SR-97: Canister Performance under Normal Disposal Conditions
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SR-97: Canister Performance under Normal Disposal Conditions. Lars Werme Swedish Nuclear Fuel and Waste Management Co. (SKB), Stockholm, Sweden Email: [email protected] ABSTRACT The SR-97 disposal canister consists of an inner load bearing cast iron component and an outer corrosion barrier of copper. The mechanical strength of the insert must be such that it withstands, with sufficient safety margins, the combined forces of hydrostatic pressure at the disposal depth (7 MPa) and the bentonite swelling pressure (7 MPa). It must also withstand an increased hydrostatic pressure during a future glaciation. This pressure increase may be as high as 30 MPa. For this load situation, no extra safety margins will be required. Uneven pressure may occur during the water saturation of the buffer and to some extent some uneven pressure may also persist after water saturation. Calculations have been performed for such design basis uneven loads and the present canister design has been shown to withstand them. Under the oxygen-free conditions at the disposal depth, copper will be thermodynamically stable. Only dissolved sulfide will be able to corrode the copper. The supply of sulfide is expected to be low and, consequently, the service life of the canister will be very long. Calculations indicate that no known corrosion processes will lead to canister failure during a shorter time than at least 1 million years. INTRODUCTION The Swedish nuclear power program will give rise to about 8000 tonnes of spent nuclear fuel by 2010. This fuel will be disposed of in granitic rock at a depth of about 500 m. A multibarrier system consisting of the fuel itself, a corrosion-resistant canister, a bentonite buffer, and the overlying rock mass will prevent leakage of radioactive substances from this repository. The canister will prevent all dispersal of radioactivity to the surrounding rock, as long as it is intact. The other barriers can retard and attenuate radionuclide dispersal to acceptable levels if the canister starts to leak. The function of the canister in the geologic repository is to isolate the spent fuel from the surrounding environment. This requirement is interpreted to mean that the canister should completely isolate the waste and that no known corrosion processes should be able to violate the integrity of the canister for at least 100,000 years. This requirement on canister integrity leads to requirements on: - mechanical strength under the conditions expected to prevail in the repository. - chemical resistance in the environment expected in the repository. To meet the requirement that other barriers should retard and attenuate radionuclide dispersal to acceptable levels if the canister begins to leak the canisters must not have any harmful effect on other barriers in the repository.
Based on generic data on the expected mechanical and chemical conditions in a deep repository, a composite canister has been designed [1]. It consists of an outer corrosion barrier of 50 mm copper and an internal load-bearing component of cast nodular
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