HELSINKI- Finland's plan to establish the world's first underground nuclear waste disposal tool a step forward on Tuesday when its builder Posiva announced a 500-million-euro ($569.55 million) investment in facilities needed for nuclear waste burial.

Posiva, owned by Finnish utilities Fortum and Teollisuuden Voima, plans to bury used nuclear fuel around 400 metres (1,312 feet) deep in Onkalo bedrock on Olkiluoto island, some 230 km northwest of Helsinki.

Finland's government has already granted a construction licence for the above-ground encapsulation plant and disposal facility Posiva needs to treat the highly radioactive waste before taking it to the underground waste repository.

"We expect to award contracts for the most significant works in the near future," Posiva President Janne Mokka said in a statement. He estimated the facilities could be operational by mid 2020s.

In Posiva's disposal process, the waste will be packed in sealed copper canisters before being transferred into tunnels and further into deposition holes lined with bentonite buffer.

Finland is not alone in trying to solve the problem of its accumulating nuclear waste.

The International Atomic Energy Agency estimates that there is now a global stockpile of around a quarter of a million tonnes of highly radioactive spent fuel in some 14 countries.

"The majority of this spent fuel remains in cooling pools at reactor sites that lack defence-in-depth such as secondary containment and are vulnerable to loss of cooling, and in many cases lack independent back-up power," Greenpeace warned in a report earlier this year.

Sweden has similar plans to store nuclear waste deep underground but the plan got postponed after an environmental court put its safety in question last year. 

Other countries, including the United States have plans to reduce the physical volume of their mounting nuclear waste but a decades-old U.S. plan to bury waste in Nevada's Yucca Mountain has been stopped by local opposition. 

($1 = 0.8779 euros)

(Reporting by Anne Kauranen Editing by Mark Heinrich) ((anne.kauranen@thomsonreuters.com; +358925166112;))