Earlier in the day, Russian forces attacked Ukraine’s Zaporizhzhia nuclear power plant near Enerhodar in the southeast of the country. Rafael Mariano Grossi, director general of the International Atomic Energy Agency (IAEA), reported that a building at the site was hit and caught fire. The fire was extinguished without causing any direct damage to the reactors and detectors continue to report normal radiation levels, Grossi told a news conference at IAEA headquarters in Vienna.
But the incident highlighted the possible dangers of a war over nuclear sites, say Grossi and others. Ukraine has 15 operating nuclear reactors, which usually produce about half of its electricity. With six reactors and a capacity of almost 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.
Nuclear experts have given Nature their assessments of possible hazards.
Damage from accidental blow is unlikely
Zaporizhzhia reactors are of modern design. Unlike the Chernobyl reactor, each is enclosed in a steel pressure vessel, which in turn is housed inside a massive reinforced concrete containment structure. (The design is called VVER – the Russian acronym for Water-Water Energy Reactor).
The plants also have several backup safety systems, says Michael Bluck, director of the Center for Nuclear Engineering at Imperial College London. He says it would be very alarming if Russian forces deliberately attempted to breach the containment structure, but catastrophic damage from an accidental hit is unlikely. “If a missile goes astray, I worry less [that]. They are very sturdy structures,” he says.
Koji Okamoto, a nuclear security researcher at the University of Tokyo, agrees. “The containment structure can have 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 hazardous material it contains is in the basement of the reactor building, protected by the remains of that building and the many tons of concrete that have been poured over it,” Rofer wrote in a blog post. The ruins of the reactor that exploded in 1986 are encased in a huge steel and concrete shell 63 meters high called the sarcophagus. “I suspect that a direct artillery hit could pierce the shell and allow a small amount of radioactive contaminants to escape, but this solid mass of molten fuel elements, containing uranium and plutonium, is unreachable,” Rofer wrote.
Spent fuel ponds pose a hazard
Most nuclear power plants – including those in Zaporizhzhia – have water ponds where spent fuel rods are kept while they are cooled. Damage to any of these pools, whether accidental or intentional, could cause the water to leak or boil. The rods would then overheat and start a fire, several observers warned.
Although not comparable to the Chernobyl disaster in 1986, such a fire could be dangerous for people near the plant, and even for those further away. “Russia should keep in mind that the prevailing winds are towards Russia,” Rofer said. Nature.
One mitigating factor is that fuel rods that have been in the pool for several weeks or months are less dangerous than they were when they started because the main carcinogenic isotope, iodine-131, quickly decays, points out Bluck. During the press conference, Grossi said that no problems had been reported with the Zaporizhzhia spent fuel ponds.
External power and cooling must be maintained
As of March 4, five of Zaporizhzhia’s six reactors had been shut down, Grossi reported. But even when shut down, a reactor still loaded with fuel is not completely risk-free. Under normal operating conditions, uranium nuclei in fuel rods fission or disintegrate, leaving behind nuclei of lighter elements. These isotopes build up over the life of the rods and many of them are highly radioactive, which continues to generate heat even after shutdown.
This means that the core of a reactor that has just been shut down must be actively cooled, which requires energy, normally taken from the network, to maintain the circulation of water around the core. “You have to remove the decay heat,” says Bluck. “If you don’t cool it down until it’s gone, the core will overheat.” If the active cooling of the reactors suddenly stops, power plants like Zaporizhzhia could face a scenario similar to that of the Fukushima Daiichi nuclear power plant in Japan, when the electricity was cut off following the earthquake and the Tōhoku tsunami of March 11, 2011, and three melted reactors.
A similar event could occur in the event of damage to systems – including pumps, heat exchangers and standby diesel generators – which provide active cooling and are outside the protective containment structure of a reactor, explains Okamoto. “Any nuclear reactor could be damaged in the event of a loss of coolants.” There are safety systems in place at Ukrainian power plants that make the reactors more resilient to such damage. “The VVER-1000 has several alternative cooling systems,” he says. “So it won’t be immediately dangerous.” However, Okamoto points out that active cooling may be required for more than ten days after a reactor shutdown. It’s unclear whether backup cooling systems could last that long.
Several experts say Nature that even if a reactor core were to melt down, it might not cause a major release of radioactive material. The main impact of such a crisis could be related to psychology and how people – including politicians and decision-makers – react. Many Europeans still remember when the Chernobyl radioactive cloud spread across the continent. “People don’t judge radiation risk well and are often more scared than they should be,” Rofer says.