Update from Chairman US Nuclear Regulatory Commission: Radiation Levels Extremely High

NRC chairman Gregory Jaczko had this to say earlier today on the dire situation as a result of the earlier explosion at Reactor 4:

In addition to the three reactors that were operating at the time of the [earthquake], a fourth reactor is also right now under concern. This reactor was shut down at the time of the earthquake.

What we believe at this time is there has been a hydrogen explosion in this unit due to an uncovering of the fuel in the spent fuel pool.

We believe that secondary containment has been destroyed and there is no water in the spent fuel pool and we believe that radiation levels are extremely high which could possibly impact the ability to take corrective measures.