What Happens If a Nuclear Power Plant Catches Fire?

Russian forces have seized Europe's largest nuclear power plant in Ukraine, damaging it in the process.

The development has caused international concern, not least because Russian shelling ignited a fire at the Zaporizhzhia NPP plant in southeastern Ukraine, sparking fears of a nuclear accident.

On Friday morning, Ukraine's state nuclear regulatory inspectorate said in an update that the fire had been put out by Ukrainian emergency services. Information on the dead and injured was not available at that time, and staff continued to work at the facility.

The safety of nuclear plants in Ukraine is highly sensitive because the country, then part of the Soviet Union, was the scene of the world's worst nuclear accident on April 26, 1986, when an explosion at the Chernobyl nuclear power station, north of Kyiv, sent a radioactive cloud over much of Europe. Russian forces captured Chernobyl last week.

The nuclear authority said that of the six nuclear power units at the Zaporizhzhia station, one was in operation at 690 MW of power. The others were either being cooled down or were disconnected from the grid. "Changes in the radiation situation have not been registered," the statement read.

However, the authority noted concerns that a loss of the ability to cool down nuclear fuel would "lead to significant radioactive releases into the environment" in an event that "may exceed all previous accidents at nuclear power plants, including the Chernobyl accident". There is also a spent nuclear fuel storage facility nearby.

Earlier, Dmytro Kuleba, Ukraine's foreign minister, urged Russian forces to cease firing at the plant so that firefighters could get through and secure it.

In a speech expressing his concern about the attack on the nuclear plant, Ukrainian President Volodymyr Zelensky said that if an explosion occurred at the facility it would be "the end for everyone. The end for Europe. The evacuation of Europe," according to the Associated Press.

Fires at nuclear power plants are dangerous and can prevent key safety systems from working correctly. A fire at the Browns Ferry Nuclear Plant in the U.S. in 1975 affected more than 1,600 electrical cables, including ones that were important for reactor safety, and operators had to perform emergency repairs on the systems before they could safely shut the reactor down.

Nuclear experts have spoken out on the current situation at the Zaporizhzhia NPP plant, with some more concerned than others.

Edward Obbard, a senior lecturer and nuclear engineering program coordinator at The University of New South Wales in Australia, told the Australian Science Media Center: "A single fire located somewhere on the same site as a nuclear power plant cannot feasibly trigger a meltdown, however if the blaze cannot be contained then the nuclear power plant at the Zaporizhzhia site will be shut down and cannot generate electricity for the people of the Ukraine.

"A greater risk to nuclear safety is a direct hit on a reactor building or spent fuel pool by explosive ordnance, which could release radioactive material if the blast is sufficiently powerful to breach containment."

Professor David Fletcher from the School of Chemical and Biomolecular Engineering at the University of Sydney, who previously worked at the U.K. Atomic Energy Authority, told the Australian Science Media Center that "the real concern is not a catastrophic explosion" but damage to the cooling system.

A nuclear reactor meltdown is a situation in which the nuclear reactor fuel gets so hot that it burns through its casing and through the containment chamber floor, causing a widespread release of radiation.

A meltdown happens if there is insufficient cooling to the nuclear reactor, such as through a water pump failure.

A nuclear hazard warning sign seen near the Chernobyl nuclear plant in Ukraine in February, 2006, which was the site of the worst nuclear power plant accident ever in April, 1986. Fears of a nuclear accident in Ukraine have been renewed this week after Russian forces seized the Zaporizhzhia NPP plant. Sergei Supinsky/AFP/Getty