Gundersen is saying that 10% of the fuel core of the Reactor 3 may be repeating a re-criticality, on and off:
"Unit 3 may not have melted through and that means that some of the fuel certainly is lying on the bottom, but it may not have melted through and some of the fuel may still look like fuel, although it is certainly brittle. And it's possible that when the fuel is in that configuration that you can get a re-criticality. It's also possible in any of the fuel pools, one, two, three, and four pools, that you could get a criticality, as well. So there’s been frequent enough high iodine indications to lead me to believe that either one of the four fuel pools or the Unit 3 reactor is in fact, every once in a while starting itself up and then it gets to a point where it gets so hot that it shuts itself down and it kind of cycles. It kind of breathes, if you will.
"I think it's a relatively significant amount – maybe a tenth of the nuclear reactor core starts back up and shuts back down and starts back up and shuts back down. And that’s an extra heat load; you are not prepared to get rid of one tenth of a nuclear reactor’s heat by pumping water in the top."
Hiroaki Koide of Kyoto University suspected an on-and-off recriticality in the Reactor 1 back in early April when TEPCO announced, supposedly erroneously, chlorine-38 was detected. Dr. Tetsuo Matsui of Tokyo University suspected recriticality in the Reactor 2 and/or the Reactor 4 Spent Fuel Pool. Gundersen suspects an on-and-off recriticality in the Reactor 3 (or any of the Spent Fuel Pools).
Below is the chart of iodine-131 detection at the Federal Office of Radiation Protection in Germany, plotting the CTBTO station data from around the world. Station 38 is Takasaki, Gunma Prefecture in Japan, 218 kilometers (135 miles) from Fukushima I Nuclear Power Plant. I always thought it was rather peculiar that there seemed to be periodic spikes in the amount of iodine-131 (also of cesium-137 but to a lesser degree) followed by a relatively calm, steady decline.
Could these spikes indicate what Gundersen is talking about? On-and-off recriticality?
