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Russian military vehicles drive through the gates of the Zaporizhzhia Nuclear Power Station in Energodar, Ukraine, on May 1.

ANDREY BORODULIN/AFP via Getty Images

None are that great to consider.

Seized in Russia’s invasion of Ukraine, the Zaporizhzhia nuclear power plant faces a day-by-day struggle to avert disaster. The plant’s long-term safety is “not sustainable,” the U.N.’s atomic energy watchdog said last week after an unprecedented visit to the facility amid continued shelling nearby.

“We are playing with fire, and something very catastrophic could take place,” International Atomic Energy Agency (IAEA) Director General Rafael Grossi this week told the U.N. Security Council. He called for a demilitarized zone around the nuclear plant, which has been repeatedly shelled over the summer and is now disconnected from outside power lines to cool its reactors.

The plant’s possible fate — whether it safely shuts down or becomes the next nuclear disaster — is still a hostage to war, say experts, and is largely in the hands of its Russian occupiers. On Friday, Grossi released a video statement saying, “the risk of a nuclear accident has significantly increased,” at the plant, due to continued shelling that has disrupted efforts to reconnect the facility to the outside power grid. He called again for Russia to demilitarize the plant, where inspectors photographed military trucks parked in turbine rooms.

“There is only one master at the plant, whatever the Ukrainians there might like to do,” said Leon Cizelj, president of the European Nuclear Society. “I would hope the Russians have enough sense, with the outside power lines down, not to rock the boat.”

Russia seized the plant in March, early in its invasion of Ukraine, in an overnight firefight seen worldwide on security cameras. Since the IAEA team, including Grossi, braved the war’s front lines last week to inspect the plant, leaving behind two inspectors on-site, the last outside power lines to the plant were cut by fires and shelling. That has left the facility, for now at least, dependent on its last working reactor for cooling power (as well as emergency diesel generators with five to 10 days of fuel). This “island mode” of operation was meant only for emergencies and to last a few hours, not days, Petro Kotin, chief of Ukraine’s nuclear energy company, told the Associated Press.

Nuclear power plants are essentially balancing acts, with pumps always cooling the hot reactors that generate electricity. Without cooling, these reactors would overheat and damage or melt their nuclear fuel rods. Even after the fuel rods are turned off, they remain hot for weeks to months, “decay heat” that requires active cooling to prevent damage that could lead to venting of radioactive gas or a meltdown.

Nuclear plants are designed to have external power for cooling with managing this balance in mind, said Jacopo Buongiorno, a professor of Nuclear Science and Engineering at MIT. A reactor that is shut off and no longer generating electricity cannot provide the power needed to keep itself cool. “That is the pickle they are in right now,” he said, at the Zaporizhzhia facility, without outside power. “Cooling must be provided even to a shutdown reactor for a long time to prevent the nuclear fuel from overheating, potentially getting damaged, and then having a release of radioactivity.”

How this cooling requirement is handled at the plant over the course of the war will shape its fate. Here are three possible outcomes.

The IAEA delegation observes damage to the Zaporizhzhia power plant during its visit this month. (Fredrik Dahl/IAEA)

There are four external power lines to the Zaporizhzhia nuclear plant from the outside electric grid, and one power line from a nearby coal plant (which has already run out of coal, according to news reports). All those lines have been disconnected or severed by shelling or due to fires over the summer, with the final outside ones cut in the last week. However, workers are trying to reconnect that line to the power grid, according to the IAEA.

“My guess is whenever they reestablish the lines, they will go back to [reactor] operations because both [Russia and Ukraine] need the electricity,” said Cizelj. Power from the plant goes to both Ukraine and Russian-occupied territories of Ukraine, where the Russian military would need electricity. Even in the midst of war, Ukraine is still a net exporter of power to Europe, he noted: “They have to get some money somewhere.”

Of the plant’s six reactors, four were shut down and are now safely cool; one is now shutting down but still warm enough to need continuous cooling. Only the last reactor, No. 6, is still operating, relying on the heat provided by the controlled chain reaction among its fuel rods to generate steam and make electricity. In a scenario where a connection to the outside grid can be restored quickly, this reactor’s operations are increased (they are now at 12 percent to 14 percent of normal in island mode) to feed electrical needs across southern Ukraine, while reactor No. 5 continues to cool, which will take weeks.

Island mode is seen as an emergency move by the IAEA, which warns that such “flexible” operation will cause wear and tear on turbines and pumps, as well as disrupt the designed radioactive decay in fuels rods, possibly damaging them, according to a 2018 report. In the U.S., the Nuclear Regulatory Commission does not allow plants to operate in island mode, according to public affairs director David Castelveter. At the Zaporizhzhia plant, it has gone on for days.

“I think this is a potentially dangerous experiment,” said Edwin Lyman, director of nuclear power safety with the Union of Concerned Scientists. “I haven’t seen anything that suggests it is an acceptable long-term option. It is typically credited as a brief stopgap measure until off-site power can be restored.”

One big danger of the reduced power production in island mode is damage to the electricity-producing turbines at the plant, which are designed to be turned by very “dry” steam heated by the reactor, said Buongiorno. The more “wet” steam produced by a reactor running at a lower level causes pitting and damage to turbine blades, a phenomenon called cavitation. “There are a whole host of conditions that degrade components when you are not running at design,” he said.

A control panel at the Zaporizhzhia nuclear power plant photographed during the IAEA visit this month. (Fredrik Dahl/IAEA)

Cizelj is more sanguine about the reactor continuing to run in island mode, saying it is possible, in theory: “Failure of one of the big pumps might force you to shut down, but as long as there is fuel in the reactor, you could keep going.”

However, even running the plant on the equivalent of a spare tire, until external power is restored, leaves the plant in the limbo it was before, he acknowledged, facing shelling of restored power lines. At the facility, a depleted Ukrainian staff works lengthened shifts at gunpoint, with reports of torture after the initial takeover, hectored by Russian nuclear technicians, according to the IAEA. “Imagine a very good driver racing with someone in the next seat yelling and distracting them,” said Cizelj. “I think this is the weakest point, the staff being stretched, and they start making mistakes.”

Ukraine’s nuclear regulator released a statement on Thursday suggesting the agency was weighing whether to shut down the plant, halting operations of the No. 6 reactor. In order to do this, Oleh Korikov, acting head of Ukraine’s State Nuclear Regulatory Inspectorate, told a news conference the plant would have to rely on 20 backup diesel generators at the plant for cooling power, according to Reuters. The generators have enough fuel only for five to 10 days of operation, according to various estimates, and would require refueling for the weeks to months needed to bring reactors 5 and 6 to a safe state where they don’t pose a risk of melting or otherwise damaging the fuel rods.

“I think the best situation would be for all six reactors to be shut down once the plant is reconnected to the grid, and to have plenty of diesel fuel on the side, so they can go for a longer time,” said Buongiorno. Even after the reactors are cooled to less than boiling temperatures, their decay heat will still need to be managed, but that is a less power-intensive and dangerous job.

In this scenario, the plant is reconnected to the grid, and Russia agrees to the shutdown, despite suspicions voiced by Ukraine that the invaders hope to switch the plant to supplying their electric grid, as a kind of war booty. (Ironically, Ukraine switched the plant to use Western fuel rods instead of Russian ones following Russia’s seizure if Crimea, complicating such a move.) The plant would need to be supplied with 100 tons of diesel fuel daily to proceed with this shutdown, according to Ukrainian officials, hard to imagine as the war nears the region of the plant, as counteroffensives rage along front lines in the last week.

The alternative, however, leaving the plant operating, leaves the possibility of the war coming to its front door with a hot reactor in operation. “We are in uncharted territory here,” said Buongiorno.

Blown-out windows at a building of the Zaporizhzhia nuclear power plant facility during the IAEA visit this month. (Fredrik Dahl/IAEA)

If an attempt is made to shut off the last reactor once power is restored or even using refueled diesel generators, the risk of a disaster recedes with each passing day as its decay heat wanes. Mismanagement of this cool-down, or perhaps more likely, an interruption of cooling power caused by shelling or running out of diesel fuel could cause a calamity more like 1979′s Three Mile Island accident — which saw a partial meltdown of overheated fuel inside the reactor — rather than the ones like the 1986 Chernobyl or 2011 Fukushima Daiichi disasters.

In this possibility, the shutdown of the No. 6 proceeds long enough under active cooling to lower the decay heat inside the reactor to the point where it would be contained by the thick steel walls and concrete covering the unit, even cooling power stops. The fuel rods inside the reactor would lose their zircon covering, releasing radioactive byproducts of nuclear reactions into the vessel or even melting depending on the heat, but staying contained. At Three Mile Island — where the 1979 accident was caused by an unnoticed loss of coolant fluid from the reactor — radioactive gases that collected in a coolant system escaped from a containment tank and reached the air. However, the radiation released in that accident seems to have stayed mainly within the facility, and the evidence for a health effect on the nearby population remains equivocal at best, and still debated in medicine. Similar venting might happen at the Zaporizhzhia facility, in this scenario.

In the end, said Buongiorno, Ukraine would lose a very expensive reactor, but workers on the site would manage the radiation risks, which would not reach the wider public.

A worst case is one where external power is shut off, diesel backups are lost and warfare comes into the facility, forcing a sudden evacuation of most of the staff. In this case, analogous to the tsunami wave that overflowed Japan’s Fukushima Daiichi plant in 2011, a sudden shutdown, or SCRAM, of the reactor would ensue without any preceding cool-down time.

In 2011, the result was three nuclear meltdowns as well as hydrogen explosions that sent radioactive gas into the air and contaminated water into the ocean. At Zaporhizhzhia, Cizelj estimates a similar disaster involving the No. 6 reactor would lead to detectable levels of radiation high enough to increase long-term cancer risks for 12 to 18 miles around the plant. But the contamination would not be high enough to cause radiation sickness or the kinds of scenes of an open reactor exposed to the air, seen at the 1986 Chernobyl accident.

“These are much safer, better-designed reactors,” said Cizelj. “We are not going to see another Chernobyl there.”

The only historical experience close to the one at the Zaporizhzhia nuclear plant was in 1991, at the Nuclear Power Plant Krško in Slovenia, which faced the threat of attack from the Yugoslav army. A battle that killed 70 people took place only a few miles from the plant, where a shutdown was considered. “There is a legend that planes were on the way to attack us and were called off at the last minute,” said Cizelj, who is a professor at the nearby Jozef Stefan Institute in Ljubljana. “But that is nothing like what is happening now. This is all new.”

There are no guarantees in warfare, he and the other experts who spoke to Grid cautioned. Any combination of the scenarios described might play out in the weeks ahead at the Zaporizhzhia plant. The installation of two IAEA inspectors at the plant is the most encouraging news from the scene, said Cizelj, providing unbiased observations: “We really need to understand what is going on there.”

That’s important because the drama of threat at the plant has at times almost obscured the reality of people fighting and dying in Ukraine every day of the war, said Buongiorno. Both sides have blamed each other for the shelling at the plant that has cut off its external power, both playing off a global fear of a nuclear accident as a tactic in the war, in his view.

“The consequences would be serious, but not as catastrophic as they have been depicted at times,” said Buongiorno. “I hate to see it to become a point of propaganda.”

He added: “The NRC’s requirements for off-site power cannot be met by an on-site source.”

Thanks to Lillian Barkley for copy editing this article.

Dan Vergano is a science reporter for Grid.

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