Onkalo, near the Olkiluoto nuclear power plant in the municipality of Eurajoki on the west coast of Finland, is the first deep geological repository for the final storage of irradiated fuel and high-level radioactive waste.
Posiva Oy, the company responsible, began construction in 2004 (with a budget of 3 billion euros) and has now completed the first five floors with tunnels totalling 1,700 metres in length (eventually 5,000 metres) with a height of 6.3 metres. It is 430 metres deep and 420 metres below sea level, which is why the name Onkalo ("cave") is so significant.
Due to the COVID-19 pandemic, the project was delayed. However, by 2023 the tunnels are expected to be ready for use and the first batches of radioactive waste are expected to be stored in 2025. Once a tunnel has been filled, the managers will seal it.
The most commonly used method for sealing and storage is the KBS-3, which consists of a mechanism of several redundant natural and artificial barriers. That is, if one fails, it does not affect the others and they continue to perform their function. The radioactive waste will be placed in cast iron containers with graphite inserted in other cylindrical containers of pure copper surrounded by a protective atmosphere of argon gas. In turn, the canisters will be surrounded by waterproof bentonite and a natural barrier of stable granite, which does not react to temperature fluctuations.
The big challenge for these containers is the corrosion caused by the oxygen dissolved in the water. Posiva, as the person in charge, explained that even if this water managed to reach the copper containers, the dissolved oxygen would have already been consumed by the bacteria and, in the hypothetical case of a possible leak, when the water reached the surface in about 10,000 years, the radioactivity would have disintegrated so much that it would no longer be a threat.
Mika Pohjonen, director of Posiva, told the media that up to 3,250 containers can be stored and will be safe "for a million years".
The area in which Onkalo is located was selected for its geological stability, as it is one of the oldest geological formations in Europe, far from any likelihood of earthquake or natural disaster. In addition, the project has been well received because, being close to a nuclear power plant that has been in operation for 40 years, the population has a greater understanding of waste management and nuclear energy.
Onkalo is the first deep geological repository, but it will not be the last as Sweden has begun construction of its own, and France, Switzerland and the UK are looking at where they could build one.
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