A Reuters report referenced on Wikipedia indicates that the Japanese gov’t said the big Fukushima explosion was a hydrogen explosion. “Hydrogen had been produced due to falling water levels in the reactor.”
Richard Black, environment correspondent at the BBC, says that the cesium found at Fukushima was outside the power buildings, and that that could indicate that the core itself was exposed to the air. Previously, I hadn’t been clear whether the presence of cesium (a waste product of mixed-oxide or MOX fuel fission) might mean that the fuel rods were exposed. Apparently it might indicate that.
Black’s view, however, is that there was no meltdown:
It appears that the reactor was shut down well before any melting occurred, which should reduce considerably the risk of radioactive materials entering the environment.
However, the detection of caesium isotopes outside the power station buildings could imply that the core has been exposed to the air.
Although Japan has a long and largely successful nuclear power programme, officials have been less than honest about some incidents in the past, meaning that official reassurances are unlikely to convince everyone this time round.
He’s referring, I think, to the Tokaimura incident (1999) and the Mihama incident (2004).