The last operating reactor at Ukraine's Zaporizhzhia nuclear power plant, which is Europe's largest, has been shut down.
The move became possible after the site was reconnected to Ukraine's power grid.
Fighting near the plant has fueled fears of a disaster like the one at Chornobyl in 1986, where a reactor exploded and spewed deadly radiation, contaminating a vast area in the world’s worst nuclear accident.
The plant -- one of the 10 biggest nuclear power stations in the world -- has been occupied by Russian forces since the opening days of the war.
Ukraine and Russia have blamed each other for shelling around the plant, which has not damaged its six reactors or spent nuclear fuel storage, but has repeatedly struck power lines and some auxiliary equipment.
While Zaporizhzhia’s reactors are protected by a reinforced shelter that could withstand an errant shell or rocket, a disruption in the electrical supply could knock out cooling systems essential for the reactors’ safety, while emergency diesel generators can be unreliable.
After the facility was knocked off transmission lines on 5 September following a fire caused by shelling, only one reactor remained operational to power cooling systems and other crucial equipment in so-called island mode.
So-called 'island mode' deemed risky
Functioning in “island mode” supplies power for the residual heat removal of the reactor cores and the spent fuel pools.
Experts say it is very unreliable, pointing out that if the diesel generators fail, a core meltdown could occur within hours.
If the reactor is already turned off, the risk depends on the time since the shutdown. The less time has passed, the more cooling is required.
While the pool containing Zaphorizhzha’s spent fuel is located inside the plant’s containment area, a serious reactor mishap would likely affect the pool as well.
Ukraine's nuclear operator Energoatom said the restoration of one of the power lines linking the plant to the country's power grid allowed engineers to shut down its last operating reactor.
Energoatom said the move was necessary to prevent a situation when the plant would have to rely exclusively on emergency diesel generators to keep the reactors cool and prevent a nuclear meltdown.
The company’s chief said that the plant only has diesel fuel for 10 days.
The International Atomic Energy Agency, the UN nuclear watchdog which has two experts at the plant, confirmed on Sunday that its last reactor was shut down after external power had been restored.
IAEA Director-General Rafael Grossi has called for a safe zone around the plant to avert a disaster, but the fighting has continued.