Nuclear Uprates

Although construction on a new nuclear plant in the U.S. hasn't occurred since the 1970s, nuclear power has added a total of 6,194 MW (equivalent of 6 large power plants) to the grid via uprates since 1977, according to the Nuclear Regulatory Commission (NRC). Exelon alone added 1,100 MW to its existing nuclear fleet from 1998 to 2008.

Pending applications nationwide reveal that uprates account for a planned 1,475 MW.
And according to the World Nuclear Association, more opportunity exists: some 3,200 MW could be added in the U.S. through 67 projects subject to NRC approval.



Uprates 101

The NRC defines an uprate as a utility refueling a reactor with either slightly more enriched uranium fuel or a higher percentage of new fuel in order to increase the power output of a reactor.
Consequently, the reactor is able to produce more thermal energy, driving a turbine generator to produce more megawatts. In order to accomplish this, components such as pipes, valves, pumps, heat exchangers, electrical transformers and generators must be able to accommodate the conditions that would exist at the higher power level.

This often requires replacing components so that systems are capable of handling higher flows.
The NRC is closely involved in power uprates, as licensees must submit an increase in power level and equipment modifications for regulatory approval."The analyses must demonstrate that the proposed new configuration remains safe and that measures continue to be in place to protect the health and safety of the public," the NRC says on its web site.

The design of every U.S. commercial reactor has excess capacity needed to potentially allow for an uprate, which can fall into three categories: measurement uncertainty recapture power uprates, stretch power uprates and extended power uprates.

The first option, measurement uncertainty recapture power uprates, are power increases that are less than 2 percent of the licensed power level. This type of uprate is achieved by implementing enhanced techniques for calculating reactor power involving the use of devices to more precisely measure feedwater flow, which is used to calculate reactor power. More precise measurements can reduce the degree of uncertainty in the power level.

Stretch power uprates are typically between 2 percent and 7 percent of the licensed power level, with the actual increase in power depending on a plant design's specific operating margin. Stretch power uprates usually involve changes to instrumentation settings but do not involve major plant modifications.

Extended power uprates are upgrades that have been approved for increases as high as 20 percent of the licensed power level. Extended power uprates usually require significant modifications to major pieces of non-nuclear equipment, such as high-pressure turbines, condensate pumps and motors, main generators and/or transformers.  (Power Engineering, November 2012, DOE-EIA)