Russian scientists have indicated that they were working on miniaturised sources of nuclear energy when a rocket engine exploded last week, increasing scrutiny of the possibility that the accident occurred while testing an experimental cruise missile powered by a small reactor.

The explosion last Thursday at a military testing ground in Russia’s Arkhangelsk region killed at least five people and caused radiation readings in neighbouring cities to spike to 20 times their normal level for half an hour.

Russia’s defence ministry said the explosion had taken place during testing of a rocket engine, but the country’s nuclear agency, Rosatom, later confirmed that several of its employees had been killed during testing of an “isotope power source in a liquid propulsion system”.

David Cullen, the director of the Nuclear Information Service in the UK, said on Monday that the view among independent experts was that the explosion appeared to have been caused by the failure of an experimental nuclear-powered cruise missile known in Russia as the 9M730 Burevestnik and by Nato as the SSC-X-9 Skyfall.

Q&A What is the 9M730 Burevestnik missile? Show Hide The Burevestnik (Storm Petrel) experimental cruise missile – called "Skyfall" by Nato – is an ambitious nuclear-powered cruise missile in development by Russia's armed forces. The concept builds on an idea originally examined by the US in the late 1950s: a nuclear-powered missile able to fly vast distances powered by an on-board reactor, tracing a complex flight path to outflank enemy defences. Essentially the idea has been to develop a missile with effectively no limitation of conventional fuel range, which could fly around the globe multiple times or trace a flight path – for instance – over the Pacific and through South America to target the US via Mexican airspace, making conventional anti-missile defences, as they are currently deployed, redundant. The US air defence network is configured on the assumption any air attack would come from the north, west, or east, but not the south. By comparison the US Tomahawk cruise missile has a range of 1500 miles. Dismissed by the US decades ago because of the dangers and technical problems involved, the missile has been pursued by Russia in the last few years along with other vehicles designed to outfox conventional defences including high-speed underwater drones and hypersonic glide vehicles. According to what little is known about the project, the missile is designed to be launched using a traditional jet propulsion motor before the nuclear power plant takes over in the air. While it would be a formidable weapon if successfully deployed, leapfrogging US missile capabilities, doubts remain among researchers as to whether its core technology is viable. Peter Beaumont

Announced by the Russian president, Vladimir Putin, in 2018, the missile is theoretically able to use an onboard source of nuclear fuel to heat fast-moving air and fly for indefinite periods. Moscow has touted the missile’s unlimited range and potential ability to penetrate US missile defences, but doubts remain among researchers as to whether its core technology is viable.

Experts in Russia’s nuclear programme have also spotted the nuclear fuel carrier ship Serebryanka near the site of last week’s explosion, where it is believed to have been taking part in a recovery effort. The vessel had previously been spotted in the waters near Novaya Zemlya, where Russia reportedly tested the Burevestnik missile in 2017.

“Our suspicion is that something went wrong during or after a Russian test of its nuclear-powered cruise missile,” wrote Jeffrey Lewis, a US arms control specialist with the Middlebury Institute of International Studies. Russia was believed to have moved Burevestnik testing to the current Nenoksa site earlier this year.

In a video statement released on Sunday evening, an official from the Russian Federal Nuclear Centre, where five employees were killed in the explosion, said the agency was working on a number of experimental technologies, including “miniaturised sources of energy using [fissile] materials”.

Vyacheslav Solovyov, the centre’s scientific director, said similar work on “small-scale nuclear reactors” is also taking place in the US. He did not say how much fissile material had been involved in the accident, or what role it may have played in the explosion.

“We are now trying to understand, we are working closely with a government commission, analysing the entire chain of events to assess the scale of the accident and to understand its causes,” Solovyov said.

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The slow release of information has heightened concerns that a major incident took place offshore from the Nenoksa missile testing site. Residents of Severodvinsk and other northern cities had reportedly stocked up on iodine, which reduces the effects of exposure to radiation. Russia has also closed a bay in the White Sea near where the accident took place for a month, indicating possible contamination or a continuing search operation.

The development of an experimental weapon such as the Burevestnik would also help to explain the secrecy around the incident, despite public concerns over radiation levels.

The US tried and failed to develop ground-hugging nuclear-powered cruise missile technology during the cold war, and questions remain as to whether the Burevestnik has a meaningful purpose, given the existence of high flying intercontinental ballistic missiles as an alternative.

Such experimental long-range cruise missiles would not have been covered by the intermediate-range nuclear forces treaty between the US and Russia that lapsed last month, because it only banned land-based cruise missiles with a range of between 500 and 5,500km (300 and 3,400 miles).

Cullen said, however, that the test failure was “symptomatic of a wider escalation in nuclear tensions stemming from a gradual breakdown in US-Russia relations” partly because “Russia is trying to rectify what it believes is a strategic disadvantage it has against the US by developing non-conventional capabilities”.