The company told the media on Saturday that its engineering and technical team had been working on the "isotope power source" for the propulsion system. On August 10, Rosatom said that five employees were killed in the accident and three more were hospitalized.
"The testers are national heroes", said Valentin Kostyukov, head of a nuclear center, which is part of Russia's state nuclear agency Rosatom. Earlier, the authorities in a nearby city said the accident had caused a spike in radiation levels but the military denied this.
The incident occurred in the far northern Arkhangelsk region during testing of a liquid propellant jet engine when an explosion sparked a fire, killing two, a defense ministry statement said.
The missile was being tested on a platform at sea when its fuel caught fire and triggered an explosion, Rosatom said in a statement quoted on Russian television.
Rosatom's statement didn't say exactly what hardware was involved, but the New York Times pointed to clues that it might have been an experimental weapon.
The ministry statement came after officials in the city of Severodvinsk, roughly 30 kilometers (19 miles) away from the test site, said on their website that automatic radiation detectors in the city "recorded a brief rise in radiation levels" around noon on Thursday.
Russian authorities had previously said that two people had been killed in the incident and that a nearby city had reported a rise in radiation levels when the rocket engine blew up at a testing site in the Archangelsk region on Friday.
Military and government authorities have given scant details about the incident, which the Defense Ministry said occurred in the Barents Sea and was one of Russia's worst submarine disasters in years.
The post was later taken down and the defence ministry said radiation levels were normal after the accident.
A number of pharmacies in the region, including chains Apteka29 and VITA, said they've experienced shortages of iodine tablets after local residents rushed to buy them following the blast.
US -based nuclear experts said on Friday they suspected the blast and radiation release occurred during the testing of a nuclear-powered cruise missile.
The blast near Severodvinsk is one of the three incidents which took place in Russian Federation this week as two explosions rocked an ammunition storage facility in the Krasnoyarsk region injuring dozens. The Russian military has, meanwhile, refuted a report that a Rosatom vessel, capable of collecting radioactive waste, was dispatched to Severodvinsk, saying it had actually been undergoing scheduled trials.