The lift is accelerating into the earth, the geologists and engineers on board stifling groans as their bodies move faster than their stomachs.

“This is the fastest passenger elevator in Europe,” says Pasi Tuohimaa, 64, a spokesman for Posiva, the nuclear disposal company that runs this facility. The lift drops 433 metres in 66 seconds before the passengers, slightly unsteadily, step into a vast network of tunnels excavated below the island of Olkiluoto off Finland’s west coast.

This is likely to be the last group of visitors to make this journey down into the bedrock. Next month Posiva will close the tunnels for a series of tests.

Then, if all goes to plan, spent nuclear fuel will be transported early next year down dedicated lift shafts before robotic machines bury the 24-tonne copper and iron canisters in the rock where they will remain for the rest of time.

One canister of spent nuclear fuel will be transported every week for the next 100 years

SUNDAY TIMES PHOTOGRAPHER RICHARD POHLE

This is the world’s first deep geological disposal facility for nuclear fuel, a concept that has been discussed by engineers and politicians for half a century. More than 20 other countries including the UK, US, France and Sweden have plans to follow suit. But the Finns have got there first.

Fiona McEvoy, 50, the head of site characterisation and research and development at the British government agency Nuclear Waste Services, is here as part of a fact-finding mission to see how a similar feat could be achieved in the UK.

She says: “It’s a watershed moment for the nuclear sector. Long-lived, dangerous waste will be locked away, safe for eternity. That is amazing.”

Fiona McEvoy

SUNDAY TIMES PHOTOGRAPHER RICHARD POHLE

Martin Walsh, 51, head of engineering at Nuclear Waste Services, also on the visit, says: “Nobody disagrees that for the legacy for nuclear waste in the UK, geological disposal is necessary.”

The most radioactive nuclear waste produced by Britain’s nuclear power stations will remain hazardous, Walsh says, “beyond our lifetime, and beyond the lifetime of our children and our children’s children”.

Martin Walsh

SUNDAY TIMES PHOTOGRAPHER RICHARD POHLE

Burying it deep in the earth is considered a “final disposal”, a solution that has been calculated to enable the radioactive waste to remain undisturbed for a nominal 500,000 years, surviving ice ages, tectonic shifts, earthquakes and sea level rise.

A safe disposal option, says Tuohimaa, fills in the “missing link” in the pro-nuclear argument. Nuclear power, he says, is consistent, secure and free of emissions. While wind and solar power are cheap and green, when the wind doesn’t blow and the sun doesn’t shine a “baseload” of consistent power is essential to stop the lights going out. Without it, net zero would be impossible.

That is why new nuclear power is central to the British government’s plans. Hinkley Point C power plant in Somerset, set to be completed in 2031, will produce enough electricity to power six million homes. Sizewell C in Suffolk, approved this year, will eventually power another six million. A fleet of small modular reactors is also planned to provide juice for the electric cars and air-source heat pumps that will be central to a decarbonised Britain.

The elephant in the room is the hazardous waste these power stations produce. The UK spends £4 billion a year transporting, storing and securing radioactive material accumulated from the past 70 years of power generation.

This includes spent uranium fuel and byproducts such as plutonium. But it also includes irradiated machinery from decommissioned facilities dotted around the UK, from Dungeness on the Kent coast to Dounreay at Scotland’s northern tip.

British visitors marvel at the “encapsulation plant”, where spent fuel is handled before being sealed

SUNDAY TIMES PHOTOGRAPHER RICHARD POHLE

Some of this waste remains at the 36 civil nuclear sites around the country. But much of it, particularly older waste, has been transported to Sellafield on the Cumbrian coast. There, in vast pools, spent fuel rods are stored underwater to contain the radiation. Other waste, such as chopped up reactor cores from plants decommissioned in the 1980s, is held in 3m by 3m stainless steel boxes filled with concrete and stacked in huge vaults. All of this, Walsh says, would be safer deep underground.

Here in Finland, the problem is slightly more straightforward. Nuclear power has been generated here since 1977 and now the country has five reactors — three of them above our heads on Olkiluoto island, and two at Loviisa on the southeast coast. The spent fuel is uniform in size and shape, and the country holds about a tenth as much waste as that built up in Britain.

But there is still a lot of radioactive material to get rid of. The two private companies that run these facilities, TVO and Fortum, jointly founded Posiva in 1995, developing this repository to dispose of their waste.

Every week, for the next 100 years, one canister of spent nuclear fuel will be transported 433m down into the earth.

Here, a robotic vehicle will carry it down one of the many disposal tunnels and carefully place it into an 8m-deep vertical hole that has been bored down into the rock, which is a particularly hard form called migmatite. “Just look at this rock,” says McEvoy, stroking the rough surface. “It is beautiful. It has been squashed, squeezed, recrystallised. It has been here 1.9 billion years. It is incredibly hard.”

Once the canister is in place the hole will be packed with bentonite, a special clay that will swell when wet, sealing the canister in place. Then, the deposit made, the robots will fill up the access tunnels over them with more bentonite clay.

Finally in the year 2120, the entire facility — the subterranean access roads, lift shafts and ventilation systems — will be filled up and sealed.

A map shows how the finished storage tunnels will look

SUNDAY TIMES PHOTOGRAPHER RICHARD POHLE

Sanna Mustonen, 55, a geologist and project leader at Posiva, has been working here for 22 years, seeing the first excavations in 2003. “Being here from the beginning and seeing it now finally happening, it’s marvellous, wonderful.

“When we started the US was well ahead, so were the Swedes and maybe the French.” She believes her team overtook them because they set a plan and simply followed it, but she admits that the support of the local population has helped. “Acceptance for the final disposal proposal was high here in Finland.”

Sanna Mustonen, centre, tells the British visitors how the tunnels will work

SUNDAY TIMES PHOTOGRAPHER RICHARD POHLE

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Vesa Lakaniemi, 56, the mayor of the Eurajoki municipality, of which Olkiluoto is a part, insists locals are quite happy to have nuclear waste below their feet. “We Finns are quite straightforward,” he says. “We trust the scientists and engineers and they tell us it is safe.”

It helps, he says, that the nuclear power station here has been a significant local employer for the past half century. “In Eurajoki there is no family who doesn’t know someone who has worked at Olkiluoto.” The plant has brought significant economic benefits to the area, including increased tax receipts and the prospect of guaranteed employment for another century while the waste is taken into the tunnels.

The three nuclear reactors, OL3, OL1 and OL2, on Olkiluoto

SUNDAY TIMES PHOTOGRAPHER RICHARD POHLE

When the council voted to approve the scheme back in 2000, it was overwhelmingly in favour, 20 votes to 7. The decision was backed the following year by the national parliament. Only three voted against the scheme versus 159 for it.

Other nations have not had quite so much luck. In the UK, plans for a similar geological disposal scheme have experienced false starts because no council has yet agreed to host a site. In June, the newly elected Reform leadership of Lincolnshire county council pulled the plug on long-running discussions to site a geological disposal site near the coastal village of Theddlethorpe.

The most likely location for a site is now off the Cumbrian coast, close to Sellafield. Nuclear Waste Services is in discussions with Mid Copeland and South Copeland community partnerships for a proposal for an access tunnel to be sunk onshore, and then run ten miles out below the seabed, where 250 miles of disposal tunnels would be dug, nearly ten times the size of the Finnish scheme.

Subject to local approval and the go-ahead of whichever government is then in power, construction is expected to start in the 2040s and start being filled in the 2050s. It will be filled with waste for 150 years before it is sealed in 2200. The lifetime cost of the UK project is estimated at up to £53 billion, compared with about £5 billion for the Finnish scheme, which at roughly a tenth of the size, serving a nation with a tenth of the population, is roughly comparable.

The speed at which progress has been made, however, is not comparable. But Walsh defends the cautious pace the British experts have taken. “The thought process, particularly around nuclear, has to be robust. You have to make sure your relationship with safety and security and the environment is sound.” He adds: “Nobody will thank us if we get this wrong.”