Nuclear radiation was detected in northwestern Russia after an explosion earlier this month.Credit: Sputnik/SPL

Rumours continue to swirl about a blast at a Russian naval base on 8 August, which killed five scientists and caused a short, unexplained spike in γ-radiation.

Information has been slow to emerge and confused by conflicting reports, but this week, Russia’s weather agency, Roshydromet, finally revealed details about the nuclear radiation that was released.

The information suggests that a nuclear reactor was involved in the blast, which lends weight to the theory that Russia was testing a missile known as Burevestnik, or Skyfall. President Vladimir Putin told Russia’s parliament in 2018 that the nation was developing the missile, which is propelled by an on-board nuclear reactor and could have unlimited range.

But because official information about the cause could be scarce, independent researchers are finding ways to glean more details about the explosion.

Nature examines the growing evidence.

What have official sources said about the blast?

The explosion happened at a military facility in northwestern Russia’s Arkhangelsk region. The region is home to Nenoksa, one of the Russian Navy’s major research and development sites.

A day after the blast, Russia’s nuclear agency, Rosatom, said that an accident happened during “tests on a liquid propulsion system involving isotopes” and later added that the incident happened on an offshore platform.

Meanwhile, Roshydromet reported a brief spike in γ-radiation at 16 times the normal level in the city of Severodvinsk, around 30 kilometres east of Nenoksa.

On 26 August, Roshydromet revealed the isotopes found in rain and air samples: strontium-91, barium-139, barium-140 and lanthanum-140.

What do we know about the scientists who died?

Rosatom named the dead scientists as Alexei Viushin, Evgeny Kortaev, Vyacheslav Lipshev, Sergei Pichugin and Vladislav Yanovsky. It’s not clear whether they were killed when thrown off the sea platform, or after being exposed to radiation.

Few details are known about the scientists’ research, which took place at the All-Russian Scientific Research Institute of Experimental Physics in Sarov. Viushin was a member of the ALICE collaboration at CERN, Europe’s particle-physics laboratory near Geneva, Switzerland, until at least 2016.

What do the isotopes tell us?

The detected isotopes of barium, strontium and lanthanum would be created in the core of a nuclear reactor, which produces energy by splitting uranium atoms in a chain reaction. These isotopes would have been released if a core exploded, says Claire Corkhill, a nuclear scientist at the University of Sheffield, UK.

Any damage an explosion might have caused to the reactor core would probably have led to the release of radioactive iodine and caesium, says Marco Kaltofen, a nuclear scientist at the Worcester Polytechnic Institute and the environment investigation firm Boston Chemical Data Corp, both in Massachusetts. An uncorroborated report in The Moscow Times on 16 August said that local doctors had traces of caesium-137 in their muscle tissue. And a Norwegian nuclear authority detected an unexplained spike in radioactive iodine-131 almost 700 kilometres away in Svanhovd after the blast. But this could be from another source: iodine-131 can be released in small quantities during the production of radionuclides for medical purposes, says Corkhill.

Boris Zhuikov, head of the Laboratory of Radioisotope Complex at the Institute for Nuclear Research of the Russian Academy of Sciences in Moscow, has an alternative explanation. His calculations show that if an explosion damaged the housing of a nuclear reactor, rather than the core, and caused a leak of radioactive noble gases — which are a product of fission — then by the time the nuclei reached the detector in Severodvinsk they would have decayed to leave precisely the isotopes observed.

But Kaltofen cautions that circumstantial evidence points to damage to a reactor core.

Does this mean that Russia was testing a nuclear-powered missile?

Some experts think so. Powering a missile is a plausible use for the huge amount of energy generated by nuclear fission, says Corkhill. Little is known about the Burevestintnik missile, but experts speculate that it could use liquid propellant to become airborne, then use a compact nuclear reactor to heat air that gets fired out the back to sustain flight — potentially for days.

Satellite images of Nenoksa taken hours before and after the blast also strongly point to a missile test, says Anne Pellegrino, a researcher at the James Martin Center for Nonproliferation Studies in Monterey, California. The images show launch infrastructure in Nenoksa that was also present at another site known to be associated with testing a nuclear-powered missile, she says. “The existence of that ship off the coast is a huge indicator,” she says.

What else could it be?

A nuclear-fission device could be part of a number of military nuclear-energy projects, says Michael Kofman, a researcher and Russia specialist at the non-profit research and analysis organization CNA and a fellow at the Wilson Center, both in Washington DC.

Kofman believes there is cause to doubt the Burevestintnik theory. He reasons that to be light enough to fly on a missile, the propulsion reactor would probably have no shielding, putting anyone around it at risk during its use. “It doesn’t make sense that Russian scientists would be standing around any sort of reactor that was being tested without adequate shielding in place,” he says. These missiles are also usually tested on land-based launchers, rather than platforms at sea, and such a test facility is visible on the coast, he adds.

This leads Kofman to deduce that the device was probably not a propulsion system for a missile. Other options include a nuclear-powered torpedo, a pressurized underwater nuclear reactor for powering undersea infrastructure, or a small reactor for space applications, he says.

What are researchers investigating?

Kaltofen is attempting to source objects such as car air filters from people who live near the blast area, to examine any radioactive elements on them. His team will compare this information to analyses of other objects irradiated by known sources, such as Japan’s Fukushima Daiichi nuclear power plant, which released significant amounts of radiation after it was damaged by an earthquake in 2011.

With enough filters, the method could work, says Corkhill, but they will need to be tested soon, before the radioactive isotopes decay.

Pellegrino’s team will look more closely at the scientists who died. The researchers will analyse the scientists’ social media, scientific publications and conference presentations, which could reveal clues about what they were working on.

The Comprehensive Nuclear-Test-Ban Treaty Organization (CTBTO), an international agency that detects nuclear-bomb tests, might also have data. It has eight stations across Russia that monitor radionuclides — but five of these had outages in the days after the blast, fuelling speculation that secretive weapons were involved. Two reporting stations have come back online and begun to backfill data, a CTBTO spokesperson told Nature.

Does the radioactivity pose a danger to the public?

The risk is low, says Zhuikov. The initial spike in γ-radiation was 16 times above background levels; by comparison, γ-radiation was 7,000 times above background levels after the 1986 reactor meltdown at Chernobyl.