Onkalo: Finnish nuclear depot destined to last 100,000 years

“Onkalo” is a Finnish word for a cave or hollow. It implies something big and deep, the end of which is unknown. A bottomless pit.

It's an apt name for a huge tomb made in Finland over the past 20 years. Onkalo, located 450 meters deep underground on Olkiluoto Island in the southwest of the country, is the world's first permanent storage site for spent nuclear fuel. So reports the BBC in an interesting article on the future of nuclear waste management.

Olkiluoto is home to three nuclear reactors, which are located next to each other by the sea. The third was opened just this year, becoming the first new reactor to deliver power to Western Europe in 15 years. These reactors, together with two others at Loviisa on the south coast, produce 33% of Finland's electricity and have provided Finland with an abundant and affordable supply of energy.

A few minutes' drive from the Olkiluoto reactors, construction of the world's first Geological Disposal Facility (GDF) for spent nuclear fuel is nearing completion. Well, Onkalo.

It is the responsibility of our generation of scientists and engineers to take up the challenge of disposing of waste, rather than leaving it to future generations – Lewis Blackburn

Onkalo cost €1bn (£860m/$1.07bn) to build and is expected to be operational in around two years. Its arrival was hailed as a watershed by many, including the International Atomic Energy Agency (IAEA). “Everyone knew the idea of ​​a geological repository for highly radioactive nuclear waste, but Finland did it,” commented Rafael Mariano Grossi, director general of the IAEA, during a visit to the site in 2020.

Other countries, including the UK, US, Sweden, France and Canada, are already investigating a similar solution, says Gareth Law, a professor of radiochemistry at the University of Helsinki who is not involved in the Finnish project. “ Finland is at least a decade ahead of all others”.

Onkalo was designed to store high-level nuclear waste that can remain radioactive for times beyond human imagination. But it has already sparked debates about whether something could go wrong. Has Finland really found the answer to nuclear waste and can it be guaranteed that it will remain safe into the deep future?

IS ONKALO REALLY THE FIRST?

Finland is the first country to build a GDF for the storage of spent fuel from civil operations.

In the United States there is already an active defense site: the Waste Isolation Pilot Plant (Wipp), located in an underground salt formation in New Mexico. But Wipp stores materials very different from Onkalo: a special category of waste from US nuclear weapons research and production programs, rather than spent nuclear fuel rods.

Several countries, including the United States and the United Kingdom, are building new reactors or upgrading existing ones to increase capacity, while others such as India, China and Russia are planning huge expansions.

The debate remains open on the safety of these plants, but a solution must also be found for the enormous quantities of spent fuel and accumulated radioactive waste, which remain dangerous for the environment and human health for hundreds of thousands of years.

This waste must be kept away from people and isolated from the environment for a period of time that is almost incomprehensible to the human mind. This problem has made the safe handling of radioactive waste one of the major problems of nuclear energy.

According to IAEA estimates, there are approximately 260,000 tonnes of spent nuclear fuel in interim storage worldwide as of 2016, most of it at reactor sites. About 70% of the world's spent fuel is found in storage basins, with the rest in concrete and steel containers called dry casks.

Few believe that this situation can be maintained indefinitely.

“We have all benefited from nuclear power for over 60 years,” says Lewis Blackburn, professor of nuclear materials at the University of Sheffield in the UK. “It is the responsibility of our generation of scientists and engineers to take on the challenge of disposing of waste, rather than leaving it to future generations.”

He claims that Onkalo enjoys public support and was created through a democratic process. “It is an outstanding achievement and a huge milestone. Finland has set an example to the world of what can be achieved with successful cooperation and transparent communication with the public.”

For decades, scientists have grappled with the problem of how to dispose of high-activity nuclear waste. Spent fuel is one of the most difficult types of waste to manage because it is highly dangerous. According to Blackburn, it produces a level of radiation that could deliver a lethal dose to a nearby person.

According to Law, the scientific consensus is that geological disposal over hundreds of thousands of years in specially designed repositories such as Onkalo, with a stable rock formation, is "the most feasible approach" for spent nuclear fuel. “We have engineering solutions for digging and tunneling, for depositing the waste and for building the barriers that we surround it with.”

The Finnish Radiation and Nuclear Safety Authority has concluded that the characteristics of the Onkalo subsoil are conducive to ensuring the safe final disposal of spent nuclear fuel.

Onkalo, like the rest of Finland, is geologically very stable and the risk of earthquakes is low. “Onkalo's rock is migmatite-gneiss: a blend of two different rock types in one rock,” explains Antti Joutsen, principal geologist at Posiva. “It is almost two billion years old and very tough.”

This is important because rock is one of the three safety barriers of the disposal concept. Furthermore, it must be stable enough to allow the construction of spawning tunnels and deep holes.

Onkalo's proximity to an existing nuclear power plant was also a factor in the site decision, Law said. “With nuclear investments, one of the problems around the world is nimby [ism], or 'not in my backyard,'” he adds. “People in this part of Finland already had nuclear power at hand and they accepted it. So putting the waste here wasn't a leap in the dark for them." (Read more about how Finland's nuclear waste project embraced foresight.)

Onkalo will receive the spent fuel rods without reprocessing them first. Some scientists argue that reprocessing – the process of separating plutonium and uranium from spent nuclear fuel that produces reusable nuclear material and high-level nuclear waste – would make them safer by reducing the overall volume of waste left. "It is possible to recover uranium and plutonium which can be used to produce new fuel," explains Marja-Siitaru Kauppi, a professor in the chemistry department of the University of Helsinki who carried out research for Posiva. About a third of the world's nuclear waste has been reprocessed and can be vitrified (turned into glass).

But others argue that reprocessing increases the risk of nuclear terrorism. Furthermore, in the long term, the vitrified slag could dissolve in contact with groundwater. Posiva itself, the company set up to build and operate Onkalo, says the reprocessing is technically challenging and expensive. It also still produces a certain amount of high-level waste, points out Johanna Hansen, research and development coordinator at Posiva. “So a disposal facility would still be needed.”

Finland decided to build a plant in Onkalo over two decades ago when the country's two nuclear producers agreed to dispose of their spent nuclear fuel in one place. The Finnish government approved plans to build the plant in December 2000, stating that “of the disposal options studied, deep underground disposal offers the best and most realistic chance of isolating high-activity nuclear waste from the biosphere and from the human habitat”.

The Onkalo repository is based on the KBS-3 concept developed by the Swedish Society for Nuclear Fuel and Waste Management, in cooperation with Posiva. The idea is to create three barriers around nuclear waste: first put it in copper containers, then wrap it in bentonite – a clay that absorbs water – and finally bury it in deep underground tunnels.

“The process begins in the encapsulation plant, which is located above ground,” said the director of the encapsulation plant. explains Hansen, who has been working on the project for more than 20 years. “Here we will place the spent fuel rods in a two-part disposal container: an inner cast iron shell and a 5cm thick copper outer shell.”

A lid is then welded onto the container, which is transported to a storage area in the disposal facility. Once the site is ready, the containers will be transferred to a horizontal deposition tunnel at a depth of 450 metres, where final disposal will take place.

Onkalo will welcome all high-activity nuclear waste produced by Finland's five nuclear power plants throughout their entire life cycle – Antti Joutsen
Initially, an elevator will lower the containers to a landing area approximately 437 meters deep. From here, a robotic transfer vehicle will take the containers to a deposition pit, Hansen says.

Five dead-end deposition tunnels have been built so far, all 350 meters long. Another 85 will be built over the years as the plant fills up. Each deposition tunnel has about 40 vertical circular holes in the floor, each 8 meters deep and 2 meters wide. In total, there is room for around 3,000 containers, one for each vertical hole.

“They will hold a total of 5,500 tons of waste,” Joutsen says. “Therefore Onkalo will accommodate all high-activity nuclear waste produced by Finland's five nuclear power plants throughout their entire life cycle.”

After each container is buried, its hole will be filled with bentonite, Hansen says. “When we have placed a container in each hole, we will fill the tunnel with more bentonite and seal it with concrete.”

Final disposal of spent nuclear fuel will begin in the coming years. Posiva estimates that it will take 100-120 years for the repository to be full. At that point the entire facility will be sealed, allowing the containers to lie, hopefully undisturbed, for at least 100,000 years, with their lethal radioactive contents sealed off from the outside world.

While the risk of earthquakes is low here, seismic activity has been taken into account, Joutsen says. “Over the next million years there will be several ice ages which will pose a risk of earthquakes. Above Onkalo there will be a layer of ice 2-3 km thick which will push the earth's crust downwards by hundreds of metres. Onkalo was built to last ."

When the ice age ends, the crust will begin to heave again — and this is when earthquakes could rupture the containers, he adds. “To prevent this from happening, we are placing them in the best possible locations – the disposal holes are in unfractured sections of the subsurface.”

This is a case of true long-term planning, so long that it even surpasses the concept of climate change. We don't know if the Finns really anticipated everything, but they certainly did their best. In 100,000 years someone will find out if they've done a good job.

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The article Onkalo: the Finnish nuclear depot destined to last 100,000 years comes from Scenari Economics .

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This is a machine translation of a post published on Scenari Economici at the URL https://scenarieconomici.it/onkalo-il-deposito-nucleare-finlandese-destinato-a-durare-100-mila-anni/ on Wed, 14 Jun 2023 09:42:12 +0000.