Transport of Gaseous 14 C in a Partially Saturated, Fractured, Porous Medium
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TRANSPORT OF GASEOUS 14 C IN A PARTIALLY SATURATED, FRACTURED, POROUS MEDIUM W. B. LIGHT, P. L. CHAMBRE, W. W.-L. LEE, AND T. H. PIGFORD Department of Nuclear Engineering, University of California, and Lawrence Berkeley Laboratory, University of California, Berkeley, CA 94720. ABSTRACT We predict the transport of 14 C from the proposed nuclear waste repository at Yucca Mountain using a porous medium model. Use of this model is justified if the Peclet number, which indicates equilibrium between gas in fractures and liquid in rock pores, is much less than unity. For the assumed release rates, maximum predicted concentrations of "4 C02 in rock near the ground surface are comparable to the USNRC limit for unrestricted areas. Furthermore, dilution near the ground surface as the "4 C0 2 enters the atmosphere will lower the concentrations by several orders of magnitude. Travel times from the repository to the surface are predicted to be hundreds to thousands of years. For a wide range of the parameters, the release rate from the source has negligible effect on the maximum concentrations at the ground surface. INTRODUCTION Radioactive gases released from nuclear waste placed in partially saturated rock would have a direct pathway to the biosphere, which presents a new problem in assessing the potential health impacts of such releases and in complying with regulations. We analyze the transport of "4 C in an unsaturated, fractured, porous medium with gas-phase advection and dispersion. Gases released into a partially saturated, fractured rock tend to move in fractures whereas vadose water is held inside the rock matrix. Strong convection flows are expected during the thermal phase of repository operation, carrying "14 C0 2 toward the ground surface. First we assess the interaction of "4 C0 2 with vadose water as a possible retardation mechanism. Then we treat the combined fracture and pore matrix as an equivalent porous medium and predict 14 C concentrations, fluxes and travel times. This analysis differs from that proposed by Ross' in that (1) we provide support for the distribution-equilibrium assumption by solving the fracture-geometry problem; (2) we do not include calcite precipitation, but allow ground-water pH to be specified by independent analysis, thereby simplifying the equations; and (3) we provide analytic solutions to the equivalent porous medium problem with constant coefficients. Unlike Amter et al.2 and Knapp, 3 who have also estimated "4 C travel-time, we include gas-phase dispersion and demonstrate its importance. We obtain essentially the same results as Knapp, but with a simpler formulation that does not predict 14 C shock fronts. MATHEMATICAL ANALYSIS We assume that 14 C is released from failed waste canisters as 14 C02(g). Because C02 dissolves readily in water, we expect much of the "4C to be retarded by dissolution into vadose water. Some 14C will react to form calcite and other minerals, but the fraction in solid phases is difficult to predict and probably not significant compared to the fraction in
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