The quoted levels are unimaginably small! Almost invisible. I wonder how they manage to detect them. Just calculated that if whole of the Earth's atmosphere was

The quoted levels are unimaginably small! Almost invisible. I wonder how they manage to detect them. Just calculated that if whole of the Earth's atmosphere was filled with I-131 with maximum reported concentration (0.5E-6bq/m3), it would be an amount of iodine that is produced with a nuclear explosion of 0.00024kt, i.e. 240kg TNT equivalent. That is so small that it hardly qualifies as a nuclear explosion - more like 'critical assembly accident'. And YES, this could be explained by a disposed nuclear reactor of an old submarine suddenly going prompt critical and blowing up! In the split second when it explodes it could release that and even much larger amount of iodine. I just couldn't imagine that levels so low are detectable. It could be also an explosion on non-disposed, fresh nuclear reactor. That can explain lack of other detectable isotopes. And that is an event too small to produce a detectable seismic signature.