- Russian troops have attacked and claimed nuclear sites in Ukraine, sparking apocalyptic warnings.
- The chance of a disastrous meltdown is still low, but higher than normal, experts told Insider.
- A meltdown would be on the scale of a Fukushima accident, not a Chernobyl-like accident.
On March 4, the world woke up to shocking images: A fire had broken out on the ground of Zaporizhzhia nuclear power plant (NPP), the largest in Europe, after artillery hit a building 300 yards from the nuclear reactors.
Nuclear reactors are physically strong, with multiple technical failsafes, but none are designed to withstand shelling.
Russia took over Zaporizhzhia NPP that day, six days after seizing the decommissioned nuclear site at Chernobyl, the infamous home of the world's worst-ever nuclear disaster.
During the fighting at Zaporizhzhia, Ukraine's minister of foreign affairs wrote: "If it blows up, it will be 10 times larger than Chornobyl!"
It was one of a series of apocalyptic warnings from Ukrainian officials, heralding nuclear disaster should the war continue to be fought in and around its nuclear sites.
Insider asked four nuclear experts to assess the risk of a catastrophic event at Ukraine's Russia-occupied nuclear sites.
They said a meltdown is very unlikely, but that a full-scale war around nuclear power plants is uncharted territory, making it impossible to totally write off the risk of a disaster.
In case of a meltdown, "then you've got like a Fukushima on your hands," Edward Geist, a policy researcher at think tank the RAND Corporation, told Insider.
"Is that as bad as a Chernobyl? No. Is that still a massive accident with huge onsite consequences? Yes, potentially."
A meltdown would be unlikely in times of peace
For a meltdown to happen at a nuclear power plant — an event that has only happened three times in history — the cooling systems would need to stop working.
One of a power plant's most important roles is to make sure that the fuel in the reactor, which can run at temperatures up to 2,000 degrees Celsius (3,632 Fahrenheit), stays cool, said Lewis Blackburn, an expert in nuclear waste materials from the University of Sheffield.
There are multiple layers of security to prevent that from happening, he said. For instance, there are backup generators to keep the water cooling the reactors if a plant gets cut off from the national power grid, which indeed happened at Chernobyl on March 9.
The other pillar of safety is the expertly-trained staff, experts said.
The systems, their backups, and the staff would all need to fail at once for a meltdown to happen, a massively unlikely scenario in a time of peace.
But the invasion has put all of these systems under simultaneous pressure, ramping up the risk of a disaster.
"All bets are off" in times of war
"We have never seen the scale of military activity around reactors like what is happening in Ukraine," Jacob Hamblin, a nuclear historian at Oregon State University, told Insider in an email.
"It is an unprecedented situation that would be difficult for any design to accommodate."
Before 9/11, no one thought it necessary to safeguard plants against active conflict or terrorism, said Geist, who wrote a book about the 1986 Chernobyl accident.
The focus of plant design, he said, was isolating the nuclear activity from the outside world — known as containment — to keep the area safe.
There was little planning for what could happen if that containment were breached from outside.
At Zaporizhzhia and Chernobyl, there is safety-critical equipment that is vulnerable to attack, Geist said. He listed backup generators, power lines, power switches, and other equipment to powers emergency cooling.
Fighting can chip away at other layers of safety, too. Essential power lines to Chernobyl and Zaporizhzhia were cut, leaving the sites more vulnerable to a power cut.
The staff at Zaporizhzhia and Chernobyl have also been working under Russian control since the invasion of the sites in extremely stressful conditions that could jeopardize their ability to react, experts said.
Blackburn, the UK expert, stressed that each power plant needs its own, well-trained staff because every site has specific features and can't be run safely simply from plans or manuals.
"It's not like the Russians can just like find somebody from that reactor that's just gonna be able to come in and like operate these reactors and have everything be hunky-dory," he said.
Fighting might also make it extremely difficult for staff to navigate the sites and respond to any failures in the system. Reports indicate that unexploded ammunition was littering the Zaporizhzhia site as of March 13.
"I'd say that is a massive risk. When you're overworked, when you're not being fed properly, you're under constant pressure, the staff can't be reasonably expected to perform at their best," Blackburn continued.
When the fire broke out at Zaporizhzhia, things could have "turned out a lot worse" if the staff had not been able to intervene, Edwin Lyman, director of nuclear power safety at the Union of Concerned Scientists, told Insider.
"You could have a propagating fire along electrical cables that could have caused a blackout," he said.
"Then all bets are off."
At Chernobyl, experts said, there's less of a concern of meltdown. Though some activity has been detected in one of the reactors recently, the last of the reactors there was shut down more than 20 years ago, so the overwhelming majority of the fuel is now cold. The exclusion zone around the plant is also pretty much uninhabited.
At Zaporizhzhia NPP, there's more risk, because the power plant is still active so there is a lot more for the staff to manage. Around 1.6 million people are live in the wider Zaporizhzhia region.
"There's a lot more possibility for some sort of serious screw up there," said Geist, the RAND expert.
Memories of Fukushima
Should a meltdown happen at Zaporizhzhia, it would be more on the scale of the 2011 Fukushima Daichi accident than a 1986 Chernobyl accident, the experts told Insider.
Unlike at Chernobyl, where the accident led to the core being exposed to the outside world, the nuclear core of Zaporizhzhia NPP is contained in a tough shell that should keep most of the radiation inside the plant, experts said.
That is unless it is severely damaged. While strong, the core hasn't been designed to withstand impact from an external explosive, said Geist.
Even without that, some radiation could escape the containment and threaten the health of the staff and local population, per the experts. A meltdown could also create a "horrible mess in the basement of the reactor," Blackburn said.
The material left behind by a meltdown, called corium, is "horrendously radioactive," he said. Clearing the corium represents a "huge engineer and scientific challenge," and is incredibly costly — work at Chernobyl has already cost about $235 billion, according to one estimate.
A meltdown might be less dangerous than a swift blast to the spent fuel
Blackburn was more concerned with another point of vulnerability: the spent fuel. This is the radioactive fuel that has been removed from the reactor and is first cooled down in ponds, then stored in dry casks once it is cold, Blackburn said.
If a rogue missile hit stores or cooling ponds at Zaporizhzhia NPP or the cooling ponds at Chernobyl NPP, "that would mean that radioactivity material could be transferred to the immediate atmosphere, which could then spread," Blackburn said.
This would be similar to what happened during the 1986 Chernobyl accident: a plume of steam picked up radioactive elements that ended up in the atmosphere. They were then carried by the wind for vast distances, potentially exposing tens of thousands of people to extra radiation. This effect, though, is difficult to measure and its extent and severity has remained a matter of debate.
Blackburn said the plume from a missile hitting spent fuel would be smaller than the one in the 1986 Chernobyl accident and therefore would likely travel less far. But lots of people could still be exposed, he said.
One of the most severe warnings about the attacks came from President Volodymyr Zelenskyy, who predicted "the end of Europe" if a nuclear accident occurred.
In response, Blackburn said: "Yeah, that is a bit sensationalist,"
"But it's not incorrect to say that if a bomb hit Chernobyl, it would be extremely bad."
Russia has nothing to gain from blowing up the sites, but accidents could happen
Blackburn said that he could hardly believe the imagery of nuclear sites being invaded in Ukraine. "That is definitely something that is in the remit of action films" rather than reality, he said.
Using any type of explosive near a reactor is "absolutely shocking. Really quite an astounding display of recklessness. That is a really stupid thing to do," he said.
Geist said he was "puzzled" that the Russian troops would risk using artillery near an active power plant. "It just seems so crazy," he said.
Both Geist and Blackburn pointed out that the Russians have nothing to gain from a nuclear accident.
Blackburn, speaking from the UK, said he felt little risk for his personal safety: "I don't think anybody outside of the people directly affected really should have any worries."
But Blackburn said he did not want to understate the risk of active fighting taking place near nuclear sites. Another nuclear disaster "at this point in history would be unfathomably bad," he said.
"So much about this invasion is unprecedented, at least in our living memory. And this is just another example of that," Geist said.
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