Nuclear Regulatory Commission Chairman Gregory Jaczko told Congress that the water in the spent-fuel pool at the Fukushima Daiichi nuclear power station’s Unit 4 has drained. As the NRC’s chairman, Jaczko’s statement is conveying not only the expert opinion of the commission and its staff, but of the United States government.

At this stage I think that heroic, last-ditch measures are being undertaken. It’s likely that the dose rates coming off Unit 4 are life-threatening and that this is a major problem for restoring water, and repairing the pool.

The accident won’t happen all at once and is likely to unfold, perhaps, over a period of weeks. The radioactive plumes will vary from the wind directions and will fluctuate. My concern is that very large inventories in plumes in the near future may arise.