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A safe solution for spent nuclear fuel

Posiva is responsible for developing and providing the final disposal solution for spent nuclear fuel needed by its owners, Teollisuuden Voima and Fortum Power and Heat. The project is based around encasing fuel assemblies in copper canisters and isolating them hundreds of meters underground in a specially excavated bedrock facility. The canisters will be surrounded with bentonite clay to protect them from harmful chemical and biological processes and rock movement.

The aim of a final repository, such as the one being developed by Posiva at Olkiluoto in Finland, is to provide a secure means of isolating spent nuclear fuel well into the future and protecting people and the environment from the radiation associated with this material.

Posiva’s solution is based on multiple technical and natural barriers. The spent fuel will be enclosed in metal canisters, consisting of a nodular cast iron insert surrounded by a copper overpack. The canisters will be placed in holes drilled into the host rock and surrounded with bentonite clay. The safety of this aspect of the solution depends for its ultimate integrity on the natural characteristics of the repository, excavated under hundreds of metres of solid, stable bedrock.

The bentonite clay to be used is a natural product, and will be produced from bentonite, an aluminium phyllosilicate formed from volcanic ash. While the original deposits of this material were laid down on the seabed, today’s deposits are normally found on dry land; Europe’s largest deposit is located on the island of Milos in Greece, and the largest worldwide in the US.

Bentonite is extracted in blocks and dried in the sun, before being ground down into a powder and used in the foundry industry, for example, as a component in mould sand. Its excellent rheological properties also see it widely used in drilling mud and geotechnical applications such as landfills.

Excellent swelling and sealant properties

Bentonite’s properties also make it an excellent choice for protecting Posiva’s copper-cased canisters. When bentonite comes into contact with water, it swells rapidly to form an impervious barrier to water ingress. A sufficiently dense layer of bentonite will also prevent microbial populations establishing themselves. Both of these factors are perfect for preventing canisters from corrosion.

Bentonite will also protect the canisters from mechanical stress. Although Finnish bedrock, dating from the ice age, is very stable, and the location selected for Finland’s final repository is free of fractures and similar weaknesses, possible movement at some point in the future cannot be completely ruled out. Bentonite offers an excellent combination of flexibility and intrinsic strength to adapt to such movement, while retaining the integrity of the facility’s internal structure.

Two alternative placement models are being investigated for the final repository to be built at Olkiluoto.
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Built to last for thousands of years

Surrounding the canisters containing the spent fuel with a natural product that has been shown to be capable of remaining stable over long periods of time like this is an important cornerstone of Posiva’s solution. Combined with the other aspects of the design, the intention is to provide a secure facility that can survive intact for thousands of years.

The bentonite used will be thoroughly screened to ensure that it contains no impurities that could impair its key properties. Extensive development work and research carried out by Posiva have generated a solid body of data on how bentonite behaves under a range of conditions, and this has been used as the basis for creating a number of long-term models.

Studies have focused on areas such as bentonite consistency, composition, and density, as well as optimum water content. Dry bentonite results in a denser mixture, but one that is also more susceptible to reacting with moisture in the surrounding environment.

Choosing the right technique

The bentonite barrier surrounding the facility’s copper canisters will be constructed from a series of cylindrical and circular blocks. These will be produced from a mixture of bentonite powder using a compression technique that results in a very dense structure.

Each block will have a diameter of 170 centimetres, and can be produced using either single-axle or isostatic compression at a pressure of 100 MPa.

In the case of single-axle compression, a ram is used to compress the material, reducing its volume to half the original. In the case of an isostatic unit, bentonite is placed in a flexible mould, which is then compressed in a pressure vessel with the help of a liquid medium. This produces a bentonite clay mixture with a density of 2,000 kg/m3.

Posiva is keeping its options open as to which of these two techniques it will select. The isostatic approach is hampered at the moment by the lack of a sufficiently large pressure vessel, and testing work to date has used smaller blocks than those that will be needed for the final repository.

In addition to bentonite-related R&D, Posiva is also continuing development work on its copper canister and underground ‘ONKALO’ characterisation facility at the Olkiluoto site. Activities at the latter are progressing, and Posiva plans to seek a construction license for the final repository itself in 2012. Under current plans, the intention is to begin disposing of spent fuel in the facility from 2020 onwards.

The different sedimentary layers of a typical bentonite deposit can be seen clearly at the Angeria mine in Algeria. Photo courtesy of S&B Industrial Minerals GmbH.
(Published in HighTech Finland 2008)