Radioisotope Fractionation and Secular Disequilibrium in Performance Assessment for Geologic Disposal of Nuclear Waste
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Radioisotope Fractionation and Secular Disequilibrium in Performance Assessment for Geologic Disposal of Nuclear Waste William M. Murphy1 and David A. Pickett2 1 Department of Geosciences, California State University, Chico, CA 95929-0205 2 Center for Nuclear Waste Regulatory Analyses (CNWRA), Southwest Research Institute, San Antonio, TX 78238-5166 ABSTRACT Two potential applications of radioisotope fractionation and decay-series secular disequilibrium in performance assessment for geologic repositories for nuclear waste are preferential radionuclide release in source term analysis and characterization of system closure as a measure of the capacity of the geologic system to isolate waste. A primary mechanism of radioisotope fractionation is selective release and mobility of alpha decay products because of nuclear recoil effects, which is evident in natural system data. Preferential release of radioisotopes from nuclear waste forms or solubility limiting solid phases could affect repository performance; however, consequences of differential radioisotope releases are neglected in performance assessments. Decay-series disequilibria are useful also to characterize open/closedsystem behavior in natural systems. Systems that are closed on time scales that are long relative to the half-lives of decay chain nuclides achieve secular equilibrium characterized by unit activity ratios among series nuclides. For geologic disposal of nuclear waste, measures focused on chemical system closure could capture the essential characteristics of the natural system with respect to radionuclide isolation and could be based quantitatively on uranium and thorium decay series secular equilibria/disequilibria. INTRODUCTION Fractionation of isotopes in natural geochemical systems is useful in interpreting geochemical release and transport processes. Geochemical fractionation of stable isotopes of a given element is due to kinetic or equilibrium mass effects during phase transitions, molecular transformations, and diffusion. These effects tend to be insignificant for heavy isotopes, including most radioisotopes of concern in geologic disposal of nuclear waste. Fractionation also occurs due to radioactivity [1]. Natural uranium and thorium decay series in systems that are closed on time scales that are long relative to the half-lives of the radionuclides achieve secular equilibrium, which is characterized by unit activity ratios among nuclides in the series. Secular disequilibrium (decay-series activity ratios other than unity) indicates open system behavior on a time scale relative to the half-lives of the nuclides. Ratios of activities of radioisotopes in a decay series are altered by processes that fractionate daughter isotopes from parents. A primary mechanism of fractionation in a decay series is selective release and mobility of alpha decay products and their daughters because of nuclear recoil effects. Progeny of alpha decay may be ejected from stable crystallographic sites or left in damaged and preferentially reactive sites. Consequentl
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