a commentary by Rik:
A recent article on Enenews ( http://enenews.com/ ) said that the melted fuel
mixture under the reactors in Fukushima was likely to be remelting.
A friend asked me to comment.
As I understand it, the corium, (a mixture of the melted nuclear fuel itself, the zirconium tubes that held the fuel pellets, and the steel racks that held the fuel bundles, all melted together) has slumped to the floor of the PCV, (pressure containment vessel) in the form of several large blobs, totalling some 130 tons per reactor. By luck rather than good management the PCVs of each of the three reactors ruptured above the level of the blobs, so they are pretty much covered by water. Several tons an hour of water is still injected into each reactor, so it must be boiling off still. So the molten corium would have formed a crust on the outside, where it is in contact with the water, but inside this crust, the corium would probably be still liquid or partly liquid, due to the heat produced by the on-going splitting of uranium atoms when struck by a neutron, in turn firing off more neutrons, and by the continued decay of the fission by-products. In other words, due to the occasional resumption of fission within the corium, it will sometimes probably again melt.
As wikipedia says: –
‘Heat is produced by the radioactive decay of fission products and materials that have been activated by neutron absorption. This decay heat-source will remain for some time even after the reactor is shut down.’
It is not that a dramatic new stage has been reached in the Fukushima disaster. The corium has been probably melting, on and off, for the whole four years.