As calls continue for an end to military activity around Ukraine's Zaporizhzhia power plant, experts are warning there is significant risk of a nuclear accident.
This week the Russian military, which has controlled the facility since March, agreed to a safety inspection by experts from the United Nations' International Atomic Energy Agency (IAEA), who arrived overnight.
Despite this, the director of Nuclear Power Safety at the Union of Concerned Scientists, Edwin Lyman, said there was a significant possibility the situation could end badly.
"It's probably a coin toss at this point," he said.
While the fate of Europe's biggest nuclear power plant has been thrust into the spotlight in recent weeks, Dr Lyman told ABC News Daily he became concerned the minute Russia set its sights on the facility in early March.
"When Russia started lobbing artillery shells at the plant and when a fire broke out, it was of extreme concern because one thing the nuclear power plant doesn't handle too well is a large fire," he said.
The fire was quickly contained, but as Russian forces took control of the plant, safety concerns only continued to grow.
Since then, there have been reports around 9,000 of the plant's staff have been forced to continue working at gunpoint, and that some have been beaten and tortured.
"There is evidence that the Russians were intimidating the staff, not allowing them to report safety issues, accusing them of being spies or saboteurs and of physical abuse," Dr Lyman said.
"These are obviously very poor conditions for the staff to work in."
Plant under attack
In the past fortnight there have been further reports of shelling of the plant, with both sides claiming the other was at fault.
Ukraine has accused Russia of using the plant as a military base to launch attacks against Ukrainian positions.
Meanwhile, Russian Defence Ministry spokesman Igor Konashenkov said this week that nine shells fired by the Ukrainian artillery in two separate attacks had landed in the nuclear plant's grounds.
While Dr Lyman doesn't believe these kinds of attacks are likely to cause a major problem for the reactors themselves, he said there is still a risk they could damage other vital parts of the plant or make it difficult to maintain the reactors.
"The most dangerous parts of the plant, like the nuclear fuel in the reactors, is contained and under a fairly strong reinforced concrete containment building," he said.
"Even if you had direct artillery fire on the containment, unless it was a sustained shelling, deliberately trying to destroy it, then it probably wouldn't cause that much damage."
However, Dr Lyman warned other parts of the plant were more susceptible to artillery fire.
"The turbine that's used to convert the hot water or the steam that's generated by the nuclear reactor into electricity are in less-protected buildings," he said.
A power plant in need of power
A greater concern than artillery fire, experts believe, is the potential for the plant to lose its offsite power connection, something that has already happened twice in the past few weeks.
While it might seem strange that a power plant's most vital input is electricity, external power is crucial in cooling the reactors to prevent them from overheating.
To reduce risk of meltdown, four of the plant's six reactors have already been put into cold shutdown since the outbreak of the war.
But because the plant is responsible for around 20 per cent of Ukraine's energy supply, shutting the remaining reactors would be a significant loss for the country.
The plant does have three external electricity supply lines, but these have all lost connection in recent weeks due to the conflict.
Last week, the company responsible for the plant, Energoatom, said fires at a nearby thermal power station had caused the nuclear plant's last remaining electricity power line to be disconnected twice.
Ukrainian President Volodymyr Zelenskyy said the plant's 20 backup diesel generators had to be "immediately activated" to avert a "radiation disaster".
"If the diesel generators hadn't turned on, if the automation and our staff of the plant had not reacted after the blackout, then we would already be forced to overcome the consequences of the radiation accident," Mr Zelenskyy said in his nightly briefing.
Dr Lyman said the fact that the site has already lost offsite power showed how precarious the situation was.
"If you lose both the offsite power and the backup diesel generators, there are other emergency measures that could be employed, but you only have a few hours to be able to set those up before the core might start to melt," he said.
Meltdown could happen in hours
One simulation of the reactors losing power showed they would have just over an hour before the cooling systems stopped working.
It predicted that the reactor would heat up so quickly that it would take less than five hours for it to break through the reactor vessel.
Even if that occurs, experts say a strong protective casing around the reactors means a Chernobyl-style disaster isn't likely.
ANU's Associate Professor and lecturer in Nuclear physics, Tony Irwin, spent time working with Russian scientists in the wake of the Chernobyl meltdown.
He said key design differences make the Zaporizhzhia plant less prone to a radiation leak, even in the case of a meltdown.
"I think it's fairly resistant to a major leak, so it's unlikely to be a Chernobyl level incident and certainly not a Fukushima level incident," Dr Irwin said.
If there is a meltdown, he said a more likely situation would be similar to the Three Mile Island incident in the US state of Pennsylvania in 1979 in which most of the radioactive material was contained inside the site.
Dr Lyman agreed the reactor's protective casing was likely to prevent a Chernobyl-style radiation leak.
"It's more likely that there would be a slower release and probably a smaller release, at least in the early stages, and therefore the consequences might be contained more to the local areas in Ukraine, maybe dozens of kilometres."
But he stressed there was no guarantee the impacts of a radiation leak would not be felt beyond the region.
"There's always some radiation that's going to spread further, and it's really unpredictable," he said.
"It depends on how much damage there is, the nature of the accident and the weather conditions."
A key issue worrying experts at the moment is whether the additional stress of staff at the plant would affect their ability to respond quickly in an emergency.
"It's a completely unacceptable situation for staff," Dr Irwin said.
"It's a terrible situation where the operators are running the plant with the Russians ordering them what to do."
Dr Lyman said that the stress the workers were under heightened the risk.
"If you have workers who are under stress, whose families may be threatened, who are feeling like they're in danger, then they may not be able to perform," he said.
"All these factors increase the risk that there could be damage to the plant that would potentially lead to a meltdown of one or more of the reactors and the spent nuclear fuel that's stored there."
Ukraine prepares for radiation leaks
The Ukrainian government has begun preparations for the possibility of a radiation leak.
In recent weeks it has run emergency drills in nearby towns and distributed iodine tablets to residents.
Iodine helps prevent radiation from amassing in the thyroid, leading to thyroid cancer; a phenomena witnessed after the Chernobyl meltdown in hundreds of Ukrainian children.
While Dr Lyman believes it is a sensible precaution, he warned it would not be enough to protect people in the case of a leak.
"In nuclear reactors, you have a sea of a soup of hundreds of different types of radioactive isotopes, all of which interact in different ways of the body," he said.
"So you can't do much about that except to either evacuate to avoid exposure or to shelter for a long time in a structure that's shielded against radiation.
"That's why the best thing is to prevent any release in the first place."
Overnight inspectors from the IAEA travelled to the city of Zaporizhzhia.
Experts from the team will remain on site to provide an impartial, neutral and technically sound assessment of the situation.
"I worried, I worry and I will continue to be worried about the plant until we have a situation which is more stable, which is more predictable," IAEA head Rafael Grossi, who personally led the mission, told reporters after returning to Ukrainian-held territory.
Dr Irwin said the best chance of getting the situation under control was for the IAEA to convince Russia to return control of the plant to Ukraine and put in place a demilitarised zone around the facility.
But he was not optimistic that was something Russia will agree to.
"Russia would very much like that nuclear power plant because it's the largest one in Europe and they could use it to power Russia," he said.
"They've got a big incentive not to give up the plant, but there's a lot of international pressure saying this current situation is completely unacceptable, so it could happen."
