A mystery explosion at a Russian weapons testing range involved radioactive materials, the authorities admitted on Saturday, as the blast’s admitted death toll rose and signs of a creeping radiation emergency, or at the least fear of one, grew harder to mask.
In a statement released at 1 a.m. Saturday, Russia’s nuclear energy company, Rosatom, said five employees had died, in addition to the two military personnel previously confirmed dead, as a result of a test on Thursday morning involving “isotopic sources of fuel on a liquid propulsion unit.”
“A bright memory of our comrades will forever live in our hearts,” the statement said.
The statement, though, shed little light on exactly what detonated on Thursday at the White Sea testing range. No use for the propulsion unit was mentioned, although President Vladimir V. Putin previously boasted that Russia has developed a nuclear engine for long-range missiles. And there was no explanation why the authorities in a nearby city had reported rising radiation levels for a brief period several hours later.
While the government has provided no full explanation of what happened, Rosatom’s statement suggested a mishap during a test of a new class of nuclear-engined weapons that Mr. Putin first spoke publicly about last year.
At the least, the statement came as the first formal acknowledgment from a central government source that radioactive materials had been involved in the accident. It offered no details on the materials used and potentially released into the environment. It said the deaths were “a result of an incident at a testing range in Arkhangelsk region.”
With information scarce, residents in cities near the accident site in Russia’s far north were taking no chances. On Friday, there was a run on pharmacies for medicines containing iodine, believed to be of some help against radiation poisoning, the Russian news media reported. Pharmacies in Arkhangelsk reportedly ran out.
A news site, Baza, released a video it said showed ambulances delivering injured people to a Moscow hospital. The vehicle doors were sealed with plastic sheeting, apparently to prevent the release of contamination from the patient’s bodies, and the drivers wore white protective suits.
A Russian maritime authority, the Administration of Western Arctic Ports, announced on Thursday that shipping would be prohibited for a month in Dvina Bay, which is in an area of the White Sea close to the military range where the explosion took place.
Then on Friday, the Russian news media reported that a specialized ship used for collecting and storing liquid nuclear waste from the country’s nuclear-powered icebreaker program had sailed into the area.
The explosion on Thursday occurred at a naval weapons testing site near the village of Nenoska that has been used for missile tests. The nuclear company’s statement did not say whether the explosion or radiation exposure had killed its employees.
Novaya Gazeta, an independent Russian newspaper, interpreted the presence of Rosatom nuclear engineers at the test site as confirming “the version that the military could have been experimenting with the newest rocket with a nuclear power unit.”
One new weapon Mr. Putin had discussed was a globe-spanning cruise missile called Burevestnik or the Petrel, named for the far-flying seabird. It would have an unlimited range thanks to a nuclear propulsion unit, he said. Mr. Putin said the device had already been tested.
“Russia’s advanced arms are based on the cutting-edge, unique achievements of our scientists, designers and engineers,” Mr. Putin said in the 2018 speech. “One of them is a small-scale heavy-duty nuclear energy unit that can be installed in a missile.”
On Thursday, Russia’s military said that the fire occurred when a liquid-fueled rocket engine exploded, but that radiation levels remained at normal background levels, contradicting reports from the municipal authorities in the nearby city of Severodvinsk.
The city government had posted a statement online saying two meters had registered “a short-term elevation in radiation” without identifying the cause. The statement was subsequently taken offline.
The state-run Tass news agency had earlier cited the Defense Ministry as offering a different assessment, saying that “there was no release of toxic materials into the atmosphere, and the radiation level is normal.”
Both Russia and the United States have ramped up development on new weapons as Cold War-era disarmament treaties have unraveled. On Aug. 2, for example, the United States formally withdrew from one keystone treaty, the Intermediate-Range Nuclear Forces Treaty, after accusing Russia of flouting the terms.
It’s unclear how far along Russia is with its new weapons. In announcing their development, Mr. Putin had shown only a cartoonish animation. It illustrated a missile looping gently around the world’s oceans and striking the United States after flying near Antarctica.
The United States abandoned publicly acknowledged research on using nuclear energy to power aviation in the 1960s.