How does the spent fuel get aerosolized and into the atmosphere? Some of the more recent comments link to a site that detected iodine-131 in Norway. Vented hydrogen/steam from partially uncovered fuel rods (e.g. Three Mile Island, Fukushima) in an active boiling water reactor is historically the most likely source (I think) but iodine-131 should be abundant. The same principle should apply to any nuclear powered submarine or ship. Other commenters point out that Russia has nuclear powered missiles and torpedoes that emit radiation. I expect the CTBTO to be able to categorize the nature of each radiation source but maybe that type of analysis takes time.