By Max Hunder and Natalia Zinets
KYIV (Reuters) -Ukraine said its Zaporizhzhia nuclear power plant was disconnected from the Ukrainian grid for the first time ever on Thursday, fueling fears of a possible mishap at Europe's largest nuclear facility that is now under Russia's control.
Ukraine's state nuclear company Energoatom said fires in the ash pits of a coal power station near the Zaporizhzhia reactor complex interfered with the power supply to the station's two last working reactors, disconnecting them from the network.
That meant that all six of the plant's reactors are not supplying energy to the Ukrainian grid, said the International Atomic Energy Agency, citing Ukraine.
Energoatom said the plant's security systems were working normally and work was under way to reconnect one of the reactor blocks to the grid.
The nuclear plant supplied over 20% of Ukraine's electricity needs before the war and its loss would pile new strain on the government, which is already bracing for a difficult wartime winter of potentially crippling energy shortages.
Energoatom, whose engineers are still operating the plant, blamed the unprecedented cutoff from the grid on Russia's invasion of Ukraine on Feb. 24.
"The actions of the invaders caused a complete disconnection of the (nuclear power plant) from the power grid - the first in the history of the plant," it said.
The White House said on Thursday that Russia should agree to a demilitarized zone around the nuclear plant, which has been in Russian hands since March.
FINAL POWER LINE
Energoatom said that despite the incident the nuclear plant was still being supplied with the power it needs for cooling by a nearby coal power station.
Grygoriy Plachkov, the former head of Ukraine's nuclear safety agency, told Reuters that nuclear power plants need electricity to cool their active reactor zones and spent fuel, as well as to ensure the operation of safety systems.
If that power link failed, the plant's diesel generators would kick in, providing electricity for as long as they have fuel, he said.
Asked if the generators would stop any meltdown of the reactors, Plachkov said: "They are supposed to (prevent a meltdown). But we have the example of Fukushima, where the generators were flooded. And right now we have soldiers on the territory of the power station … who can guess their intentions?"
An energy official who declined to be identified told Reuters that the two reactors that had been disconnected were in fact already being powered by Soviet-era diesel generators.
A spokesperson for Energoatom denied that was the case.
Russia and Ukraine have accused each other of shelling the site, fuelling international fears of the potential for a disastrous nuclear accident.
That has prompted calls for an urgent mission of the International Atomic Energy Agency to the site.
Officials from the U.N. nuclear watchdog are "very, very close" to being able to visit Zaporozhzhia, IAEA Director-General Rafael Grossi said on Thursday.
(Reporting by Max Hunder and Natalia Zinets; additional reporting Pavel Polityuk and Andrea Shalal; Editing by Catherine Evans, Tom Balmforth, Angus MacSwan and Marguerita Choy)