SC Nuclear Repair Gains Worldwide Look
By: Steve Waldrop
Officials at the V.C. Summer Nuclear Station say that their daily routine has changed considerably, all because of a crack less than 3 inches in size. General Manager Greg Halnon says that he knows the complex repair work to the station's "Alpha Hot Leg" is being watched by plant scientists throughout the country.
The trouble began when engineers found boric acid corrosion near the weld of a 15-foot-long stainless steel pipe. The tube brings water of 620 degrees from the reactor containment area to a steam sector. The plant has been closed since October 7. Construction was started on the plant in 1974.
How do you repair a 2 1/2 inch thick, 30-inch diameter pipe section that carries 400,000 gallons of super-hot radiated water and weights a ton a foot? Nuclear officials have never been faced with this kind of problem before. Halnon has welcomed calls with suggestions from scientists and managers around the world.
Tests showed the pipe was leaking about 0.3 gallons per minute, which is well under the federal allowable level of one gallon per minute at which action is required. A routine walk through of the plant, done every 18 months, showed stalagmite-like rivulets of boron corrosion on a welded section from when the pipe was laid in 1977. Boron is used to regulate the fission process.
Halnon and his engineers have searched for a root cause of the leak, and while the study is not complete, he believes the cause of the crack to be the type of weld that was used more than 20 years ago. "The rest of the system was not compromised," he said.
A decision was made to cut out a 12-inch section of the pipe surrounding the leak. Framatome Technology Inc. was contracted to do the job. Approximately 60 people were sent in to perform the repairs.
It took about five days for the reactor system to be drained. Then a circle of yellow lead shields, weighing close to 100 tons, was placed around the reactor. Workers cut the pipe and lifted it with winches and chains about 8 feet to the main platform. 10 feet of stainless steel replacement pipe was purchased and sent to Framatome in Lynchburg, VA., for testing and preparation and then sent to South Carolina, while specialists were training for the job on a mock-up near the reactor.
A narrow-groove welding process was used to prepare the four surfaces. No smoke, sparks or debris is discharged during the 10-day process. "It's like a blue strobe light when they're working," Halnon said.
The old weld as made of a material called alloy 600. It looked like the defect came from the material, and not properly handling the specific sequencing of welding. The new material is a combination of nickel, chromium and iron and is called Incanel and is very much like steel.
A specially designed robot will be used to help the final inspections for the nondestructive examination phase. Halnon hopes the plant and its 600 employees will be able to return to full operation by the second week in January.
The Federal Nuclear Regulatory Commission has sent teams to Summer to monitor progress and plans. And the NRC and plant officials have met to discuss the analysis of the flawed pipe section.