Posiva Oy is preparing to submit the operating licence application for a spent nuclear fuel geologic repository located at Olkiluoto, Finland. The repository construction licence has been granted in 2015 and construction of the repository has begun in January 2019. Construction of the encapsulation facility near the repository has also started.
Safe disposal is achieved by the KBS-3 concept, developed in collaboration with Posiva's Swedish counterpart, SKB. The KBS-3 concept consists of disposal canisters (a cast iron insert and a copper overpack), surrounded by swelling bentonite and disposed in granitic bedrock. The copper overpack ensures containment of radioactive materials and its performance against corrosion is a key driver for the design of the engineered barrier system and the repository overall.
The operating license application is supported by a safety case addressing the long-term performance of the canisters against various loads, including corrosion, over 1 million years. The impact (or lack thereof) of the evolving environmental conditions on copper corrosion is based on a conceptual and/or deterministic assessment of the possible corrosion depth due to different types of corrosion. The effect of the initially trapped oxygen, temperature, radiation, groundwater chemistry (e.g. chloride and sulfide) as well as the combination of chemical and mechanical loads are taken into account when assessing the canisters' lifetime. Posiva is also expanding its applied corrosion knowledge base by studying the effect of chemical disturbances created during repository construction on localised and stress corrosion cracking.
Posiva has also recently commissioned a full-scale, in-situ, barriers installation test (called FISST) in the ONKALO®[1] underground characterisation facility at Olkiluoto. The test consists of a 50 m tunnel with two deposition holes containing one canister each and surrounded by swelling clay material (bentonite). The canisters are heated by electric resistances simulating the decay heat from the spent nuclear fuel. The tunnel is backfilled with swelling clay and sealed by a reinforced concrete plug. The evolution of temperature, relative humidity, buffer displacement and swelling pressure is monitored. Gases will also be sampled periodically in the tunnel and analysed to shed light on their evolution during the very early evolution after the installation of the plug.
Copper corrosion in pure oxygen free water has been investigated (e.g. micro- and nano-scale characterisation of copper surfaces in presence of water vapour) in response to questions raised during the Swedish licensing process. The tests commissioned by Posiva confirm and complement the results obtained by SKB.
[1] ONKALO® is a registered trademark of Posiva Oy.
Spent nuclear fuel, disposal, corrosion of disposal cansiter, copper