The Near-Field Transport Code Tullgarn and its USE in Performance Assessment
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THE NEAR-FIELD TRANSPORT CODE TULLGARN AND ITS USE IN PERFORMANCE ASSESSMENT
Patrik Sellin and Nils Kjellbert Swedish Nuclear Fuel and Waste Management Company, Stockholm, Sweden. ABSTRACT
The near-field radionuclide migration code Tullgarn has been developed for performance assessment purposes. As a part of the PROPER-code package it has been successfully applied in the SKB 91 safety analysis. The features and processes included in the code are: Radioactive chain decay Different canister failure mechanisms (copper corrosion from sulphide attack, steel corrosion, internal overpressure and initially defective canisters) Spent fuel dissolution. The model is based on the assumption that the dissolution rate is proportional to the cc-dose rate Transport calculations are done with a resistance-network model. Tullgarn calculates the stationary release of radionuclides from a defect in the canister through the buffer and out into a fracture in the rock or up to the damaged zone under the deposition tunnel. Tullgarn can be used as a stand-alone model for near-field release calculations or as a submodel in an integrated assessment. In the SKB 91 analysis, Tullgarn gave the source term to the far-field model. INTRODUCTION
Nuclear activities must be conducted in an acceptable manner with respect to safety and radiation protection. The safety of the activities is assessed by means of performance and safety assessments. The Swedish Government's approval of KBS-3' in 1984 has established that a method exists today for the final disposal of spent nuclear fuel and that method fulfils society s demands for safety and radiation protection for the public. In the KBS-3 design (Figure 1) the spent fuel is placed in copper canisters, which are then filled with lead. The canisters are deposited one by one in holes drilled in the floor of a system of drifts in the rock at 500m depth. The space between the canister and the rock is filled with highly compacted bentonite clay. The system of storage drifts is regular with a distance of 25m between the drifts. The distance between the deposition holes is 6m. In the years 1989-1992 an integrated site-specific safety analysis called SKB 912 was carried out. The intention of the analysis was to produce a part of the background material required for the siting of a final repository for spent nuclear fuel. The primary objective was to explore the following questions: What importance do the site-specific characteristics of the bedrock and the hydrological regime around the repository have for the overall safety? What relative importance do different site-specific characteristics have for safety? How can placement and design of the repository be adapted to conditions on the site in order to take advantage of the safety barriers offered by the bedrock? Since these questions all relate to the site geology much more effort was put into geohydrological modelling than to near-field modelling in SKB 91. However, to get a good understanding of the relative importance of geological factors one must have a
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