Coupled Process Modeling and Waste-Package Performance
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COUPLED PROCESS MODELING AND WASTE-PACKAGE PERFORMANCE B. P. MCGRAIL AND D. W. ENGEL Pacific Northwest Laboratory a), P. 0. Box 999, Richland, WA 99352
ABSTRACT The interaction of borosilicate waste glasses with water has been studied extensively, and reasonably good models are available that describe the reaction kinetics and solution chemical effects. Unfortunately, these models have not been utilized in performance assessment analyses, except in estimating radionuclide solubilities at the waste form surface. A geochemical model has been incorporated in the AREST code to examine the coupled processes of glass dissolution and transport within the engineered barrier system. Our calculations show that the typical performance assessment analyses using fixed solubilities or constant reaction rate at the waste form surface are not always conservative or realistic predictions of radionuclide release. Varying the transport properties of the waste package materials is shown to produce counterintuitive effects on the release rates of some radionuclides. The use of noncoupled performance assessment models could lead a repository designer to an erroneous conclusion about the advantages of one waste package design or host rock setting over another.
INTRODUCTION Most current performance assessment models for the engineered-barrier system (EBS) utilize a fixed solubility limit or constant dissolution rate of the waste form as conservative boundary conditions for near-field transport calculations. The transport equations can then be solved either analytically' or numerically. 2 With these models, changing the transport properties of the system, such as the diffusion coefficient, tortuosity, or porosity, affects radionuclide release rates in a straightforward manner consistent with the structure of the governing transport equations and has no effect on the assumed solubilities or dissolution rate of the waste form. With a glass waste form, special problems arise in the application of these models because the glass is a metastable solid that cannot be in equilibrium with an aqueous solution. Hence the use of equilibrium thermodynamics to calculate a fixed solubility for use in performance assessment studies is inherently problematic. In this paper, we present results from a geochemical model, which has been implemented in the Analytical Repository SourceTerm (AREST) code,3 that more appropriately describes the coupling between glass/water chemical reactions and transport across the engineered-barrier system. MODEL DESCRIPTION The AREST code is intended to provide a quantitative, probabilistic assessment of the performance of the individual barriers of the overall EBS. AREST contains three major components: 1) waste package containment (WPC), 2) waste package release (WPR) , and 3) engineered-system release (ESR).
(a)Operated for the U.S. Department of Energy by Battelle Memorial Institute under contract DE-AC06-76RLO 1830. Mat. Res. Soc. Symp. Proc. Vol. 294. ('1993 Materials Research Society
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The WPC module simulates the
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