Ukraine's Zaporizhzhia nuclear power station — the largest of its kind in Europe — has come under fire in the war in Ukraine and has been seized by Russian forces.
A fire broke out in a building at the plant on Thursday night (local time) as Ukrainian defenders and Russian troops fought for control of the area.
Local mayor Dmytro Orlov said the fire was caused by heavy Russian shelling and called it a threat to world security.
The Ukrainian military later said that the fire had damaged part of a reactor building but that the plant was still operating safely after being captured by Russian soldiers.
Nuclear monitors said there was no change to radiation levels.
Earlier, Ukraine's President warned the fighting around the plant could cause a possible nuclear disaster that could affect large parts of Europe.
Here's what we know about the Zaporizhzhia nuclear power plant, and what could happen next.
What is at Zaporizhzhia?
Zaporizhzhia is the biggest nuclear power plant in Europe and accounts for one-quarter of Ukraine's power generation.
The power plant has six Russian-designed reactors. It was first commissioned in 1984, two years before the Chernobyl disaster, and has since been expanded and upgraded.
The plant is situated in Enerhodar, a city on the banks of the Dnieper River, a few hundred kilometres from the Black Sea in south-eastern Ukraine.
Russia has already captured the defunct Chernobyl plant, about 100 kilometres north of Kyiv, which spewed radioactive waste over much of Europe when it melted down.
The Zaporizhzhia reactors are of a different and safer type, analysts said.
What's the latest?
Russian troops have been attacking Enerhodar as part of a drive to cut the rest of Ukraine off from the sea.
Local emergency services said Russian shelling caused a fire in one of the plant's training buildings, but said they extinguished it on Friday morning.
Soon after the fire was put out the regional military administration said Russian forces had taken control of the site.
The military released a statement saying Russian attacks had caused some damage to the compartment of the plant's number 1 reactor, but had not affected its safety.
US Energy Secretary Jennifer Granholm said Zaporizhzhia's reactors were "protected by robust containment structures" and being "safely shut down".
Background radiation levels remained unchanged at the plant, Russia's RIA news agency cited a plant spokesman as saying.
What's been the response so far?
While the fighting raged, Ukrainian Foreign Minister Dmytro Kuleba called on Russia to end its attacks near the power station.
He warned that if the plant blew up, the effect would be "10 times larger than Chernobyl".
In an emotional speech in the middle of the night, Ukrainian President Volodymyr Zelenskyy said he feared an explosion would be "the end for everyone. The end for Europe. The evacuation of Europe".
"Only urgent action by Europe can stop the Russian troops," he said.
"Do not allow the death of Europe from a catastrophe at a nuclear power station."
UN nuclear watchdog chief Rafael Grossi also called on Russian and Ukrainian troops not to fight in the area.
"Director-general Grossi appealed for an immediate halt to the use of force at Enerhodar and called on the military forces operating there to refrain from violence near the nuclear power plant," the International Atomic Energy Agency said in a statement.
The American Nuclear Society backed up reports from a White House official saying radiation levels remained within natural background levels.
"The real threat to Ukrainian lives continues to be the violent invasion and bombing of their country," the group said in a statement.
US President Joe Biden also urged Russia to cease its military activities in the area and allow firefighters and emergency responders to access the site.
What is the danger of a meltdown?
The attack intensified fears about waging a modern war near nuclear reactors.
Nuclear expert Maria Rost Rublee told the ABC that problems would start if any of the plant's six reactors lost their connection to the external grid.
If the Zaporizhzhia reactor lost connection to the external grid and diesel back-up generators failed, the ensuing nuclear disaster could potentially be worse than Chernobyl, Dr Rublee said.
"This is quite dangerous. It's exactly what we were warning about. This is the first time we've had active war carried out among active nuclear power plants.
"The real worry here is that firefighters and plant operators, because of the active war going on, won't be able to get to the plant to take care of it the way it needs to happen."
University of Sydney expert David Fletcher said the plant's cooling systems would be the focus.
"At present it seems as though it is only ancillary buildings that have been damaged by Russian missiles," he said.
"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. It was this type of damage that led to the Fukushima accident."
What could happen if a reactor is seriously damaged?
Dr Rublee said any potential meltdown would have massive implications not just for Ukraine, but for the rest of Europe, including Russia, and even Australia.
"This is nuclear radiation being spread all across the region," she said.
"It also affects Australia, because nuclear radiation cannot be contained by space and so it won't be as bad as it will be for neighbouring regions, but that goes into the atmosphere and that radiation affects all of us."
