Out of the fog of mis information issued by nuclear authorities, a hazy awareness of the danger of re criticality of displaced fuel lingers.
What is the actual disposition of the fuel ? Noone knows.
How often has recriticality occurred amid the destroyed, displaced and molten fuel ?
Wikipedia reports the situation reguarding reactor Number 2 as follows:
Concerns over re-criticality
On 1 November 2011 TEPCO said that xenon-133 and xenon-135 was detected in gas-samples taken from the containment vessel of reactor 2, in a concentration of 6 to 10 (or more) parts per million becquerels per cubic centimeter. Xenon-135 was also detected in gas samples collected on 2 November. These isotopes are the result of nuclear fission-reaction of uranium. Because the short half-lifes of these gases: (Xe-133: 5 days Xe-135: 9 hours), the presence could only mean that nuclear fissions were occurring at some places in the reactor. Boric-acid was poured into the reactor in an attempt to stop the fission-reactions. No significant change in temperature or pressure was found by TEPCO, so there was no sign of large-scale criticality. The reactor-cooling was continued, but TEPCO would examine the situation at reactor 1 and 3 also. Professor Koji Okamoto of the University of Tokyo Graduate School made the comment that localized and temporary fission might still happen, and that the melted fuel could undergo fission, but the fuel was probably scattered around. Neutrons from radioactive materials could react with the uranium fuel and other substances. Self-sustaining chain reactions were unlikely, thanks to the huge amounts of boric acid that were poured into the reactor. According to Okamoto, these neutrons should be closely monitored to make sure fission did not happen, because when the fission-reactions were not controlled, it would be impossible to reach a state of “cold-shutdown”. Therefore it was needed to locate all molten fuel in and outside the reactor-vessel.
On 3 November 2011 TEPCO said that the tiny amounts of xenon-135 detected in the reactor’s containment vessel atmosphere came from spontaneous nuclear fission with curium-242 and curium-244, substances that were present in the nuclear fuel. A critical fission would have caused much higher concentrations of xenon isotopes. These reactions would occur constantly, and did not lead to criticality in the melted fuel of reactor 2. All assessments would be sent to NISA for reevaluation.
The detection of xenon on the afternoon of 1 November by TEPCO was reported to NISA in the night. The next day, 2 November just past 7 a.m., NISA informed the Prime Minister Yoshihiko Noda’s secretary about the possibility of critical reactions in reactor 2. Two hours later at 9 a.m. prime minister Edano learned the news. At a press-conference, the Chief Cabinet Secretary Osamu Fujimura revealed that Minister of Economy, Trade and Industry Yukio Edano sent a strong reprimand to Hiroyuki Fukano, the chief of NISA, because NISA failed to report the incident immediately to both himself and the Prime Minister’s Office, and that NISA waited almost a day after the find was done. Fujimura said, “I have been told that NISA decided not to report the incident until the following morning because the agency didn’t believe it was a dangerous situation.” Wikipedia http://en.wikipedia.org/wiki/Fukushima_Daiichi_nuclear_disaster_%28Unit_2_Reactor%29
What the current situation is regarding the fuel, how often the fuel has gone critical and produced more fission product to be emitted from the plant, remains a mystery.
The plant continues to present an on going danger.