The South Texas Project (STP) Unit 1 reactor is back on-line and operating at full power after an extended shutdown. The unit reached 100 percent power at 2 a.m. today.

Unit 1 had been off-line since March 26 when it was shut down for refueling and maintenance, which were expected to take three weeks. However during a routine inspection April 12, a small amount of residue was found on two instrument tubes that penetrate the bottom of the reactor vessel. Tests showed that the substance was boric acid from reactor coolant. Technicians found a total of 153 milligrams of the residue, about the size of one-half of an aspirin tablet, on the two tubes.

The bottom of the identical Unit 2 reactor was inspected in December 2002 and had no residue. The reactor remained online and operating at full power while STP repaired Unit 1.

"The site has done remarkable work in resolving several difficult issues, including some never encountered before," said Joe Sheppard, president and CEO of the STP Nuclear Operating Company. "We deployed technology in May that didn't exist in April," Sheppard noted.

The first challenge was to determine the scope of the problem, which had not been seen before in the industry. Residue had been found on instrument tubes in the tops of reactors, but never on the bottom of one. STP called in and conferred with experts from the Electric Power Research Institute, the Institute of Nuclear Power Operations, other nuclear power plants and nationally recognized engineering consulting firms.

Extensive testing determined that the two affected tubes each had a tiny crack through which the residue had slowly seeped over the course of several years. The tests were repeated on the other 56 instrument tubes, and no cracks were found in any of them. The reactor vessel was examined ultrasonically, and the test showed that the 5.4-inch-thick steel vessel was in good condition.

Next, STP managers and engineers analyzed several possible ways to repair the tubes. They chose a method that has been used to replace tubes in the bottoms of coolant system pressurizers, where the pressures and temperatures are equivalent to those in reactors.

Tom Jordan, STP's vice president of engineering, said, "We took an approach which had already proven successful in comparable applications." In addition, the repair technique previously had been reviewed and approved by the federal Nuclear Regulatory Commission.

The repairs were carefully rehearsed to minimize the amount of time workers spent beneath the reactor. STP had a full-scale mock-up of the bottom of a reactor vessel, complete with all instrument tubes, built in pie-shaped sections that were assembled on site. The practice sessions reduced radiation exposure during repair work by nearly 50 percent.

The repair itself took less than two weeks. The affected tubes were cut out and replaced with ones made of a more durable alloy than available when the reactor vessel was built in the 1970s. The new tubes were secured by welding them to large nickel-chromium pads which had been welded onto the outside of the vessel. All welds were tested and verified.

The Unit 1 project required new procedures and tools. Ways to perform various tests underwater had to be devised, and an underwater nozzle plug to facilitate testing had to be developed. In addition, special tools were designed and fabricated on site or built by vendors to STP specifications.

"This was one of the greatest technical challenges STP has ever faced," Plant General Manager Ed Halpin emphasized. "At every step, we had to overcome major obstacles and find solutions to new and complex problems. In every instance, the team succeeded. Their performance has been truly outstanding."

The repair as well as most of the testing was done by Framatome ANP, which has extensive experience in reactor products, services and maintenance. The battery of tests it conducted indicate that the cracks probably were caused by manufacturing flaws. That may be confirmed when tests of metallurgical samples taken from the tubes are completed in two months.

Throughout the project, STP kept federal regulators, state and local officials, and community leaders informed of test results and the progress of repairs. Plant officials gave daily briefings to the Nuclear Regulatory Commission's resident inspector on site, and worked closely with a special inspection team sent by the agency. STP also held weekly teleconferences with regulators, submitted numerous reports and other documents to them, and met three times with officials at NRC Headquarters in Rockville, Md.

The NRC held a fourth public meeting July 28 in Bay City to announce the findings of its inspection team. NRC Acting Deputy Region IV Administrator Dwight Chamberlain said during the session, "We're very satisfied with the actions you've taken." Three days later, STP was formally notified that the federal agency had completed its review of the Unit 1 repairs, allowing the reactor to return to service.

STP initiated restart procedures as soon as it received the NRC letter. The process entailed several phases. Systems and equipment were tested at each stage to ensure they worked properly.

STP's twin reactors produce more than 2,500 megawatts of electricity, enough for more than one million homes in Texas. The plant is managed by the STP Nuclear Operating Company and owned by AEP Texas Central Company, Austin Energy, City Public Service of San Antonio, and Texas Genco, LP.

SOURCE: Texas Genco, L.P.