Regulators and congressional investigators clashed Wednesday over a new report warning that in the event of an accident at a nuclear plant, panicking residents from outside the official evacuation zone might jam the roads and prevent others from escaping.

The report by the Government Accountability Office, which acts as the investigative arm of Congress, challenges a three-decade-old fundamental of emergency planning around American nuclear power plants: that preparations for evacuation should focus on people who live within 10 miles of the site.

The GAO found that people living beyond the official 10-mile evacuation zone might be so frightened by the prospect of spreading radiation that they would flee of their own accord, clog roads, and delay the escape of others. The investigators said regulators have never properly studied how many people beyond 10 miles would make their own decisions to take flight, prompting what is called a “shadow evacuation.”

As a result, the GAO report says, “evacuation time estimates may not accurately consider the impact of shadow evacuations.”

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