Iodine-131 levels from sewage sludge have spiked in September and October in Zushi City and Chiba prefecture, Japan. Zushi is southwest of the Fukushima Daiichi plant, past Tokyo. Chiba is south of Daiichi.
Fukushima Diary has a post about the elevation on iodine in Date, Japan in July.
High levels of radioactive iodine are due to increasing criticalities in the underground coriums. In this post on September 9, I pointed out a dangerous situation had developed at the plant. The elevated levels of iodine measured at these locations appear to be due to this event. Iodine has also spiked in French river water recently.
The nuclear chain reactions going on under Daiichi are not just emitting iodine, but a host of other radioactive substances. These reactions are also heating up the coriums, which means they are moving downward faster. They are more likely to hit large pools of underground water. Water slows down the neutrons from the criticality, and they become thermal neutrons, which increase the criticality. This increases the likelihood of hydrovolcanic eruptions or other events which are caused by radioactive substances encountering water.
I see to no reason for this to stop, in the coming decades, or for hundreds or even thousands of years.