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I just finished reading the NRC letter. Two statements caught my eye:

1) "All U.S. nuclear power plants are built to withstand external hazards, including earthquakes, flooding, and tsunamis, as appropriate."

How can we be certain that the measures implemented in the US are appropriate? We have not even yet fully uncovered the causes for the reactor accidents at Fukushima Daiichi nuclear power station.

If we examine the excerpt of the seismic scram log published for unit 1 for example (posted by edano yesterday upload.wikimedia.org ), the shutdown of unit 1 proceeded as envisaged during the first minutes after the quake. According to Tepco, the situation turned to the worse, after the tsunami disabled the diesel generators depriving the reactor safety systems of AC power.

The diesels' failure may represent one major cause underlying the accident. However, the reactor safety systems of unit 1, as well as of units 2 and 3, are supposedly designed such that the reactor can be cooled and its core covered with water for eight hours, using DC battery power and decay heat alone. Both were still available at Fukushima after the loss of AC power. Despite, lack of coolant caused fuel damage at unit 1 within four hours. How can the NRC claim that reactors of similar design are safe in the US, considering that we do not fully understand the causes instrumental to the catastrophic reactor failures at Fukushima?

2) "The NRC believes that it is highly unlikely that a similar combination of events such as those which occurred in Japan could occur in the United States..."

Since a different combination of similar events cannot be ruled out, this belief seems unwarranted.

US reactors of similar design as at Fukushima Daiichi nuclear power station should be permitted to operate only at 70 percent of their full electrical power output, until a clear understanding of the causes for the three catastrophic reactor failures is reached, most notably because each failure was anticipated to occur only once in ten-thousand years or less.