Every
year, a few nuclear power plants ask the NRC for permission to boost
their power output in what’s called an “uprate.” The principles behind
an uprate are fairly simple.
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.
for more details on uprates. For an earlier post on the uprate at Nine Mile Point nuclear power plant, go
here .
Scott Burnell
Public Affairs Officer
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