Radioactive sensors suddenly go offline two days after mysterious nuclear explosion in Russia

Russia has insisted there is nothing to worry about after the operator of a series of radiation monitoring stations went offline following a mysterious nuclear explosion at a missile testing site.

The Comprehensive Nuclear-Test-Ban Treaty Organisation (CTBTO), headquartered in Austria, said on Monday its two Russian sites closest to the blast went offline days after, soon followed by two more.

The revelation fuelled suspicions Russia had tampered with them, but Kremlin spokesperson Dmitry Peskov said on Tuesday he was confident government agencies in charge of the radiation monitoring stations did their job properly.

He added he did not have precise information about whether and how those stations transmitted data.

Russia's state nuclear agency, Rosatom, has acknowledged nuclear workers were killed in the explosion on 8 August, which occurred during a rocket engine test near the White Sea in far northern Russia.

The explosion caused a spike in radiation in a nearby city and prompted a local run on iodine, which is used to reduce the effects of radiation exposure.

Russian authorities have given no official explanation for why the blast triggered the rise in radiation. US-based nuclear experts have said they suspect Russia was testing a nuclear-powered cruise missile vaunted by president Vladimir Putin last year.

"We're ... addressing w/ station operators technical problems experienced at two neighbouring stations," Lassina Zerbo, head of the CTBTO, said on Twitter overnight.

The CTBTO's International Monitoring System includes atmospheric sensors that pick up so-called radionuclide particles wafting through the air. Mr Zerbo said data from stations on or near the path of a potential plume of gas from the explosion were still being analysed.

While the CTBTO's monitoring network is global and its stations report data back to headquarters in Vienna, those stations are operated by the countries in which they are located.

The two Russian monitoring stations nearest the explosion, Dubna and Kirov, stopped transmitting on 10 August, and Russian officials told the CTBTO they were having "communication and network issues", a CTBTO spokeswoman said on Monday.

"We're awaiting further reports on when the stations and/or the communication system will be restored to full functionality."

It is not clear what caused the outage or whether the stations were tampered with by Russia.

Mr Zerbo also tweeted a simulation of the explosion's possible plume, showing it reaching Dubna and Kirov on 10 and 11 August, two and three days after the explosion.

The spokeswoman said later that two more stations, Bilibino and Zalesovo, stopped transmitting data on 13 August. Bilibino is in far eastern Siberia, outside the map of the simulated plume that Mr Zerbo tweeted. But that simulation also showed the plume reaching Zalesovo on 13 August.

"About 48 hours after the incident in Russia on August 8, these stations (Dubna and Kirov) stopped transmitting data. I find that to be a curious coincidence," said Daryl Kimball, executive director of the Arms Control Association, a Washington-based think tank.

He and other analysts said any Russian tampering with monitoring stations would be a serious matter but it was also likely to be futile as other CTBTO or national stations could also pick up telltale particles.

"There is no point in what Russia seems to have tried to do. The network of international sensors is too dense for one country withholding data to hide an event," said Jeffrey Lewis, director of the East Asia Non-Proliferation Program at the Middlebury Institute in California.

Rosatom has said the accident, which killed five of its staff, involved "isotope power sources".

The CTBTO's monitoring system comprises more than 300 seismic, hydroacoustic, infrasound and radionuclide stations dotted around the world that together are aimed at detecting and locating a nuclear test anywhere.

Its technology can be put to other uses, however, such as when the CTBTO passed on data from two hydroacoustic stations to Argentina in 2017 to help locate the wreckage of a missing submarine.

Additional reporting by Reuters