The closing last March of five nuclear plants because their plumbing wasn't built to withstand strong earthquakes has led to a finding that 29 other nuclear plants in the United States need better pipe supports.

Of the 29 plants surveyed by the Nuclear Regulatory Commission, 25 are operating and four are awaiting licenses to begin operation. None of the 29 will be ordered to close, but all will be required to build up the supports around their piping to prevent rupture in case of an earthquake.

"What we're talking about here is strengthening pipe supports so the pipes won't start swinging in an earthquake," Darrell Eisenhut, acting director of the NRC's division of operating reactors, said in an interview.

"In many cases, we're finding that the piping is supported up instead of down, things like that."

Eisenhut said that the plant owners are strengthening the pipe supports without being told to. He said that at least two plants, the Brunswick plant of Carolina Power & Light Co. and the Pilgrim plant of Boston Edison Co., have been shut down while their owners strengthen pipe supports.

"We anticipate the plants will notify us and tell us what action they're taking" Eisenhut said. "We've given everybody 120 days to have all this work done."

Among the faulty pipe designs being found, Eisenhut said, are cases where supports are undersized, installed the wrong way or not attached to the plant walls in the right way. In several plants, shock absorbers in pumps and valves had no supports, meaning they could fail if a strong earthquake started them in motion.