Creep of the copper canister in the Swedish program
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Creep of the copper canister in the Swedish program. William H Bowyer Consultant to the Swedish Nuclear Power Inspectorate (SKI) ABSTRACT The Swedish programme for disposal of high level nuclear waste includes the development of a container which comprises a cast iron load bearing canister contained in a 50 mm thick copper corrosion shield. The temperature of the outside of the canister is likely be up to 100°C, and repository processes may lead to long term loads of up to 50MPa. Creep of the copper is therefore an issue. SKB (The Swedish Nuclear Fuel and Waste Management Co.) have been conducted tests on three materials, OF copper containing 10ppm sulphur (OF1), OF copper containing 6ppm sulphur (OF2) and OF copper containing 6ppm sulphur and 50ppm phosphorus (OFP). In order to improve confidence in extrapolation of test results to practical temperatures and stresses, it is desirable to support the extrapolation procedures with a physical model. Life predictions made using the Frost and Ashby model [5] together with a simplification of the Cocks and Ashby model [6] provide good agreement with published experimental data for OF2 and OFP materials. Extrapolation of this data using the model leads to life predictions of 12,000 years for the OF2 material and 120,000 years for OFP material. The prediction for OFP depends on an assumption that an observed strengthening mechanism conferred by phosphorus at high stresses and temperatures, is equally as effective under repository conditions. OF1 material fails after a relatively short life and by a different mechanism to OF2 and OFP. The change in mechanism may be explained, using the model [6], for cases where segregating species reduce surface energy of grain boundary voids. It is suggested that in this case the segregating species is sulphur. INTRODUCTION The Swedish programme for disposal of high level nuclear waste includes the development of a container which comprises a cast iron load bearing canister contained in a 50 mm thick copper corrosion shield. It is intended that the integrity of the canister will be maintained for at least 100,000 years. The temperature of the outside of the canister is likely be up to but not above 100°C, and repository processes may lead to long term loads of up to 50MPa. [1]. Creep of the copper is therefore an issue. Short-term tests (up to 3 years) have been conducted By SKB on three candidate materials. These are OF copper containing 10ppm sulphur (OF1), OF copper containing 6ppm sulphur (OF2) and OF copper containing 6ppm sulphur and 50ppm phosphorus (OFP) [2,3]. Empirical methods have been used to extrapolate the test results to beyond 100,000years [2,3,4]. In this work the published test data has been extrapolated using physical models in an attempt to improve confidence in prediction of creep performance under the expected service conditions. The models employed have been the Frost and Ashby model [5] for steady state creep rates and the Cocks and Ashby model for creep fracture processes [6].
MINIMUM CREEP RATES Published values
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