It said the salt water was mixing with radioactive rods in tanks at the Andreeva Bay facility, and could set off a chain reaction whose fall-out could spread across northern Europe in a worst-case scenario.

"I can state officially that there have been no situations which might threaten the environment," a Rosatom official told Reuters on Friday.

Nils Boehmer, an atomic physicist and head of Bellona's Russian section, said the report in Rosatom's Atomnaya Energiya publication warned that salt water was causing uranium particles to fall off rods and settle on the bottom of tanks.

The concentration of such particles was not known, he said, but according to the Rosatom report if it topped 5-10 percent, it could trigger an uncontrolled chain reaction.

"This could lead to the release of a lot of energy over a short amount of time and contaminate a 10-kilometre radius around the facility," Boehmer told Reuters.

Danger would grow exponentially if reactions spread from one tank to another within the site -- the largest for radioactive waste and spent nuclear fuel from Russia's Northern Fleet.

"In such a case, the radioactive fallout could be higher and affect northern Europe to a greater degree than the region was hit by the Chernobyl disaster," Boehmer said.

Leaks from Andreeva Bay facilities have already led to radioactive contamination of the nearby fjord, Ballona said.

"We know how big the powder keg is, but we don't know how long the burning fuse is," Bellona's Alexander Nikitin, a former Russian navy engineer jailed in the 1990s for exposing Russian nuclear waste practices, told Norwegian daily Aftenposten.