Sweden today began construction of a terminal storage facility for spent nuclear fuel that will be the second of its kind in the world, where nuclear waste will be stored for 100,000 years.

How to store the deadly radioactive waste until it is made safe is a question that has dogged the nuclear industry since commercial nuclear reactors began operating in the 1950s.

Finland is the only country that is close to completing a permanent repository.

“It is difficult to appreciate the importance, for Sweden and for the climate transition, of the fact that the construction of a final storage site has started,” said Environment Minister Romina Purmohtari. “They said it wouldn’t work, but it is working.”

The World Nuclear Association estimates there are about 300,000 tonnes of spent nuclear fuel globally that needs disposal. Most of it is stored in cooling ponds near the reactors that produced it.

In addition to the nuclear fuel already produced, many countries in Europe and around the world are planning to build new reactors to provide electricity for the transition from fossil fuels.

The final storage facility at Forsmark, about 150 kilometres north of Stockholm on Sweden’s east coast, will have 60-kilometre-long tunnels buried 500 metres below the surface in an old rocky substrate 1.9 billion years old.

It will be the final home for 12,000 tons of spent nuclear fuel, which will be enclosed in 5-meter-long, corrosion-resistant copper capsules that will be packed in clay and buried.

The storage facility will receive the first waste in the late 2030s but will not be filled until around 2080 when the tunnels will be filled and closed, the Swedish Nuclear Fuel and Waste Management (SKB) said.

The process, however, could be delayed. MKG, a Swedish non-governmental organisation dealing with nuclear waste, has filed an appeal in a Swedish court seeking further safety checks.

It said research by Sweden’s Royal Institute of Technology showed that the copper capsules could corrode and leak radioactive elements into groundwater.

“We have room to wait ten years to make a decision, given that this is something that should be safe for 100,000 years,” said Linda Birkenthal, president of MKG.

The Forsmark repository will cost about 12 billion kroner ($1.08 billion) and will be covered by the nuclear industry, SKB said.

It will have space in which all waste produced by Sweden’s nuclear power plants can be stored.

However, it will not have the capacity to store fuel from reactors to be built in the future. Sweden plans to build 10 more nuclear reactors by 2045.