What actually has happened at Fukushima Daiichi Nuclear Power Plant number one:
The 'explosion' and fire was in a turbine hall, not a reactor or within a containment building or vessel.
Two employees and two subcontractors were injured at the time of the big Honshu Quake yesterday. One employee received a years 'safe' dose of radiation. Which was probably around the same figure as some Medical Tomography scans deliver (Around 1-2Rem, with 0.5Rem dose for a standard X-ray).
The backup diesel generators for the reactor circulation got swamped by the Tsunami and stopped working. At the time of writing, replacements are being installed.
Speculation in the lamestream is rampant about Meltdowns and 'Chernobyls', but the Japanese reactors are Boiling Water reactors, which once shut down begin the long slow process of cooling. They don't melt down.
Oh. No more panic there then. Hi ho, time for morning coffee.
Updated midnight PST 12th/13th March 2011.
Lots of speculation still about 'meltdowns', but as the reactors have been shut down, the slow cooling of reactor cores has begun. Yes, according to the press releases, there have been some problems with getting water into the reactors, but considering they've just had a massive tranche of seawater sloshing about after a Tsunami and major earthquake, I'd say the Engineers had everything at reactors one and two under at least some sort of control.
Update: H/T Depleted Cranium. Explosion at Number 3 reactor (Not in the containment structure) Possibly due to unvented Hydrogen. As an aside; to the commenter who told me I didn't know what a containment building looks like, I spent part of my early career working in and around Coal, Gas and Nuclear power stations on fire alarm / suppression systems. FYI; I do know the difference between a turbine hall and a containment building. Also between main breakers and transformers, smoke stacks and cooling towers. Funny how so many people don't, eh?