While speaking of the atomic testing in North Korea, CTBTO has inadvertently disclosed information on Fukushima.
One slide shows Xenon 133 peaks in Takasaki Station. Or xenon 133 is a fission product. The corium of Fukushima seems to know criticality phases in 2014 and 2015 (detection beyond normal = red triangles)
Which also explain why traces of Iodine-131 have been repeatedly found at various locations in Japan every year since 2011. Iodine-131 being a very short life radionuclide, it should not be present anymore after March 2011.
The highest peaks of xenon 133 were in May, June July 2015, corresponding to the Iodine-131 detections in sewage sludge in May 2015
Source: Fukushima Diary
Source : CTBO
Credits to Pierre Fetet & Paolo Scampa for these informations.