Ukraine crisis: How dangerous was Russia's nuclear plant strike?

Europe’s largest nuclear power plant was hit allegedly by Russian shelling early Friday, sparking a fire at one of its six reactors and raising fears of a disaster that could affect all of central Europe for decades, like the 1986 Chernobyl meltdown.

Concerns faded after Ukrainian authorities announced that the fire had been extinguished, and while there was damage to the reactor compartment, the safety of the unit was not affected.

But even though the Zaporizhzhia nuclear plant is of a different design than Chernobyl and is protected from fire, nuclear safety experts and the International Atomic Energy Agency warn that waging war in and around such facilities presents extreme risks.

One major concern, raised by Ukraine’s state nuclear regulator, is that if fighting interrupts power supply to the nuclear plant, it would be forced to use less-reliable diesel generators to provide emergency power to operating cooling systems. A failure of those systems could lead to a disaster similar to that of Japan’s Fukushima plant, when a massive earthquake and tsunami in 2011 destroyed cooling systems, triggering meltdowns in three reactors.

As claimed by Ukrainian officials, the reactor that was hit was offline, but still contained highly radioactive nuclear fuel. Four of the other six reactors have now been taken offline, leaving only one in operation.

The reactors at the plant have thick concrete containment domes, which would have protected them from external fire from tanks and artillery, said Jon Wolfsthal, who served during the Obama administration as the senior director for arms control and nonproliferation at the National Security Council. 

Another danger at nuclear facilities are the pools where spent fuel rods are kept to be cooled, which are more vulnerable to shelling.