During the 2nd week in January 2017, slightly elevated levels of iodine-131 were observed over northern and western Europe. The levels were measured during a temperature inversion, along with elevated levels of naturally occurring radioisotopes.
The deployment of an WC-135 (Constant-Phoenix) air monitoring aircraft from Mildenhall Royal Air Force Base in the UK, to further monitor elevated levels of iodine-131 in the atmosphere, led to initial speculation that Russia had carried out a nuclear test or an unplanned event had occurred in that region.
Iodine-131 isotope is frequently used in medical procedures and is produced by neutron irradiation of tellurium or by nuclear fission of uranium-235. Its half-life is a little over eight days (8.02d), so its detection suggested it was produced around end December 2016 to the beginning of January 2017.
The iodine-131 detected over Europe was particulate; meaning that it was adsorbed onto microscopic dust particles. The fact that it was measured along with elevated levels of naturally occurring radioisotopes suggests that it is from medical uses, perhaps lofted in particles (Gaussian plume) from spray processing. Alternatively, a facility producing medical isotopes may have had a small release; although this should have been reported, and there seems no concentration that would suggest such a release.
At least two facts point away from nuclear testing as the source. A nuclear test, even a relatively small one, would give a seismic jolt that the Comprehensive Nuclear Test Ban Treaty (CTBT) Organisation’s International Monitoring System would detect. No such event was detected or by other seismologists. Additionally, other isotopes from fission would have been detected by air sampling. No others have been reported.
The Russian nuclear test site is on Novaya Zemlya island, not far from where the iodine-131 was first reported. Construction has been active at Novaya Zemlya, and there has been speculation that Russia is considering restarting nuclear testing. Without confirming seismic and isotopic data, a nuclear test is improbable.
Also not too far from the initial detection is the Russian nuclear submarine base at Murmansk. Old submarines have been partially decommissioned and operational boats regularly serviced at the base. It is possible that the iodine-131 was released here, but the lack of other fission isotopes makes this less likely.
The deployment of a WC-135 Constant Phoenix aircraft, are fitted for detection of atmospheric monitoring equipment to gather evidence of nuclear tests, releases. Two of these aircraft remain active, and are rotated to air bases around the world, initially due to concerns regarding Novaya Zemlya, which predate the iodine-131 detection. Since levels have returned to normal, there is no further requirement to check at altitude.
Another possibility for detection of the iodine-131 would be a release from a civil nuclear reactor, although, again, one would expect to see other fission products present.
Until there is more evidence, the current general conclusion is that medical iodine-131 probability released into a sewage system / plant, behaving as a natural radionuclide during a temperature inversion (i.e. becoming slightly more concentrated) was the likely cause.
Update: The CTBTO issued a statement on 20 February 2017, stating that no other fission isotopes have been observed. The statement also conveys that iodine-131 has not been observed above local historical levels since this event