A four-year-long intercontinental odyssey culminates this week in replacement of the largest steam generators in a U.S. nuclear powerplant. The generators were fabricated in Italy, shipped via the Panama Canal to the head of the Gulf of California, offloaded in Mexico and conveyed via country roads through northern Mexico and southern Arizona to the Palo Verde generating station by workers and supervisors speaking at least four different languages. The operation will be repeated in 2005 and again in 2007.

CHANGE OUT New steam generators are a tight squeeze. (Photo courtesy of Arizona Public Service Co.)

The replacement operation is occurring during a refueling and maintenance outage that began Sept. 27 and is due to be finished in mid-December. The two steam generators in 1,270-Mw Palo Verde Unit 2 were moved out of the containment building Oct. 23 and 25. Their replacements were moved in Oct. 27 and 29.

Arizona Public Service Co., Phoenix, launched the $230-million operation to design, fabricate, transport and install the new units with a 1999 fabrication order placed with Ansaldo Energia (now Ansaldo Comozzi), Milan, Italy. In 1999, APS also awarded a $4-million fixed-price contract to Michael Baker Jr. Inc., Moon Township, Pa., to provide engineering, permitting and construction oversight services for the transport operation.

The new steam generators were designed somewhat larger than the old ones, which entered commercial service in 1986, because of a need for more heat-transfer area. The Inconel 600 material of the old generators’ 11,012 tubes was not holding up. "We found this material starts to crack. We’ve been losing megawatts for a couple of years now," reducing generation capacity by about 2%, says Ron Pontas, APS installation project manager. The new generators have 12,580 Inconel 690 tubes. Pontas says the new material has been on the market for about a dozen years, and appears to be more resistant to corrosion. But because its heat-transfer coefficient is lower, "it needs a longer tube bundle to make up," he adds. So the new generators are 3.5 ft taller, with a girth 14 in. greater in the evaporator section, than the old ones to accommodate the added length and tube bundles.

Ansaldo completed fabrication and shipped the generators in August 2002, using transport contractor Fagioli S.p.A., Parma, Italy. After a 35-day voyage by heavy-lift ship, they arrived in the deepwater port of Guaymas, Mexico, in late September. There they were transshipped to a barge for final delivery to Puerto Peñasco, at the head of the Gulf of California.

The original generators came through Peñasco two decades ago, but development since then has reduced the space necessary for the new ones. So Michael Baker installed a sheetpile dock 7 miles away and dredged a 500-ft channel an average of 3 to 5 ft deep to accommodate the barge’s 15-ft draft and 20-m width. Docking originally was scheduled for June, when humid weather and longer high-tides would make offloading easier. But a delay in fabrication pushed delivery out three months, and antinuclear protesters occupied the docking site for four days before they were removed, costing another month’s delay because of a lost tide window, says Colin Olness, Baker’s project manager. The delays didn’t affect the outage dates, but did mean the offloading would occur in a less-than-optimal season. In addition, "We lost a lot of the dredging that we were maintaining," says Olness.

On a calm day, Nov. 3, 2002, the generators, already sitting on "self-propelled modular transporters" powered by four diesel-driven hydraulic power packs, rolled deliberately off the barge to a staging area. "They're similar to the units that move the space shuttle," says Olness. In the staging area they were configured for road travel with a full complement of 208 tires on 26 axles. One week later, the first generator set out on the final 200-mile leg of the trip to Palo Verde, 40 west of Phoenix. Completing that leg would take two more months.

The route had been prepared to receive the 1,087-ton loaded rigs. Protection was required for 426 structures, including wood culverts, concrete culverts, bridges, a canal and railroad crossings. Exhaustive research exhumed records going back to the 1930s, allowing Baker to assess the load-carrying capacity of roadways and bridges and devise strategies to improve them. But many records simply weren’t available. "In Mexico, we just shored or overbridged on all of them," says Olness.

Overbridging consisted of placing an I-beam, 50 ft long and 10 ft wide, over existing bridges deemed to need the protection. Shoring was required as well to prepare for the load. Contractor Precision Heavy Haul (HQ tk) leapfrogged the steam generators, which were moving at an average speed below 5 mph, placing the overbridging ahead, then moving it to the next bridge after the load had passed.

One of the most challenging obstacles was Interstate 8 in Gila Bend, Ariz. "There is a bridge on I-8 that crosses abandoned Phelps Dodge railroad tracks," says Olness. "We negotiated an agreement to use the railroad right-of-way, then tore out the unused track and excavated underneath the Interstate bridge until we achieved the 30-ft clearance we needed."

The first generator, 75 ft long and 24 ft dia, was moved across the U.S.-Mexico border at Lukeville, Ariz., and parked. The transport team picked up the second generator and delivered it on Jan. 2 to Palo Verde, then returned to the border to bring up the second generator. The transporting operation was complete Jan. 21. Actual time in transit for each generator was 17 days, but the schedule was extended because of holiday breaks at Thanksgiving and Christmas and the staged movement of the first generator, which was not part of the original plan, says Olness.

In March 2003, San Francisco-based Bechtel began field work to prepare the steam generators for installation. Already on the job since 1999 doing design packages for the work, the contractor in March established facilities, prefabricated piping and components, prepared the generators’ joints for welding, qualified welders and performed training on mockups. In August, construction began on a runway to feed the old generators out of the 24-ft-dia hatch and to feed the new ones in. The size of the hatch was one factor that limited the growth in the design size of the new generators. Just 2 in. of clearance was left when the evaporator section’s girth was expanded to accommodate the added tube bundles.

The workforce in the last week of October was 733, close to its peak for the project, says a Bechtel official. Except for an incident when an auxiliary crane blew a seal, costing "a couple of days overall," the work has gone without a hitch, he says. Bechtel is planning its work with the goal of completing demobilization Dec. 19, before the Christmas holiday.