Based on informatio I’ve been able to get from NEI, it appears that cooling has successfully been restored to the reactor cores. As of 12:30 pm EST, the Unit 1 containment had successfully been vented to reduce pressure and TEPCO was making preparations to vent the Unit 2 and 3 containments. Apparently, there was a hydrogen explosion inside the Unit 1 containment earlier today, but neither the containment or reactor vessel were damaged. This explosion is an indication that there is probably significant fuel damage( but not neccessarily melting). Hydrogen is produced from a Zirconium – water reaction within the core, which is expected when the clad overheats in the presence of steam. Hydrogen is also produced by dissasociation of water into hydrogen and oxygen gas during the fission process. However, with the reactor shut down, the most likely source of the hydrogen is a Zirconium water reaction.
The NEI news release also stated that the cores were being cooled with Boric Acid and seawater. Boric acid is a neutron poison (Boron absorbs neutrons). The fact that TEPCO is using seawater mean that they understand that they probably have significant fuel damage, and won’t probably be operating these reactors anytime soon, if ever. If the cores were undamaged, they would never introduce seawater, with all of it’s impurities into the reactors.
The IAEA (International Atomic Energy Agency) has classified this accident on th International Nuclear Event Scale (INES) lower than the TMI accident in 1979.
So despite the fear mongering of the press, it appears that there won’t be a meltdown with large scale contamination and loss of life like Chernobyl.