Many nuclear power plant workers in besieged Ukraine have been at their posts around the clock since last Thursday when the Russian invasion began, making them prey to fatigue and human error as war reaches its seventh day and troops close in on their facilities, the International Atomic Energy Agency has said.

At a special emergency meeting on Wednesday, the IAEA’s general director, Rafael Rossi, called on Russian troops not to interfere with nuclear facility workers doing their jobs.

“In this context, it is also imperative to ensure that the brave people who operate, regulate, inspect and assess the nuclear facilities in Ukraine can continue to do their indispensable jobs safely, unimpeded and without undue pressure,” he told his IAEA colleagues Wednesday, according to the World Nuclear Association.

“I want to emphasize there is nothing normal about the circumstances under which the professionals at Ukraine’s four nuclear power plants are managing to keep the reactors that produce half of Ukraine’s electricity working,” he added.

The Russian defense ministry on Wednesday send a letter to the IAEA claiming that its troops control the territory surrounding Ukraine’s sprawling Zaporzhe nuclear plant, whose six Soviet-built VVER-type pressurized water reactors make it the biggest nuclear power station in Europe.

Rossi that, saying the Russians “have the physical control of the perimeter, including the village where most of the employees live.”

But German Galuschenko, Ukraine’s energy minister on Thursday pushed back against that claim, telling Nastoyaschoe Vremya, Radio Free Europe’s television network, that: “The plant is well protected. A takeover by external forces will not take place.”

Footage from Enerhodar, the city where Zaporozhe is located, showed that hundreds of unarmed plant workers had erected barricades against Russian troops on the roads leading to the plant. Those interviewed characterized themselves as a “human shield” against an incursion.

Galuschenko called on the IAEA, which is a body of the United Nations, to lobby for establishing no-fly zones in the skies over Ukraine’s four nuclear power plants.

Situated 225 kilometers up the Dnepr River from the Black Sea, the Zaporozhe plant lies astride Russia’s main invasion lines. The South Ukraine nuclear plant and its three VVER reactors sits near the city of Mykolaiv 570 kilometers to Zaporozhe’ west.

Ukraine’s Rivne and Khmelnytskyi nuclear plants – which have four and two VVER units, respectively – are in the country’s far western region, which so far has seen little Russian troop activity.

The difficulties facing Ukraine’s nuclear workers was brought into high relief Wednesday when Ukraine’s nuclear regulator posted a public birthday greeting to Valentin Geiko, the 60-year-old head of shift at the decommission Chernobyl nuclear power plant, where Russian forces have held workers since taking over the plant on Friday.

Geiko, wrote the regulator “for the past six days has been in charge of the security of the nuclear facilities at the Russian-military occupied plant. He can’t hand over his shift and can’t leave his post.”

Site of the world’s worst nuclear disaster in 1986, the Chernobyl site and its irradiated exclusion zone houses 20,000 spent nuclear fuel assemblies that require constant monitoring.

The Russian invasion of Ukraine represent the first time a country with an extensive and established civilian nuclear industry has been invaded, which alarms the world’s foremost nuclear experts. That’s because, despite the robust steel defenses that shield reactors vessels, nuclear plants are simply not designed to sustain the kind of damage to infrastructure wars can inflict.

“In peacetime, the most severe threats are severe weather,” Edwin Lyman, an expert on nuclear proliferation and nuclear terrorism, told the Bulletin of Atomic Scientists.

Of special danger are military attacks that could deprive Ukraine’s plants from cooling the highly radioactive fuel in their cores, or the spent fuel they store in open pools. Should plants lose access to the power grid, they would have to rely to back-up diesel generators and their finite supply of fuel to continue cooling operations.

“That’s the situation they faced at Fukushima, in Japan, in 2011, where the plant lost both off-site and on-site power,” Lyman told the Bulletin. “In that case, there were very few means the operators had to try to keep the fuel from melting down, and the result was three core meltdowns. So it is critical that you keep cooling, however you can.”

According to Lyman, each of Zaporozhe’s reactors has three backup generators supplied with a week’s worth of fuel.

But the human error factor is harder to compensate for – which is all the more worrying now that Ukraine’s nuclear workers are being forced to work such grueling, non-stop and stressful hours.

“If the enemy controls the plant and all the access points, are the personnel who were not on duty, are they going to report to work?” said Lyman. “And if not, then you have the shifts that are there now, they ordinarily will not have to work, nonstop, and can’t work nonstop.”

Given that Ukraine’s nuclear plants were designed by the Soviet Union, it might be easy enough for Russia to send in relief personnel to spell the exhausted Ukrainian nuclear staff. But Lyman told the Bulletin that that adds yet another variable to an already difficult situation.