The Zaporizhia Nuclear Power Plant

Ukraine’s Zaporizhzhia Nuclear Power Plant, which has been attacked and seized by Russian forces.Credit: Dmytro Smolyenko/Ukrinform/ZUMA

On 4 March, Russian forces attacked Ukraine’s Zaporizhzhia Nuclear Power Plant near Enerhodar, in the country’s southeast. Rafael Mariano Grossi, the director-general of the International Atomic Energy Agency (IAEA), reported that a building on the site had been hit by a missile and had caught fire. The fire was put out without having caused any direct harm to the reactors, and detectors continue to report normal radiation levels, he said in a press conference at the IAEA’s headquarters in Vienna.

But the incident has highlighted the possible dangers of warfare around nuclear sites, he and others say. Ukraine has 15 working nuclear reactors, which ordinarily produce roughly half of the country’s electricity. With 6 reactors and a capacity of nearly 6 gigawatts, Zaporizhzhia is the largest nuclear power plant in Europe. It is now occupied by Russian forces, as is Chernobyl — the site of the worst nuclear accident in history, which took place in 1986.

Nuclear experts gave Nature their assessments of the possible dangers.

Damage from an accidental hit is unlikely

The reactors at Zaporizhzhia have a modern design. Unlike the Chernobyl reactor, each is enclosed in a pressurized steel vessel, which in turn is housed inside a massive reinforced-concrete containment structure. (The design is called VVER — the Russian acronym for water–water energetic reactor.)

The plants also have multiple safety back-up systems, says Michael Bluck, director of the Centre for Nuclear Engineering at Imperial College London. He says that it would be very alarming if the Russian forces were deliberately trying to breach the containment structure, but that catastrophic damage from an accidental hit is unlikely. “If a missile goes astray, I’m less worried about [that]. These are very robust structures,” he says.

Koji Okamoto, a nuclear-safety researcher at the University of Tokyo, agrees. “The containment structure may have a resistance to normal weapons,” he says.

And even at the site of the Chernobyl disaster, the risk of accidental radioactive releases is limited, according to Cheryl Rofer, a retired nuclear scientist based in Santa Fe, New Mexico. “The dangerous material in it is in the basement of the reactor building, protected by the remains of that building and many tons of concrete that have been poured over it,” she wrote in a post on her blog. The ruins of the reactor that exploded in 1986 are enclosed in a massive 63-metre-tall steel and concrete shell called the sarcophagus. “I suspect that a direct artillery hit could breach the shell and allow a small amount of radioactive contaminants to escape, but that solid mass of melted fuel elements, containing uranium and plutonium, is inaccessible,” she wrote.

A screen grab showing a view of Zaporizhzhia nuclear power plant during a fire

A screenshot from footage showing a fire at the Zaporizhzhia plant on 4 March.Credit: Zaporizhzhia Nuclear Power Plant/Anadolu Agency/Getty

Spent-fuel ponds are a hazard

Most nuclear power plants — including that at Zaporizhzhia — have pools of water where spent fuel rods are kept as they cool down. Damage to one of these pools, whether accidental or intentional, could cause the water to leak out or boil off. The rods would then overheat and start a fire, several scientists have warned.

Although not comparable to the Chernobyl disaster, such a fire could be hazardous to people in the vicinity of the plant, and even to those further afield. “Russia needs to keep in mind that the prevailing winds are towards Russia,” Rofer tells Nature.

One mitigating factor is that any fuel rods that have been in the pool for several weeks or months are less dangerous than they were at the beginning, because the main cancer-causing isotope — iodine-131 — decays quickly, Bluck points out. At the IAEA press conference, Grossi said that no issues had been reported with Zaporizhzhia’s spent-fuel ponds.

Outside power and cooling must be maintained

As of 4 March, five of Zaporizhzhia’s six reactors had been shut down, Grossi reported. But even powered down, a reactor that’s still loaded with fuel is not completely without risk. Under normal operations, uranium nuclei in the fuel rods undergo fission, or break up, leaving behind nuclei of lighter elements. These isotopes accumulate during the lifetime of the rods, and many of them are highly radioactive, continuing to produce heat even after the reactor is shut down.

This means that the core of a reactor that has just been shut down must be actively cooled, which requires power, normally taken from the grid, to keep water circulating around the core. “You have to remove the decay heat,” Bluck says. “If you don’t cool it until it’s gone, then the core will overheat.” If the reactors’ active cooling suddenly stopped, plants such as Zaporizhzhia could face a scenario analogous to what happened at the Fukushima Daiichi Nuclear Power Plant in Japan, when power was cut off in the aftermath of the 11 March 2011 Tōhoku earthquake and tsunami, and three reactors melted down.

A similar event could occur if there is damage to the systems — including pumps, heat exchangers and back-up diesel generators — that provide active cooling and are outside a reactor’s protective containment structure, says Okamoto. “Any nuclear reactor could be damaged when coolants are lost.” There are safety systems in place at Ukrainian plants that make the reactors resilient to this damage. “VVER-1000 has several alternative cooling systems,” he says. “So it will not immediately be dangerous.” However, he points out that active cooling might be required for more than ten days after a reactor is shut down. It is unclear whether back-up cooling systems would be able to last that long.

Several specialists told Nature that even if a reactor core were to melt down, it might not cause a large release of radioactive materials. The main impact of such a crisis could be related to psychology and how people — including politicians and policymakers — react. Many Europeans still remember the days when Chernobyl’s radioactive cloud spread over the continent. “People do not judge the risk of radiation well, and they are much more frightened, frequently, than they need to be,” Rofer says.