Chemical Durability of Copper Canisters under Crystalline Bedrock Repository Conditions
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CHEMICAL DURABILITY OF COPPER CANISTERS UNDER CRYSTALLINE BEDROCK REPOSITORY CONDITIONS ROLF SJOBLOM*, HANS-PETER HERMANSSON** AND ORJAN AMCOFF***
*Swedish Nuclear Power Inspectorate, Box 27 106, S-102 52 Stockholm, Sweden **Studsvik Material AB, S-611 82 Nyk6ping, Sweden ***University of Uppsala, Institute of Geology, Department'of Mineralogy and Petrology, Box 555, S-751 22 Uppsala, Sweden ABSTRACT
In the Swedish waste management programme, the copper canister is expected to provide containment of the radionuclides for a very long time, perhaps millions of years. The purpose of the present paper, is to analyse prerequisites for assessments of corrosion lifetimes for copper canisters. The analysis is based on compilations of literature from the following areas: chemical literature on copper and copper corrosion, mineralogical literature with emphasis on the stability of copper in near surface environments, and chemical and mineralogical literature with emphasis on the stabilities and thermodynamics of species and phases that may exist in a repository environment. Three main types of situations are identified: (1) under oxidizing and low chloride conditions, passivating oxide type of layers may form on the copper surface; (2) under oxidizing and high chloride conditions, the species formed may all be dissolved; and (3) under reducing conditions, non-passivating sulfide type layers may form on the copper surface. Considerable variability and uncertainty exists regarding the chemical environment for the canister, especially in certain scenarios. Thus, the mechanisms for corrosion can be expected to differ greatly for different situations. The lifetime of a thick-walled copper canister subjected to general corrosion appears to be long for most reasonable chemistries. (It is assumed that the canister has no defects from manufacturing and that the bentonite buffer is intact). Localized corrosion may appear for types (1) and (3) above but the mechanisms are widely different in character. The penetration caused by localized corrosion can be expected to be very sensitive to details in the chemistry. INTRODUCTION AND BACKGROUND
The aim of the Swedish programme', 2 for the development of a system for the direct disposal of spent nuclear fuel is to contain the radionuclides for a very long time. One important strategy to achieve this aim is the application of the multibarrierprinciple. This principle is applied in the Swedish reference disposal system', KBS-3, which is being developed by The Swedish Nuclear Fuel and Waste Management Company (SKB). In this system, the canisters are, deposited in holes drilled through floors of tunnels with compacted bentonite clay in between the canisters and the surrounding crystalline basement rock. Barrier functions are provided by the spent fuel itself, by the canister, by the bentonite backfill and by the surrounding rock. In the Swedish programmel' 2 , a great emphasis is put on a long-life canister. The reference canister consists of an inner one made of steel and an outer one made o
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