http://www.guardian.co.uk/world/2011/mar/29/japan-lost-race-save-nuclear-reactor
The Guardian is quoting Richard Lahey as a person who has worked at Fukushima who indicates that the change in radiation probably signals that the bottom of the containment vessel has been ablated and the molten fuel is now sitting on the concrete floor below the reactor bubbling its way through the concrete.
Because I always look at sources and motivations, who is Richard Lahey and is he a credible authority on nuclear reactors?
He is associated with Renssselaer Polytechnial Institute in Troy, New York.
Here is his brief bio:
Lahey joined the Rensselaer faculty in 1975 and has served as chairman of the Department of Nuclear Engineering and Science, president of the Faulty Senate, and dean of Engineering. He is a member of the National Academy of Engineering and the Russian Academy of Sciences and is a fellow of the American Nuclear Society and the American Society for Mechanical Engineers. He has received numerous honors, and he consults frequently with industry and government organizations. He has supervised 44 doctoral and 30 master’s students.
It is a reasonable hypothesis. Since they are now reading trace amounts of plutonium contamination, that can come only from the nuclear fuel itself, not the cladding. The plutonium is either coming from the spent fuel pools- in which case there would be a very large amount of contamination since the fuel pools are outside containment, or it’s coming from one or more of the reactor cores that have lost integrity.
So now the Japanese have taken this accident down a path that physicists have argued about for years. Millions of dollars have been spent on computer analysis of this scenario. Will the melted core spread out over the concrete mat and slowly cool, or will it punch through the mat?