In early January, 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.
This, along with the deployment of an American WC-135 aircraft to the Mildenhall Royal Air Force Base in the UK, has led to speculation that the Russians have carried out a nuclear test. This is highly unlikely for several reasons.
The results in the first paragraph, along with a bit of other information, suggests the origin of the iodine-131. That isotope is frequently used in medical procedures. I have experienced one myself; a benign thyroid tumor producing too much thyroid hormone was shrunk by the intense but localized radiation. It is produced by neutron irradiation of tellurium or by nuclear fission of uranium-235. Its half-life is a little over eight days, so its detection means it was produced within the last month or so.
When people (or sometimes animals) are treated with iodine-131, they must stay from others for a few days to avoid irradiating them. By then end of three days, they are safe, partly because of the isotope’s half-life, but also because the isotope is eliminated from the body into urine. That urine goes down the toilet into the sewage system, or perhaps in kitty litter. The amounts in the sewage system are diluted enough that they are not dangerous to workers.
The iodine-131 detected over Europe was particulate; that probably means that it was adsorbed onto microscopic dust particles. If it were pure, it would be gaseous. 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 from spray processing of sewage. Alternatively, a facility producing medical isotopes may have had a small release; 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) Organization’s International Monitoring System would detect. No such event was detected by them 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 the Russians are 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 junked at sea and operational ones are serviced. 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, fitted for detection of atmospheric evidence of nuclear tests, appears to be routine. The two that are now active are rotated to air bases around the world. Because of concerns about Novaya Zemlya, which predate the iodine-131 detection, it is not surprising that one would be deployed to the UK. Since levels have returned to normal, there is not much reason for it to check them at altitude, although it would not be surprising if that were done. Results may or may not be released; unlike the International Monitoring System results, these are the property of the United States government.
Another possibility would be a release from a civilian nuclear reactor, but, again, one would expect to see other fission products as well.
Until there is more evidence, my conclusion is that medical iodine-131 from sewage plants, which is becoming more common, behaved as natural radionuclides do during a temperature inversion and became slightly more concentrated.
Update: The CTBTO has issued a statement that no other fission isotopes were observed. The statement also says that iodine-131 has not been observed above local historical levels. So my conclusion stands. h/t to Jeffrey Lewis.
Top Photo: A WC-135W Constant Phoenix aircraft performs touch ‘n go landing exercises Feb. 12 at Offutt Air Force Base, Neb. (U.S. Air Force photo/Josh Plueger)