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Russia has bestowed posthumous awards on five nuclear experts and "national heroes" who died in a mysterious explosion at sea during a rocket engine test, authorities said on Sunday.
Officials have been drip-feeding information about the blast on a platform in the White Sea off northern Russia that caused a radiation spike in a nearby city.
US-based nuclear experts said they suspected the explosion occurred during the testing of a nuclear-powered cruise missile vaunted by President Vladimir Putin last year.
The rocket's fuel caught fire, causing it to detonate and knock several people into the sea.
"The testers are national heroes," said Valentin Kostyukov, head of a nuclear centre, which is part of Russia's state nuclear agency Rosatom.
"These people were the elite of the Russian Federal Nuclear Centre and have tested under some of the most incredibly difficult conditions," he added, according to a statement from Rosatom.
The five, who worked for the centre based at the closed city of Sarov, would be given state awards, Kostyukov said, without specifying what. Sarov's administration announced two days of mourning, saying the experts died while "performing the task of national importance."
Rosatom named the five as: Alexei Vyushin, Evgeny Koratayev, Vyacheslav Lipshev, Sergei Pichugin and Vladislav Yanovsky.
Though the Defence 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.
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.