A Coupled Chemical-Mass Transport Submodel for Predicting Radionuclide Release from an Engineered Barrier System Contain
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A COUPLED CHEMICAL-MASS TRANSPORT SUBMODEL FOR PREDICTING RADIONUCLIDE RELEASE FROM AN ENGINEERED BARRIER SYSTEM CONTAINING HIGH-LEVEL WASTE GLASS. *B. P. McGrail, M. J. Apted, D. W. Engel, and A. M. Liebetrau, Battelle, Pacific Northwest Laboratories, P. 0. Box 999, Richland, WA 99352 ABSTRACT A mechanistic model describing a dynamic mass balance between the production and consumption of silicic acid was coupled to a near-field mass transport model to predict the dissolution kinetics of a high-level waste glass in a deep geologic repository. The effects of interactions between an iron overpack and the glass are described by a time-dependent precipitation reaction for a ferrous silicate mineral. The kinetic model is used to transform radionuclide concentration-versus-reaction progress values, predicted from a geochemical reaction path computer code, to concentration-versus-time values that are used to calculate the rate of radionuclide release by diffusive mass transfer to the surrounding host rock. The model provides for both solubility-limited and kinetically limited release; the rate-controlling mechanism is dependent on the predicted glass/groundwater chemistry. INTRODUCTION The Power Reactor and Nuclear Fuel Development Corporation of Japan (PNC) is conducting studies on the feasibility and safety of a deep geologic repository for the permanent disposal of high-level waste. Battelle, Pacific Northwest Laboratories (BNW), through the Performance Assessment Center for Engineered Barriers Program, is assisting PNC in its assessment of the waste package and near-field performance of such a repository. The performance of the repository will be determined by the combined performance of the geologic setting and the multiple engineered barrier system (EBS). Because the properties of the EBS can be controlled, tested, and validated, PNC is emphasizing the development of models that describe the performance of each EBS component under anticipated repository conditions. Principal components of the PNC conceptual design include a borosilicate glass waste form, a 25-cm-thick cast steel container and a bentonite clay packing material.Ell The purpose of this paper is to describe modifications made to the waste package r[lease (WPR) module in the Analytical Repository Source-Term (AREST) code, [j which was developed by BNW for the U.S. Department of Energy, to accommodate the borosilicate glass waste forms and engineered barrier materials being considered by PNC. THEORY The AREST code simulates coupled chemical-thermal-hydrologicalmechanical processes to predict the containment performance and controlled release of radionuclides from the EBS. The WPR module in the AREST code is structured around the mass transport theory developed ky Chambr6, Pigford, and coworkers at the University of California, Berkeley.L•J Time-dependent radionuclide release rates are presently calculated with AREST assuming a spherical waste package configuration surrounded by a packing material embedded in a porous, semi-infinite host rock. A solubility
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