Compliance Assessments of Hypothetical Geological Nuclear Waste Isolation Systems with the Draft Epa Standard

Abstract

Estimates of potential radionuclide releases from HLW storage facilities in geologic formations are an integral part of the technical basis for the regulation of nuclear waste disposal. We have attempted to assess the importance of the variation of several variables and model assumptions to the calculation of radionuclide discharge from hypothetical repositories. The conceptual models of the repository sites are consistent with our current understanding of the characteristics of the geologic environments being studied by the Department of Energy. It must be stressed that we have not attempted to accurately model any real sites; at the present time, the available data are not sufficient for this purpose. Large uncertainties exist in the characterization of the solubilities and sorption of radionuclides, in the description of the regional and local hydrogeology and in the mathematical treatment of contaminant transport in the presence of fracture flow and matrix diffusion. The fraction of waste that could be accessed by breaching the repository, the effects of mixing processes within the facility and the probability of human intrusion are not known. We have completed preliminary calculations of potential radionuclide discharges from hypothetical repositories in tuff and bedded salt [1,2]. Our theoretical approach and computational methods are illustrated in this paper with examples for a repository in volcanic tuff. We also briefly discuss the importance of radionuclide source term assumptions to the calculations for a bedded salt repository.

Radioactivity, Geochemistry, and Health

“…Uncertainties in properties of the engineered and natural barriers are incorporated into the risk assessment by using ranges and probability distributions for the parameter values (K d s and maximum aqueous radionuclide concentrations) in Monte Carlo simulations, by regression equations to calculate sorption and solubility limits from sampled geochemical parameter ranges, and by the use of alternative conceptual models (see Section 11.6.4.3.2.2). Representation of the probabilistic aspects of geochemical processes in risk assessment is discussed in Siegel et al (1983bSiegel et al ( , 1992, Chen et al (2002), and Turner et al (2002). Simplifications in solubility and sorption models used in performance assessment calculations for the WIPP (DOE, 1996a,b,c) and the proposed HLW repository at Yucca Mountain as reviewed in Siegel and Bryan (2003) and in Section 11.6.3.2.2 of this chapter.…”

“…Sorption of a radionuclide may vary drastically over postulated flow paths and over time at Yucca Mountain. One approach to represent this variability in performance assessment calculations is to sample K d s for transport equations from a probability distribution based on experimental measurements as discussed above (Siegel et al, 1983a;Wilson et al, 2002). Another approach is to calculate a range of K d s from thermodynamic data for a range of groundwater compositions (Turner and Pabalan, 1999;Turner et al, 2002).…”

Environmental Geochemistry of Radioactive Contamination

“…Databases of K d values that can be used to estimate pdf's for various geologic media are summarized by Barney (1981a,b), Tien et al (1985), Bayley et al (1990), U.S. DOE (1988), McKinley and Scholtis (1992), Triay et al (1997), the U.S. Environmental Protection Agency (1999a,b) and Krupka and Serne (2000). Methods used to specify pdf's for K d s for use with sampling techniques such as Latin Hypercube Sampling (Iman and Shortencarier, 1984;Helton and Davis, 2002) have been described by Siegel et al (1983Siegel et al ( , 1989, Wilson et al (1994) and Rechard (1996). Serne and Muller (1987) describe attempts to find statistical empirical relations between experimental variables and the measured sorption ratios (R d s).…”

“…Uncertainties in properties of the engineered and natural barriers are incorporated into the risk assessment by using ranges and probability distributions for the parameter values (K d s and maximum aqueous radionuclide concentrations) in Monte Carlo simulations, by regression equations to calculate sorption and solubility limits from sampled geochemical parameter ranges, and by the use of alternative conceptual models. Representation of the probabilistic aspects of geochemical processes in risk assessment is discussed in Siegel et al (1983Siegel et al ( , 1992, Chen et al (2002) and Turner et al (2002). Simplifications in solubility and sorption models used in performance assessment calculations for the WIPP and the proposed HLW repository at Yucca Mountain, respectively are described below.…”

Environmental geochemistry of radioactive contamination.

“…Changes in mineralogy and transient concentrations of competing and complexing ligands may cause the sorption at any point along the flow path to change. One approach to represent this variability in performance assessment calculations is to sample K d s for transport equations from a probability distribution based on experimental measurements as discussed above (Siegel et al, 1983;Wilson et al, 2002). Another approach is to calculate a range of K d s from thermodynamic data for a range of groundwater compositions.…”