Radioactive substances could be released from Ukraine's Chernobyl nuclear power plant because it cannot cool spent nuclear fuel after its power connection was severed, the country's state-run nuclear company Energoatom said on Wednesday.
Russian forces have captured the plant and Energoatom said fighting had made it impossible to immediately repair the power line to the plant.
It said a radioactive cloud could be carried by wind to other regions of Ukraine, Belarus, Russia and Europe.
Nuclear reactors use fuel rods. These are made from uranium, which is processed into ceramic pellets and stacked together into sealed metal tubes.
When they’ve made their energy and can no longer keep the chain reaction going which is needed to produce energy, fuel rods are removed and put in deep ponds of water for cooling. This requires constant cooling because the spent rods still produce heat through radioactive decay.
The pumps pushing cool water over the rods need power - and this is what has been severed.
“With no power supply, this water could slowly evaporate, potentially resulting in contamination of the building by low levels of radioactive isotopes,” Professor Claire Corkhill, Chair in Nuclear Material Degradation at the University of Sheffield, says.
Why has Russia invaded Ukraine and can Putin be stopped? Listen to the ITV News 'What You Need To Know' podcast
Energoatom said there were about 20,000 spent fuel assemblies
Critically, how much heat a fuel rod continues to put out once it’s spent depends on how much energy it actually used in the reactor and how long ago it left the reactor.
And this is what matters with this situation now - and what some scientists say makes it different to Fukushima, which also had an on-site power failure.
In Fukushima the rods had been used not that long before the incident, so were still quite “hot”.
The last nuclear reactor unit at Chernobyl was shut down over 20 years ago.
And because of this, some scientists say that while concerning, the main threat is to people working inside the plant and maybe the local area - not the rest of Europe.
The last reading from Chernobyl shows no "worrying levels", says Europe Editor James Mates
According to Mark Wenman, a nuclear expert at Imperial College London, this means the heat produced by the fuel in the storage ponds will have substantially reduced over the 20 to 30 year period.
“The fuel storage ponds are also very deep and would likely take weeks for the water to boil down even without cooling pumps active. This should hopefully allow enough time for the power to cooling systems to be restored,” he said.
Dr Mark Foreman, a nuclear chemist at Chalmers University of Technology, Sweden said even if the cooling ponds dry out the consequences will “be far less than either the Chernobyl accident of 1987 or the more recent Fukushima accident”.
“It is also important to note that drying out of the ponds will not cause a nuclear reaction or explosion to occur,’ he says.
But for those working at the plant there are risks.
Without power the ventilation systems at the plant would also not be working, exposing staff to dangerous doses of radiation, Energoatom said.
Dr Foreman agrees that a loss of ventilation will make it difficult to manage radioactive dust.
“I strongly suspect that conditions for the workers will get worse.
"It may become much harder for workers to enter some parts of the site without full protective clothing, they may also have greater difficulty in changing in and out of their protective clothing,” he says.
On Tuesday, the UN nuclear watchdog, the International Atomic Energy Agency (IAEA), said they were no longer receiving data from the safeguarding systems monitoring nuclear material at the radioactive waste facilities at the plant.
These systems keep track of nuclear material.
“It is also concerning that communications with the IAEA to the plant are being lost so it will be far more difficult to get up to date live information on the ongoing situation,” Mark Wenman says.