The Atomic Energy Act originally established the length of a U.S. commercial nuclear reactor license as 40 years and made it renewable for another 20 years. The U.S. Nuclear Regulatory Commission has stated that it bases the length of these licenses (and the 50+ renewed licenses granted to date) not on any particular technical limitation but on whether the plant meets current safety requirements. Does this mean there could be reactor life after 60?
In the late 1990s, Constellation Energy served as the tip of the spear in the nuclear industry’s efforts to pursue the relicensing of plants to extend their operation from 40 years to 60 years. Its Calvert Cliffs plant received the first license renewal from the U.S. Nuclear Regulatory Commission (NRC) in March 2000 (Figure 1). Constellation may soon be in the vanguard again, as nuclear plant owners are now evaluating what it might take to extend the life of existing units beyond 60 years, possibly to 80 or 100 years.
|1. Industry first. In March 2000, Constellation Energy’s Calvert Cliffs Nuclear Power Plant was the first U.S. nuclear plant to earn a 20-year operating license extension from the Nuclear Regulatory Commission (NRC). Its two reactors, which originally went online in 1975 and 1977, are now licensed to operate through 2034 and 2036. Source: NRC|
Constellation and many of its utility brethren are engaged in research through the Electric Power Research Institute’s (EPRI’s) Long-Term Operations Project to support decision-making on whether or not life extension for a given nuclear plant is technically sound and whether the benefits and costs of modernization and advanced technology could justify investment for long-term operation. Such informed decision-making is critical because nuclear plant owners could spend up to $1 billion or more per plant on capital investments to enable long-term operations.
For example, the $400 million that Omaha Public Power District invested in its Fort Calhoun plant in the early 2000s to replace and refurbish most major components and systems will enable that plant to operate safely and reliably through its 60th birthday in 2033. Notably, however, the investments were also intended as a strategic down payment on even longer-term operations, possibly to 80 years and beyond. As nuclear plants age, more and more plant owners are beginning to weigh the risks and rewards of such decisions.
License renewal beyond 60 years will require technical work to prepare for the regulatory scrutiny associated with long-term operations. Although the NRC has developed criteria for extending licenses to 60 years, new and perhaps more stringent criteria will be required when considering operation past 60 years. By identifying and addressing the range of technical issues involved in long-term operations, nuclear plant owners and their regulators can engage in more timely and fruitful discussions of potential barriers to extended operation. Research and development (R&D) activities can identify potential issues of regulatory concern, collect data to characterize potential problems posed by the issue, and devise approaches for mitigating or eliminating any degradation that could endanger the safe operation of a plant.
Plant demonstrations will play an important role in characterizing issues affecting long-term operations and subsequently in demonstrating mitigation actions and new technology capabilities. EPRI, the U.S. Department of Energy (DOE), and Constellation have established a multi-year collaborative effort to investigate aging concerns at the Ginna (Westinghouse pressurized water reactor [PWR] design) and Nine Mile Point Unit 1 (General Electric boiling water reactor [BWR] design) nuclear plants, which are both more than 40 years old. The sidebar briefly discusses life extension differences between the two designs.
EPRI also recognizes the importance of broad stakeholder collaboration. Such engagement ensures alignment on R&D priorities, reduces duplication of effort, and optimizes available research funding. To coordinate research activities, EPRI maintains close communication with its global membership, the NRC, DOE, the Nuclear Energy Institute, and international entities such as the Materials Aging Institute. For example, EPRI coordinates research efforts with the DOE, whose Light Water Reactor Sustainability Program is pursuing R&D in several complementary technical areas.
For its Long-Term Operations Project, EPRI is identifying and prioritizing research activities that target generic nuclear industry issues and reflect input from nuclear plant owners and stakeholders worldwide. Five of the key emerging research areas are discussed in the following sections.
One chief area of concern is how concrete in the reactor containment building will perform over the long term. Concrete structures will degrade under continuous exposure to water, boric acid, radiation, and high temperatures. At some point, degradation will cause the concrete to fail.
What is not yet known is whether 80 or 100 years of exposure is enough to limit the concrete’s ability to perform its functions during a design-basis accident. No experimental or field evidence to date indicates that concrete’s properties would be appreciably lessened. However, additional data are needed to gain a better understanding of applicable degradation mechanisms and to develop inspection and testing methods that can confirm these indications.
The Materials Aging Institute, EPRI, and Oak Ridge National Laboratory are collaborating to identify critical issues, characterize materials properties, and develop computational materials science on concrete aging. The project is also investigating new nondestructive evaluation techniques, forensic concrete examination methods, and prognostic modeling tools that could be used to determine the remaining useful life of the concrete. If inspections and aging models reveal no significant causes for concern, life extension decisions can be made with greater technical confidence.
Out of this work will come a suite of tools that can be used by plant owners to evaluate concrete characteristics on-site. The first version is scheduled for release in about three years. Though that may seem a long time, speed is limited by the frequency of plant outages. Because nuclear plants only shut down every one and a half to two years, there are only two outage opportunities to test the guidelines in those three years. Some of the characterization methods will first be tested at the Constellation Ginna and Nine Mile Point plants during their spring 2011 outages.
A nearer-term 2011 goal is to systematically examine the various degradation phenomena now experienced in operating plants and then compile an Aging Reference Manual that clearly defines the physics of degradation processes in the context of how such degradation manifests itself in the field. The manual will contain a framework for identifying at-risk concrete structures and applicable degradation mechanisms.
As nuclear plant owners consider the long-term operation of nuclear power plants, a larger population of capital assets can be assessed in terms of possible refurbishment or replacement. Greater awareness of the capital costs, operating costs, and performance improvements associated with a particular refurbishment would enable facility owners to make decisions that maintain or enhance the safety, reliability, and economic performance of the nuclear plant. Moreover, because such decisions cannot be made in a vacuum, the ability to compare these costs and benefits across multiple systems or pieces of equipment is critical to achieving a truly integrated life-cycle management approach.
Replacement or refurbishment plans must consider multiple components and typically must follow a phased implementation strategy. Plant owners can’t just take a plant down for six months to make the desired modifications; they have to incorporate modifications over several outages to maintain the high availability expected of nuclear plants.
For example, pursuing long-term operations at one particular plant may hinge on the decision to replace its steam generators—a huge project costing hundreds of millions of dollars. Because steam generators were designed for the originally expected 40-year life of the plant, there are typically no containment building openings large enough to remove an old generator and bring in a new one. Openings, therefore, need to be cut through the shell (Figure 2). When deciding to install new generators, it may make sense to add capacity to the plant, which could entail new condensers, transformers, and low-pressure turbines as well.
|2. Hole in the wall. SGT LLC, a URS Washington Division-AREVA NP joint venture, completed a steam generator replacement at the Florida Power & Light St. Lucie Unit 2 reactor in late 2007. A temporary opening in the concrete shield wall and steel containment vessel was required to remove and replace the old steam generators and reactor heads. Courtesy: SGT|
This one decision cascades through all plant systems. Because changes cannot all be made in one outage, operators have to carefully identify the sequence of actions that will minimize negative performance impacts and deliver the earliest return on investment. Myriad decisions that can be made along the way in the timing, capital expenditures, and replacement sequence translate into millions of options with their own unique advantages and disadvantages. This is clearly not something that can be optimized using a pen and calculator.
To aid plant owners, EPRI is working with the French utility EDF to investigate several life-cycle management tools that can evaluate such cascading technical and economic issues. Central to these tools is a knowledge base of selected large capital assets such as reactor internals, spent fuel pool, torus, structural supports, coatings, transformers, and buried piping. This database will contain both cost data and technical data, such as end-of-life experience for certain types of equipment under certain operating conditions. If a nuclear plant operator pushes a piece of equipment closer toward the end of its life without replacement, there is an increasing chance of catastrophic failure, and the models will show the likely effects of early replacement versus the risk and impact of a failure.
Accompanying the knowledge base will be a methodology and set of analytical tools that simulate a range of possible technical, economic, and environmental conditions to guide owners in making decisions about their nuclear facilities. This will enable nuclear plant owners to answer questions such as: Which capital assets should be replaced or refurbished? At what point in their life cycle should they be replaced? What will the project costs be?
A late-2010 product emerging from this life-cycle management research will be a compilation and evaluation of potentially life-limiting issues at nuclear plants and their likelihood of affecting long-term operations beyond 60 years.
As nuclear plant structures, systems, and components age, the monitoring and analysis of condition- and performance-related data can provide information critical to cost-effective maintenance and life-cycle management. By centralizing these functions, nuclear plant owners can gain access to diagnostic and prognostic functions that would enable better decision-making across the nuclear facility. Although most nuclear plant systems and equipment perform extremely safely and reliably, two pressures are causing the nuclear industry to step up its monitoring capabilities.
The first is that aging-related failures can lead not only to large unexpected repair expenses but also to unplanned plant downtime. Because the nuclear business case relies on high availability, unexpected failures are particularly damaging. As plants age, operators need to be prepared to recognize failure precursors early enough to schedule repairs or system modifications during the next planned outage.
The second driver is that advances in information technology have enabled more sophisticated, automated online monitoring than was previously available. Monitoring of passive components such as pipes and pressure boundary components for signs of long-term aging is a robust approach to equipment health management. For example, research is investigating the use of electrical sensors on pipes to detect corrosion and erosion and the use of microscopic strain gauges to detect movement in metal that could indicate cracking.
Online monitoring of active and passive components will utilize both condition- and performance-related data to guide maintenance and life-cycle management. EPRI and others are developing monitoring tools that use anomaly detection, equipment failure signature analysis, and prognostic capabilities. These tools are being built one piece at a time so that plant operators can use the applicable tools as soon as they become available, rather than waiting for a complete system to monitor the entire plant.
One piece of equipment ripe for more effective online monitoring is the power transformer. When transformers fail, they often fail catastrophically, resulting in major downtime and peripheral damage. Although several methods for monitoring transformer health exist, they have not been assembled into an integrated system. EPRI’s Long-Term Operation Project is pursuing such integration and will be assessing the integrated systems’ capabilities in an industry pilot application.
The pilot test will include infrared technology to look for hot spots, sniffers to detect trace chemicals in the air that indicate materials degradation inside the transformer, and acoustic monitoring. Using sophisticated computer software to look for patterns among the data from the different types of sensors, operators will be able to detect anomalies that are not otherwise detectable. This will give early warning so the replacement can be planned for the next outage.
Risk-Informed Safety Margin Characterization
New challenges related to nuclear plant safety will emerge as plants age. These challenges could derive from a change in regulatory policy, a change in perceived risk due to new or different external threats, or an event at one or more operating plants. The ability to better assess the risks from these challenges will be critical to long-term operations.
Current methods for characterizing and quantifying safety and risk are conservative and have been validated over a narrow range of operating and accident conditions. They are also difficult and time-consuming to apply and do not explicitly address sources of uncertainty. Improvements addressing these limitations will include a modeling and simulation environment ready to tackle new challenges associated with aging issues, design and operational changes, and emerging plant conditions or events.
EPRI is collaborating with the DOE’s Light Water Reactor Sustainability Program at the Idaho National Laboratory (INL) to address this issue in several ways. First, INL is developing a next-generation safety analysis tool that uses better computational engines, better results visualizations, better treatment of uncertainty, and better validation. This tool will not only more accurately confirm the plant’s capability to meet its license requirements but will also consider the effects of aging and plant improvements in a simulation environment designed to enhance safety.
Second, EPRI is developing a probabilistic risk assessment (PRA) tool that includes a complete spectrum of hazards and initiating events. With advanced computational methods, results visualization, and connectivity to plant information, this tool will enable efficient and effective online and outage risk management.
Third, EPRI and INL are investigating a new paradigm for safety assessment that complements, but does not replace, the existing licensing basis and PRA assessments. The method will directly calculate the available safety margin for a scenario of interest. This safety margin would be a more direct measure of safety than confirmation of licensing commitments or estimation of accident risk. The method would employ the advanced probabilistic risk assessment and deterministic safety analysis tools described above. Users will be better able to measure the safety effects of new instrumentation systems or changed operating procedures that would make the plant more reliable, or they could see the safety impact of new fuel designs that could increase power or operating flexibility.
Development of these tools and pilot projects to validate their capabilities will continue through much of the decade.
Materials of construction for critical piping and reactor components are what separate radioactive process streams from adjacent plant areas, including worker dose exposure areas and the broader environment. As nuclear plants age, monitoring and maintaining the integrity of such materials is paramount to safe operation.
For example, irradiation of the reactor internals or corrosion of reactor vessel penetration welds can hamper reliable plant operation and necessitate increased inspection frequency. Improved understanding, prediction, and mitigation of aging and degradation in primary system metals would significantly enhance the ability of nuclear plant operators to plan for long-term operations.
Research efforts conducted in this project will address multiple plant operations needs. Advanced inspection and test methods will enable characterization of the chemistry and microstructure at the molecular scale, and the resulting data will enable the formulation and validation of crack initiation and propagation models. Degradation models will enable prediction of remaining useful life and the development and testing of mitigation methods.
EPRI maintains an ongoing awareness of the state of industry knowledge with respect to degradation mechanisms for reactor materials. This knowledge base—captured in a “living” document called the Materials Degradation Matrix—identifies and prioritizes gaps where R&D is required to resolve materials aging issues in specific plant components. The Materials Degradation Matrix currently encompasses gaps related to plant operation through 60 years. Recognizing that longer plant operation could introduce new degradation mechanisms, EPRI and the nuclear industry are collaborating to revise the matrix so that it incorporates known and potential R&D gaps through 80 or more years of operation.
Some of the strategic issues on materials degradation related to long-term operation that are addressed in this revision include:
- Impacts of increase in the end-of-life neutron fluence
- Increase in fatigue cycles
- Late-in-life increase in cracking susceptibility
- Long-term instability of surface stress improvement
- Steam generator fouling and corrosion
- Environmental effects on fracture resistance
- Stress corrosion cracking of nickel-based alloys and austenitic stainless steels
Another area where focused R&D is needed relates to irradiation-assisted stress corrosion cracking of stainless steels, commonly used in all light water reactors. The behavior of such materials when exposed to another 20-plus years of neutron fluence is not well known; characterizing the crack initiation and propagation mechanisms will enable nuclear plant owners to more confidently assess whether and when reactor components should be replaced or repaired.
Where repair of irradiated materials is called for, advanced welding methods may be needed. Materials that are irradiated with neutrons for many years become resistant to welding, can become brittle when welded, and in many applications require underwater welding. Through the Long-Term Operations Project, EPRI and INL are developing hybrid and solid-state weld techniques that can overcome these challenges. Their development requires advanced predictive and simulation analysis, creation of representative samples in test reactors, and validation of the welding methods on these sample materials.
To Extend or Not to Extend
Decisions regarding nuclear plant operation beyond 60 years are not as far away as one might think. Although the oldest U.S. reactors will not reach 60 years for almost another two decades, their owners will need to know within the next four to eight years whether they will be able to rely on existing nuclear assets in meeting projected demand. As a result, research must progress now to ensure that the technical basis is in place for making sound decisions regarding the safe and reliable operation of these plants over the long term.
— John Gaertner ([email protected]) is a technical executive for the Electric Power Research Institute.