WASHINGTON — The chairman of the Nuclear Regulatory Commission acknowledged Thursday that more should have been done to thoroughly investigate a tip that security guards routinely took naps while on the job at a Pennsylvania nuclear plant.

It wasn't until a videotape of guards sleeping in a "ready room" at the Peach Bottom plant in south-central Pennsylvania surfaced several months later that the NRC announced in September a special investigation.

The tip was in the form of a letter, which the NRC received last spring from a former employee who was writing on behalf of current employees. After the NRC received the letter, it allowed the Exelon Corp., owner of the plant, to investigate the allegations. In doing so, Exelon found no evidence that guards were taking nap breaks.

NRC Chairman Dale Klein testified Thursday during a Senate Environment and Public Works subcommittee hearing that one reason may be because there was collusion by the guards to sleep during their shifts. He said the letter's author asked not to be contacted, and Klein said the agency made a mistake by honoring that request.

"We were not as rigorous as we should have been," said Klein, adding that the NRC is taking action to prevent similar problems at all nuclear plants.

After the videotape surfaced, Exelon Corp. fired the Palm Beach Gardens, Fla.-based Wackenhut Corp. as its security guard provider at Peach Bottom and its other plants.

While acknowledging there was wrongdoing, company and nuclear oversight officials at the hearing sought to minimize the risk that the sleeping guards posed.