|By Brian Schimmoller, Contributing Editor|
Stress corrosion cracking in nuclear power plants has resulted in significant maintenance expenses for repair and replacement, and continues to challenge materials management programs across the industry. Much can be done through materials selection and water chemistry to avoid such cracking, and proactive physical mitigation with welding is also a viable option.
Pressurizer heaters are used in pressurized water reactors to provide the heat required to maintain pressure during transient conditions. These heaters are inserted in pipe penetrations into the vessel called heater sleeves. The sleeves are welded to the vessel during original fabrication, and the heater is welded to the sleeves. The two welds comprise the pressure boundary for the system and are susceptible to stress corrosion cracking. If cracking occurs in the welds, the heater sleeves can leak, potentially leading to unplanned unit downtime.
Over the past several years, Constellation Energy Nuclear Group (CENG) has pioneered the development, testing, and application of a proactive welding process to repair heater sleeves. Initial application of the process occurred in 2012 at CENG’s Calvert Cliffs nuclear plant in Maryland. The process reduces welding time by 80 percent and weld volume by 90 percent; it also enhances worker safety through improved radiation protection.
CENG received the Nuclear Energy Institute’s “Best of the Best” Top Industry Practice (TIP) Award for this accomplishment earlier this year. In conferring the award, the NEI noted that because the operational efficiencies achieved through the process also yield cost savings, the innovation has the added potential of saving electric utilities—and their customers—hundreds of millions of dollars in future uses.
“We considered several options for this issue, including vessel replacement and lower hemisphere repair,” said Lennie Daniels, senior project manager at CENG. “After detailed analysis, our plant configuration made the repair option more viable.”
CENG devised a first-of-a-kind weld repair. The new tooling and the processes had to be developed and proven to meet ASME requirements. The tooling – which included weld heads, machining equipment, and nondestructive evaluations (NDE) equipment – had to be built to work inside small-bore piping, with the ability to function reliably 30 feet off the floor. NDE consisted of remote visual inspection via cameras mounted directly on the weld head.
“Process control was extremely important,” said Daniels. “Each weld head had to be placed in a specific location, and a specific number of weld layers had to be made to ensure compliance with the codes and to meet the required thickness for the repair process.” Once the welds were deposited, they had to be machined to meet the surface finish requirements for the NDE inspections. The entire process was controlled to ensure proper alignment of the heater sleeve to the support plates. If alignment was not maintained, the heater could not be inserted into the pressurizer.
In all, CENG repaired 119 heater sleeve locations at Calvert Cliffs, and no leakage occurred. The project was completed a day ahead of schedule and at less cost than the other options considered.
One of the main benefits, according to Daniels, was that the tooling was designed to be operated remotely, which means that both the welding and NDE could be perfomed from outside the protected area. This minimizes the need for workers to be directly exposed to radiation. In fact, the project came in far below planned dose levels, about 35 percent lower than the project goal. Personnel contamination incidents were 75 percent lower than project goals.
The first-of-a-kind weld process is not expected to be a “one and done” application. Daniels believes the technique could be adapted to other components in a nuclear plant, such as bottom-mounted nozzles.
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