It's why the Geneva convention prohibits the targeting of nuclear power facilities.

And why, whether it was intentional or not, last night's incident, in the words of the International Atomic Energy Agency (IAEA), puts us in uncharted territory.

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Thankfully, last night's missile strike on the Zaporizhzhia nuclear plant missed anything that could have caused a release of radioactive material. But it was a close call.

If a missile had hit the reactor buildings themselves, the facilities that store spent nuclear fuel, or even the electricity supply to the plant, the situation could have been very different.

The six VVER model reactors at the Zaporizhzhia complex (all of Ukraine's 15 operational reactors are of this design) are inherently safer than the RBMK reactors at Chernobyl. It was one of these that melted down and caught fire in the 1986 disaster.

The fire in the reactor core burned for days, releasing huge amounts of radioactive material over the immediate area and a plume of shorter-lived radioactive elements across Europe.

'Worst case scenario could be more similar to Fukushima disaster'

Fortunately, that kind of scenario is unlikely now.

VVER reactor cores are contained within extremely strong steel pressure vessels, and those are contained within a concrete building typically built to withstand accidental impact - such as an aircraft flying into them.

If their cooling systems failed due to loss of power or damage to their structures, the worst case scenario could be more similar to what happened at the Fukushima nuclear complex in 2011, with an explosive failure of the reactor building releasing significant, but far less, radiation into the environment.

If a reactor was deliberately targeted, the outcome is less certain, but almost certainly more dangerous.

Nuclear sites 'must remain out of harm's way'

However, it would cause as much difficulty for the invading Russian forces as it would for Ukraine.

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."

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Today, in response to the incident, the IAEA made it clear, the situation in Ukraine cannot continue as it is, if the country's reactors are to remain safe.

"It is time for action," director-general Rafael Grossi said.

He said he has offered to personally visit Ukraine with a team of experts to work with both Ukrainians and Russians to ensure nuclear sites remain out of harm's way.