Arest-CT: A New Chemical Transport Code for Waste Disposal System Performance Analysis
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AREST-CT: A NEW CHEMICAL TRANSPORT CODE FOR WASTE DISPOSAL SYSTEM PERFORMANCE ANALYSIS
B. P. McGrail, C. I. Steefel, J. A. Fort, D.W. Engel, and S.B. Yabusaki Pacific Northwest Laboratory(a), P.O. Box 999, Richland, Washington, 99352
ABSTRACT A new computer code, Analysis of RadionuclidE Source-Term with Chemical Transport (AREST-CT), is described in this paper. The code is being designed to support performance assessment analyses of engineered systems for subsurface isolation of hazardous and radioactive wastes. Radionuclide releases from an engineered system are modeled by solving governing equations describing conservation of water mass, air mass, thermal energy, and chemical species mass. As such, the AREST-CT code will be capable of simulating radionuclide release and transport in a non-isothermal, unsaturated-saturated setting. Constituitive equations are implemented that describe corrosion of iron-based container materials, glass, and spent fuel waste forms. The governing equations are solved in a two-dimensional domain using an integrated finite-volume method. A third-order total variation diminishing (TVD) numerical scheme is evaluated to minimize numerical oscillations and dissipation of steep concentration gradients in advection-dominated transport problems.
INTRODUCTION Subsurface disposal of hazardous, low-level, and high-level radioactive waste is being considered in many countries as a means to safely sequester these wastes for periods of tens to hundreds of thousands of years. The hydrologic and geochemical properties of the soil or rock at a site can provide a natural barrier or resistance to transport of contaminants. Because of heterogeneity in natural geologic media, uncertainties in water flow rates and physical properties, and extrapolation over long distances and times, most countries allocate considerable importance to engineered barrier materials such as waste form, container, and backfill to mitigate releases from the site. The primary functions of engineered materials include
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isolation of the waste from contact with water for as long as possible minimization release rate from the waste form retardation of contaminant transport by adsorption chemically conditioning the water contacting the engineered materials.
To evaluate the performance of engineered materials in meeting specific design objectives, process models are being developed in many countries that describe the corrosion of container materials, hydrolysis of glass waste forms, oxidation/dissolution of spent fuel, and adsorption on backfill materials, etc. The process models generally tend to be mechanistic because of the need to extrapolate over geologic time periods. For example, current dissolution models for spent (a)Operated for the U.S. Department of Energy by Battelle Memorial Institute under Contract No. DE-AC06-76RLO 1830. Mat. Res. Soc. Symp. Proc. Vol. 353 0 1995 Materials Research Society
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fuel relate the dissolution rate of the spent fuel matrix to specific chemical properties of the water conta
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