Ukrainian government officials said on Wednesday that damage from Russian forces had left the former Chernobyl nuclear power plant ‘disconnected’ from outside electricity, leaving the site of the worst nuclear accident in history dependent on energy diesel generators.

The International Atomic Energy Agency, the United Nations nuclear energy agency, said on Wednesday it saw “no critical impact on the security” of the complex.

The American Nuclear Society, a professional group, agreed. “The loss of potency is a serious matter but it does not pose a threat to the public,” he said in a statement.

But officials warned the situation around the plant, where there was an explosion and fire at one of the reactors in 1986, was still very worrying.

The plant has not produced electricity since the last of its four reactors shut down in 2000, but if its generators stop working, it could affect storage operations for large quantities of radioactive nuclear waste.

Since the plant was captured by Russian forces shortly after the invasion began last month, the IAEA said there had been interruptions in the flow of data it automatically receives from radiation monitors and other plant sensors.

A total loss of power would completely cut off that feed, leaving agency experts with little knowledge of what’s going on there except what could be gathered using portable devices. On Tuesday, the IAEA said it had lost communications with its sensors at the plant.

Chernobyl’s most hazardous waste is found in two places.

As is common practice in the nuclear power industry, spent fuel from the four reactors is stored in water ponds that dissipate the heat produced during the radioactive decay of the fuel. When fuel is newly removed from a reactor, there is a lot of decomposition and therefore a lot of heat, so power plants need energy to run the pumps that circulate storage water to remove the excess heat.

The IAEA said the spent fuel assemblies at Chernobyl – there are more than 20,000 of them – are old enough and dilapidated enough that circulation pumps are not needed to keep them safe.

“The heat load of the spent fuel storage pool and the volume of cooling water contained in the pool are sufficient to maintain efficient heat removal without the need for electrical power,” the agency said.

The other main source of nuclear waste is the ruins of the destroyed reactor itself. About 200 tons of fuel remain there, in a lava-like mixture with melted concrete, sand and chemicals that were dumped on the reactor during the disaster. This mixture ends up in the remains of the reactor. Certain parts of it are totally inaccessible and have only been studied by drilling them.

A functioning reactor requires pumps that circulate water around the core, keeping it cool and moderating the nuclear reaction to prevent a meltdown. There is no cooling water in the chaotic and confused remains of the reactor, so the loss of power would not affect them.

But in recent years, there have been incidents in which nuclear reactions have started spontaneously in pockets of these fuel-containing materials, leading to spikes in radiation levels. Without monitoring – of humidity in addition to radiation levels – workers would not know if a new incident occurred.

Since 2017, the destroyed reactor has been covered with a large vaulted structure, intended to confine the waste and guard against any radiation release. The structure is also intended to allow work to evacuate waste to long-term storage.

The facility only obtained an operating license from the Ukrainian authorities last year, so work had only just begun and will take decades. There are several large cranes and other specialized equipment to allow crews to work safely. Without electricity, most, if not all, of this work could not take place.

On Wednesday, the Russian Energy Ministry said Belarus, whose border is not far from the Chernobyl zone, was working to restore power to the complex from its own grid.

William J. Broad and Ivan Nechepurenko contributed report.