Radiation sensors in Stockholm have detected higher-than-usual but still harmless levels of isotopes produced by nuclear fission, probably from somewhere on or near the Baltic Sea, a body running a worldwide network of the sensors said on Friday.
The Comprehensive Nuclear-Test-Ban Treaty Organisation (CTBTO) oversees a network of hundreds of monitoring stations that use seismic, hydroacoustic and other technology to check for a nuclear weapon test anywhere in the world. That technology can, however, be put to other uses as well.
One of its stations scanning the air for radionuclides - telltale radioactive particles that can be carried long distances by the wind - detected unusually high levels of three radionuclides earlier this week: caesium-134, caesium-137 and ruthenium-103.
The Stockholm monitoring station "detected 3isotopes; Cs-134, Cs-137 & Ru-103 associated w/Nuclear fission @ higher[ ] than usual levels (but not harmful for human health)", CTBTO chief Lassina Zerbo said on Twitter on Friday evening.
Zerbo's post included a borderless map showing where the particles might have come from in the 72 hours before they were detected - a large area (here) covering the tips of Denmark and Norway as well as southern Sweden, much of Finland, Baltic countries and part of western Russia including St Petersburg.
"These are certainly nuclear fission products, most likely from a civil source," a spokeswoman for the Vienna-based CTBTO said, referring to the atomic chain reaction that generates heat in a nuclear reactor.
"We are able to indicate the likely region of the source, but it's outside the CTBTO's mandate to identify the exact origin," she added.