“I believe Russian leaders know of the seriousness of the problem, but in wartime, in the midst of war, would soldiers on the ground systematically and accurately understand these concerns and strictly ensure that they are addressed?” Zhao said. “That’s greatly uncertain.”

03:04

Ukraine says Russia has taken control of Zaporizhzhia Nuclear Plant

Ukraine says Russia has taken control of Zaporizhzhia Nuclear Plant

Citing information from the Ukrainian nuclear regulator and plant operators, IAEA director general Rafael Mariano Grossi said a projectile of an unknown type hit a training centre next to the Zaporizhzhia Nuclear Power Plant’s six reactors on Thursday and caused a fire that was eventually put out.

Footage streamed on the plant’s website showed fire and smoke billowing from a building as projectiles and flares landed at night.

“The physical integrity of the plant has been compromised with what happened last night,” Grossi said. “We, of course, are fortunate that there was no release of radiation and that the integrity of the reactors in themselves was not compromised.”

He said Ukrainian staff were still operating the plant, but the Russian military had control of the site on Friday morning. Two people in charge of security had been injured.

Only reactor No 4 was operating, and that was running at 60 per cent capacity, he said.

Russia denied responsibility for the attack.

Zhao, from Carnegie’s nuclear policy programme, said a timely shutdown and safe cool-down of the reactor were essential in maintaining the safety of the plant. It was also important that water continued to be supplied to cool nuclear waste repositories.

“In the case of a fire, it boils down to the electricity and water supply to the cooling system, as well as the access afforded to emergency services,” he said.

02:06

Ukraine nuclear power plant, Europe’s largest, is on fire after Russian attack

Ukraine nuclear power plant, Europe’s largest, is on fire after Russian attack

David Fishman, a Shanghai-based senior manager at power sector consultancy The Lantau Group, said the Russian control of the Zaporizhzhia plant also threatened the power supply in Ukraine.

“A big important portion of Ukraine’s power supply is controlled by the invading force, their adversary,” Fishman said. “So Ukraine has to worry about that.”

According to Energoatom, the Ukrainian state enterprise that runs the country’s four nuclear power stations, the Zaporizhzhia plant generates 40-42 billion kilowatt-hours, accounting for a fifth of the country’s average annual electricity production.

The shelling of the power plant prompted China to urge Ukraine and Russia to ensure the safety of the nuclear facilities and prevent further escalation.

“China will continue to closely monitor how the situation develops and calls on all relevant parties to remain calm and exercise restraint,” foreign ministry spokesman Wang Wenbin said.

Meanwhile, British Prime Minister Boris Johnson has spoken to Ukrainian President Volodymyr Zelensky about Zaporizhzhia and will be seeking an emergency United Nations Security Council meeting to raise the issue, according to Johnson’s office.

A UN resolution adopted in 1990 says the General Assembly “recognises that an armed attack ... on a safeguarded nuclear facility ... would create a situation in which the Security Council would need to act immediately”.

However, Zhao said diplomatic pressure had not visibly changed how the war was fought in Ukraine.

“Whether it’s at the United Nations Security Council or the special General Assembly session, there was clear international pressure on Russia’s political acts,” he said.

“But it did not visibly affect frontline warfare. So such pressure is not very likely to immediately change the situation surrounding nuclear power plants.”

However, Paul Dibb, an emeritus professor of strategic studies at the Australian National University, said it was unclear whether the strike on the plant was deliberate.

“The central question is: was this a deliberate, nasty attack by Putin?” he said. “I wouldn’t put it past him, frankly, in his current mood. Or was it an error and was he into something else?”

Dibb said there were signs that Nato had been trying to avoid provoking Russia since Putin put his country’s nuclear forces on higher alert.

He said the US decided on Wednesday to delay a regular test launch of the Minuteman-III intercontinental ballistic missile to avoid further escalation of tensions.

“If Russia’s reading on a medium scale of nuclear alert, you don’t want to provoke them,” Dibb said.

Pentagon spokesman John Kirby said at the time that the US was taking steps to reduce the risk of miscalculation at a “moment of tension” between it and Russia.

The now-decommissioned plant of Chernobyl, the site of the world’s worst nuclear disaster, has also come under Russian control.

While local officials have reported elevated levels of radiation there, nuclear experts have said a repeat of the 1986 Chernobyl disaster is unlikely given the Zaporizhzhia plant’s improved reactor containment design.