In mid June, NRC said the cause of the excessive wear of the tubes in the steam generators at San Onofre Nuclear Power Plant could be a faulty computer modeling by the manufacturer, Mitsubishi Heavy Industries, which underpredicted the velocity of the steam and water.
Mitsubishi Heavy Industries sold 4 replacement steam generators to Southern California Edison. Each has over 10,000 tubes, instead of 4,000 or so for a standard steam generator that the company manufactures. Mitsubishi Heavy Industries is one of the top manufacturers of steam generators in the world.
From 10News.com (7/13/2012):
Tube Wear At San Onofre Blamed On Settling, Vibration
SoCal Edison Says Wear On Steam Generator Tubes Did Not Reach Federal Limits
SAN ONOFRE, Calif. -- The wear in the walls of steam generator tubes that forced the shutdown of the San Onofre Nuclear Generating Station earlier this year was caused by a settling of support equipment and vibrations, Southern California Edison announced Friday.
A leak from a tube forced the utility to take Unit 3 off-line in January, and thinning was found in a large number of them, which are less than two years old.
The plant's other unit had already been closed for routine maintenance. Neither has been returned to service.
SCE reported to the Nuclear Regulatory Commission last week that most of the wear in the tube wall was less than 20 percent, below the federal safety threshold of 35 percent. A tube over the limit must be plugged.
The cause of the wear on the tubes was vibration, which is unusual, and settling of the support structures, which sometimes occurs, SCE spokeswoman Jennifer Manfre said. She said the number of tubes affected by the settling was higher than normal.
"We're using this information and additional detailed data collected through testing to develop our repair plans according to best practices and industry standards, particularly the data on the unexpected tube-to-tube wear," said Pete Dietrich, senior vice president and chief nuclear officer.
SCE reported that in Unit 3 -- where the leak occurred -- 1,806 tubes showed wear of some type and 807 tubes were ultimately plugged. Of those, 381 tubes were plugged for wear of more than 35 percent and the rest as a precaution.
In Unit 2, 1,595 tubes showed wear of some type and 510 tubes were ultimately plugged. Six were for showing wear of more than 35 percent and the rest for preventative measures.
The units each have thousands of steam generator tubes.
An anti-nuclear group called Friends of the Earth released a statement Thursday, claiming that the utility and the NRC were down-playing the extent of damage. The FOE statement calls the steam generator problems at San Onofre the "most severe" of any nuclear plant in the United States.
San Onofre has more than three and a half times the number of steam tubes plugged as a safety measure than at 31 other similar U.S. reactors combined, according to the group. They contend that the number of tubes plugged in Unit 2 -- which did not have a leak -- is nearly five times that of any other reactor.
The San Onofre plant generates about 20 percent of the power for San Diego Gas and Electric. The utility is importing power to cover for the San Onofre shutdown, much of it coming over a transmission line from Imperial County that recently began operating.
Well, if each unit has two steam generators which have 10,000 tubes each, there are 20,000 tubes. If the number of tubes plugged in Unit 2, 510, was five times that of any other reactor in the US, what I'd like to know is the total number of tubes in the steam generator(s) in a typical PWR in the US. (Does anyone know? I'm being lazy on weekend...)
And yes, the tubes got worn out in 2 years instead of 20...