Apologies for poor video. Good information, though.
New York Times has a related article:
A training manual developed by the companies that operate this type of reactor and dated 2009 refers to the possibility of “creep rupture,” in which molten core material begins seeping through a hole in the vessel and creates a bigger hole as it works; the document says the molten core material can “ablate” a bigger hole. It can then burn through the steel at the bottom of the drywell and interact with the concrete, producing carbon monoxide and hydrogen, which could react explosively.
Some engineers have theorized that if a core melted down and concentrated at the bottom of the vessel, it could melt through the vessel and then burn through the concrete of the foundation. One element of such an event would probably be a resumption of the nuclear chain reaction, in a molten mass in which no control would be possible because there would be no control rods to slide smoothly between neatly arrayed bundles of fuel.
Other experts say that a resumption of the chain reaction would be difficult or impossible with the type of fuel in use at Fukushima Daiichi.