The Biogeochemical Transport Code Drink: a Mechanistic Description.
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TIlE BIOGEOCHEMICAL TRANSPORT CODE DRINK: A MECHANISTIC DESCRIPTION. P. HUMPHREYS, T. JOHNSTONE, D. TRIVEDI, AND A. HOFFMANN, British Nuclear Fuels plc, Warrington, England.
ABSTRACT DRINK is a 2D research code, which simulates the long term evolution of shallow, trenched, LLW disposal sites with significant groundwater flow. It employs a finite difference solver and is built around a geochemical transport model into which various functional units are interfaced. These units describe sorption, corrosion, microbiology, radionuclide decay, colloids, mineral precipitation and dissolution and gaseous release. Each of these units is described here, along with a ID simulation of uranium migration. INTRODUCTION Radionuclide migration from low level waste (LLW) disposal sites is of interest to all parties involved in LLW management. The complexity of the relevant interactions, coupled with time scales in the order of thousands of years, makes the prediction of radionuclide behaviour difficult. Mathematical modelling provides a practical option for investigating this behaviour and providing insight into radionuclide release. DRINK is a 2D research code, which simulates the long term evolution of shallow, trenched, LLW disposal sites with significant groundwater flow. It is essentially the embodiment of a conceptual model of how such sites develop, which has been constructed through applying current scientific understanding to data on site inventories and liquid and gaseous effluent streams. DRINK employs a finite difference solver and is built around a geochemical transport model into which various functional groups are interfaced. These groups describe: sorption, corrosion, microbiology, radionuclide decay, colloids, mineral precipitation/ dissolution and gaseous release. Radionuclide migration is controlled through precipitation/dissolution reactions and the partition of species between the liquid phase and sorption onto either stationary components, such as the waste matrix or engineered barriers, and colloidal particles. Radionuclide speciation is determined by the prevailing geochemistry, with changes to pH and Eh being determined through interactions between microbial activity and the site chemistry.
CODE DESCRIPTION Trench Geometry. DRINK models trenches as interconnecting homogenous cells sitting on a variably permeable base. Each cell is subdivided vertically into saturated and unsaturated zones with 2D spatially variable flow occurring in the lower saturated zone. Flow in the unsaturated zone is not explicitly modelled and is represented by a source term input into the top face of the saturated zone. This allows trench infiltration to be included, and Mat. Res. Soc. Symp. Proc. Vol. 353 0 1995 Materials Research Society
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lysimeter' data on unsaturated zone radionuclide release and modification to infiltrate chemistry to be accommodated. Trench base permeability is determined by the user as a boundary condition. Unsaturated zone porosity provides each cell with a discrete head space. Starting Conditions and
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