Atoms: Beyond Maintenance

October 10, 2023, 9:42AMNuclear News

Maintenance can mean no more than keeping the status quo. But plant maintenance programs at U.S. nuclear power plants do that and then some. Even the programs are themselves maintained and improved—and so a long-lived nuclear fleet has improved with time. Read on.

Ready to “repower” nuclear?

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Do nuclear plant outages need rebranding?

The typical outage at a nuclear power plant—refueling, maintenance, and large or small component replacements or upgrades—sets up a plant to keep reliably producing power for 18 to 24 months.

In the world of wind energy, a wind turbine overhaul is not an outage—it’s a “repowering,” and it unlocks tax incentives for older wind projects.

According to the Land-Based Wind Market Report: 2023 Edition, released in August by the Department of Energy’s Office of Energy Efficiency and Renewable Energy, a repowered wind turbine might have major components replaced and its rotor diameter expanded. The 13 wind projects that were “partially repowered” during 2022 had a median age of 10 years and a total capacity of 1.7 GW before and 1.8 GW after repowering, for a gain of just about 100 MW. While repowering wind turbines can increase energy production, according to the report, “The ability of partially repowered wind projects to access the [production tax credit] has been the primary motivator for the growth in partial repowering in recent years.”

Downtime

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Data source: NRC, Reactor Operational Experience Results and Databases (nrcoe.inl.gov)

The Nuclear Regulatory Commission tracks the annual criticality status of all nuclear reactors in operation, including fleetwide sums of critical reactor years and shutdown years. This graph takes those data and shows how the percentage of total reactor years that pressurized water reactors and boiling water reactors spent in shutdown in each year between 1987 and 2022 has dropped over time (with notable exceptions following the Fukushima Daiichi accident in 2011). Clearly, we never tire of charting the nuclear power industry’s two-decade-long maintenance success story.

Common Defense

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Common-cause failure (CCF) events—a failure of two or more components, such as pumps or valves, during a short period of time due to a single shared cause—were once a significant factor holding nuclear capacity factors down and causing unplanned outages. The primary defense? Maintenance.

Analysis of past CCFs (collected in the report CCF Parameter Estimations, 2020 Update, prepared by Idaho National Laboratory staff for the NRC and published in 2022) points to maintenance program modifications as the most common defensive strategy to prevent the recurrence of particular CCFs. As modifications have improved already excellent maintenance programs, there has been a steady and significant reduction in CCF events, now about one-tenth the number that occurred in the late 1990s.

Performance patterns

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Source: Sofia D. Hamilton et al., “How does wind project performance change with age in the United States?” Joule 4 (2020); doi.org/10.1016/j.joule.2020.04.005.

Nuclear power plant maintenance is not optional, and its benefits are plain—nuclear capacity factors have increased across the fleet and have stayed up for over two decades, with older reactors among the top performers.

The wind energy industry has a different pattern. According to the Land-Based Wind Market Report, “Capacity factor data suggest performance decline with project age, though perhaps mostly once projects age beyond 10 years.”

Research published by Lawrence Berkeley National Laboratory in 2020 puts it more plainly, concluding that “a performance drop occurs when plants lose eligibility for production tax credits,” and the “most surprising aspect of these results is the abrupt decline in performance found after 10 years of operation. . . . The coincidence in timing between the performance drop and the 10-year window of eligibility for the federal PTC is notable. It suggests that maintenance and operating strategies change when projects lose access to the sizable tax benefits afforded by the PTC.”

Wind turbine degradation, it turns out, may be “a function of weighing the costs of maintenance against the value of operation of a wind project.”



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