First of all, a plant’s NRC license sets a limit on how much heat the reactor core can generate. A hotter core creates more steam, which then runs a turbine at a higher rate to generate more electricity (we’ll get to how much more electricity in a moment). The basic heat limit is a key part of figuring out how the plant’s safety systems would protect the public in an emergency situation. The NRC carefully analyzes uprate requests to ensure the plant’s systems will continue to function properly at the proposed higher power level.
There are three uprate categories, starting with an increase of only a percent or two. A plant does this by installing more accurate instruments to determine how much water is going into the reactor. Plant operators must be sure they’re sending the correct amount of water through the core to maintain the proper power level. Since older instruments are slightly less accurate, a pre-uprate power level will be slightly lower to ensure safety. If plant operators are more certain how much water’s going into the core, they can be more certain how much power the core generates and therefore a slightly higher power limit is still safe and appropriate.
The second category is a “stretch” uprate, which increases the power level between two and seven percent. This is done by changing some instrument settings, along with redesigning the core to use slightly more of the type of uranium that undergoes a chain reaction. This redesign increases the core’s output.
An “extended” uprate, the third category, adds from seven to as much as 20 percent to a plant’s power output. It’s the most complex of the uprates, since it involves improving major systems such as pumps, turbines and generators, as well as using a revised core design. Updating so many systems with new parts requires the plant to do a lot of careful analysis to show the uprate can be done safely.
Several factors come into play when figuring out how much more electricity a nuclear power plant will produce after an uprate. Updating the non-nuclear side of the plant, such as the generator itself, can improve the plant’s ability to transform the reactor’s heat into electricity. A nuclear power plant also needs electricity to run its systems, and that comes from the transmission grid, offsetting some of the plant’s output. The plant’s needs can remain relatively steady after an uprate, however, so the increased output goes straight to the grid.
Putting all this together means that if a plant increases its reactor core output by 15 percent, for example, its net electricity output can actually increase a little more than 15 percent.
The NRC’s uprate reviews last at least nine months, and normally take about a year and a half for extended uprates. Our evaluations cover much more than changes to the fuel or large components such as pumps and turbines. We examine the uprate’s impact on the plant’s instruments and control systems, and we also look at how the increased power level comes into play for analyzing possible accidents and their consequences. We’ll also ensure the plant will appropriately revise its procedures and train its personnel.
Over the past few decades, uprates have safely and appropriately added the electricity generating equivalent of approximately seven new reactors to the U.S. power grid.