UPDATE 1-Finland expects to begin to bury nuclear waste in 2022

* Nuclear waste firm asks for key government licence

* Plans to bury encased spent nuclear fuel for 100,000 years (Rewrites headline, lead, adds quote, detail)

HELSINKI Dec 28 (Reuters) - Finnish Posiva on Friday asked for government permit to construct a final repository for used nuclear fuel, planned to be the first site in the world to start burying capsulated nuclear waste.

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

The spent nuclear fuel is planned to be buried for at least 100,000 years in encased capsules of copper and cast iron and surrounded by buffers of bentonite clay, that swells if it gets in contact with water. The clay will prevent the spread of radioactive material into the bedrock.

"We believe it will take just short of two years (to process the application), so that in 2014 we would have the permission in our hands and in 2015 we could begin the construction," Posiva Chief Executive Reijo Sundell told a news conference.

He added the final disposal is now estimated to begin around 2022.

Onkalo repository will take waste only from reactors in Finland, which has four and plans to have seven by around 2020.

Sweden and France also have plans to start storing waste underground from 2025. (Reporting by Terhi Kinnunen; writing by Jussi Rosendahl; editing by James Jukwey)