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Securing safe final disposal

Founded in 1995, Posiva is responsible for the final disposal of spent nuclear fuel in Finland, and for research and other activities in the field of final disposal. Designing, manufacturing, and testing a final disposal canister have been an important part of this work.

Based on the work carried out by Posiva, spent fuel assemblies from Finland’s nuclear power plants at Olkiluoto and Loviisa will be encapsulated in metal composite canisters. These will feature an outer copper shell tightly enclosing an inner nodular cast-iron insert. The copper shell will protect the insert against the corrosive effect of groundwater and act as radiation shielding; while the insert will provide mechanical strength.

The fuel assemblies from Olkiluoto will be packed in 4.8-metre canisters weighing 20.9 tonnes before they are filled. The canisters for the fuel assemblies from Loviisa will be slightly smaller, at 3.6 metres long and weighing 16.1 tonnes. Both types of canisters will have an outer diameter of approximately one metre and will be capable of holding 12 fuel assemblies.

The canisters will be placed in holes drilled in the bedrock many hundreds of metres below the surface at a final repository. The canisters will then be surrounded with bentonite clay, to protect them from direct water flow and minor bedrock movement.

Placing the canisters deep in the bedrock in this way will isolate the waste from events on the surface and prevent unintentional penetration of the facility by man.

When the last canisters have been placed in the repository, the encapsulation plant will be decommissioned, the tunnels filled with a mixture of bentonite and crushed stone, and the shafts leading to the repository closed off.

Stringent requirements

The requirements laid down for the canisters are extremely stringent, and the canisters have been subjected to extensive tests.

Posiva has carried out trials to establish the most appropriate casting parameters and to ensure that the mechanical and other properties of the inserts meet requirements. Tensile tests and microscopic analysis have been used to measure the mechanical properties, ductility, and strength of sample castings.

Electron beam welding has been used to establish the best means for sealing the lid of the copper canisters. Particular attention has been given to guaranteeing weld quality and weld integrity.

In addition to test manufacturing and research, Posiva is continuing to investigate new technologies to ensure that the final design of the canister produced when mass production begins represents the best technology available at the time.

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> Tiina Jalonen
(Published in High Technology Finland )