The new TEPCO press release provides additional insight into the current situation. Out of all the power plants, one that seems to be the most problematic is unit 1.
All 6 units of Fukushima Daiichi Nuclear Power Station have been shut down.
Unit 1(Shut down)
- Reactor has been shut down. However, the unit is under inspection due to the explosive sound and white smoke that was confirmed after the big quake occurred at 3:36PM.
- We have been injecting sea water and boric acid which absorbs neutron into the reactor core.
Other sources seem to confirm that the reactor containment vessel has not been breached which allows the second step highlighted above: i.e. flood the containment with water to continue the cooldown of the decay heat. The use of borated water is unusual in that boron is never used in Boiling Water Reactor's (BWR, looks like a GE design) coolant. Boron is a neutron poison that stops any chain reactions very quickly as it has a very large cross section for thermal neutrons (the particles enabling chain reactions).. Some of that boron may come from a US source. I am guessing that they want to be on the safe side besides cooling the reactor core. That also means that this reactor will be out of business for a long while. One can also read about the two other units:
Unit 2(Shut down)- Reactor and Reactor Core Isolation Cooling System have been shut down.Current reactor water level is lower than normal level, but the water level is steady. After fully securing safety, we are preparing to implement a measure to reduce the pressure of the reactor containment vessels under the instruction of the national government.Unit 3(Shut down)- Reactor has been shut down and we continue injecting water by High Pressure Core Injection System. After fully securing safety, we are preparing to implement a measure to reduce the pressure of the reactor containment vessels under the instruction of the national government.- Currently, we do not believe there is any reactor coolant leakage inside the reactor containment vessel.
If I read this correctly, the current main concerns are therefore for units 1 and 2 with unit 1 being the most concerning. From their description, it looks like they have the means of cooling these units.