Ukraine’s power network is at “heightened risk of catastrophic failure” after Russia’s missile and drone attack on Sunday, Greenpeace has warned, raising fears about the safety of the country’s three operational nuclear power stations.

The strikes by Moscow were aimed at electricity substations “critical to the operation of Ukraine’s nuclear plants” and there is a possibility that the reactors could lose power and become unsafe, according to a briefing note prepared for the Guardian.

Shaun Burnie, nuclear expert at Greenpeace Ukraine, said: “It is clear that Russia is using the threat of a nuclear disaster as a major military lever to defeat Ukraine. But by undertaking the attacks Russia is risking a nuclear catastrophe in Europe, which is comparable to Fukushima in 2011, Chornobyl in 1986 or even worse.”

The pressure group called on Russia to immediately halt its attacks on Ukraine’s energy grid and for the International Atomic Energy Agency (IAEA) to deploy permanent monitors in substations critical to the country’s nuclear plants. The IAEA conducted one inspection in late October, but has not committed to return.

Though Greenpeace is an independent organisation, it maintains contact with Ukraine’s government. Official Ukrainian sources contacted by the Guardian acknowledged Greenpeace’s technical analysis of the crisis.

In 1986, Ukraine was the location of the world’s worst nuclear disaster, when a faulty design led to an explosion and destruction of a reactor at Chornobyl. Thirty people died within a month, and radioactive material spread into Ukraine, Belarus and Russia and to a lesser extent into Scandinavia and Europe.

On Sunday night and early morning, Russia unleashed a barrage of more than 210 missiles and drones aimed at electricity generation and transmission targets around the country. Hours later, Ukrenergo, the country’s main electricity provider, announced nationwide rationing to help the system recover.

Explosions were heard in the cities of Kyiv, in Odesa and Mykolaiv in the south, in Kryvyi Rih, Pavlohrad, Vinnytsia in central Ukraine and Rivne and Ivano-Frankivsk in the west. Explosions were also heard near Ukraine’s border with Moldova where Ukraine’s grid connects with its neighbour and into the rest of Europe.

Though the attacks are not thought to have directly targeted Ukraine’s three remaining operational nuclear power plants, at Rivne and Khmelnytskyi in the west, and the South Ukraine plant, Greenpeace says Russia was deliberately trying to increase the stress they are under by targeting substations that they are linked to.

On Sunday, the IAEA reported that main power lines from four substations to three nuclear power stations were cut, and that at the Khmelnytsky plant monitors on site “heard a loud explosion”. Two power lines into Rivne became unavailable and output was reduced in six of the nine operational nuclear reactors at the three sites.

The three sites account for about two-thirds of Ukraine’s electricity because previous attacks by Russia have destroyed most of the country’s coal and oil-fired plants, while some of the country’s hydro facilities have also been damaged.

A particular concern is that “severe damage to Ukraine’s electricity system, including substations, is causing major instability”, Greenpeace said, which could mean the extended loss of external power to the reactors. Cooling of reactor and spent fuel requires power, whose stable supply is at risk, the environmental group added.

In the event of a loss of supply, Ukraine’s reactors have on-site diesel generators and batteries to provide essential electricity supply with enough fuel for seven to 10 days, but if fuel cannot be maintained or power be restored the consequences could lead to a nuclear disaster, Greenpeace said.

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“Loss of cooling function at one or more reactors would inevitably lead to nuclear fuel melt and large-scale radiological release,” Greenpeace said in its brief. “Most at risk are the people and the environment of Ukraine, but there is the potential for much of Europe and beyond to be severely impacted,” it added, depending on the wind direction at the time.

Prior to Sunday’s bombing, Britain had already accused Russia of engaging in nuclear blackmail at a meeting of the OSCE a fortnight ago. Its 57 members include Russia, so it is one of the few international forums where western countries can engage with Moscow.

“We have also heard Russia threaten Ukraine in this room that it could turn off 75% of its remaining electricity by hitting just five targets,” the UK said in a statement delivered at a meeting in Vienna on 7 November.

“This could only be a reference to Ukraine’s nuclear power plants. Such threats are unacceptable. As is the risk to Ukraine’s nuclear power plants of an unreliable power supply due to Russia’s sustained attacks against Ukraine’s grid.”

British sources indicated they believed that Ukraine’s energy generation had been reduced to about one-third of its pre-war capacity in the spring, though repairs over the summer had improved that figure back to 50%.

The impact of the latest bombing on generation remains unclear, though Ukraine’s energy ministry said on Tuesday that 9GW of power had been lost in 2024, equal to “the peak consumption of countries such as the Netherlands or Finland”.

At the beginning of the war, Russian forces captured Ukraine’s fourth nuclear power plant, the Zaporizhzhia facility, which houses six reactors. The site, on the frontline on the Dnipro river, remains occupied though the reactors are in cold shutdown.