Laser Welding Technology to Put Kewaunee Power Plant Back On-Line
Elizabeth M. Lockyer
KEWAUNEE, Wis. -- Throughout the nuclear power industry, steam generators have met their enemy, and its name is corrosion. Now, a new laser welding technology may prove to be the saving grace of corroded steam generator tubes in general and for the Kewaunee Nuclear Power Plant in particular, which has been out of service since Sept. 20.
Nuclear power plant steam generators use low-temperature mill-annealed Inconel Alloy 600 tubing that is susceptible to both inside- and outside-diameter stress corrosion cracking. Some plants like Kewaunee had repaired cracked tubes by inserting smaller-diameter tubing that was mechanically expanded (sleeves). However, over a period of time, such sleeved tubes started to crack because of the high stresses induced in the tube by the mechanical process.
Before its application at the Kewaunee plant, laser welding was used primarily for installing new sleeves. Today, following on the heels of laser welding repairs at the Doel-4 Power Plant in Doel, Belgium, the technology is being applied to bring the Kewaunee Nuclear Power Plant back on-line.
"Part of the reason for the prolonged shutdown at Kewaunee is that we have only three Nd:YAG laser systems to cover all the plants we service," said Bala R. Nair, manager of service technology development at the Nuclear Services Div., Westinghouse Electric Corp. in Madison, Pa. "This year, we were already committed to large sleeving programs at three power plants when we received the call from Kewaunee. This necessitated their waiting more than three weeks until a laser system was freed from one of those sites."
Nair explained the procedure, "The energy source for the welding is a 1000-W pulsed Nd:YAG laser manufactured by Lumonics Ltd. of Rugby, UK. The laser energy is delivered through four fiber optic segments about 50 m in length. An optical multiplexer, which is located at the end of the third fiber optic segment, permits switching of the laser beam to one of four beam paths. This enables welding to be performed in up to four steam generators in parallel on a time-shared basis."
A 15-m-long weld-head conduit transmits laser power and shield gas for welding to the weld head, which includes a replaceable mirror that directs the laser beam to the weld area.
All of the laser-welded sleeving operations are performed remotely using the Westinghouse nuclear service robot, ROSA III, and no manned entry of the steam generator is required.
"Westinghouse is now looking at an extension of this technology that we call Direct Tube Repair," said Nair. "With this technique, no sleeve is required. Rather, the process restores degraded steam generator tubes by the controlled melting of the tube wall from the inside using a laser beam. In addition, the melted metal's corrosion resistance of the weld is improved by addition of a filler metal. We hope to be able to use this technology next spring."
Nair sees applications for types of laser-weld technology in more than half the pressurized water reactor power plants in this country and in many European plants in France, Belgium and Sweden that are of the Westinghouse design.
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