How dangerous was Russia's attack on the biggest nuclear power plant in Europe?

A Russian missile strike on Europe's largest power plant in Ukraine has sent shockwaves across the world, sparking fears overnight of a disaster that could affect the continent for decades to come.

Russian troops are now in control of the Zaporizhzhia nuclear plant, in the eastern city of Enerhodar - the second plant they have seized control over in Ukraine after storming Chernobyl.

The shelling sparked a fire at the plant raising fears of a disaster that could have disastrous consequences for Europe, with Ukrainian President Volodymyr Zelenskyy warning "Russia had risked an explosion six times the magnitude of Chernobyl".

Ukrainian officials announced no radioactive material had been released, radiation levels remain normal, and all six of the plant's reactors appear to be in tact.

While much of Europe was asleep, a Russian shell hit the continent's biggest power plant, reports ITV News Correspondent John Ray

Three Ukrainian troops were killed and two others injured in the attack, according to a state nuclear company.

Though concerns were eased over a nuclear disaster, fears grew over how far President Vladimir Putin is willing to go to win the war against Ukraine, as world leaders condemned the "reckless" attack on the plant amid warnings the worst is yet to come from Russia.

Nuclear power is a significant energy source for Ukrainians and the country is heavily dependent on its 15 plants. The attack appears to be another sign Russia is trying to cut off resources to Ukrainians.

What happened?

After taking the strategic port city of Kherson, Russian forces moved into the territory near Zaporizhzhia and attacked the nearby city of Enerhodar to open a route to the plant late on Thursday.

The mayor of Enerhodar, Dmytro Orlov, said a Russian military column had been seen heading towards the nuclear facility and that loud shots rung out across the city.

Overnight, a "projectile" hit a training building close to one of the plant’s reactor units, "causing a localised fire that was later extinguished," said The International Atomic Energy Agency (IAEA).

The plant's spokesman, Andriy Tuz, told Ukrainian television that initially, firefighters were not able to get near the flames because they were being shot at.

Credit: Viktor Buchnev

Ukrainian authorities later announced Russia had seized control of the nuclear plant.

But the IAEA said it continued to be operated by its regular staff.

"The safety systems of the plant’s six reactors had not been affected and there has been no release of radioactive material," said the IAEA. "Radiation monitoring systems at the site are fully functional."

But the agency said the plant's operator "has reported that the situation remains very challenging" and staff have not yet been able to access the whole site "to assess that all safety systems are fully functional".

Most experts saw nothing to indicate an impending disaster.

What could have happened?

The reactor, close to where the missile struck, was reportedly offline, but still contains highly radioactive nuclear fuel.

Two of the other six reactors have now been taken offline, while only one is operating at 60% power and two others are being held "in reserve" in low power mode, according to the IAEA.

"It highlights the potential dangers of a war in this country where there are 15 nuclear reactors," ITV News Correspondent Dan Rivers reports from Ukraine

The reactors at the plant have thick concrete containment domes, which would have protected them from external fire from tanks and artillery, said Jon Wolfsthal, who served during the Obama administration as the senior director for arms control and non-proliferation at the National Security Council.

However, he added: “We don't want our nuclear power plants to come under assault, to be on fire, and to not have first responders be able to access them."

Another danger at nuclear facilities are the pools where spent fuel rods are kept to be cooled, which are more vulnerable to shelling and which could cause the release of radioactive material.

One major concern, raised by Ukraine's state nuclear regulator, is that if fighting interrupts power supply to the nuclear plant, it would be forced to use less-reliable emergency diesel generators to provide emergency power to operating cooling systems.

A failure of those systems, or if they ran out of fuel, could cause a station blackout that would stop the water circulation needed to cool the spent fuel pool, said Najmedin Meshkati, an engineering professor at the University of Southern California.

This could lead to a disaster similar to that of Japan's Fukushima plant, when a massive earthquake and tsunami in 2011 destroyed cooling systems, triggering meltdowns in three reactors.

“That is my big - biggest concern,” said Mr Meshkati, who has studied both the Chernobyl and Fukushima disasters, raising the concern also voiced by Wolfsthal and others.

David Fletcher, a University of Sydney professor in its School of Chemical and Biomolecular Engineering, who previously worked at UK Atomic Energy, noted that even shutting down the reactors would not help if the cooling system failed in such a way.

“The real concern is not a catastrophic explosion as happened at Chernobyl but damage to the cooling system which is required even when the reactor is shut down,” he said in a statement.

"It was this type of damage that led to the Fukushima accident.”

What concerns remain?

Ukraine is heavily reliant on nuclear energy and has 15 reactors at four stations that provide about half of the country's electricity.

James Acton, the co-director of the Nuclear Policy Program at the Carnegie Endowment for International Peace, said the simple key to keeping the facilities safe was to immediately end any military operations around them.

“Under normal circumstances, the likelihood of a reactor losing power and of the emergency diesel generators being damaged and of not being repaired adequately quickly is very, very small,” Acton said.

“But in a war, all of these different failures that would have to happen for a reactor to become damaged and meltdown — the likelihood of all of those happening becomes much more likely than it does in peacetime.”

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Mitsuru Fukuda, a professor at Nihon University in Tokyo and expert on crisis management and security, said the Zaporizhzhia attack raises broader questions for all countries.

“Many of us did not expect a respected country’s military would take such an outrageous step,” he said.

”Now that (Russian President Vladimir) Putin has done it, not only Ukraine but the international community, including Japan, should reevaluate the risk of having nuclear plants as potential wartime targets.”

At an emergency meeting of NATO and EU foreign ministers to discuss the crisis in Ukraine, the military alliance's secretary-general Jens Stoltenberg said the attack “just demonstrates the recklessness of this war and the importance of ending it".

US President Joe Biden, British Prime Minister Boris Johnson and others called for an immediate end to the fighting there as they warned the attack could have caused a catastrophic disaster across the continent.

Boris Johnson spoke to Ukrainian President Volodymyr Zelenskyy shortly after 3am, highlighting the concerns of world leaders over the attack on Zaporizhzhia, Political Reporter David Wood says

Downing Street called the situation in Zaporizhzhia “gravely concerning”, while Foreign Secretary Liz Truss echoed Mr Stoltenberg's concerns about the "reckless" attack.

Following a conversation with Ukrainian Prime Minister Denys Shmyhal, the IAEA director appealed to all parties to “refrain from actions” that could put Ukraine's nuclear power plants in danger.

Mr Shmyhal called on western nations to close the skies over the country's nuclear plants.

“It is a question of the security of the whole world!” he said in a statement.

Ukraine is also home to the former Chernobyl nuclear plant, where radioactivity is still leaking.

It was taken by Russian forces in the opening of the invasion after a fierce battle with the Ukrainian national guards protecting the decommissioned facility.

The IAEA earlier this week appealled to Russia to let the Chernobyl staff “do their job safely and effectively" after Ukrainian officials said they have been held by the Russian military without rotation and are exhausted.

The IAEA's director general  Rafael Mario Grossi said he remained "extremely concerned" about the situation at the Zaporizhzhia nuclear power plant, its staff and the attack on it.

"Firing shells in the area of a nuclear power plant violates the fundamental principle that the physical integrity of nuclear facilities must be maintained and kept safe at all time," he said.

During fighting on the weekend, Russian fire also hit a radioactive waste disposal facility in Kyiv and a similar facility in Kharkiv.

Both contained low-level waste such as those produced through medical use, and no radioactive release has been reported, but Mr Grossi said the incidents should serve as a warning.

He said: “The two incidents highlight the risk that facilities with radioactive material may suffer damage during the armed conflict, with potentially severe consequences."