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The explosion killed five scientists and led to a nearby village being temporarily evacuated.
Russian state organisation Roshydromet has identified four radioactive substances after samples were taken from the city where an explosion happened a few weeks ago.
The substances were identified as 'technogenic radionuclides' - strontium-91, barium-139, barium-140 and lanthanum-140.
Norwegian nuclear safety expert Nils Bøhmer said these findings confirm that it was a nuclear reactor that exploded.
He told The Barents Observer: "The presence of decay products like barium and strontium is coming from a nuclear chain reaction. It is a proof that it was a nuclear reactor that exploded."
Russian state nuclear agency Rosatom said at the time that rocket fuel caused the explosion, writing that the accident occurred 'during tests on a liquid propulsion system involving isotopes' in a statement on 10 August.
However, experts say the four radioactive substances identified by Roshydromet are inconsistent with this explanation.
Andrei Zolotkov, a chemist who worked on Russia's nuclear icebreaker fleet for 35 years, told The Guardian the kind of 'propulsion system' Rosatom described 'uses just one radionuclide, and during its decay, and it cannot produce these kinds of isotopes'.
There have been widespread fears of radiation sickness following the blast. As many as 60 medics who treated the victims of the explosion were sent to Moscow for medical tests.
However, the radioactive substances Roshydromet identified have short-lived effects, with half-lives ranging from just 83 minutes to 12.8 days, according to Business Insider.
Rosatom's statement on 10 August read: "Five Rosatom staff members died and a further three people were injured in a tragic accident that took place during tests on a liquid propulsion system involving isotopes at a military facility in Arkhangelsk region.
"We offer our deepest condolences, and all possible support, to the families and friends of those who died.
"Those injured have been admitted to hospital and are receiving treatment. Our thoughts are with the loved ones of all those affected."
The five engineers who were killed during the incident were named as Alexey Vyushin, Yevgeny Koratayev, Vyacheslav Lipshev, Sergey Pichugin and Vladislav Yanovsky.
The Guardian contacted Rosatom to ask if there had been a reactor blast, and were referred back to the organisation's original statement.
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