According to news reports, officials say that a buildup of hydrogen gas released from the containment vessel mixed with oxygen causing the reactor building to explode.

Officials say the containment vessel does not appear to be compromised at this time, but there is evidence that fuel rods are melting in the core and they are having difficulty controlling it. Officials have expanded the evacuation radius to 20km, or 12+ miles.

Officials also say radioactive steam was released into the environment in an earlier attempt to reduce pressure in the reactor. Radioactivity levels inside the reactor building control room were reportedly 1000 times normal levels prior to the explosion.

U.S. experts, sounding like industry shills, appeared on CNN and other media throughout the day yesterday saying there wasn't anything to worry about and that officials would get the reactors under control. Clearly they were wrong.

Other news reports say, however, that this is different from Chernobyl because the Japanese (and U.S.) reactors have containment systems that weren't present at Chernobyl. Hopefully they are right.

• Japan Times: Four injured; meltdown feared as nuclear plant spews radiation

• Bloomberg: Japan Tries to Cool Unstable Atomic Reactor, Thousands Evacuated