Quick-change Artists

By Joel Drucker

Nestled in Arizona’s picturesque Sonora Desert about 60 kilometers from Phoenix, the Palo Verde Nuclear Generating Station is the largest nuclear power site in the United States, its three units producing electricity for some 4 million people. So when the two aging steam generators in Unit 2 needed replacing, it was imperative to do the job quickly and efficiently. In the final months of 2003, Bechtel—the world leader in steam generator replacement (SGR)—did exactly that.

As their name implies, steam generators use heat from a nuclear power plant’s reactor core to create steam, which drives the plant’s turbines to create electricity. Each steam generator contains thousands of alloy tubes through which hot radioactive water is pumped under high pressure. This process prevents the water from boiling. As the pressurized water flows through the tubes, it heats nonradioactive water outside the tubes to make steam.

A typical steam generator has more than 3,000 alloy tubes. The tubes were expected to last for 40 years, but metallurgy and water chemistry issues have significantly shortened their useful life. When approximately 15 percent of these tubes have corroded, a plant suffers from significant maintenance costs and the inability to maintain full power.

Over the last 20 years, there have been 34 replacement projects at power plants throughout the United States. Bechtel has handled 27 of them, setting records for the lowest radiation exposure, shortest replacement outages, and best employee safety.

Having performed design and construction at Palo Verde in the mid-1970s, Bechtel is very familiar with the plant’s configuration. Although the new steam generators at the plant are functionally identical to the original ones, they include design enhancements, such as improved tubing materials and water recirculation piping, that will minimize tube degradation. These upgrades will add 36 centimeters to the diameter of the lower shell and 104 centimeters to the height, making the generators the largest ever installed. The new generators weigh 726 tonnes apiece, about 41 tonnes more than their predecessors.

“The larger generators made construction more complex,” says Bechtel Project Manager Frank McDougall. “We modified many components, including steel platforms, pipes, pipe supports, and instrumentation.”

The replacement operation coincided with a refueling and maintenance outage that began September 27. The two steam generators were moved out of the Unit 2 containment building on October 23 and 25, and the new ones were moved in on October 27 and 29. The plant was back online in mid-December, 79 days after the initial shutdown.

The installation of the steam generators was handled quickly and efficiently—but planning required nearly a decade. Arizona Public Service Company commissioned a study in 1995 to identify the best methods for replacing its steam generators. Not until two years later was the project approved. Ansaldo-Camozzi, a heavy machinery manufacturer, was brought on board in 1998 to design and fabricate the replacement steam generators in its Milan, Italy, shop. That took more than three years. The generators were shipped to Arizona in a complex logistical operation that required an ocean voyage, flat-bottomed barges, and special land transporters that took the equipment via the Panama Canal and up through Mexico.

Whereas many other Bechtel projects start from the ground up, in the world of SGRs, it’s necessary to work amid radiological conditions and space constraints that change as the project evolves. Because access is so tight, welding can be particularly difficult. Workers also must contend with hot pipes, and dust and smoke must be controlled. All of this puts a premium on safety and planning.

“Because SGRs take place inside a confined area,” says McDougall, “it’s kind of like working in a space capsule. You don’t have the luxury of throwing more and more people at a problem. You have a 24-meter-tall generator, and every three meters you have different things that need to be done—welding pipes, installing valves, etc. You don’t want grinding sparks flying around or dropped tools falling into other work areas. Getting the right people in the right places at the right time is a multilayered chess game.”

McDougall says Palo Verde is one of the most complex SGR projects Bechtel has ever undertaken. A typical SGR has to focus on only one aspect of engineering, such as civil or mechanical. At Palo Verde, it was necessary to devote considerable resources to both of these areas, as well as to electrical aspects. On the civil front, the SGR team had to cut out and replace two concrete walls as well as modify platforms and main steam pipe restraints. Bechtel engineers made more than 140 radiographic testing welds, added two new piping systems, and rerouted all of the existing piping connected to the steam generators. The electrical work included temporary power installations associated with most SGRs as well as extensive removal and restoration of conduit, cabling, and instrumentation.

The payoff, though, is tremendous. Each of the new generators at Palo Verde will produce an additional 55 megawatts—more than the production of a small, natural gas–fired power plant. That means Palo Verde will be able to increase its power output by 3 to 5 percent.

Bechtel estimates that steam generators will have to be replaced at more than a dozen nuclear power plant reactors in the United States between now and 2008.

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