Imagine you are the owner of a four-door sedan, but want to drive something more powerful. What are your options? New cars are out of your price range, but you can try to soup-up your old car — new instruments, a higher grade fuel, or maybe better tires.
The nuclear industry is facing similar questions. With the many obstacles in the way of building new power plants, owners are devising ways to improve power generation in existing plants.
When the Nuclear Regulatory Commission (NRC) licenses a commercial nuclear power plant, it sets limits on the maximum operational power limit for the reactor core. This is the not necessarily the maximum power limit the core and plant can be operated at, but instead is a power level that the NRC has determined is the safest maximum for that particular plant. NRC’s approval is needed to raise a U.S. nuclear plant core’s maximum power limit — also known as a “power uprate.”
There are two considerations to keep in mind when seeking to raise a plant’s power limit: The ability of the core to produce more heat to generate power, and the limitations of the plant structure to safely accommodate the increased heat and steam flow.
There are three main methods of power uprates used in the United States:
1. Measurement Uncertainty Recapture (MURs)
In the first two cases, a power uprate may not require any major modifications. Measurement Uncertainty Recapture (MURs) power uprates implement improved techniques and instrumentation for recalculating power. As an illustration, when operating a vehicle, the more precise your instrumentation, the more efficient your ability to calculate speed and the faster you can drive without exceeding the speed limit. Similarly, the more precise the instrumentation and the more efficient the operator’s ability to calculate heat, the closer the reactor can operate to exceeding its own structural safety limit.
Because reactor power is the result of nuclear reactions between sub-atomic particles, it is very difficult to measure. Instead, secondary parameters are used to measure reactor power. By measuring the results of the energy—or heat—generated by the nuclear reactions, the reactor power can be estimated. The secondary parameters commonly used to measure reactor power are feed water flow rate and enthalpy of the steam and feed water. We can pretty accurately measure temperature, pressure, and volume.
What is a little more difficult to accurately measure is flow rate — specifically, feed water flow rate. To measure feed water flow, we use a device called a venturi meter. Over time, constant water flow builds corrosion in the nozzles, which restricts flow and makes the meter reading less accurate. One method of uprating is to replace old venturi meters with more advanced feed water flow meters. The old meters had an accuracy of ±2 percent. Today’s meters have an accuracy of ±0.5 percent, hence replacing the meters gives the plant another 1.5 percent increase in feed water flow which translates into a corresponding power increase. This is why an MUR can only uprate the plant ≤ 2 percent.
To return to our car example, this would be the equivalent of when vehicle manufactures began exchanging the old mechanical speedometers for electronic speedometers. This technology allows plant operators to more accurately measure reactor power and, thus, operate closer to power limits without going over by eliminating power uncertainty.
2. Stretch Power Uprates (SPUs)
The second method of power uprate is stretch power uprates (SPUs). Stretch power uprates require adjustments to instrumentation settings, operating procedures, technical specifications and/or setpoints, and are good for between a 2 percent and 7 percent increase in power generation. These adjustments allow the maximum power limit to be raised to within the design capacity of the plant. To distinguish this from MURs, SPUs allow for an increase in the operational reactor power limit, where an MUR merely allows us to operate closer to the designated power limit because it reduces “measurement uncertainty.”
Using our car example, an SPU would be like working with your vehicle’s manufacturer to determine what the true maximum RPM for a manual transmission would be in different gears, and then resetting the gauge so that the “red-zone” is smaller and starts at a higher RPM than the presumable, ultra-cautious limit set by the manufacturer for mass production.
The NRC usually sets conservative maximum power limits, which may not be the maximum power achievable by the core. The core may not require changes to accommodate the power uprate, which is often the case for MURs and SPUs. Instead, the limitations may lie solely on the plant side. If increasing the power potential of the core is desired, a plant may be refueled with slightly more enriched uranium or a higher percentage of new fuel.
3. Extended Power Uprates (EPUs)
The third type of power uprate is the extended power uprate (EPUs). If modifications to the core are put in place, the plant itself may need modifications to accommodate the increased energy. The thermal energy created by the core is used to heat water and create steam. The steam is then used to turn a turbine and generate electricity. The steam is then condensed back into feed water and repeats the cycle. Therefore, an increase in power leads to more heat, more heat transfer, more steam, greater flow, more stress on the turbines, pipes, valves, pumps, heat exchangers, etc. This means that components capable of enduring conditions at the new power levels are necessary. The EPUs can lead to power uprates as high as 20 percent above the previous operating limit.
Domestic and International Uprates
The NRC has approved 154 uprates since uprates began being used in the 1970s. This has resulted in approximately 21,105 MWt (megawatts thermal) or 7,035 MWe (megawatts electric). This generating capacity is equivalent to about seven new reactors. Considering that the nuclear industry supplies one-fifth of the nation’s electricity, uprating remains an attractive option due to the comparatively low cost.
- Westinghouse reports, as of September 2011, that they have completed MURs on 34 American pressurized water reactors (PWRs), five European PWRs and two Asian PWRs.
- Westinghouse reports, as of September 2011, that they have completed SPUs on 42 American PWRs, two European PWRs and four Asian PWRs.
- Westinghouse reports, as of September 2011, that they have completed EPUs on 13 American PWRs and 10 European PWRs.
- Switzerland has undergone uprates on five of its reactors for an increase of 13.4 percent.
- The United States has approved more than 140 uprates since 1977.
- Spain is currently involved in a program to uprate its nine reactors by up to 13 percent.
- Finland has uprated two of its plants.
- Sweden has uprated all three of its plants
As nuclear plants construction faces resistance around the world, it is worth considering whether more power can be generated from existing resources. Uprates are one way to make power generation more efficient and powerful than before.