it's produced in two ways, mainly. both of them are inside nuclear reactors. it can be produced from Tellurium being irradiated and then decaying, or it can be produced as a daughter-product from fission of plutonium, uranium and / or thorium. thing is tho, if there was a major leak from any reactors, including the ones at Tchernobyl, I-131 isn't the only radioisotope that would be produced. many, many others would also be produced and scattered at the same time.

it is, however, used as a medical isotope to treat thyroid cancers, which, probably not surprisingly, is the same type of cancer that it causes. this is because iodine naturally gathers around the thyroid glands. the important point tho is that it's used as a medical isotope. here's a website from Australia that talks about it's uses. if I-131 is discovered in the environment and other, longer lived radionucleonides aren't found with it, the most likely source for it would be a leak from a hospital or some other facility like it.

this is also completely beside the point. the point is that the accusation of I-131 being detected is hyperbolic and based on little more than speculation. for a start, only particulate traces were found, which means only a few particles. that wouldn't irradiate an ant. to continue with, looking at a map of where it was detected, the most likely candidate for it's origin is somewhere in France or Germany, or even Poland considering the levels detected there are about 15x anywhere else, and considering it wasn't found with other fission products, most likely a pharmaceutical facility rather than a nuclear one. the sniffer plane was planned to be transferred to England on that date for months already, and it didn't detect anything.

not to mention the fact that if you listen to the entire audio clip linked above, five minutes in they start talking about how humans are from mars, and towards the end they talk about UFOs as if aliens are real. it's a conspiritard podcast, where the conspiritard host from a different conspiritard podcast is interviewed with the most softball questions imaginable.

credibility = zero

EDIT - oh yeah, and if the russians were doing some tests and had detonated anything, the world would know. the USA has satellites capable of detecting the very unique double-flash produced by a nuclear explosion (the first flash is the radiation, then it goes a little bit dim because the fireball is opaque, then as the shockwave expands and it goes transparent again there's a second flash as the really bright centre of the explosion is visible again) AS WELL AS underground tests. they've also got this which can hear nuclear explosions. if the russians were doing anything, the americans would be screaming bloody murder, to put it politely. the USA also isn't the only country that has nuclear monitoring infrastructure. even with Trump in Putin's pocket, the world would know.

this is all just fearmongers and doomsayers attempting to spread their paranoia around, just like they've been doing since the early 1970s when the oil industry funded the founding of Greenpeace. after 45+ years of this shit, tactics of the conspiritards are well known and acknowledged. oh? there hasn't been a nuclear accident that killed anyone for over a decade? better generate some more fear then, because countries are looking at the carbon footprint of every other form of electricity generation and going "maybe nuclear is how we save the planet" again for the first time since the 1980s. i'd say "fuck the fearmongers" but i really don't wish them a good time, or that they'd breed. fucking idiots whinging and bitching about a possible distopic future (that, if you look at the facts, isn't actually possible) and completely ignoring that we're creating a far, far worse distopic present and future by keeping going the way we are now. they can go to hell. unfortunately, they are going there, and they're taking the rest of us with them.

EDIT 2 - let me know if you want me to provide more evidence to back this up.