By Bob Rucker, Freelance Writer
As nuclear power plants age, modifying or replacing their major components can contribute greatly to their efficient operation. One of the most challenging opportunities for such an activity is the replacement of a pressurized water reactor’s steam generators. Steam generators are large heat exchangers that convert heat from a plant’s reactor into steam to drive turbine generators and produce electricity.
At Southern California Edison’s 2,200 MW San Onofre Nuclear Generating Station (SONGS), midway between Los Angeles and San Diego, Bechtel has completed steam generator replacement (SGR) projects at Units 2 and 3, with the Unit 3 SGR completed in December 2010. Units 2 and 3 have been online since 1983 and 1984, respectively.
As designers and builders of more than half of the commercial nuclear power plants in the United States, and operating services provider at many others, Bechtel and its Unit 3 team, led by project manager Mark Romano, drew on decades of lessons learned and management tools, as well as the company’s own variations on more common industry methodology.
Bechtel’s project team uses a specially engineered lift and runway system to remove an old steam generator from Southern California Edison’s San Onofre Nuclear Generating Station’s Unit 3 containment building in 2010.
From Romano’s perspective, perhaps the team’s most effective tool for ensuring quality and safety and managing the planned outage was a tool that Bechtel pioneered for major modification work and that has become one of the more common tools used in outage planning, vertical slice reviews (VSRs). Bechtel’s team made extensive use of VSRs as they planned the Unit 3 work.
“The VSRs created for Bechtel and our customer an opportunity to jointly identify and leverage vital lessons learned from the Unit 2 SGR that we completed in early 2010 and from previous SGRs Bechtel has performed. This proved invaluable as we embarked on the Unit 3 work,” Romano said.
How VSRs Work
The VSR is a process that can be used to help managers efficiently levelize resources, identify critical and near-critical paths for both the project work and the overall outage, plan for safety and develop the logistics to be used during a planned outage. The VSR captures all activities performed at a given time during the outage, grouped by specific days during the outage and major work areas.
In a complex process, Bechtel lifted old steam generators inside the containment building, prepared them for removal, then pulled them through an opening in the containment wall to a lift system and transporter. The same transporter and lift system positioned and raised the new steam generators to the opening.
At SONGS Unit 3, these work areas included the construction opening in the containment vessel, the work required to detach and reattach the steam generators for the reactor coolant system, major commodity removal and replacement and movement of the old and new steam generators out of and into the containment building. Bechtel’s VSR process documented the start, progress and conclusion of thousands of work steps and activities.
Using the VSR document as a blueprint for discussion, the project’s shift outage manager led team meetings that spanned three days. The first meeting occurred approximately five months before the start of the outage. The meetings brought together craft general foremen, superintendents, field engineers, design engineers, subcontractors, operators, procurement specialists, chemists, health physicists, facilities maintenance personnel and others in a single room where the team’s VSR document was pinned to a wall. The shift outage manager then guided discussions of the team’s work to date, short-term objectives and resource allocations and longer-term schedule and resource concerns.
“The meeting was an opportunity to elicit innovative solutions to schedule challenges, build teamwork and motivate individuals with stretch goals and highly specific personal task lists,” said Cecelia Hernandez, Bechtel’s construction opening task manager.
As each meeting progressed, the team identified and assigned action items for resolution. As the outage approached, the team held additional VSR meetings at approximately two-month intervals.
from Outage to Pre-outage
The initial VSRs identified some especially valuable and timesaving lessons learned in two key areas: removing concrete to create the containment opening and removing the containment’s tendons.
Discussions during the Unit 3 VSRs revealed that time could be saved by revising the design of the work platform used to remove tendons, and by removing two tendons during a pre-outage work window instead of during the outage itself. Other lessons noted in the Unit 3 VSR, such as setting up certain work areas and degreasing tendons months before the outage, paid additional premiums. In fact, moving a number of tasks from the outage to the pre-outage window—all gleaned from the Unit 3 VSRs and lessons learned—were crucial to saving the Unit 3 team more than 90 critical-path hours.
|Steam generator replacements at Units 2 and 3 will enable the San Onofre station to operate until its current license expires in 2022, and set the stage for an anticipated 20-year license renewal.|
Also based on its Unit 2 experience, Bechtel proposed using hydro-demolition to create the 28-foot-diameter opening in the containment’s four-foot-thick concrete wall, through which the old and new 641-ton steam generators would be transported. Hydro-demolition uses remotely guided robots equipped with nozzles to “wash away” concrete with a stream of water concentrated at 20,000 psi. Because hydro-demolition’s impact is more focused and easier to control than hydraulic hammers, it’s much easier to avoid damaging embedded rebar or liner plate, and those commodities can typically be reused.
Although not new to the nuclear industry, the challenge of hydro-demolition lay in scheduling work to be performed prior to and during the outage around an operating nuclear station. This scheduling became a successful team effort of Bechtel and SCE. Relying on Bechtel’s experience at other plants and on early VSRs, the team rewrote procedures, expedited the approval of engineering designs and calculations, performed constructability reviews and adjusted its procurement and vendor mobilization efforts.
The Bottom Line
Ultimately, by shifting aspects of the work from the planned outage to the pre-outage work window and by using hydro-demolition (as learned from Unit 2 and reviewed during the VSRs) Bechtel was able to remove and replace both of Unit 3’s generators and restore the construction opening to its original condition with new steel and concrete within the planned duration.
A crane lowers one of San Onofre’s old steam generators to a transporter after its removal from Unit 3’s containment.
Of course, every steam generator replacement has its own requirements, since the design of nearly every U.S. nuclear plant is unique. But the lasting lesson from Bechtel’s two consecutive SONGS SGR projects is that effective tools and processes, like lessons learned and VSRs, are crucial to timely completion of major equipment work during a planned outage.
“The takeaway is that VSRs should not be limited to SGR projects,” Romano said. “They are a valuable process for any project that requires integration of multiple work scopes and multiple work groups. In the nuclear industry, I wouldn’t execute an outage without performing one.”
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