat dissipation, source term mineralsolubilities increase. The rate of release of U and other species iscontrolled by a series of processes: transport of oxidants and flux ofwater; oxidative dissolution of spent fuel; uranyl mineral precipitation;uranyl mineral dissolution or transformation; and radionuclide transport.Decreasing diffusion and reaction rates and increasing uranyl mineralsolubilities with decreasing temperature may lead to a change with time fromsolubility to transport or reaction rate as a source term controllingmechanism. Preservation of large quantities of uranyl minerals formed byoxidation of uraninite and radiometrie ages of secondary uranophane at thenatural analog site at Peña Blanca indicate that oxidative alteration ofuraninite was fast relative to transport of U away from the deposit. Thesuccessive formation of schoepite and uranophane in natural settings whereuraninite has been oxidized may represent a paragenesis reflectingincreasing temperature or increasing incorporation of environmentalcomponents. In contrast, diminishing temperature conditions in a repositorysource area could lead to the reverse sequence of mineral formation.

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References

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