The announcement on Wednesday that electricity had been cut to Ukraine's Chernobyl nuclear plant has revived concern over the effect Russia's invasion may have on it's neighbour's nuclear installations.

But experts have cautioned that for now the lack of electricity at Chernobyl -- the site of the world's worst nuclear disaster -- does not pose a major security risk at the plant itself.

But last week's shelling at Zaporizhzhia nuclear power plant -- Europe's largest -- was a reminder that the stakes could get much higher if one of Ukraine's four active nuclear installations is hit.

- What happened? -

On February 24, Russia invaded Ukraine and seized the defunct Chernobyl plant, site of the world's worst civil nuclear disaster in 1986, which killed hundreds and spread radioactive contamination west across Europe.

According to a statement from Ukraine's energy operator Ukrenergo on Wednesday, "because of military actions of Russian occupiers", the plant at Chernobyl "was fully disconnected from the power grid".

Ukrenergo added that military operations meant "there is no possibility to restore the lines".

The country's nuclear inspection body SNRIU said "emergency diesel generators have been activated in order to power important security systems" and should be able to last for 48 hours.

However, after this period "cooling systems of the storage facility for spent nuclear fuel will stop", Ukrainian Foreign Minister Dmytro Kuleba said in a tweet.

The Chernobyl site comprises four decommissioned reactors -- including one encased in a giant sarcophagus -- as well as stores of nuclear waste.

More than 200 technical staff and guards remain trapped at the site, now working under Russian guard.

Telephone communications with the site are no longer possible.

The UN atomic watchdog, the International Atomic Energy Agency (IAEA) has called on Russia to make sure that staff are able to rest and rotate through shifts as normal.

- What is the risk? -

The reactor involved in the accident doesn't itself pose a problem, Karine Herviou, deputy director general of France's nuclear safety institute IRSN told AFP.

"That reactor core does not need a cooling system," she explained.

But what of the 20,000 rods still in the plant's spent fuel pool?

Given the time that has passed since the accident in 1986, the heat load and the volume of the cooling pools are together "sufficient for effective heat removal without need for electrical supply", says the IAEA.

The agency said on Wednesday it saw "no critical impact on safety" at Chernobyl thus far.

Even if electricity isn't re-established after 48 hours, Herviou says that "from what we know of the installations there isn't a danger of radioactive emissions".

In such a scenario, she points to studies done after the 2011 Fukushima accident in Japan which indicate there would be "a slow rise in temperature to around 60 degrees Celsius but no uncovering of the fuel rods".

"The water will not be brought to boiling point", she says.

- What about the active plants? -

A power outage would "cause more problems" at Ukraine's four active power plants, says Herviou.

"There you would absolutely have to ensure the cooling of the fuel in the reactor core or in the cooling pools," she says.

"The heat that needs to be removed is much greater" than in Chernobyl, she explains.

Generators could support the systems for seven to 10 days.

But beyond that a lack of electricity would risk "a Fukushima-type scenario with a meltdown in the core reactor".

Since the beginning of the Russian invasion IAEA Director General Rafael Grossi has called for the utmost caution around Ukraine's nuclear sites.

He has offered to go to Ukraine in order to negotiate with both sides on a way of securing the safety of the country's nuclear installations.

"This time, if there is a nuclear accident, the cause will not be a tsunami brought on by Mother Nature," he told an IAEA meeting earlier this week, alluding to the cause of the Fukushima accident.

"Instead, it will be the result of human failure to act when we knew we could, and we knew we should."

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