Ukrainian nuclear plant set on fire. Should we worry about another Chernobyl?

Russian troops have seized Ukraine’s Zaporizhzhia nuclear power plant after overnight shelling sparked a fire near one of the facility’s reactors, which burned for several hours.

World leaders have condemned the “reckless” attack on the plant, the largest nuclear facility by capacity in Europe, after a projectile struck an auxiliary training building just 490 feet (150 meters) from one of the plant’s reactor units, starting a fire that blazed from early morning until around 6:20 a.m. local time Friday (March 4).

Firefighters successfully put out the fire, with no damage reported to reactors or safety systems, and radiation hasn’t exceeded baseline levels, according to the International Atomic Energy Agency. Employees have continued to work on the site after its capture by Russian forces, the Ukrainian state inspectorate for nuclear regulation said in a statement. Ukraine’s nuclear agency said that three soldiers had been killed and two wounded by the attack.

Related: 5 weird things you didn’t know about Chernobyl

“The plant’s staff continues to work on power units, ensuring the stable operation of nuclear facilities,” Energoatom, Ukraine’s nuclear power operator, wrote on Telegram. “Unfortunately, there are dead and wounded among the Ukrainian defenders of the station.”

The Zaporizhzhia plant, which is situated in southeastern Ukraine near the city of Enerhodar, produces enough energy to supply 4 million households with electricity, covering roughly one-fifth of Ukraine’s population. 

The attack on the plant, which came on the ninth day of Russia’s invasion of Ukraine, has raised concerns about the security of Zaporizhzhia and Ukraine’s three remaining operational nuclear power plants. The Ukrainian state inspectorate for nuclear regulation has warned that “loss of the possibility to cool down nuclear fuel will lead to significant radioactive releases into the environment,” which could “exceed all previous accidents at nuclear power plants, including the Chernobyl accident and the accident at the Fukushima Daiichi nuclear power plant.”

“Russian people, I want to appeal to you: How is this possible? After all, we fought together in 1986 against the Chernobyl catastrophe,” Ukrainian President Volodymyr Zelenskyy said in a televised address on Friday. U.S. President Joe Biden called the Ukrainian president in the early hours of the morning to speak about the situation at the plant, according to the White House. The two leaders urged the Russian government to halt military activity around the area and allow firefighters and emergency responders to enter the plant.

Some experts, however, think that making close connections between Zaporizhzhia and the Chernobyl disaster could be a mistake.

Large parts of the Chernobyl exclusion zone, which lies about 60 miles (100 kilometers) north of the capital, Kyiv, have been closed off since the disastrous meltdown of Ukraine’s Chernobyl nuclear power plant in 1986. In that disaster, two explosions inside the plant’s reactor flipped its 2,000-ton (1,800 metric tons) lid like a coin, blanketing the surrounding 1,000 square miles (2,600 square kilometers) with radioactive dust and reactor chunks. Following evacuation and the dousing of the nuclear fire — which cost many firefighters their lives — the reactor was sealed off and the area was deemed uninhabitable by humans for the next 24,000 years. After fighting broke out there last week, the zone is also occupied by Russian forces, Live Science previously reported. 

Despite the frightening surface similarities between the two events, Zaporizhzhia’s reactors are much safer than the ones at Chernobyl, according to nuclear scientists. Unlike Chernobyl’s RBMK-1000 reactors, Zaporizhzhia uses more modern pressurized water reactors, which require significantly less uranium fuel in the reactor core, thus limiting the likelihood of a runaway chain reaction. Two layers of protection — a steel-reinforced concrete outer containment unit and a 8-inch-thick (20 centimeters) steel inner vessel — also surround the reactor. Both layers are designed to withstand earthquakes and explosions. Pressurized water reactors also shut down automatically in the event of an emergency. 

Nonetheless, a direct shell hit to the outside of a reactor could still be dangerous, according to Robin Grimes, a professor of materials physics at Imperial College London. Puncturing the Zaporizhzhia reactors’ twin shells wouldn’t lead to an explosion like at Chernobyl, he says, but it would still release a lot of dangerous material.

“It is not designed to withstand explosive ordinance such as artillery shells,” Grimes said in a statement. “While it seems to me unlikely that such an impact would result in a Chornobyl-like nuclear event, a breach of the pressure vessel would be followed by the release of coolant pressure, scattering nuclear fuel debris across the vicinity of the plant and a cloud of coolant with some entrained particles reaching further.” 

Despite being much safer than those at Chernobyl, Zaporizhzhia’s reactor cores still contain a lot of highly radioactive fuel, and this is not the only source of concern. Environmentalists and nuclear experts have long warned that the plant’s spent nuclear fuel rods, cooling in acres of open water pools and standing in open-air yards behind the site, could produce catastrophic airborne plumes of radiation if struck by a stray shell or missile.

On the day before the blaze (March 3), crowds of local residents and employees of the nuclear facility attempted to block Russian troops from advancing toward the plant by setting up improvised barricades, but after some resistance, the Russian troops broke through.

Edward Obbard, a nuclear engineering program coordinator at the University of New South Wales in Australia, said that the greatest impact of the attack is not the risk of nuclear fallout but rather the loss of electricity to the Ukrainian people and the likelihood of much more fighting.

“The availability of nuclear power is vital to energy security in multiple European States and particularly in Ukraine,” he said in a statement. “All nuclear-related risks to people in the vicinity, even in very worst case scenarios, pale in comparison to the very direct and lethal hazard of continued fighting on the ground.”

Originally published on Live Science.

Ben Turner is a U.K. based staff writer at Live Science. He covers physics and astronomy, among other topics like tech and climate change. He graduated from University College London with a degree in particle physics before training as a journalist. When he’s not writing, Ben enjoys reading literature, playing the guitar and embarrassing himself with chess.