French nuclear agency (ISRN) on February 13th that it was first detected six weeks prior to that.
Iodine-131 (131I), a radionuclide of anthropogenic origin, has recently been detected in tiny amounts in the ground-level atmosphere in Europe. The preliminary report states it was first found during week 2 of January 2017 in northern Norway. Iodine-131 was also detected in Finland, Poland, Czech Republic, Germany, France and Spain, until the end of January.Anthropogenic, of course, means that it's typically only created by human efforts. Certainly, it could have come from a fission bomb, but there are global networks to detect nuclear tests and they don't report anything suspicious. Where else does it come from? It turns out that this isotope is routinely produced for medical testing. Could it be a lab accident?
Let's ignore that for a moment and talk numbers. The ISRN report says
In France, particulate 131I reached 0.31 µBq/m3 and thus the total (gaseous + particulate fractions) can be estimated at about 1.5 µBq/m3. These levels raise no health concerns.What's a µBq/m3? It gets a bit dense with details for a bit here, so I'm going to lift most of this explanation from PJ Media's Charlie Martin. That abbreviation represents a rate, and means 1 micro (millionth of) 1 Bequerel per cubic meter. A Bequerel is a measure of how much radioactive material there is. 1 Bq means that one atom is decaying every second. Saying 1.5 micro Bequerel implies one millionth of one atom is decaying but that just can't happen; atomic decay only applies to whole atoms. To turn that into a whole number, you have to multiply the whole thing by 2 million, which says 3 atoms are decaying in 2 million cubic meters per second.
Two million cubic meters is a big volume, but does it help you visualize it better if I say 3 atoms are decaying in 70.63 million cubic feet? Does it help if I convert that to 528 million gallons? Neither of those help me visualize this large number, but Charlie Martin used an interesting example. Remember seeing the picture of a German airship called the Hindenberg? It had a volume of 200,000 cubic meters. That's convenient because it means 10 of these Zeppelins would have 3 atoms of Iodine 131 decaying.
Banana Equivalent Dose (BED), an indication of the amount of radioactivity contained in a typical banana. There's a little unit shuffling here (we're not talking cubic meters, after all), but it turns out that a BED is 15 Bequerels, or five times as much radioactive decay as in 10 Hindenbergs.
The French ISRN stated almost immediately the levels were so low that this 131I is not a risk to health, and we see it's about equal to eating 1/5 of a banana. Those agree since we know bananas aren't dangerous, except possibly calorically, if you have 27 bananas in one pie all in one sitting.
The question, then, is still "where did it come from?" 131I is actually an important medical isotope, used for treatment mainly of thyroid disease -- thyroid cancer or Grave's disease. When it's used, the patient basically eliminates it through urine, and yes, the urine has to be treated as low-level radioactive waste, but inevitably a little bit escapes, especially places where maybe they aren't quite as careful as in the United States. Then it gets into the atmosphere, possibly from spray treatments of sewage, or something that creates an aerosol.
It almost certainly wasn't from a nuclear weapons test, and there's a good discussion of why here. In short form, other signs would have been detected.
The most likely scenario, then, is that some patients were given 131I for a thyroid disease, and went home. Within a few hours, they urinated most of it into the sewer system, where some of it seems to have been discharged into the environment. Maybe something malfunctioned at the sewage facility. The radioactive decay will be gone within a couple of months (8 half lives would be 64 days) but was never a health hazard. The only reason we know about it is that we can detect radiation at such absurdly low levels.