Finland is on the cusp of burying spent nuclear fuel in the world's first geological tomb, where it will be stored for 100,000 years.

The pioneering project has been hailed as both a watershed moment for the long-term sustainability of nuclear energy and "a model for the entire world."

At some point either next year or in early 2026, highly radioactive spent nuclear fuel will be packed in watertight canisters and deposited into bedrock more than 400 meters below the forests of southwest Finland.

The durable copper canisters will be isolated, separated from humans and kept underground for thousands of years.

"Onkalo," which is the trademark name of the long-term disposal facility, is the Finnish word for a small cave or pit. It is an apt name for the repository, which sits atop a warren of tunnels and is situated next to three nuclear reactors on the island of Olkiluoto, approximately 240 kilometers from the capital of Helsinki.

Established in 1995, Posiva is tasked with the responsibility of handling the final disposal of spent nuclear fuel rods at Onkalo. The Finnish company is jointly owned by nuclear power company TVO and utility Fortum.

"Basically, the Onkalo project is that we are building an encapsulation plant and disposal facility for spent fuel. And it's not temporary, it's for good," Pasi Tuohimaa, head of communications for Posiva, told CNBC via videoconference.

Tuohimaa said the first-of-its-kind geological disposal facility had received a lot of interest from industry players, citing what he described as a nuclear "renaissance" in recent years and an energy crisis that gripped Europe and parts of Asia from mid-2021 through to late 2022.

"Having a solution for the final disposal of spent fuel was like the missing part of the sustainable lifecycle for nuclear energy," Tuohimaa said.

The Onkalo project has stoked debate about whether anyone can guarantee the long-term safety of spent nuclear waste and the extent to which atomic power should be used in the fight against the climate crisis.

Nuclear energy currently provides about 9% of the world's electricity, according to the World Nuclear Association.

As it's low-carbon, advocates argue that nuclear energy has the potential to play a significant role in helping countries generate electricity while slashing emissions and reducing their reliance on fossil fuels.

Some environmental groups, however, say the nuclear industry is an expensive and harmful distraction to cheaper and cleaner alternatives.

"I work both in nuclear waste disposal and nuclear accidents and I have experienced the best and worst of what the nuclear industry can offer," Gareth Law, professor of radiochemistry at the University of Helsinki, told CNBC via videoconference.

"Clean energy, cheap energy, good baseload but then I have seen the bad side too, accidents, waste creation and the problems that we have there," he continued.

"To have a country now that's demonstrating that you can actually take this very dangerous waste that's going to be here for 100,000-odd years into the future, and we actually have a disposal solution for it, I think that shows it can be done."

Law described the Onkalo project as a "big milestone" for both Finland and the international nuclear power industry.

"Posiva are very correct in selling this as a world first. It is going to be the first repository to take spent nuclear fuel and dispose of it in what I think is going to be a very safe and robust manner into the future."

Law said that while many countries will want to follow in Finland's footsteps when it comes to the geological disposal of spent nuclear fuel, the Nordic country is "at least a decade" ahead of neighboring Sweden, the next country that's likely to achieve such a feat.

"Scientifically and engineering-wise, it is a very difficult thing to put it into place and to enact, but also politically, it is very, very difficult to get the impetus to do this disposal scenario," Law said.

"There are many countries in the world that are still very much in the planning stages and even just trying to find somewhere to put the waste. So, the fact that Finland [has] built a repository now and in the next year or two we're going to be operating it and start the disposal process … I don't want to call it a miracle, but it wouldn't be a bad way of framing it in the global context."

The Onkalo project is based on the so-called "KBS-3" method developed by the Swedish Nuclear Fuel and Waste Management Company, which is working on what could be the world's second final repository.

KBS-3 is based on a multi-barrier principle, where several engineered barriers seek to ensure the long-term safety of the spent nuclear fuel. In practice, it means that if one of the barriers were to fail, the isolation of the radioactive waste is not compromised.

"It is a way to showcase that such a small nation sometimes is able to solve one of humankind's maybe top 20 problems or challenges," Finnish Climate Minister Kai Mykkänen told CNBC via videoconference.

"As we have seen during the past 10 years, nuclear seems to be required in a very important way for the green deal in Europe … but especially if we want to see Asia and the U.S. get rid of fossil electricity production," he added.

Asked whether the Onkalo project could be seen as a solution to the sustainability of nuclear waste, Mykkänen replied: "Yes, definitely."

He added, "I am sure that the clear majority of the Finnish population, and also an even larger population near to Onkalo, they are seeing it in a similar way. People really see it as a solution that replaces more harmful energy."

Mykkänen said he hoped the Onkalo project would be "a model for the entire world."