How Worried Should We Be About Zaporizhzhia?

Ukrainian President Volodymyr Zelensky sounded the alarm in June and again in July over a possible plan by the Russians to sabotage Ukraine’s Zaporizhzhia nuclear power plant, potentially triggering a nuclear catastrophe. For months, experts and policymakers have been debating the likelihood of a nuclear accident or deliberate sabotage at Zaporizhzhia. How real is the risk?

The plant has been caught in the crossfire of the Ukraine war since the Russians took control of it in the early days of the invasion last year, and both sides have since accused the other of plans to sabotage the plant. Russian forces have stationed military equipment around the site, using it as a de facto exclusion zone free from any threat of incoming Ukrainian fire, and Zelensky recently claimed Russian forces placed “objects resembling explosives” on the rooftops of several reactors. The International Atomic Energy Agency (IAEA), a U.N. nuclear watchdog, has been monitoring the site for any evidence of mines or explosives since Zelensky’s claims, but the agency said on July 12 that it has yet to find evidence of sabotage, though it has not been granted access to the roofs of two reactors.

The fate of the plant keeps coming up in the war of words between Ukraine and Russia. Kremlin propagandists recently suggested they might have to blow up the plant to force Kyiv to the negotiating table; Russia recently allegedly blew up a massive dam and hydroelectric power plant in southern Ukraine that was essential to the Nova Kakhovka reservoir, which supplied water to the plant. The IAEA said Zaporizhzhia has “sufficient water for some months” and is exploring backup options including constructing wells that can replenish cooling water essential to the safety of the plant. Ukrainian health officials also said late last week that they are studying worst-case scenarios for a radiation release and have readied nearly 200 hospitals to treat civilian casualties.

Some experts and organizations such as the American Nuclear Society (ANS) have opted for caution, reassuring people that the power plants are “robust, hardened pieces of critical infrastructure; built to withstand natural and man-made hazards.” Its stance is that there is not much need for concern when there are currently no signs of sabotage. Almost all the reactors have also been in so-called cold shutdown for months, which reduces the risk of any sudden catastrophic event.

But other experts warned of underestimating the risks of significant radioactive release if either an attack were to occur or if there were a failure, deliberate or otherwise, in the plant’s cooling system.

“The fact remains that if Russia or any other entity wanted to sabotage this operational nuclear plant, there are multiple ways in which they could likely achieve a significant radiological release, and to try to pretend that’s not even a possibility is, I think, doing a disservice to the Ukrainian people,” said Edwin Lyman, director of nuclear power safety at the Union of Concerned Scientists.

All six reactors at Zaporizhzhia have been shut down for about 10 months, which means the nuclear chain reactions responsible for creating nuclear energy have been halted. However, the byproducts of these reactions, called residual heat, are still radioactive and require great attention and management, experts said. 

Five of the six reactors are operating under a cold shutdown, which means these reactors are kept below boiling point. If the plant loses off-site power and subsequently its cooling system, operators would have more time than in a hot shutdown situation to reestablish cooling before having to worry about radioactive release. 

Zaporizhzhia currently has one reactor in hot shutdown mode to supply steam to the plant for various safety reasons and to help keep components of the plant operating. A hot shutdown means the reactor is above boiling point and highly pressurized, increasing the risk for a release of radioactive material if any such damage were to occur or cooling were to be disrupted, experts said.

“If you open the bottle of still water like in a cold shutdown state, there will be nothing happening, while opening the shaken can, [representing a] pressurized system like in a hot shutdown state, would result in release of some part of the can,” said Attila Aszodi, a professor at the Institute of Nuclear Techniques of the Budapest University of Technology and Economics.

Experts including those at the IAEA and ANS said that bringing all units into a cold shutdown would help reduce the risk of radiological release: a boiler could make steam just as easily as that sixth reactor.

“The IAEA experts are strongly encouraging the ZNPP to investigate all possible options to install an external boiler to generate the steam required, which would enable the site to bring all units into a cold shutdown state,” the IAEA said.

Even with the robust infrastructure that provides additional protection to the plant, “the risks with the war are high enough, [and] it is therefore absolutely reasonable to minimize technological risks in the current situation,” Aszodi said.

Shelling from Russian forces could again knock out power lines and cut off the site’s electricity, which the plant’s cooling system relies on. In October, the plant lost its off-site power due to shelling and had to rely on backup diesel generators at least three times that same month. But Lyman said diesel generators should not be considered a long-term solution because of their reliance on fuel supply and need for manual maintenance.

“The electrical system is really probably the Achilles’ heel, because if you lose electrical distribution, which is what happened with Fukushima, then so much of the equipment would have to be operated manually, and that would really make things harder,” Lyman said. The meltdown in 2011 at Japan’s Fukushima nuclear power plant after a tsunami was the world’s last major nuclear accident.

All reactors require continuous cooling and, therefore, electricity, regardless of their shutdown status. Reactors in cold shutdown give operators a few days to restore cooling rather than just a few hours, which would be the case if it were in hot shutdown, Lyman said. But it could take less than a few days if other issues simultaneously occur such as leaks, ruptures, or deliberate sabotage.

“The Russian side knows the plant very well. They know where the weak points are,” Aszodi said. “Systematic shelling of the reactor building would be bad, but destroying the cooling and electricity supply is easier.”

And experts are worried that even a couple of days may not be enough when the staff is greatly reduced and lines of command are unclear. Thousands of workers have managed to escape before Russian forces took control last year, leaving only a portion of what was once a 12,000-person workforce at the site, according to the International Labor Organization. Reports of torture and abuse have also raised concerns about the working conditions of existing operators at the plant. 

“Our sympathy and thoughts are always with the workers. These are engineers just there to do the work,” Jacopo Buongiorno, a member of the ANS Rapid Response Taskforce and nuclear engineering professor at the Massachusetts Institute of Technology, said. But he and ANS maintain that the remaining workers are sufficient to handle any issues at the plant and disruptions to the cooling system. And even if the Russians deliberately sabotaged the plant, the radiological consequences would be “very low,” according to Buongiorno, with a radioactive dispersal of at most a 2-kilometer radius.

“There is still a significant amount of radioactive material at the site, and you can blow up the site and some radioactivity is going to come out,” Buongiorno said. “But the real question that we should ask ourselves is what are going to be the radiological consequences? Is it really going to be a cloud of radioactive material hovering over Europe and killing scores of people? Short answer is no.”

But holding a nuclear power plant hostage is one less reason to put one’s guard down, experts including Aszodi said.

“I don’t think that any of the parties to this conflict would have the interest to destroy the plant, or make real damage, or release radioactivity from the plant,” Aszodi said. “But unfortunately, we have seen a lot of irrational events and actions in this war. Irrationality is the main risk, actually.”