I have a degree in nuclear engineering. But not a practicing nuclear engineer.. but let me chime in.
This is a serious accident, no doubt. But the reactor is a boiling water reactor and as such it is inherantly more safe than say Chernobyl which was a graphite moderating reactor. Graphite burns at high temperature and the russian containment system blew.
In a boiling water reactor, the water is used to moderate the neutrons which is what causes the nuclear reaction. With the water gone, it can't generate any more nuclear fission but there's latent heat in there which is sufficient to melt the primary containment vessel.
If that happened, then it will melt it, pressure would relieve itself into the secondary containment, which is a concrete structure. The core may still be very hot and melt into the floor, but I'd say vast majority of the nuclear material would be contained.
The second containment vessel is large enough to expand any pressure generated by the breach of the primary containment.
Now the caveat is they can design all this stuff but they never put this sort of emergency into practice.. so this is real life case of whether human engineering did the right stuff or they didn't design it for some fallacy somewhere (ie something they never thought about how it would behave). I see more problems in real world engineering from the fact people can't think up everything...
It would be a shame for the japanese to have been nuked twice in WW2 then nuked again