Effects on radionuclide concentrations by cement/ground-water interactions in support of performance assessment of low-level radioactive waste disposal facilities

Abstract

C o p k r o f h b r t r y~m d r t n d v d r u n d h 8~m m r h t h . N R C~~r AbstractThe U.S. Nuclear Regulatory Commission (NRC) is developing a technical position document that provides guidance regarding the performance assessment of low-level radioactive waste (LLW) disposal facilities. This guidance includes considerations associated with the chemical environment of the vault disposal system and the effects this system may have on the release and mobility of radionuclides. Because the disposal system will contain cementitious materials as structural, waste form, andor bacld'ill materials, the geochemical properties of pore waters buffered by reactions with cement will be very different from those waters associated with the local soil and geology. This environment therefore needs to be considered within the source term calculations if credit is taken for solubility limits andor distribution coefficients for -dissolved radionuclide concentrqtions within disposal units. Geochemical modeling may be used to assess potential chemical conditions in concrete vault disposal units and associated effects on aqueous speciation, solubilities, and sorption of radionuclides that are released from the waste form.In support of NRC7s development of this technical position, two literature reviews were done on information related to the chemical environments associated with the interaction of water with cementitious materials. One review was conducted on methods and associated solid-phase assemblages used to model the composition of pore water resulting from reaction with cementitious materials used in a disposal vault. This literature review also included related information on experimental studies of cemendwater systems, natural analogue studies of cement and concrete, and radionuclide solubilities experimentally determined in cement pore fluids.Based on the results of .this review, geochemical modeling was used to demonstrate the calculation of conservative maximum concentrations for dissolved americium, neptunium, nickel, plutonium, radium, strontium, thorium, and uranium with respect to key geochemical input parameters for two ground-water environments associated with the disposal system. These environments include 1) a cement buffered system, wherein the leachate pH is controlled at values above 10 by the effective buffering capacity of the concrete; and 2) a ground-water buffered system, wherein the leachate pH and related solution parameters are dominated by the local ground-water system.Another literature review was completed on the available data for the sorption potential of selected LLW radionuclides onto "fresh" cement/concretewhere the expected pH of the cement pore waters will equal or exceed 10. The review included data for the radionuclides americium, inorganic carbon, chlorine, iodine, lanthanide elements, niobium, nickel, neptunium, plutonium, radium, strontium, technetium, thorium, and uranium. Based on information gleaned from the literature, a database was developed of preferred minimum distribution coeff...

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