More than 100 medical workers who helped victims of the mysterious blast at a military testing range underwent checks, officials said on Friday, adding that one was found with a low amount of radioactive caesium-137 in his muscle tissue.
The Federal Medical and Biological Agency concluded with a "fair degree of probability" the isotope, which can de deadly in high doses, came from something he ate and that it was not threatening to the man's health.
The statement followed Russian media reports claiming dozens of medical workers were exposed to radiation.
The 8 August incident at the Russian navy's range in Nyonoksa on the White Sea killed two servicemen and five nuclear engineers and injured six. It was followed by a brief rise in radiation levels in nearby Severodvinsk, but the authorities insisted it posed no danger.
Reports claimed medical teams at the Arkhangelsk city hospital had not been warned they would treat people exposed to radiation and lacked elementary protective gear. They said Russia's security agency forced the medical workers to sign non-disclosure papers.
The workers, who spoke on condition of anonymity fearing official reprisals, said many doctors and nurses felt angry about the authorities putting their lives at risk by concealing the vital information.
Kremlin spokesman Dmitry Peskov shrugged off the reports, questioning the veracity of anonymous sources. He also alleged certain forces which he did not name could be interested in making false allegations about radioactive threats.
The changing and contradictory accounts of the incident by Russian officials drew comparisons to Soviet attempts to cover up the 1986 explosion and fire at the Chernobyl nuclear power plant in Ukraine, the world's worst nuclear disaster.
The Russian defence ministry at first denied any radiation leak in the incident even as the authorities in nearby Severodvinsk reported a spike in radiation and advised people to stay indoors and close windows. Terrified residents rushed to buy iodine, which can help reduce risks from exposure to radiation.
Russia's state weather and environmental monitoring agency said the peak radiation reading in Severodvinsk on 8 August briefly reached 1.78 microsieverts per hour in just one neighbourhood — about 16 times the average. Peak readings in other parts of Severodvinsk varied between 0.45 and 1.33 microsieverts for a couple of hours before returning to normal.
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The Comprehensive Nuclear Test Ban Treaty Organization said earlier this week that several Russian radiation monitoring stations went silent shortly after the explosion. Observers saw that as part of an effort to conceal the radiation data, which could help determine the technology that was being tested at the time of the explosion.
President Vladimir Putin has hailed the victims, saying they were doing "very important work for the nation's security", but kept silent on what type of weapon they were testing.
Some, including US president Donald Trump, said the explosion occurred during tests of Russia's prospective Burevestnik nuclear-powered cruise missile, which was codenamed "Skyfall" by Nato.
Additional reporting by AP