Last Thursday’s explosion killed caused radiation readings in neighbouring cities in north-western Russia to spike 20 times their normal level.
Russia’s defence ministry initially said the explosion had taken place during testing of a rocket engine.
Its fuel reportedly caught fire, causing it to detonate and knock several people into the White Sea in the sub-Arctic Arkhangelsk region.
However, the country’s nuclear agency Rosatom later confirmed it was an “isotope power source in a liquid propulsion system”.
Vyacheslav Soloviev, scientific director for the institute developing the weapon, also said Russia is working on power sources using “radioactive materials, including fissile and radioisotope materials”.
Experts believe Russian scientists were testing the 9M730 Burevestnik, dubbed SSC-X-9 Skyfall by NATO.
The nuclear powered cruise missile was unveiled by Russian President Vladimir Putin in a state of the nation address last year.
Putin believes the Burevestnik’s nuclear propulsion would give it “unlimited” range.
But Mark Galeotti, a leading Russia analyst and researcher at the Royal United Services Institute (Rusi), told the BBC: “There is a lot of scepticism about whether the Burevestnik will ever see the light of day.”
“The best (thing) for their memory will be our further work on the new weapons”
Alexei Likhachev, head of nuclear agency Rosatom
He added: “There is speed versus the weight of the system, and the risk of a missile that spews radioactive exhaust wherever it goes.
“These new systems have their origin in Soviet times – they’ve been taken off the shelves and given new investment.”
Rosatom’s head Alexei Likhachev said of the victims: “The best (thing) for their memory will be our further work on the new weapons.
“We are fulfilling the task of the motherland, its security will be reliably ensured.”
He heads the Sarov nuclear centre, a secret Cold War-era facility responsible for Russia’s hydrogen bomb arsenal.
The five fatalities were named as:
Alexei Vyushin, a designer and software specialist.
Yevgeny Korotayev, a senior electrical engineer.
Vyacheslav Lipshev, head of the scientific testing team.
Sergei Pichugin, a testing engineer.
Vladislav Yanovsky, deputy head of the scientific testing department.
Three other engineers were injured in the blast and are now recovering in hospital.