The nuclear plant attack puts us in uncharted territory – what is the worst-case scenario?

The Zaporizhzhia nuclear power plant in southern Ukraine is the largest in Europe.

It is one of the country’s four active nuclear plants, which combined provide around half of its total electricity.

The plant was hit by Russian forces in the early hours of Friday morning, but Ukrainian officials said the fire had been extinguished by dawn.

Although the International Atomic Energy Agency (IAEA) has confirmed no essential equipment was damaged in the attack, experts fear indiscriminate Russian shelling could result in a similar incident, which could spark a nuclear disaster.

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Here Sky News looks at what we know about the attack so far and the risks that remain amid the Russian invasion.

What happened in Zaporizhzhia?

Ukrainian authorities say a fire broke out the Zaporizhzhia nuclear power plant after it was shelled by Russian forces in the early hours of Friday morning.

Local officials claim that at first, firefighters were unable to access the site because they were being blocked and risked being shot at by Russian troops.

But eventually a team of 40 people and 10 units were able to get through and the fire had been extinguished by 5am local time.

Three Ukrainian troops were killed in the explosion, state utility company Energoatom confirmed.

The Zaporizhzhia plant is home to six nuclear reactors.

Experts say the reactor that was hit by shelling was offline at the time and while its compartment was damaged, no essential equipment was affected.

In a Facebook statement the Inspectorate for Nuclear Regulation said: “Systems and elements important for the safety of the nuclear power plant are in working condition.”

It also confirmed there has been no change in radiation levels.

Four of the other six reactors at the site have been taken offline, according to reports, leaving just one in operation.

Previously Petro Kotin, head of Energoatom, which operates all four of Ukraine’s nuclear plants, told the United Nations that there are mitigations in place for a “special period” such as a “bombing attack”.

He said: “In such a case the plant is shut down and unloaded until the threat is eliminated.”

Experts have also stressed that nuclear plants have protective domes for their reactors, which are designed to withstand plane crashes.

Around midday local time on Friday, Russian officials claimed they had taken control of the plant – as they have already done with the one in Chernobyl.

How reliant on nuclear is Ukraine?

There are four active nuclear power plants in Ukraine, which are located in the south and west of the country and made up of a total of 15 reactors.

Ukraine is heavily reliant on nuclear energy, which produces half of its overall electricity, according to the World Nuclear Association.

All four plants are run by the state utility company Energoatom.

But Ukraine’s nuclear infrastructure is heavily dependent on Russia, which designed all of its reactors and provides most of its fuel.

The country has looked increasingly to the West for nuclear services in recent years, with the American firm Westinghouse commissioned to build at least four new reactors at existing sites.

And weeks before the invasion, Energoatom had said it aimed to become fully independent of Russian nuclear fuel within two years.

Is there still a risk of a nuclear disaster in Ukraine?

In the first week of the invasion, Russia took control of the defunct Chernobyl nuclear facility in northern Ukraine – the site of the world’s worst nuclear disaster in 1986.

According to Energoatom, Ukrainian staff at Chernobyl are now being held hostage at the site and forced to work under Russian orders.

But while small amounts of radioactivity are still leaking at Chernobyl, experts claim Ukraine’s active nuclear plants pose far more of a risk.

Professor David Fletcher, who previously worked at UK Atomic Energy, explained: “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.”

Nuclear reactors can be shut down for safety reasons, but that requires electric power.

If during heavy shelling any of the four plants have their electricity supply cut, they would be forced to rely on emergency diesel generators to power a shutdown and keep reactor fuel cool.

The generators can be extremely unreliable and if they stop working, can result in a meltdown.

“It was this type of damage that led to the Fukushima accident,” Professor Fletcher, who now works at Sydney University, added.

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The 2011 incident in Fukushima, Japan, was the worst nuclear disaster since Chernobyl.

In the immediate aftermath of the attack, Ukrainian foreign minister Dmytro Kuleba said that if a similar incident happened at Zaporizhzhia “it will be 10 times larger than Chernobyl”.

With radioactive waste disposal facilities also hit in Kyiv and Kharkiv in the first week of the invasion, Ukrainian and global leaders have called on Russia not to fire anywhere near them.

James Acton, the co-director of the Nuclear Policy Program at the Carnegie Endowment for International Peace, said in a blog post: “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.

“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.”

There are also concerns that, like in Zaporizhzhia, emergency service workers may be blocked from dealing with an incident should one occur.