Nuclear waste vault safe for 100,000 years

Onkalo tunnel is located at a depth of more than 400m underground, specializing in storing highly radioactive waste from nuclear power plants, expected to operate from 2024.

Onkalo, the world's first permanent nuclear waste vault, lies beneath the forest on Olkiluoto, an island off the west coast of Finland. Within a few years, spent reactor fuel rods encased in large copper drums and giraffe-sized will be transported here by elevator. Next, the self-driving vehicle will take them to one of dozens of dead-end tunnels in an ant-like network in the bedrock.

The spent uranium fuel rod will be sealed in a corrosion resistant copper container.

After 30-40 copper barrels are buried at the bottom of the tunnel, the burial hole will be sealed with bentonite, a clay that absorbs water. Engineers also filled each tunnel with bentonite and sealed it with concrete. The copper barrels will lie there for 100,000 years, even as the climate warms in the coming centuries ushering in the next Ice Age. According to Antti Mustonen, a geologist working on the project, the nuclear waste cellar is located in stable bedrock 430 meters above the ground and 420 meters below sea level.

Two of Finland's four reactors are located in Olkiluoto. Once a new reactor in Olkiluoto connects to the grid later this year, nuclear power will account for more than 40% of the country's electricity. However, nuclear power also comes with a limitation that used uranium fuel rods are very hot and highly radioactive. Fuel rods can be immersed in a water bath to cool for decades or armored and concrete for dry storage. Either way, above-ground storage is susceptible to breakdowns, leaks or mismanagement for thousands of years of hazardous waste existence, says Budhi Sagar, a former nuclear expert. at the Southwest Research Institute.

Construction work inside the cellar.

Without long-term measures, waste will pile up. Finland had about 2,300 tons of waste in 2019. 263,000 tons of spent fuel are in storage facilities around the world, the International Atomic Energy Agency (IAEA) estimates in its report. this year.

Posiva, the company that develops and manages Onkalo, started looking for a site to build the bunker in the 1990s. From dozens of potential sites, they narrowed the list down to four with different geological features. The final option lies between Olkiluoto and the area around Loviisa. In 1999, Posiva proposed choosing a site to build Onkalo.

The bedrock at Onkalo has been mostly stable for billions of years, according to geologists, despite evidence of earthquakes in the past 10,000 years as the glacier shrank at the end of the last ice age. Posiva's team predicts no major earthquakes in the region until after the next ice age. Onkalo lies between two parallel fault zones, about 800 m apart. If an earthquake occurs, activity will be concentrated along two existing fault lines. They will absorb movement and nothing will happen in the area in between.

But earthquakes are not the main threat. According to Sarah Hirschorn, director of geosciences at the Canadian Nuclear Waste Management Organization (NWMO), the only way for objects to move from storage to the ground and affect people is with currents. country. That means underground storage should be in the layer of clay, salt or hard crystalline rock, because they have small holes that do not communicate with each other, so it is difficult for water to penetrate. In Onkalo, the nearly 2 billion-year-old bedrock is mostly gneiss, a hard rock that forms under high temperature and pressure.

Even without the hollow structure, the rock layer can still contain cracks, and Posiva had to map them to avoid them as workers dig deeper. It is the fissure that controls the movement of the water, says geologist Neil Chapman, an adviser to Finland's Radiation and Nuclear Safety Authority (STUK). If any large cracks are found during drilling, the hole will not be used. If it got into the bunker somehow, the water would have to seep through the bentonite and copper to reach the spent nuclear fuel.

After arriving on Onkalo, the used fuel will be processed at the packing plant. In a stainless steel room surrounded by a 1.3-meter-thick concrete wall, the robot will drain the remaining water on the fuel rod from the storage pool, sealing it inside a cast iron box housed in a copper container. Argon gas will be injected between the box and the container to provide an inert atmosphere, then the copper tank is tightly welded. Copper has a slow rate of erosion. When groundwater reaches the depths of the Onkalo, a chemical reaction or bacteria consumes all the oxygen.

In addition to the copper barrel, the surrounding bentonite material also prevents radioactive leakage. This mineral not only repels water, but also prevents microorganisms from reaching the surface of the copper barrel. Microorganisms can pose a threat because they metabolize sulfate in groundwater and turn it into sulfides, causing copper to slowly corrode. Posiva notes this possibility, but the company's calculations show that even with the sulfide concentration increased, the copper barrel still has a lifespan of more than 100,000 years. If all of the above safety measures fail, the leaked waste must still pass the final hurdle. After taking decades to reach the surface, radiation levels will drop.