Technicians at the San Onofre Nuclear Generating Station have taken more than 1,317 steam generator components out of service because they had worn down so much that they could no longer function safely.
“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,” Senior Vice President and Chief Nuclear Officer Pete Dietrich said in a release. “Safety continues to be the guiding principle behind all the work we are doing.”
Dietrich told Patch after a June Nuclear Regulatory Commission meeting that plant officials hadn't yet made a decision on whether to restart the plant at all. Officials have confirmed that the troubled generating station will be closed at least through August.
The worn components are tubes that carry super-heated, highly pressurized radioactive steam. The thousands of tubes within the steam generator exchange heat from the radioactive water that circulates around the nuclear fuel rods and transfers it to boil pure water that makes steam. The steam turns turbines to make electricity.
It was a small leak in one of these tubes in January that revealed the extensive wear throughout the generators in both units of the plant -- each unit has two.
Southern California Edison identified tube wear caused by three major factors, according to the release.
- Anti-vibration bar wear: This was caused by a metal anti-vibration bar meant to hold the thousands of U-shaped tubes in place. Especially in the Unit 3 generator, this bar wasn't holding the tubes tight enough, allowing them to vibrate and wear down.
- Tube support plate wear: The tube support plate is a metal plate inside the generator with thousands of holes that allow the tubes to pass through it. The plate is another support structure meant to stabilize the highly pressurized tubes, but the tubes vibrated against it, causing wear.
- Tube-to-tube wear: This wear was caused by tubes rubbing against one another.
Two minor categories of wear were also included:
- Retainer bar wear: Retainer bars serve a similar function as the anti-vibration bars but have a different configuration.
- Wear caused by a foreign object: The foreign object wear appeared only in Unit 2 and was caused by a piece of welding material about the size of a quarter rubbing against two tubes.
As a result of the wear -- mostly less than 20 percent, according to the release -- plant technicians took the following action:
- In Unit 2, 1,595 tubes showed wear of some type, and 510 tubes were ultimately plugged — six tubes for showing wear of more than 35 percent and the rest for preventative measures.
- In Unit 3, 1,806 tubes showed wear of some type, and 807 tubes were ultimately plugged — 381 tubes for wear of more than 35 percent and the rest for preventative measures.