First of all, a disclaimer. NOTHING has been confirmed yet. Repeat – nothing confirmed yet. UPDATE***-NISA CONFIRMS REACTOR MELTDOWN ***-link
What is confirmed is an explosion at the Japan Fukushima Daiichi nuclear power plant. According to reports from workers on-site, it was the containment building for the reactor.
Again, not confirmed.
UPDATE***Chief Cabinet Secretary confirms that containment building damaged. This is, in fact, the protection should a full reactor meltdown occur and breach the vessel – link
If this is correct, it could mean the worst case scenario. If the containment building has been compromised and a reactor meltdown does occur, the vessel housing the reactor would likely be compromised due to the molten fuel and we could have another Chernobyl on our hands.
***UPDATE 9:05 PM EST***– It’s become clear at this point that at a minimum, a reactor meltdown has occurred. We don’t know yet if it is a full meltdown. We do know the containment building was compromised due to a hydrogen explosion that completely blew the roof off. The cesium leak confirms the fuel rod melting. Only time will tell just how extensive the leak is and ultimately how many other reactors also fail. The worst case scenario would be a vessel breach below the reactor leading to water and soil contamination if the floor of the containment building doesn’t hold. Also you could have a fire leading to Chernobyl-like air contamination. No nuclear explosion is possible.