The head of Russia’s state nuclear corporation has expressed concerns about the spread of the novel coronavirus to three of its so-called “nuclear cities,” including one that houses a top-secret research institute that helped develop the Soviet nuclear bomb.

The cities hold a fabled place in Russia’s nuclear industry, which is managed by the state-controled Rosatom corporation. Most of them are closed to foreigners and even most Russians require special permission to enter them because of the top-secret facilities that many of them house.

In his most recent video appearance, Rosatom chief Alexei Likhachev said a special delivery of ventilators and personal protective equipment for medical workers has been sent to the city of Sarov and other closed towns where dozens of cases have been reported.

“This (pandemic) creates a direct threat to our nuclear towns,” Likhachev said in the video address on the Rosatom website ­­­– a communication method he has embraced since the beginning of the pandemic. “The situation in Sarov, Elektrostal, Desnogorsk is today particularly alarming.”

His remarks come as Russia reports a total of 155,370 cases of coronavirus infection and 1,451 deaths, making Russian the seventh most infected country, having surpassed China, Turkey and Iran last week.

The spread of the coronavirus has posed special challenges to the worldwide nuclear industry, where small groups of highly trained specialists are required to safely run reactors and manage nuclear fuel and radioactive waste services in close quarters.

In Russia, Rosatom has moved to sequester its nuclear technicians onsite at the plants and facilities where they work to minimize their exposure to carriers of the coronavirus.

Rosenergoatom, which is the utility subsidiary of Rosatom, hasn’t made clear precisely how many Russian nuclear workers have been put in isolation or ordered to shelter at their plants. But Rosatom controls a sprawling network of reactors, laboratories, commercial structures and fuel fabrication facilities that employ some 250,000 people.

Russia’s 11 commercial nuclear power plants operate a total of 38 nuclear reactors. Rosatom also has 36 power units at different stages of implementation in 12 countries around the world. It is currently constructing seven reactors overseas: two each in Bangladesh, Belarus and India, plus one unit in Turkey.

In a sense, Russian nuclear power and scientific facilities are uniquely designed to handle outbreaks of the virus. The 10 so-called “closed administrative territorial formations,” which were formed as part of the Soviet nuclear weapons program, are nearly entirely off limits for foreigners as well as most Russians.

The cities hosting Russia’s commercial nuclear plants, while less strictly separated from the rest of the country, are nonetheless surrounded by checkpoints and often require foreign visitors to get special permission to enter them.

As of last week, said Likhachev in his address, there were 47 infected personnel among the corporation’s ranks.

In the city of Sarov alone, there have been 23 cases of Covid-19, Likhachev said – 7 of them Rosatom employees.

Known formerly as Arzamas-16, Sarov didn’t even appear on maps until the breakup of the Soviet Union in 1991. It remains home to the All-Russian Scientific Research Institute of Experimental Physics, which made headlines last year when five of its scientists died in a mysterious radiological explosion at a weapons testing site in the Russian Arctic.

The institute, which is now run by Rosatom, is an important part of Russia’s nuclear military complex. Likhachev indicated in his address that the workers who tested positive are affiliated with the institute.

Other cities important to Russian nuclear research that have been impacted by the coronavirus include Obninsk, site of Russia’s first commercial nuclear reactor, Udomlya, where the Kalinin Nuclear Power Plant is located, and Zheleznogorsk, a closed city in Siberia that used to produce weapons grade plutonium for Soviet nuclear missiles.

The first Russian nuclear power to isolate itself in order to minimize the exposure of its worker to the virus was the Beloyarsk nuclear power plant 1,800 kilometers to Moscow’s east, where technicians have been living in an onsite dormitory since late March. The general order for the rest of Russia’s nuclear plants to sequester their reactor and safety technicians followed from Rosenergoatom on April 3.

Things have not always gone according to plan. In late April, several of the 270 workers isolated at the Rostov nuclear plant took to social media to complain they were being treated like “cattle.”

One worker complained on Instagram that there were too few toilets, too many non-functional showers, and that the onsite dormitory was littered with dead cockroaches. Upper level management at the plant, meanwhile, had been accorded much better conditions.

Rosatom later said that it had rectified the difficulties reported by the Rostov plant workers.