Many more tubes are worn in the generators at a shuttered California nuclear plant than just those tied to a radiation leak earlier this year, newly released data reveal.

Nearly 9 percent of the tubes in the Unit 2 and Unit 3 reactors at San Onofre Nuclear Generating Station exhibit some wear, information posted Friday on the Nuclear Regulatory Commission website shows.

Plant operator Southern California Edison Co. said the majority of the tube wall thinning "is not unusual in new steam generators and is part of the equipment settling in."

But an activist whose demand for the data led to its release called the wear worrisome.

"It's very alarming," said Dan Hirsch, president of Committee to Bridge the Gap, a nuclear safety advocacy group. "These reactors are crumbling. The new steam generators are falling apart long before they should, and yet Edison has been reluctant to reveal the extent of the damage and is pushing to start up with crippled steam generators that haven't been fixed."

Unexpectedly fast tube wear was cited as one of the reasons NRC ordered the continued closure of San Onofre, located about 60 miles north of downtown San Diego. The plant has been shuttered since a Jan. 31 radiation leak.

Stress tests conducted by investigators after the leak found that multiple tubes were at risk of rupture, the first time in the history of the industry that more than one tube at the same facility has failed a pressure check.

Generators with the damaged tubes were installed as replacements in 2009 and 2010 at a cost of $670 million. They are supposed to last 40 years.

NRC says that there is no timetable for reopening the plant, which supplied power to about 1.4 million homes. Edison, responding to NRC's confirmatory action letter, is preparing a report detailing the causes of the leak and what it plans to do to prevent future occurrences. The utility said it will not seek to restart San Onofre until it is safe to do so.

Edison said that it released the information on tube wear because of Hirsch's request at a June 18 public meeting. At that session in San Juan Capistrano, Calif., Hirsch stood up and asked for the total number of tubes with wear greater than 10 percent (Greenwire, June 19). He refused to sit until Peter Dietrich, Edison's senior vice president and chief nuclear officer, agreed the data would be forthcoming.

The data show wear to a total 3,401 tubes in Unit 2 and Unit 3.

Edison said, however, that tube wear falls into different categories and that most of the wear that the company reported to NRC is not problematic because it occurred on support structures. The bulk of the wear in those places was less than 20 percent, Edison said.

Tubes in those areas are expected to degrade some, spokesman Scott Andresen said.

"The tubes wear to a certain point, and then they find -- I guess in layman's terms it would be a sweet spot -- and operate that," Andresen said, comparing it to a "new pair of shoes."

"That's kind of what some of the wear on these tubes is an indication of," Andresen added.

There were three major categories of wear, Edison said in a statement: "anti-vibration bar wear, tube support plate wear and tube-to-tube wear." The data it gave to NRC also included "two minor categories of wear," retainer bar wear and wear due to a foreign object.

"The foreign object wear, also not unusual in new steam generators, was only found in Unit 2 and was caused by a piece of welding material about the size of a quarter rubbing against two tubes," the statement said.

The most serious category is tube-to-tube wear, Andresen said. That is the wear that investigators said contributed to the radiation leak.

The Unit 3 generator has 811 tubes with "tube-to-tube wear" greater than 10 percent, according to the data given to NRC. In Unit 2, there are two tubes with degradation exceeding that level. There are 19,454 tubes in each generator.

Andresen noted that any tube with wear of 35 percent or greater has been "plugged," or taken out of service. In the Unit 2 generator, there are six plugged tubes, while in Unit 3, there are 381 no longer operating.

Hirsch, however, said it is problematic that San Onofre already has plugged such a high number of tubes. The plant can shut down 8 percent before it must reduce operations.

"They've plugged 400 tubes, yet have [a total of] 3,400 tubes damaged," Hirsch said. "They want to start up with something on the order of 90 percent of the damaged tubes not plugged and just hope that they don't get worse and hoping they don't blow."

Hirsch also pointed out that there are a large number of tubes that have 20 to 34 percent wear. There are 372 in that category in Unit 3 and 141 in Unit 2.

Andresen said that he did not know how many of those tubes were close to the 35 percent mark but noted again that tubes commonly wear to a degree and then stop.