The failed missile test that killed five Russian scientists on the White Sea last week involved a small nuclear reactor, Russian officials said Sunday.
In a video interview published late on Sunday, Russian officials at the nuclear research institute where the scientists had worked said the accident had caused a two-fold rise in radiation levels that had only lasted an hour.
Vyacheslav Solovyev, one of the officials at the institute in the closed city of Sarov, did not spell out exactly what the nuclear experts had been working on, but said the institute worked on nuclear-powered energy sources.
He said its work served both civilian and military ends.
"One of the lines (of research and development) is the creation of sources of thermal or electric energy using radioactive materials, including fissile materials and radioisotope materials," he said.
"These developments are also actually happening in many countries. The Americans last year...also tested a small-scale reactor... Our center also continues to work in this direction."
Valentin Kostyukov, head of the nuclear center, which is part of state nuclear agency Rosatom, said a year of careful work had been carried out ahead of the test and that a state commission was investigating what went wrong.
The nuclear experts battled to control the situation, but were unable to prevent the accident, Kostyukov said. He called them "national heroes" and said the institute had asked for them to be given posthumous state awards.
"These people were the elite of the Russian Federal Nuclear Centre and have tested under some of the most incredibly difficult conditions," he added.
Russia's state nuclear company Rosatom said the accident occurred while a missile was being tested on an offshore platform in the far northern Arkhangelsk region. It said that the missile used "isotope power sources." Fuel exploded and the blast blew staff into the sea, it said.
Though the Defense Ministry initially said no change in radiation was detected after Thursday's explosion, local officials in the nearby city of Severodvinsk said radiation had briefly spiked, without saying how high.
Anxious local residents stocked up on iodine, used to reduce the effects of radiation exposure.
Norwegian authorities said they increased radiation monitoring in response to the incident, but did not detect anything abnormal. Finland did not expect to be affected by the blast, due to southerly winds and distance, the direct of Finland's Radiation and Nuclear Safety Authority, Pia Vesterbacka, told Bloomberg on Monday.
Professor Jeffrey Lewis of the Middlebury Institute of International Studies said his "working hypothesis" was that the blast was related to the 9M730 Burevestnik nuclear-powered cruise missile, known by NATO as SSC-X-9 Skyfall.
In February, Putin said that tests on the Burevestnik were going successfully.
A state commission is investigating the accident.
Moscow has a history of secrecy over accidents, most notably after an 1986 explosion at the Chernobyl nuclear power plant in the former Soviet republic of Ukraine, which is regarded as the worst nuclear mishap in history.