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For a pressurized-water reactor, replacing steam generators usually keeps the unit offline for about 75 to 80 days when the work is combined with a normal refueling outage. Documented projects range from 63 days 13 hours to about 121 days, so a two-and-a-half-month planning assumption is reasonable, while a complex project can approach four months.
Those figures describe the entire outage—from shutdown and defueling through testing, fuel loading and authorization to restart—not merely the time spent lifting out and installing the generators.
The practical outage range
The most useful broad benchmark is approximately 75 days. A Powers Engineering filing in an Environmental Protection Agency docket cites a 75-day average for a steam-generator replacement outage. Diablo Canyon environmental documentation gives a 75–80-day duration for an outage that included steam-generator replacement.
Real projects show why that number should be treated as a planning range rather than a promise. Callaway’s 2005 outage, which replaced four steam generators, lasted 63 days and 13 hours. A project-management case study estimated approximately 95 days for Catawba. The U.S. Energy Information Administration recorded 121 days for the 2022 Watts Bar Unit 2 refueling outage that included steam-generator replacement.
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On that evidence, a reasonable public answer is roughly 60 to 120 days, with 75–80 days as the central benchmark. The shorter figure reflects an exceptionally well-executed project; the longer figures include more conservative planning or a larger amount of work in the same outage.
| Project | Total offline time | Generators | Plan or actual | Concurrent work and qualification |
|---|---|---|---|---|
| Callaway, 2005 | 63 days 13 hours | Four | Actual | Outage including replacement; Ameren Missouri described it as a world record at the time for replacing four generators in an outage. |
| Diablo Canyon | 75–80 days | Not stated | Documented outage duration | Steam-generator replacement combined with the refueling outage. |
| Catawba | Approximately 95 days | Not stated | Project estimate | Planning figure; the source does not establish an actual final duration. |
| Watts Bar Unit 2, 2022 | 121 days | Not stated | Planned refueling-outage duration | Replacement was part of the refueling outage; it is not presented as a universal actual duration for every project. |
The comparison is not an apples-to-apples performance ranking. Unit design, access arrangements, the amount of refueling and maintenance performed at the same time, and whether the number is a plan or a final result all change the total.
Why the replacement takes weeks instead of days
A steam generator is a large heat-exchanger vessel between two systems. Reactor coolant transfers heat through thousands of tubes to water on the secondary side, where steam is produced for the turbine. The vessel is connected to large-diameter primary and secondary piping, structural supports, insulation, instruments and safety-related systems. Replacing it is therefore a major plant modification, not a simple component swap.
The reactor must normally be shut down and defueled first, which is why utilities schedule the work during a refueling outage. The replacement sequence then combines heavy construction, nuclear-quality welding, inspection, testing and regulatory controls.
1. Shut down and defuel the reactor
The unit is taken offline and the reactor is placed in the refueling configuration. Fuel is removed so workers can perform work inside containment and on the reactor coolant system under the procedures required for the outage. This front-end work is part of the outage clock even though it is not a steam-generator lifting operation.
2. Create the access opening and heavy-load path
Steam generators are too large to remove through ordinary equipment doors. The outage team must establish a route for the old vessels and the replacements, which can require opening or modifying the containment structure. Cranes, transport fixtures, temporary supports and load-control procedures are set up before a vessel is moved. The geometry of the containment and the available heavy-lift arrangement can be a defining schedule constraint.
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3. Disconnect piping, supports and services
Workers separate the generator from reactor-coolant piping and from the secondary steam and feedwater systems. They also disconnect supports, insulation, drains, vents, instrumentation and other attached services. These interfaces must be documented and controlled because each one has to be restored in the correct configuration.
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After the connections are cut and the load path is ready, the old vessels are moved out of containment. The lift itself may be relatively short compared with the preparation and restoration work around it. Rigging, clearances, temporary structures and radiological and industrial controls determine how quickly the move can proceed.
5. Set and weld the replacements
The new generators are positioned, aligned and connected. Large welds on the primary and secondary boundaries require qualified procedures, controlled work conditions and time for inspection. Any repair or rework discovered during welding can affect the critical path.
6. Perform nondestructive examination and inspections
Welds and other safety-significant work are examined using the required nondestructive methods. Inspectors verify that the completed joints, supports, tubing-related components and installed services meet the applicable design and quality requirements. Hold points can prevent the outage from moving to the next phase until results are accepted.
7. Restore insulation, instruments and supports
Once the pressure-boundary work is accepted, crews reinstall insulation and reconnect instruments, cables, drains, vents, supports and operating equipment. The replacement is not complete until the surrounding systems are returned to their design configuration.
8. Test, reload fuel and restart
The plant performs pressure tests, leak checks, functional tests and system walkdowns. Operators then reload fuel and work through the controlled startup sequence. Licensing and regulatory authorization, as well as completion of required documentation, are part of the return-to-service process. These final activities explain why the reactor can remain offline after the physical installation appears finished.
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Why replacement outages are longer than ordinary refueling outages
Steam-generator replacement is commonly about twice as long as an ordinary refueling outage because it adds a large construction project to the normal fuel-cycle work. A standard refueling outage already includes defueling, inspections, maintenance, repairs, fuel loading and startup testing. Replacement adds containment access, heavy lifts, major piping cuts, high-integrity welds, examinations and system restoration.
Utilities often combine other planned maintenance with the same shutdown to avoid taking the reactor offline again. That strategy can improve long-term availability, but it also means the longest or most uncertain task can determine the return date for the entire unit.
What determines whether the outage is 65 days or 120 days?
Reactor and containment design
Different plants provide different routes for removing a generator. The size and geometry of containment, the location of equipment, and whether a large opening can be made and restored efficiently affect both preparation and restoration time.
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Number and arrangement of generators
Replacing four vessels requires more lifts, more piping interfaces and more welds than replacing a smaller set. The physical arrangement also matters: access and interference between components can make one plant faster or slower than another even when the generator count is similar.
Concurrent refueling and maintenance scope
The published duration may include only the replacement and routine refueling work, or it may include a substantial package of inspections and maintenance. The outage ends only when all critical work is complete, so unrelated emergent work can become schedule-driving.
Weld and examination results
Welding and nondestructive examination are sequential activities. A weld repair, a failed examination or additional inspection can require work to be repeated before testing begins. This is why a schedule with little contingency can move quickly from an on-time forecast to a late return.
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Licensing and regulatory hold points
Safety-related modifications require documented reviews and approvals. A plant may be physically ready but unable to restart until required tests, records and regulatory hold points are complete.
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Inspection can reveal conditions that were not visible during planning. Equipment failures, damaged supports, unexpected contamination or access problems can add work. The final forecast should therefore be updated as inspections and tests produce results, rather than treated as fixed when the outage begins.
Planned duration versus actual return date
A planned outage duration is an estimate, not a guarantee. The Catawba figure of approximately 95 days is explicitly a project estimate, while the Watts Bar Unit 2 figure of 121 days is reported as a planned 2022 refueling-outage duration. Callaway’s 63-day-13-hour result is an actual historical accomplishment, not a promise that another plant can match it.
Koeberg Unit 2 illustrates the operational decision behind schedule changes. After a final review indicated a high likelihood of returning later than planned, the utility deferred the steam-generator replacement to a later outage to protect electricity supply during the winter-demand period. Deferral can be preferable to starting a high-risk project when the forecast no longer supports a safe, reliable return date.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.What a 75-day schedule usually includes
The phrase “75-day replacement outage” should be read as a total plant outage. A simplified schedule has overlapping work fronts rather than one task followed by another:
- Early outage: shutdown, defueling, containment preparation and installation of temporary lifting and support equipment.
- Removal phase: cutting and disconnecting piping and services, moving the old generators and preparing the interfaces.
- Installation phase: setting the new vessels, alignment, welding and reconnection.
- Verification phase: nondestructive examination, pressure and leak testing, instrumentation checks and system walkdowns.
- Startup phase: fuel loading, controlled reactor startup, functional testing and authorization to return to power.
Work can proceed in parallel, but tasks on the critical path cannot be skipped. A delay in a primary-boundary weld, for example, can hold up testing and startup even if refueling work is already complete.
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How to estimate a named reactor’s outage
For a specific unit, do not use the 75-day benchmark without checking the project configuration. The minimum information needed is:
- Confirm that the reactor is a pressurized-water reactor and identify the unit.
- Count the steam generators being replaced and note their size and arrangement.
- Find the planned refueling-outage start and target return dates.
- Determine how containment will be opened and how the heavy loads will be moved.
- List major concurrent maintenance, inspection and refueling work.
- Check for licensing reviews, inspection hold points and required startup tests.
- Compare the utility’s original plan with its latest forecast as work progresses.
If those details are unavailable, present 75–80 days as a broad benchmark and 60–120 days as a documented project range. Do not describe either figure as a guaranteed schedule for a current outage; current return-to-service dates require confirmation from the plant operator or regulator.
Does replacement guarantee years of continued operation?
No. Steam-generator replacement addresses the condition of those components, but it does not guarantee that the plant will operate for a particular number of years. The Nuclear Regulatory Commission reports that corrosion and cracking in older steam-generator tubing led to replacements at many facilities; the first U.S. replacement occurred in 1980.
San Onofre replaced its steam generators in 2010 and 2011, then later shut permanently after additional tube-degradation problems. That history shows why a replacement project must be evaluated together with the new design, fabrication quality, operating controls and subsequent inspection results.
Bottom line for outage planning
For a pressurized-water reactor replacing steam generators during refueling, plan on approximately 75–80 days offline. A highly efficient project can finish in about two months, as Callaway did in 63 days 13 hours, while a larger or more complex outage can run to roughly three or four months, as the Catawba estimate and Watts Bar Unit 2 schedule illustrate. The decisive factors are the plant’s access and lifting configuration, the number of generators, concurrent work, weld and inspection results, and any licensing or emergent findings.
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