Figured I should start a thread regarding this, it's rather serious. Canaris asked me what kind of reactor it was, and figuring it out was a pretty involved process, but I'm rather convinced I've tracked it down.
The Boiling Water Reactor is one I like to call a Single Cycle reactor because it has just one water-pump cycle connecting the rods to the turbines. I'm pretty sure this is the kind of reactor used at Fukushima because I've heard concerns that the turbines could be contaminated (which can happen in a single cycle).
To compare here is a Pressurized Water Reactor, or a Dual Cycle reactor. It utilizes two water-pump cycles, so that the water that touches the rods never actually touches the turbines. This also helps contain the reactor should anything go wrong.
Thus far no official "meltdown" has been declared, but they have evacuated the immediate area because it looks like some radioactive steam was ejected to alleviate pressure in the reactor housing block. Well.. that and the explosion, the cause of which I'm still not sure.
EDIT:
The Chernobyl reactor was a graphite (or carbon) moderated reactor. It falls under "Thermal Reactors" and it can be classified as a Boiling Water Reactor, but carbon moderated reactors can also be classified as their own group. So it wouldn't be fair to say "Fukushima was a BWR, just like Chernobyl! That's why it melted down!"