Yesterday, NRC announced it was sending a team of additional inspectors to the San Onofre Nuclear Generating Station to more fully understand why steam generator tubes are failing a pressure test. The tubes were isolated to Unit 3, which was safely shutdown on Jan. 31 when the tube leak was identified. There was no risk to the employees, the public, or the environment during the pressure test and both units are currently powered down.
In order to understand the situation a little better, think of a steam generator as a large egg filled with water and thousands of long, thin metal tubes in a U-shape formation attached at the bottom of the egg. The steam generator removes heat from the nuclear reactor as the hot radioactive water that surrounds the fuel rods passes through these tubes. As the hot water travels through the tubes, the rest of the water in the egg becomes steam. The steam is transferred to the turbine building where it forces the turbine to move and creates electricity.
A steam generator tube failing is a problem because radioactive water that passed over the nuclear reactor and into the steam generator may escape into the created steam through a hole in the tube. Then, the radioactive steam could end up at the turbine and eventually may escape to the environment.
The NRC inspectors will take a very comprehensive look at this Unit 3 condition, including looking at the manufacturer’s steam generator design, construction and shipment to the site located near San Clemente, California. While other steam generators have shown wear after one cycle of operation, the level of wear at Unit 3 is more than expected.
Currently, the plant’s operator is continuing pressure testing and so far seven tubes have failed. In addition to the resident inspectors, an NRC steam generator expert is on site and inspecting the ongoing testing. A public exit meeting will be scheduled in the coming weeks where the team will share their preliminary results.
Lara Uselding Public Affairs Region IV
U.S. NRC Blog 
The licensee has a quality assurance program in place to ensure quality fabrication and construction, complying with construction codes and standards. The licensee inspects the equipment upon receipt and does a pre-service examination prior to putting the tubes in service. The 19,000 plus tubes are individually tested using electromagnetic testing. The NRC inspection team will hold a public exit meeting with the licensee to discuss preliminary findings and will also issue a written report within 30 days of completion of the inspection.
Lara Uselding
i am just curious as to whether or not these steam generator tubes undergo high resolution x-ray examination prior to final assembly to identify any questionable areas of fragility that may breech the patency of these assemblies. I’m quite sure the quality control has to be second to none when assembling a reactor generator plant. i would like to know what the outcome was if at all possible.