Demand for Nuclear
There are many factors present today that support the resurgence of nuclear power throughout the world. Such factors include the increasing reliance on global oil sources, the increasing price of oil and gas as fuel sources, and the need for generating power that is environmentally clean. In the United States, which has not seen a new nuclear power station begin commercial operation since 1993, the opportunity for new nuclear build has gained significant momentum. During the interim (1993 to present) you would have been surprised if you saw a newspaper headline that stated, “U.S. Builds Three 1362 MWe Nuclear Power Plants to Support Nationís Energy Needs.” Although this never occurred, the U.S. Nuclear Regulatory Commission (US-NRC) has approved new generation via power uprate projects at existing nuclear power plants. In fact, power uprates have contributed over 4000 MWe of new generation in the U.S. since 1993, with another 2600 MWe expected by 2010. This is equivalent to five large nuclear power stations. Power uprates have also been successfully implemented in other parts of the world, primarily Europe, where the focus has been on plant modernization. Power uprates will never compete fully with the capacity of a new build, but they are an important component of an existing utilityís strategy for a safe, clean and reliable energy future.
Each existing nuclear plant has additional power that can be added to the electricity grid after completing necessary analysis, modifications, and licensing steps with the nuclear regulator. There are generally three types of power uprates to be considered:
• Stretch Power Uprate (SPU) Ė which yields up to 7% thermal power uprate through additional analysis of existing safety margins in the plant, usually with no hardware changes required.
• Measurement Uncertainty Recapture (MUR) power uprate that usually nets a 1.5% thermal power uprate through improved feedwater flow measurements.
• Extended Power Uprate (EPU) - which can result in 20% or more thermal power uprate through analysis of safety margins at higher powers and implementation of necessary plant modifications.
All three of these uprate types can be implemented at both Boiling Water Reactors (BWRs) and Pressurized Water Reactors (PWRs), although we will see later that the EPU can be more economical for the BWR given its single-cycle design. Figure 1 shows the history of power additions through power uprates in the U.S. since 1993. The economics of power uprate are very straightforward and take advantage of the minimal increase in Operations and Maintenance (O&M) expense as a result of the power uprate and only a small increase in fuel cycle costs. The remaining factor in the power uprate economics is related to the costs to modify the plant and the engineering, licensing and training costs. With estimated costs of US$1,000/installed kW or less for the up front costs, it is easy to understand why a power uprate is an attractive project for a utility.
Stretch Power Uprates (SPU)
Early on, the SPU was the dominant project type in the U.S. primarily due to the ease of implementation and low cost. For most nuclear power plants, original design analyses were performed at 105% of rated thermal power to provide added margin. With this design margin also came margin in the basic steam cycle design and, therefore, only instrumentation setpoint changes are typically required with an SPU. This clearly results in the lowest US$/installed kW up-front costs, and since 1993 over 40 SPUs have been approved by the NRC resulting in an additional 2000 MWe of capacity.
Measurement Uncertainty Recapture (MUR)
In the late 1990ís several developments spurred renewed interest in power uprates and resulted in an increased rate of plant projects to take advantage of the economic benefits of power uprates. State-of-the-art ultrasonic feedwater flow measurement devices were developed and installed at numerous nuclear power plants to provide a more accurate determination of reactor thermal power, which is important to safety analysis. Since reactor power uncertainty is often a dominant parameter in many safety analyses, the use of reduced uncertainties allows operation with increased operational margin to plant and fuel design limits and a small increase in the licensed thermal power. The first MUR uprate was approved by the US-NRC in 1999 and since then over 30 MUR uprates have been approved resulting in an additional 500 MWe of capacity.
Extended Power Uprates (EPU)
At the same time, fuel design evolutions were being introduced with significant improvements in fuel performance leading to the ability to design with higher fuel enrichments and operate to higher exposures. During the 1990ís many BWRs shifted to 24-month operating cycles as a result of the improved fuel performance and the ability to load the core during a refuelling outage with sufficient reactivity to maintain adequate margins to limits throughout the cycle. This improvement in fuel capability also opened the door for vendors and utilities to explore EPU projects and determine the feasibility of uprating beyond the Stretch and Measurement Uncertainty Recapture power levels. This feasibility was most readily achieved in the single-cycle BWR design.
BWRs are operated with constant reactor vessel steam dome pressure and the turbine control valves are designed to maintain these conditions. Although it is feasible (and has been done) to increase the reactor vessel steam dome pressure as part of a power uprate, this results in major evaluations of the primary system. GE developed an alternative approach for power uprate in which the reactor vessel steam dome pressure is maintained at its pre-uprate conditions and the turbine is modified to operate efficiently at the new conditions. This is referred to as Constant Pressure Power Uprate. This approach greatly simplifies the primary system impact and possible hardware modifications required to support the uprate. The major modifications required for an EPU are primarily related to balance-of-plant equipment such as the high-pressure turbines, condensate pumps and motors, main generators, and/or transformers.
The most limiting component for a PWR is typically the steam generator and given the cost to replace a steam generator, it is more difficult to implement an EPU without a parallel steam generator upgrade. For plants where plans are in place to replace the steam generator for other reasons, it would be beneficial to evaluate the potential of an EPU in the future such that sizing of the steam generator could take this into consideration.
Economically, EPUs are the most significant uprates, and in fact they are also significant compared to other power sources. Conservatively, an EPU is equivalent to a 150 MWe wind farm, one heavy-duty simple cycle gas turbine, one small hydro facility, or two small natural gas combined cycle (NGCC) plants. Figure 2 shows how favorably EPUs compare relative to other generation sources given the current prices of fuel. EPUs represent the most challenging power uprate due to the wide range of design and licensing basis impacts and the need for plant modifications. However, the larger payback from an EPU in terms of MWe has resulted in 13 approvals in the U.S., adding over 1500 MWe of nuclear capacity since 1998.
Nuclear plants are designed to operate for a specified lifetime and the systems and components are generally designed for a lifetime that is at least equal to that of the plant (nominally 40 years). Consider the “bathtub curve” in Figure 3 that shows system/component failure rates as a function of time.
Once systems/components operate beyond the infant mortality phase they generally enjoy a reliable “useful life” until the end of plant life. With the implementation of license renewal, or Plant Life Extension (PLEX), at many reactors throughout the U.S and the world, the likelihood of major systems/components entering into the ďwearoutĒ period is increased. In addition, operation at power uprate conditions can increase the duty on major systems/ components primarily due to higher steam flows, fluence and temperatures experienced at the uprated conditions.
For example, flow-induced vibration (FIV) has been identified as a key area where increased duty is expected following a power uprate. Several plants have experienced damage to their steam dryers that has lead to an extensive program to improve the understanding of the effects of uprates on these components. This program includes extensive in-plant data collection, the development of scale model test facilities to study components susceptible to FIV and improvements in analytical techniques for evaluating loadings on reactor internals. These issues have demonstrated the need for a comprehensive review of plant conditions prior to implementation of a power uprate and advanced testing and analytical techniques to address the consequences of changes in operating conditions. Although the above examples did not result in degraded safety performance, they had a direct impact on the ability to achieve the uprated conditions in a reliable manner.
In response to the effects that EPUs and PLEX can have on plants and to assist in general Plant Life Management (PLIM), GE has developed Performance 20SM an approach to integrate the challenges posed by these changes to the plant design and licensing basis. In the past, EPU evaluations were focused on ensuring safe operation of the plant, satisfying all regulatory requirements and making changes necessary to accommodate the increase in reactor vessel steam flow. These evaluations typically relied on the plantís current Updated Final Safety Analysis Report (UFSAR) and system design specifications, documents and design basis information. Performance 20SM not only integrates all safety and regulatory requirements, but also assesses operating histories for the plant(s) involved in a Power Uprate project.
Performance 20SM implements a collaborative approach between utilities and GE to focus on both the design and the current operating condition to address reliability, equipment obsolescence, life extension and general plant performance in addition to increasing the output of the plant. GE evaluates both the safety margins and operational margins to help select the optimum target power level through a “pinch point” analysis. This analysis identifies those plant components that must be replaced or modified and the associated economics to allow pursuing the next power level and the above-mentioned life extension goals. When combined with the EPU pinch-point analysis, it helps avoid the redundant modification risk previously discussed by ensuring that changes are implemented in a staged and efficient manner (e.g., avoid making reliability-based modifications today that will require additional changes in several years due to the implementation of an EPU).
Even without EPU, Performance 20SM provides a valuable snapshot of the power plantís conditions relative to the original design margins and can be used as part of a general life cycle management program for reliability purposes. This program compliments a plantís existing reliability programs that may already be underway. The approach combines GEís design and fleet expertise with the plantís operating experience. With an integrated Performance 20SM approach, long-term planning to address reliability, life extension and power uprates can be implemented efficiently.
As mentioned, an important aspect of EPUs is the major modifications that are required, such as those associated with the reactor recirculation system, the reactor pressure vessel (RPV) internals, controls and instrumentation and the turbine-generator. While the design changes are potentially major in scope, GE has significant previous design and implementation experience in modifying the system configurations. This experience extends to the redesign and replacement of high performance reactor internals into existing reactors that involves cutting, removal, installation, and welding of structures inside and attached to the RPV.
The nuclear industry is enjoying its highest level of public support in years. Countries around the world are embracing nuclear power as a necessary part of an integrated energy policy designed to meet the needs of a growing world while at the same time promoting a safe and clean environment for the future. The nuclear industryís response to these times should not be solely focused on the addition of new capacity through new builds but should continue to evaluate the existing fleet of nuclear power plants for opportunities to increase their output. Power uprates are a near-term option that allow utilities to “install” as much as 20% of a “new” plant in a fraction of the time and at a fraction of the cost of a true new plant. Through processes like Performance 20SM utilities can be confident that the impacts of Power uprates and life extension will be addressed to ensure that the new megawatts are safely and reliably generated.