Results of an Aqueous Source Term Model for a Radiological Risk Assessment of the Drigg LLW Site, UK
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Figure 1. Plan view of the disposition of the Drigg Trench and Vault disposal systems (solid lines), and coincidence with the DRINK finite difference grid (dashed lines). THE DRINK SOURCE TERM MODEL The DRINK model utilises the BNFL biogeochemical reaction Generalised Repository Model (GRM) [1,2] to simulate the evolving geochemistry of the Drigg trenches and vaults. GRM considers kinetically controlled steel corrosion and microbial induced cellulose degradation reactions. The products of these processes are used to determine an evolving redox condition, taking account of kinetically controlled microbially mediated redox reactions between redox product species and species in groundwater (e.g. SO 4 ), and minerals in soils (e.g. Fe(OH) 3 ). Redox potential (pe) is calculated by using standard mass action equations [3] considering the most oxidising couple. The resulting pe is used as a constraint for equilibrium speciation and mineral equilibrium calculations by a routine based on PHREEQE [4], which determines the pH and master species concentrations, including those radionuclides which are solubility controlled. GRM describes the 2-dimensional lateral groundwater flow in the saturated zone by means of a finite difference solver. The discretisation of the finite difference grid used in the DRINK model is shown in Figure 1. Vertical flow is considered on a cell by cell basis and is used to simulate the release of radionuclides from the unsaturated zone to the saturated zone. In DRINK, sorption is modelled using a distribution coefficient (Kd) which is selected taking into consideration the simulated geochemical model, and the types of sorbant surfaces present in the Drigg trenches and vaults. Radioactive decay is considered on a cell basis for dissolved, sorbed and precipitated phases, and for the unsaturated zone. BIOGEOCHEMICAL EVOLUTION OF THE DRIGG SITE Chemical conditions in the Drigg trenches is simulated to vary over a period of around 1000 years. During this time degradation of cellulose and steel corrosion result in the establishment of conditions more reducing and acidic than the local groundwater. Figure 2 shows examples of the time evolution of concentrations of solid and dissolved species. In the DRINK model the first stage of cellulose degradation is the hydrolysis of cellulose modelled by a first order kinetic reaction with a pH dependent hydrolysis constant [2]. The computation of the cellulose concentration in Figure 2a includes the presence of hydrolysis in the saturated zone, and the transfer of cellulose and other materials from the unsaturated zone resulting from settlement. In 130
effect cellulose in the saturated zone is replaced by that in the unsaturated zone until around 100 years when the unsaturated zone is depleted of cellulose. After this time an exponential decrease in cellulose concentration is simulated. Iron corrosion is modelled by a zero order kinetic reaction. In the saturated zone iron increases (Figure 2b) in concentration because of transfer from the unsaturated zone as a res
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