Since the arrival of nuclear power in the 1950s, more than 400 reactors in 31 countries have produced about 430,000 metric tons of spent nuclear fuel, and until now no one has developed a permanent solution for disposing of it. The best solution would be to put spent fuel in an underground facility that can contain it safely for millennia. Spent fuel is a major threat to human health for at least 10,000 years and remains radioactive for several million. The repository is being built by Posiva Oy, a Finnish company set up by two of the country’s nuclear operators in 1995 to find a way of permanently storing spent nuclear fuel. Researchers have also expressed concerns about potential corrosion of storage containers, specifically the copper canisters.