Effect of the degree of compaction of bentonite on the general corrosion rate of copper

Since 1996 VTT Industrial Systems has co-ordinated a research programme, that has been funded by Finnish and Swedish radiation and nuclear safety authorities, concentrating on the durability of copper canisters in different corrosive environments. According to the Scandinavian final disposal vault concept, a copper canister is an essential barrier for the release of radionuclides. It can be damaged in use in two different ways, either mechanically or due to corrosion. The mechanical failure may be caused for example by the tectonic movement or uneven wetting and following swelling of bentonite. The corrosion failure, on the other hand, may be caused by three different mechanisms, namely general corrosion, pitting corrosion and stress corrosion cracking. The factors affecting the occurrence of different corrosion failures have been studied extensively especially in Finland, Sweden and Canada during the last decade. As a result, some factors (e.g. stress corrosion caused by the nitrites) have been excluded, but others, such as the localised corrosion caused by sulfides, are still to be studied. The present work concerns the effect of the degree of bentonite compaction on the general corrosion rate of copper in a final disposal vault environment of radioactive waste. The feasibility of two sensors - an on-line corrosion rate monitoring sensor and a modified reference electrode, has been tested with respect to copper corrosion in MX80 bentonite saturated with highly saline groundwater in conditions closely simulating those in a nuclear waste repository. In addition, weight loss coupons were used to allow a comparison of the on-line and off-line corrosion rate measurement techniques. Two different degrees of compaction of bentonite were used in tests which extended for one month. The corrosion rate measured with the on-line probe shows a high value initially, and tends towards very low values after a roughly ten hour exposure. Based on both the weight loss and on-line measurements increasing the degree of compaction slows down the initial corrosion rate. The developed novel reference electrode design enables the measurement of the corrosion potential of copper and the redox potential both in the groundwater and in the compacted bentonite. The measurement results and their indication with regard to the corrosion processes are discussed in more detail in the paper. Both sensors have shown the expected stability up to at least one month of operation. Thus they can be used for reliable long-term corrosion rate and electrode potential measurements in the presence of compacted bentonite.