It just got worse. From AP:
Japanese officials say the nuclear fuel rods appear to be melting inside all three of the most troubled nuclear reactors.
Chief Cabinet Secretary Yukio Edano said Monday: "Although we cannot directly check it, it's highly likely happening."
Some experts would consider that a partial meltdown of the reactor. Others, though, reserve that term for times when nuclear fuel melts through a reactor's innermost chamber but not through the outer containment shell.
We can only hope that the team that did not test for an 8+ magnitude earthquake did a better job at fortifying the containment shell which is the only thing separating radiation from the outer world.