You don’t know what you don’t know. That expression truly applies to nuclear safety rules in the United States that do not adequately weigh the risk a single event would knock out electricity from the grid and from emergency generators, said Nuclear Regulatory Commission (NRC) officials.

A task force created after the accident at the nuclear plant, Fukushima Dai-ichi, delivered an oral progress report to the five-member commission last week. In that session, commission officials said they learned some of the safety equipment installed at American nuclear plants over the years, including hardware added after the Sept. 11, 2001, terrorist attacks, does not undergo as diligent a process of maintenance or inspection as original components do.

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A crucial reason for the extensive damage to the Fukushima plant’s reactors was the loss of electricity needed to run water pumps and to reposition valves. The American nuclear industry argues its reactors are ready to cope with that kind of emergency.

But Charlie Miller, the chairman of the task force, said studies by safety experts in the United States analyzed the risk of losing electricity from the grid or from on-site emergency generators, but not both at the same time.

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Steven P. Kraft, an executive of the Nuclear Energy Institute, the industry’s trade association, speaking after the meeting said that in the past it was “not considered credible” that a single event could knock out both supplies. In view of recent events, he said, it is time to prepare for the possibility of an extended blackout.

One of the commissioners, George E. Apostolakis, pointed out existing safety analyses also assume electricity will be come back within four or eight hours after a power cutoff, but blackouts on the grid often last far longer. “Why do we still assume things that are now, in retrospect, unrealistic?” he asked.

The task force, appointed in April, should complete its investigation in August but is periodically updating the commission. In another finding, it warned emergency vents added to American reactors to protect against a hydrogen explosion after an accident might not function, just as they proved inoperable in Fukushima.

Another challenge is the commission’s inspectors have not undergone training to evaluate the condition of a variety of hardware or review procedures adopted as extra precautions after the Sept. 11 attacks.

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