What are the safety risks at Ukraine's nuclear power plants?

But the Russians have now captured Zaporizhzhia in Ukraine, and the rulebook is changing.

A week prior to that, the invading army had also captured the area around the decommissioned Chernobyl nuclear power plant.

The capture of both plants mean we're in uncharted territory. What are the risks, and what are experts concerned about?

Russian forces captured Chernobyl on February 24. Heavy Russian machinery in the Chernobyl exclusion zone has kicked up some radioactive dust.

But the site is being monitored by the International Atomic Energy Agency, and there isn't yet cause for alarm.

"The radiation levels aren't much above background; I don't think it's a big problem," says Tony Irwin, technical director of SMR Nuclear Technology and chair of Engineers Australia's Sydney division nuclear engineering panel.

Of more concern is what will happen if the Russians continue to limit movement in the area. At the moment, several hundred Ukrainian workers and guards are trapped at the site, unable to be relieved.

"The annual fire season's approaching," says Irwin.

"If the fire brigades were prevented from getting to bushfires, that would again cause a radioactive hazard. It's not going to be a major hazard, but it's all undesirable."

Irwin says that even if power were cut off from Chernobyl for a long time, the remaining fuel is so old that it no longer presents a significant danger.

According to the World Nuclear Association, any radioactive leaks would be local, and wouldn't spread to other parts of Europe.

But the inability of staff to leave and enter the site means that ongoing maintenance and safety work can't be done.

"Of course there's no routine maintenance," says Irwin.

"There's no progress with using the New Safe Confinement, which is over the damaged reactor. Testing of that was completed in August 2021. They're expected to start dismantling the reactor and working on it. And of course, that's all stopped at the moment."

So the invasion isn't presenting any major radiation threats at Chernobyl, but it is delaying the ultimate long-term storage of the plant's No. 4 reactor, which exploded in 1986 in what's considered history's worst nuclear-power accident in terms of casualties and cost - estimated at US$68 billion up to 2019.

Zaporizhzhia, the largest nuclear power plant in Europe, was seized on March 4.

A fire ignited by Russian shells broke out in a training building on the site, but it didn't damage any key equipment and was extinguished. (For more on the fire and what nuclear power plants are prepared for, read this article.)

Russian forces are now controlling the plant, but allowing Ukrainian staff on and off the site to operate it.

"That's a very big power plant - it was pretty clear there that they were just trying to seize it [and] didn't want to damage it," says Associate Professor Nigel Marks, from Curtin University's School of Electrical Engineering, Computing & Mathematical Sciences.

He adds that Russia has "really crossed a line" in this capture.

"It's never happened before, that the military has surrounded and taken over an operating nuclear power plant. That's a remarkable thing to do."

While the plant's capture is an unprecedented move, it's probably not un-premeditated. The plant was initially built in the 1980s under the purview of the Soviet Union, of which Ukraine was then a member-state, and there are similarly designed plants in Russia and the rest of Eastern Europe.

"It is their design, and they would have known exactly what they were doing," says Marks.

"It's not as though they're coming into an area where they can't read the knobs on the dials. And for sure, this has been planned for a long time.

"They would have known precisely what they're looking to do and how to how to do it."

Marks also says that there's nothing unique to Zaporizhzhia's design that might flummox unfamiliar management.

"With these big plants, there's immense standardisation," he says.

"It's the only way you get economies of scale. So it's not as though each one has its own custom build or anything crazy like that."

Irwin doesn't think Russians will be doing anything novel with Zaporizhzhia.

"All they're really going to do is say when it's operating and when it's shut down," he says.

This is, of course, very damaging to Ukraine as well - it's limiting the country's electricity supply.

This was likely Russia's logic in seizing the plant. Ukraine isn't currently connected to any other power grids in Europe.

"They can switch cities on and off, they can squeeze cities," says Marks.

"It's not particularly warm there at the moment and people need electricity for heating, so you can drive the civilians out of the cities."

While not impossible, it's unlikely that Russians in charge of the plant will make mistakes with nuclear-contamination results. Zaporizhzhia's capture presents many more pressing threats to Ukraine than that of radioactivity.

What about nuclear risks as the war goes on?

As the invasion continues, it's possible there will be more fighting around Chernobyl, Zaporizhzhia, or other nuclear power plants.

The consequences of breaches at these plants would be dire - but the chance of this happening is very low.

As seen with the fire at Zaporizhzhia, all nuclear power plants are built with multiple redundancies, serious fire-proofing, and extra electricity supplies.

None of these things eliminate the risk of a disaster, but they do make it very unlikely.

"Having been a reactor operations engineer for many years in the UK, I feel really sorry for the people trying to operate the plants safely," says Irwin.

"They're under a lot of pressure."

"Obviously you want a reactor to be operated in a very calm environment," says Marks.

"The idea that there's gunfire around the plant or that there's a fire from something, even if it's in a training building, it's incredibly disruptive to everyone's mindset [...] It's a crazy, crazy thing to have happened.

"I think it was always extremely unlikely that anything terrible would happen, but at the same time, highly concerning."

Marks says that even if there were a nuclear disaster, it would be unlikely to spread beyond Ukraine.

"It's not good, but compared to all the other things that are happening in Ukraine, it's pretty modest."

The Internationl AEA's Director General, Rafael Mariano Grossi, has outlined "seven pillars" of nuclear safety and security in response to this crisis:

Grossi said that several of these pillars had been put at risk by Russia's capture of Zaporizhzhia.

"It is high time to stop an armed conflict from putting nuclear facilities at severe risk, potentially endangering the safety of people and the environment in Ukraine and beyond," he said.