Investigations of Resistance Variations in Electrically Heated Canisters for Deep Repository
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,QYHVWLJDWLRQVRI5HVLVWDQFH9DULDWLRQVLQ(OHFWULFDOO\+HDWHG&DQLVWHUVIRU'HHS 5HSRVLWRU\ Baohua Zhu1, Hans-Peter Hermansson1, Rolf Sjöblom2 and Gunnar Ramqvist3 1 Studsvik Nuclear AB, SE-611 82 Nyköping, Sweden 2 Tekedo AB, Spinnarvägen 10, SE- 611 37 Nyköping, Sweden 3 Swedish Nuclear Fuel and Waste Management Co, Äspö Hard Rock Laboratory, SE-572 95 Figeholm, Sweden $%675$&7 At the end of 2001, measurements indicated that the resistance between the conductors had fallen in SKB’s (Svensk Kärnbränslehantering AB) electrically heated test canisters. Electrochemical impedance spectroscopy (EIS) was employed together with chemical analysis of gas compositions and corrosion products to investigate the causes of the resistance decrease inside the canister. The measured system is complicated and the evaluation not straightforward. However, it seems that the plug connecting the external electrical power supply to the heaters in the canister is a critical component and that a moisture film around the electrical contacts of the plug may explain the resistance decrease. It is suggested to continue to monitor the EIS spectra with time to gain more information about the cause of the resistance drop. ,1752'8&7,21 According to the Swedish programme, spent nuclear fuel will be disposed of in a repository in crystalline rock. The technical barriers will comprise a bentonite buffer together with a composite canister having an outer vessel of copper for protection against corrosion and an inner one of steel for mechanical integrity. In the Äspö Hard Rock Laboratory (HRL), full-scale tests on canisters are going on as part of a programme to demonstrate and understand the function of different components of the repository system. Another aim is to show that a high quality can be achieved in design, construction and operation of the repository. The experience gained from over almost twenty years of operation in a prototype repository at HRL will be used in planning the next stage of expansion of the repository. The prototype repository will provide a demonstration of the integrated function of the repository and provide a full-scale reference for test of predictive models concerning individual components as well as of the complete repository system. The prototype repository project is cofunded by the European Commission and comprises six full sized canisters with internal electrical heaters to simulate the waste. A section with four canisters and bentonite was fully installed during 2001. This section has been back-filled and plugged and is presently in operation and the test is planned to continue during at least 10 and possibly up to 20 years. Two additional canisters will be installed in another section. In another so-called canister retrieval test, one fullscale deposition hole has been installed for the purpose of testing technology for retrieval of canisters after the bentonite buffer has become saturated. This Retrieval Canister, which includes electrical heaters, and its surrounding bentonite buffer were installed in 20
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