Radioactive substances to go underground for million years in Finland Sky News reports from world's first tomb for nuclear waste

The world's first geological tomb for nuclear waste is rapidly taking shape more than 400 metres below the forests of Finland.

Batches of lethally radioactive uranium will start arriving within two years for burial in the warren of tunnels carved into the bedrock.

Other countries, including the UK, are considering plans to build their own geological disposal facilities, which should safely isolate the 260,000 tonnes of highly radioactive waste that has accumulated worldwide since the dawn of nuclear power in the 1950s.

Sky News was given rare access to the site, called Onkalo, which means "cavity" in Finnish. It is built next to three nuclear reactors on the country's southwestern coast.

Security was so tight in the aftermath of the sabotage of the Nord Stream gas pipeline that there was a strict no-filming rule above ground.

But we were taken down the 5km access road that winds through the bedrock, so deep that our ears popped.

At the bottom, further tunnels fanned out. So far five have been completed, but up to 100 could be built over the coming decades, stretching to more than 40 miles in all.

Our guide was Sanna Mustonen, a geologist and senior project manager for Posiva, the company that runs the facility.

She said the bedrock was formed almost two billion years ago and has remained intact ever since.

"The rock itself, like in the whole area of Finland, is very stable," she said.

"We have old rock. We don't have continental plates nearby, so we don't have any earthquakes, seismicity, or things like that."

Like other countries, Finland stores spent nuclear fuel above ground in shielded bunkers while it seeks a long-term solution.

But Mika Pohjonen, managing director of Posiva, said it would be irresponsible to leave such dangerous waste where it could fall into the wrong hands.

He told Sky News: "If you look at history, 300 years back, how many wars have there been in Europe, for example?

"On the surface, the interim storage needs active measures from humans, the building needs to be heated, the spent fuel must be cooled, and there must be security around it.

"If you look a generation forward you cannot really see that that kind of arrangement would be risk-free enough."

Various solutions to the nuclear industry's waste problem have been suggested, including launching it into deep space, burying it in an ocean trench and dropping it into a fissure in the Earth's crust.

They've been dismissed as unfeasible, expensive or environmentally risky.

"Safe for one million years"

Instead, Posiva will encase spent nuclear fuel in double-layered metal canisters that will slot into holes bored in the floor of the tunnels.

To keep them dry, they will be swaddled in bentonite, an absorbent material used in cat litter.

More bentonite will be used to backfill the tunnels, which will be plugged into concrete.

When the complex is full in around a century's time, with perhaps as many as 3,250 canisters, it will be sealed up and all traces removed above ground.

"It will be safe for one million years," Mr Pohjonen said.

"There may not be humans here anymore because in that time there will be ice ages or [this area will be] underwater but this is designed to keep it out of the biosphere."

Dummy canisters are already buried in bentonite and surrounded by sensors.

Some scientists have warned water could corrode the metal, become radioactive and then rise to the surface over millennia.

But Posiva says the multiple barriers keep the waste in and water out. And if there was a leak in a highly unlikely worst-case scenario, modelling shows that by the time any water reached the surface in 10,000 years the radioactivity would have decayed so much that it would not be a threat to life.

Finland's progress has been watched closely by other countries. Sweden has begun construction of its own deep geological disposal site. France, Switzerland and the UK are further behind.

A shortlist of four possible sites in Cumbria and Lincolnshire has been drawn up.