Saw this post via twitter. It goes through what has happened, the safety measures, why they failed etc etc.
The plant came close to a core meltdown. Here is the worst-case scenario that was avoided: If the seawater could not have been used for treatment, the operators would have continued to vent the water steam to avoid pressure buildup. The third containment would then have been completely sealed to allow the core meltdown to happen without releasing radioactive material. After the meltdown, there would have been a waiting period for the intermediate radioactive materials to decay inside the reactor, and all radioactive particles to settle on a surface inside the containment. The cooling system would have been restored eventually, and the molten core cooled to a manageable temperature. The containment would have been cleaned up on the inside. Then a messy job of removing the molten core from the containment would have begun, packing the (now solid again) fuel bit by bit into transportation containers to be shipped to processing plants. Depending on the damage, the block of the plant would then either be repaired or dismantled.
The explosion was apparently caused by an abundance of caution.
At some stage during this venting, the explosion occurred. The explosion took place outside of the third containment (our “last line of defense”), and the reactor building. Remember that the reactor building has no function in keeping the radioactivity contained. It is not entirely clear yet what has happened, but this is the likely scenario: The operators decided to vent the steam from the pressure vessel not directly into the environment, but into the space between the third containment and the reactor building (to give the radioactivity in the steam more time to subside). The problem is that at the high temperatures that the core had reached at this stage, water molecules can “disassociate” into oxygen and hydrogen – an explosive mixture. And it did explode, outside the third containment, damaging the reactor building around. It was that sort of explosion, but inside the pressure vessel (because it was badly designed and not managed properly by the operators) that lead to the explosion of Chernobyl. This was never a risk at Fukushima.
Worth a read.
Update: For some balance: Rob at The Standard keeps it classy.
On top of the quake and tsunami aftermath, Japan is already dealing with evacuations and the effects of the radiation leaked so far. If the last ditch efforts (using sea water for cooling) fail then:
If the temperature inside the reactor continues to rise and reaches roughly 2200C, the uranium fuel pellets will start to melt. From there, melted fuel will eat through the bottom of the reactor vessel, then the floor of the damaged containment building. At that point, the uranium and radioactive byproducts of the nuclear reactions will start escaping into the environment.
At some point, the walls of the reactor vessel will melt into a lava-like pile, slump into any remaining water on the floor, and could cause an explosion much bigger than the one caused by the hydrogen, enhancing the spread of radioactive contaminants. …
In Auckland, Dr Krofcheck said that if the Fukushima Daiichi accident became a meltdown and released large amounts of radiation, “I’m sure it would not be a major problem for New Zealand. Most of it would be confined to the Northern Hemisphere, and most certainly, Japan itself.”
We’re looking at a second Chernobyl.
Could go either way I guess…