Finland has granted an operating licence for the world’s first permanent underground repository for spent nuclear fuel. The government handed Onkalo, in western Finland, the licence on Thursday, waste management company Posiva said.

Although the move does not allow the opening of the facility, it is expected to start operations soon, the firm said. However, 22 years after construction began, no exact date has been decided yet.

“This is a great and historic day for both Posiva and Finland,” the company’s Chief Executive Officer Ilkka Poikolainen said in a statement.

“Before final disposal begins, Posiva will finalise the commissioning stage, carrying out the necessary tests and inspections which STUK [the Finnish Radiation and Nuclear Safety Authority] also needs to verify and approve.”With space for 6,500 tonnes of uranium, Onkalo, which means “cave” in Finnish, will provide permanent storage for spent fuel from Finland’s five nuclear reactors.

The fuel will be enclosed in copper canisters that are highly resistant to corrosion inside the geological repository. The canisters will then be lowered into holes drilled into the bedrock, which will be filled with bentonite clay and finally sealed with a steel-reinforced concrete plug.

The waste will remain highly radioactive for tens of thousands of years. After about 100,000 years, the levels will be about the same as the uranium ore of which the fuel is made, according to Posiva.

Countries have been wrestling with what to do with dangerous nuclear by-products since the first plants were built in the 1950s. Currently, most of it is in temporary storage.

Final repositories are being built in other countries, including neighbouring Sweden and France, but Finland is expected to be the first to open its underground storage facility. The cost of the Onkalo site has been estimated at $1.16bn.