The attack on Europe's largest nuclear power plant (NPP) by Russian troops has sparked global fears of a nuclear disaster.
Experts say an attack on nuclear material at the Zaporizhzhia nuclear power plant in Ukraine could release cancer-causing radioactivity, at least in Ukraine itself.
Some of the possible scenarios during an attack on a nuclear power plant include a direct attack, such as a missile attack that damages the reactor, a blockage in a pool where spent nuclear fuel cools, or an attack that disrupts the power supply to the plant and affects the systems that keep the reactor's nuclear core cool.
"There are several scenarios that could get very bad," said Allison Macfarlane, former chair of the US Nuclear Regulatory Commission, quoted from the Bulletin of the Atomic Scientists, Monday (7/3/2022).
Robert Rosner, a physicist at the University of Chicago and former chair of the Bulletin Board of the Atomic Scientists, said the biggest risk in his view was a blockage in the spent fuel pool containing many spent fuel assemblies.
"Used-fuel pools are often less protected than the reactor itself. A missile hitting the spent-fuel pool would be very bad news," Rosner said.
Reflecting on Chernobyl and Fukushima
As soon as news of the Russian attack broke, some officials in Ukraine speculated that a crisis bigger than the 1986 Chernobyl nuclear disaster was brewing.
However, there are a number of differences between the Zaporizhzhia nuclear power plant and the Chernobyl nuclear power plant. Zaporizhzhia has a highly reinforced retaining structure for its reactor, whereas the Chernobyl reactor does not have such a structure.
Also, the newer Zaporizhzhia nuclear reactor does not use graphite as a material to moderate the nuclear reactions in its core, while the older Chernobyl reactor design does use graphite, which ignites when the core overheats. It was the graphite fire at Chernobyl that sent highly radioactive material into the atmosphere and then across Europe.
Yet another nuclear power expert, MV Ramana of the University of British Columbia, did not think that Ukrainian President Voldymyr Zelensky was too concerned when he speculated that the attack on Zaporizhzhia could be worse than Chernobyl.
"Anything could have happened, right? In retrospect, you might look and say, 'Oh, they didn't actually target the reactor.' But we don't really know. If the battles were going in a different way, it's very possible they hit something much more sensitive," he said.
However, Macfarlane said, an attack on a nuclear reactor would cause big problems. "If you hit the reactor vessel itself, and punch a hole in it, we will lose water, that means we lose coolant and melt the fuel. I assume this can happen and then the nuclear power plant will experience a massive radiation release," he said.
Another scenario, a nuclear plant requires a constant power source to maintain a system that cools the nuclear fuel in the reactor and the spent fuel pool by circulating water. The plant has a diesel generator to provide backup electricity, if needed.
Outages that shut off offsite power to generators and backup generators can cause major problems leading to radiation release.
"If the grid goes out, then usually there is a diesel generator on site, and this supplies backup power," said Ramana. "But if these are occupied through military action, one can easily imagine these facilities also being damaged so there may not be any cooling at all. And that could lead to destruction, as we saw in Fukushima," he said.
Macfarlane is also concerned about the overall effect the war in Ukraine has on the people who work at the country's nuclear plants. Even plants that are shut down require people to continually ensure that various systems, including those that keep nuclear fuel cool, are operational.
"Factory operators during wartime may decide to go and protect their families. Even if they continue to work, under stress, they may not get enough rest. These reactors require a lot of care and attention. You can imagine a lot of different scenarios that create problems," he explained.
Zaporizhzhia is currently in Russian hands. But there are still three nuclear power plants in Ukraine that have not been taken over by the Russian military. Experts are also worried about this.
"This is uncharted territory. We have never had so many reactors in a battle zone. The act of attacking a reactor should not be included in any military plans," Ramana concluded.