Because the half life of xenon-133 is 5.25 days and that of xeon-135 is 9.14 hours, criticality is very likely to have occurred just before the gases were analyzed.Although more than seven months have passed since the start of the nuclear fiasco, clearly the reactor has not yet been stabilized. Tepco's plan to achieve "cold shutdown" of the Nos. 1, 2 and 3 reactors by the end of this year may face difficulty.The fact that Tepco cannot deny the possibility of criticality irrespective of its scale is a grave situation. The conditions are similar in the Nos. 1, 2 and 3 reactors. It is thought that nuclear fuel in them melted and has collected in the bottom of both the pressure and containment vessels.[...]
After claiming that possible criticality was indicated by release of various isotopes of Xenon gas TEPCO has now backtracked to say it is only a spontaneous fission reaction of curium (a less serious condition). So what is really true? TEPCO has had a tendency to lie, downplay and delay releasing news concerning their crippled Fukushima Daiichi nuclear power plant so it is likely that something more serious is going on than what they are attempting to present to the public. For example, per the ENENEWS article Krypton-85 up over 14,000% in one day at Reactor No. 2 — Kr-85 used to detect “plutonium separations” the existence of Krypton-85 in Fukushima Daiichi reactor 2 is indicative of plutonium fission.