The Zaporizhzhia nuclear power plant has been disconnected from Ukraine’s national grid for the first time in nearly 40 years of operation, the country’s nuclear power operator, Energoatom, has said.

The final power line connecting the plant to the grid was cut twice by fires at the ash pits of a nearby coal-fired power plant. Three other lines had already been taken out in months of fighting.

Disconnecting the plant raises the risk of catastrophic failure of cooling systems for its reactors and spent fuel rods, which run on electricity.

There have been growing international concerns about safety at Europe’s largest plant, where only two of six reactors are operational. Russia has stationed military vehicles and is running a campaign of terror against largely Ukrainian workers who still run the plant, which has been occupied by Russian forces since the early days of the war.

“The actions of the invaders caused a complete disconnection of the Zaporizhzhia nuclear power plant from the power grid – for the first time in the history of the plant,” Energoatom said on Thursday.

If all external connections go down, it must rely on diesel-fuelled generators for power. If they break down, engineers only have 90 minutes to stave off dangerous overheating.

During the outage, the plant still received supplies of electricity from one remaining backup line connecting the plant to the nearby conventional power plant, Energoatom said. There were three of these lines before the war, but two have been cut.