The failed missile test that ended in an explosion killing five scientists last week on Russia's White Sea involved a small nuclear reactor, according to a top official at the institute where they worked.

The institute is working on small-scale power sources that use "radioactive materials, including fissile and radioisotope materials" for the Defence Ministry and civilian uses, Vyacheslav Soloviev, scientific director of the institute, said in a video shown by local TV.

The men, who will be buried on Monday, were national heroes and the "elite of the Russian Federal Nuclear Centre," institute Director Valentin Kostyukov said in the video, which was also posted on an official website in Sarov, a high-security city devoted to nuclear research less than 400 kilometers east of Moscow.

The blast occurred on August 8 during a test of a missile that used "isotope power sources" on an offshore platform in the Arkhangelsk region, close to the Arctic Circle, Russia's state nuclear company Rosatom said over the weekend. The Defence Ministry initially reported two were killed in the accident, which it said involved testing of a liquid-fuelled missile engine. The ministry didn't mention the nuclear element.