Russian and Ukrainian forces are locked in a standoff at the Zaporizhzhia nuclear power plant, raising fears across Europe and the specter of Chernobyl. Shelling near the strategically located plant — which both sides have blamed on the other — has increased the risk of a serious accident, and families are fleeing the area in the face of a possible nuclear catastrophe.

Zaporizhzhia is Europe’s largest nuclear power plant, and it provides electricity to Ukraine and to several European countries. Its location on the Dnipro River makes it a critical target for Russian forces, which have controlled the plant since March. Despite Russian forces allegedly turning the plant into a military installation, Ukrainian operators still manage the safety and daily operations of the plant, under significant duress.

Multiple parties, including UN Secretary General Antonio Guterres, have called for the immediate demilitarization of the plant, citing the potential for a serious and widespread nuclear disaster. However, Ivan Nechayev, deputy director of the Russian foreign ministry’s information and press department, claimed that such a de-escalation “will make the plant even more vulnerable.” Russian officials also claimed in a letter to the UN that Ukraine was planning a “provocation” there Friday, according to the TASS state-run news agency — a claim Ukraine countered, saying Russia planned to disconnect the plant from the Ukrainian grid and connect it to Russia’s power grid, Reuters reported. Thus far, neither incident has come to pass.

The global concern about Zaporizhzhia’s security isn’t surprising, especially with the 1986 Chernobyl nuclear disaster still a presence. Furthermore, that concern isn’t unfounded; there are real possibilities for problems at the plant, ranging from alarming to cataclysmic.

The biggest risk to Zaporizhzhia is a power outage

While there are many things that could go wrong at Zaporizhzhia, “The likelihood of an intentional attack on the [plant] that leads to a major nuclear disaster is low,” Ivanka Barzashka, founder and co-director of the King’s Wargaming Network at King’s College London, told Vox via email. “Moscow would have a lot to lose and nothing to gain from such an outcome, given the reactor’s proximity to Russian forces and population.” Furthermore, the plant is built to withstand direct attacks, as it’s constructed with reinforced concrete.

The real risks to the facility are more likely to be due to human error, accidental shelling, or a lack of electricity to cool the nuclear material, according to Matthew Bunn, the James R. Schlesinger professor of the practice of energy, national security, and foreign policy at Harvard’s Kennedy School of Government.

“The biggest concern is [the] cooling of a nuclear power plant,” Bunn said. “In general, to avoid an accident at a nuclear power plant, you need to keep the reactor core under water, and the spent fuel and the spent fuel pool under water so they’re continuously cooled.” That cooling process requires electricity, which now comes from Ukraine’s external power grid. The Fukushima nuclear plant disaster in Japan, for example, occurred because of a tsunami which cut off-site electricity to the plant and destroyed the generators, making it impossible to cool the facility even though the reactor had undergone emergency shutdown.

However, as Bunn told Vox, a number of those lines have already been cut, increasing the possibility that Zaporizhzhia might have to rely on diesel-powered generators to support the cooling process. It’s unclear how much fuel those generators have, given that Russian forces have reportedly been siphoning off the fuel for their own purposes, Bunn said. “Diesel’s a highly sought commodity in any war zone,” he said. “There are supposed to be days of diesel at the site; we don’t know whether that’s still true or not.” The Ukrainian nuclear agency Energoatom said on Friday that Russian forces were seeking diesel to fuel the generators in the case of power loss, according to Reuters.

In a worst-case scenario, the plant could lose power and the pumps circulating water to cool the reactor core and spent fuel pool would shut down. The heat that the reactor core and the spent material generate would then boil the surrounding water until it evaporates, exposing the reactor core “within hours,” Bunn said. “The fuel would then start to melt. Even if you shut the reactor down, some people refer to it as, ‘the fire that doesn’t go out’ — the fuel still generates a lot of heat from the radioactive decay of the split atoms, what are called the fission products, in the fuel.”

However, a spent fuel fire — what Bunn referred to as the “very very worst case” — is unlikely given that there’s just not as much of it at Zaporizhzhia as there are at other sites; that’s because Zaporizhzhia used to send spent fuel to Russia for storage and reprocessing there. “That really only happens when you have fuel that’s pretty closely packed and really hot, having been released from the reactor fairly recently,” he said.

Even if the electricity supply holds, shelling could damage the facility, causing water to leak out of the plant and upsetting the cooling process. Alarmingly, the ongoing shelling has already done damage to the plant — including near a substation which prompted one of only two operating power lines to shut down on August 5.

As Bunn told Vox, the human element is critical in maintaining the plant’s safety. “The Ukrainian operators have been operating essentially at Russian gunpoint for months,” Bunn said. “[They are under] enormous psychological stress; many of them have sent their families away, they’re exhausted. Under those conditions, the possibility of human error in operating the plant is ever-present. They have been doing a heroic job, but people under stress make mistakes.”

Operators at the plant who have been able to speak to outside sources paint a harrowing picture. “What is happening is horrific and beyond common sense and morality,” plant staff wrote in a Telegram channel, according to the BBC. “The psychological situation is difficult,” a worker called Svitlana told the BBC. “Soldiers are walking everywhere with weapons and everyone is actually kept at gunpoint.”

Why is Zaporizhzhia so important?

Zaporizhzhia is important to both Ukraine and Russia because it’s a crucial energy supply. And while UN chief Gutteres declared Friday that the electricity the plant provides is Ukraine’s to do with as it pleases, it’s not unlikely that Russia would want to connect that power source to its own grid, as Ukraine has warned.

“A safe and reliable nuclear energy source connected to the Crimean power grid would significantly benefit Russia,” Barzashka told Vox. Crimea is connected to the Russian power grid; Ukraine was, as well, before the war, but quickly switched over to the European power grid after the invasion, according to IEEE Spectrum.

The Zaporizhzhia power plant sits on the very northern border of Russian-controlled territory, and in addition to its utility as a power source, Russia is also “using the plant as a shield against any possible Ukrainian attempt to retake the area,” as the Atlantic Council’s resident fellow for security research Ruslan Trad wrote.

Ukrainian President Volodymyr Zalenskyy has called the strategy “blackmail with radiation,” accusing Russia of using the potential for nuclear disaster to prevent Ukrainian forces from retaking the surrounding territory.” About 500 Russian soldiers are there, according to Energoatom head Petro Kotin, and footage showing Russian military vehicles in the plant’s turbine hall emerged Friday. “Russia is launching attacks from this reactor, arguably in contradiction to international law, because they know the Ukrainians won’t fire at the reactor because of the huge danger,” Bunn said. “That is reckless, illegal, and is an outrage.”

Thus far, Russia has refused calls to demilitarize the plant and create a safety zone around it, saying that the Russian troops positioned there are a safeguard against nuclear disaster.

Bunn, as well Nikolai Steinberg, a former chief engineer at Chernobyl, maintain that the reactor should have been shut down months ago, with Steinberg calling its continued operation “a crime.” However, Bunn explained, “they have remained operating because they’re very, very important to the Ukrainian grid, and the Ukrainian government has been making money selling electricity from these reactors into the European market.”

Ukraine — not a wealthy country to begin with — has suffered significant economic hardship due to the war, so it’s held financial hostage in a sense to its nuclear industry, particularly given the sanctions against Russian fuel imports and serious climate-related problems with other European energy sources. Should Russia divert Zaporizhzhia’s energy to Crimea and Russia and away from Ukraine and Europe, it would cause significant ripple effects and drive up energy prices in the European market. “Russia stealing a nuclear-power plant is a problem for Europe,” Suriya Jayanti, the former energy head at the US Embassy in Kyiv, told the Wall Street Journal.

Shutting the plant down at the beginning of the invasion, though challenging for energy security in Ukraine and Europe, would have allowed the reactors to cool, making nuclear disaster less likely. However, Russia reportedly lacks the ability to recruit its own operators with the technical capacity and willingness to run the plant in an active war zone, so officials wouldn’t agree to take the plant offline for fear they couldn’t restart it.

“I fear that, because of the importance of this site, we probably have not seen the end of fighting near this reactor, and I fear that Russia is not going to agree to the sensible UN proposal for a demilitarized zone at this site,” Bunn said.

How do we mitigate disaster — now, and in the future?

Ukrainian experts are warning of potentially cataclysmic consequences should there be an accident at Zaporizhzhia, including radioactive leakage that could kill thousands, displace 2 million, and cause radiation pollution in an area triple the size of Ukraine, according to IEEE Spectrum. A senior nuclear safety expert with the Ukrainian National Academy of Sciences, Olena Pareniuk, warned that Zaporizhzhia could cause the first magnitude-8 nuclear accident; for context, the meltdowns at Chernobyl and Fukushima were graded magnitude-7 on the International Nuclear and Radiological Event Scale.

Given that shutting down the reactors doesn’t seem like an option as of right now, Bunn said the most pressing action is to stop the shelling. “Operating reactors in a war zone is just not a good idea,” he said. The next step is to allow a monitoring team from the International Atomic Energy Agency (IAEA) to enter the plant. Multiple parties including Guterres and French President Emmanuel Macron are urging Russia to do so; Macron announced Friday that Russian President Vladimir Putin had agreed to let a team in, but didn’t offer further details.

Sending the IAEA monitoring team would likely pause hostilities, since the combatants would probably be unwilling to risk harming civilians on a high-profile mission, and allow the experts to gather independent information about the status of the plant and its workers.

That’s not a sustainable fix, though, according to Bunn; “The IAEA just isn’t set up for sending teams that just stay for months at a time,” he said. “The most plausible scenario there is, send an IAEA team, make arrangements for people and equipment to come in and out more regularly,” then sending monitoring teams in on a monthly basis, he said.

Ultimately, the world needs better agreements and treaties surrounding the status of nuclear facilities in conflict. For example, a 1977 amendment to the Geneva protocol forbids attacks on civilian nuclear power plants and other infrastructure, but the Russians withdrew from that agreement in 2019.

A more favorable scenario, Bunn said, would be widespread adoption of the treaty between India and Pakistan, who have agreed not to attack each others’ nuclear facilities and even clarify annually which facilities are to be avoided.

Among the main reasons there aren’t solid agreements banning attacks on nuclear power plants? United States policy. “The United States has wanted to maintain the option of attacking nuclear reactors, partly for nuclear nonproliferation reasons. It is not unusual for a state launching a nuclear weapons program to claim — when it’s actually a military reactor — that it’s a civilian reactor,” Bunn said. In 1994, for example, the US weighed attacking a North Korean reactor with the understanding that it was contributing to weapons proliferation, but ultimately declined to do so.

The US, Bunn said, should at least agree to the position that functional, internationally-monitored reactors should be protected from attack. The status quo “makes it very difficult for the United States to get up on a high horse about this situation.”