It is said that it is physically impossible for the nuclear material in any of the Fukushima reactors to melt through the containment vessels. Despite a rumor of a crack in one of the vessels, nuclear power experts have maintained that it is impossible that there could be such a crack. Nonetheless, a US based GE-connected nuclear engineered who has ties to the Fukushima facility has boldly asserted that he thinks that the core in reactor 2 has “melted through the bottom of the pressure vessel … and at least some of it is down on the floor of the drywell.”
Richard Lahey was head of safety research for this kind of reactor for GE at the time that they installed the units at Fukushima. He has told his analysis to The Guardian. You can read it here.
NHK news service has not mentioned this, nor has the International Atomic Energy Agency. This is utterly unconfirmed and probably not true. It is, after all, impossible.
But if it is true, this places the situation at Fukushima on the high side of the TMI tickmark on the scale of badosity. The core at Three Mile island got all messed up but it did not breach its containment.