Finland is on the verge of opening the world's first permanent underground repository for nuclear waste, a major step in solving one of the biggest challenges linked to nuclear energy. The facility, called Onkalo, has been built deep beneath a forest in southwestern Finland and is designed to safely store highly radioactive nuclear waste for around 1,00,000 years.

Located in Eurajoki, the repository lies 433 metres below the ground. Visitors reaching the site descend through a lift inside a cave until it stops at the underground storage level. The repository has been carved into bedrock that is around 1.9 billion years old, creating a stable environment for the long-term storage of radioactive material.

The project aims to address a problem that countries around the world have struggled with for decades. Every nuclear power plant produces radioactive waste that remains hazardous for tens of thousands of years. Since nuclear power plants first began operating in the 1950s, governments have searched for safe and permanent ways to dispose of this waste. Most countries currently store spent nuclear fuel in temporary cooling pools, a solution that is not meant for long-term disposal.

Finland's Onkalo facility could become the first permanent answer to this challenge. The country's nuclear safety authority is expected to issue its final approval this month. Once that approval is granted, an operating licence can be issued, allowing the repository to begin functioning.

"We hope to start operations at the end of this year, or most probably early next year," said Philippe Bordarier, chief executive of nuclear operator TVO, told Science Alert news.

The storage process has been designed with multiple layers of protection. Above ground, spent nuclear fuel rods will first be sealed inside thick copper canisters. These canisters are specially designed to resist corrosion over extremely long periods of time. After sealing, they will be transported underground and lowered into holes drilled into the floors of the tunnels.

The canisters will then be surrounded by bentonite clay, a natural material known for its sealing properties. When bentonite clay comes into contact with water, it expands and creates a protective barrier around the waste containers. This additional layer is intended to prevent the movement of radioactive material and strengthen the repository's safety system.

The facility has enough capacity to store 6,500 tonnes of uranium. This amount is sufficient to hold all the spent nuclear fuel produced by Finland's five nuclear reactors. Waste will continue to be placed in the repository for about 100 years. After the storage process is completed, the entire underground vault will be permanently sealed.

The radioactive waste stored inside Onkalo is expected to remain underground for roughly 1,00,000 years. Scientists estimate that this is the amount of time required for the waste to decay to levels comparable to natural uranium ore found in the earth.

According to experts involved in the project, ensuring safety over such an enormous timescale has been the primary goal from the beginning.

"Basically, it needs to be safe forever," said Lauri Parviainen, a chemist at Posiva, the waste management company behind the project.

Despite the extensive planning, scientists have identified certain risks that could affect the repository in the distant future. One concern is the possibility of corrosion affecting the copper canisters over very long periods. Another concern is the impact of earthquakes that could occur during future ice ages.

However, researchers and engineers working on the project say years of studies and safety assessments have produced encouraging results. Repeated evaluations have consistently indicated that the repository can safely contain the radioactive waste over the long term. The findings have strengthened confidence in Onkalo's design and its ability to provide a lasting solution to the nuclear waste problem.

If approved and launched as planned, Onkalo will mark a historic milestone in nuclear waste management. The Finnish project could also serve as a model for other countries searching for permanent ways to handle radioactive waste that remains dangerous for thousands of years.