Aging is a universal phenomenon. Over time, everything and everybody must face the inexorable deterioration that aging causes. The majority of nuclear power plants in the world have been operating for 20 years or more and many plants are facing equipment and system degradation that is affecting the economic operation of their facilities. Over that same time period, materials properties have been improved that offer longer life with reduced corrosion susceptibility, safety requirements have increased, and welding and NDE techniques have improved. Utilities, whether considering extending the service life of their plants or looking for improved operations in increasingly competitive markets, are looking to life cycle management to assist them in making the right decisions.

A Proven Process

Life cycle management (LCM) is a proven, effective process for extending plant life in an integrated and cost-effective manner that prioritizes and allocates resources to the critical systems, structures and components. The normal operation of a plant or component exposes it to temperatures, mechanical loads, and environmental issues that result in aging mechanisms such as irradiation, fatigue, corrosion, fretting, cracking, etc. Over time, operation is affected.

Managing those factors that contribute to aging and physical degradation can have a dramatic impact on a nuclear utility's bottom line. Millions of dollars can be saved in avoidable operation and maintenance costs over the extended operating life of the nuclear fleet. In fact, LCM programs completed at four US plants resulted in long-term savings of $15M (15.5 Euros) to $30M (29.5 Euros) per plant.

Life Cycle Management is a Balancing Act

LCM is a balancing act between short and long-term strategic goals and the extended life of the plant. It encompasses a complex and interconnected group of features including financial models, long-term aging strategies, preventive maintenance programs and obsolescence planning tools. The key is a complete understanding of the aging processes, the life-limiting situations, and the establishment of thresholds. This knowledge, applied to the definition and qualification of suitable corrective and preventive actions, facilitates the risk/benefit analyses that result in the most cost-effective decisions.

Conclusion

Experience, historical data on individual plants over time, and a thorough understanding of the aging mechanism and physical degradation and their effect on components and systems are the primary tools for life cycle management. Combining the accumulated knowledge and experience of French, German and US engineering, manufacturing and R&D provides a complete set of LCM tools ranging from those dedicated to the detailed evaluation of a given aging process to those providing global plant management support to electricity utilities. Framatome ANP has been helping customers manage their assets from the time the plant was built to the present. The result is a set of powerful management tools that draw on lessons learned and detailed data that have saved customers millions of dollars in unplanned outages and maintenance costs while maximizing plant value. (See the following case studies.)

RPV Integrity Assessment: Indispensable for Safe and Reliable Long-Term Plant Operation

The integrity of the reactor pressure vessel (RPV) is essential for safe and reliable plant operation and especially for plant life extension. The residual life of the RPV can determine the overall life of the whole nuclear power plant. Advanced techniques are used to verify the RPV integrity throughout the planned or prolonged life of the plant and to provide a good basis for cost-effective implementation of preventive and corrective measures, if necessary. The following two examples of RPV safety integrity assessments use fracture mechanics tools.

Example 1:

A substantial part of the PWR-RPV integrity assessment is related to the pressurized thermal shock (PTS) analysis. The safety of the RPV during a loss of coolant accident (LOCA) has to be proven over the entire life of the component. Framatome ANP is a leader in performing RPV safety analyses, a multidisciplinary effort that involves, among other things, highly sophisticated and detailed thermal-hydraulic analyses and structural analyses including fracture mechanics assessments. This work has been performed by Framatome ANP for RPVs in the following plant design types: Siemens, EDF 900 MWe plants, Westinghouse, and Russian VVER 440 MWe and 1000 MWe plants.

Example 2:

The allowable defect size is a major criteria for the safety assessment of RPVs and thus for lifetime prediction of this component. For the RPV main coolant pump nozzle of the Swedish BWR nuclear power plant Forsmark 1, new devices had to be developed for the non-destructive examination (NDE) because of limited accessibility. To calibrate these new devices, fracture mechanics calculations were performed to minimize non-destructive testing. Allowable defects, postulated to be located in the weld of the pump nozzle at the bottom of the BWR RPVs, have to be safely detected by NDE inspection. To achieve realistic conditions, the residual stresses must be defined as necessary input for these fracture mechanics calculations. Framatome ANP has developed comprehensive models, based on numerical tools, to simulate the welding process using the real welding parameters. The number of weld beads and the appropriate mechanical boundary conditions with adjacent materials must be considered. Based on the outcome of the fracture mechanics assessment, the maximum allowable defect sizes are conservatively defined. The NDE method can be tailored to specific needs, and the optimal NDE inspection intervals can be derived.

Temperature field in the RPV wall and the resulting stresses in the nozzle region during cold water injection in the primary circuit, (temperatures in °C; stresses in MPa)

Preventing Thermal Fatigue in Nuclear Components

Preventing thermal fatigue begins during the design phase, particularly for safety-related components and those subjected to severe loading conditions. A list of loading conditions to which the various systems may be subjected is established early in the design phase. An in-service analysis is performed to ensure that design assumptions are correct under operating conditions. If there is a significant discrepancy, the design assumptions must be reassessed. Designs based on analysis of potential load conditions have been proven to prevent fatigue crack initiation, however unforeseen complex loading and insufficient monitoring can lead to a risk of fatigue damage.

The cracking of an elbow in the residual heat removal (RHR) system at Civaux 1 is an example of unforeseen loading. This system includes a mixing area for the hot fluid from the reactor coolant system and a fluid cooled by an exchanger, designed to evacuate the maximum residual heat from the core, following a power operation. Fatigue cracks appeared in this system when it was used for long periods of time during unit startup.

To determine the cause of the damage to avoid the risk of such an incident affecting other systems of similar design and to adjust the basic surveillance and preventative maintenance programs, both the loading applied to the component and the component material resistance must be analyzed. Tests or digital simulations using software such as Star-CD to determine local fluid temperature distributions and Systus software to identify the resulting structural stresses are used to evaluate the loading applied to the component. These analyses lead to suggestions for improved operation, or improved design in the event of equipment replacement.

The structure's resistance capacity depends on both the material of which it is made and the manufacturing process used (surface condition, residual stresses, etc.). Metallurgical assessments and fatigue test programs performed on typical specimens can highlight such effects, leading to a better understanding of the remaining service life of existing structures and the precautions to be taken during the manufacture of replacement equipment for an extended service life. Assuring the absence of fatigue on equipment, in particular under high-cycle thermal fatigue situations, is critical to the operation of nuclear power plants. It requires multi-disciplinary skills (thermal-hydraulics, mechanical engineering, materials, inspection, repairs, etc.) to control the risk of damage over the long-term.

Configuration of the N4 residual heat removal system zone affected by fatigue and internal degradation of the elbow highlighted by liquid penetrant examination.

Example of a simulated fluid mixing zone and determination of the resulting stresses

LCM on One System Leads to Big Savings

One of Framatome ANP's successful life cycle management (LCM) projects focused on a US plant's turbine controls. Following a complete assessment of the plant's operating systems, it became clear that significant operating improvements and cost savings potentially could be achieved by targeting the plant's existing electro-hydraulic turbine control (EHC) system.

Following Framatome ANP's LCM procedures, the team identified the EHC system boundaries and examined its critical equipment and components.

The system's operation and maintenance history was reviewed and then compared with other similar systems at other plants to uncover any anomalies.

Benefits of LCM

• Reduces unplanned outages due to equipment failure
• Reduces operating costs
• Mitigates risks of components critical to power generation
• Improved equipment reliability and availability
• Prioritization of competing options for capital while meeting emergent needs
• Prioritization of plant modifications and planning for implementation
• Estimation of capital upgrades and development of a long-range plant improvement plan

Analysis

An engineering analysis of the most critical components was completed to determine its susceptibility to failure. For example, some vital components such as relay cards seemed to indicate a high susceptibility to failure that could cause the system and the plant to be taken off line. The current analog system was obsolete and in fact, the existing EHC system's vendor had indicated that it no longer would provide technical support after 2005. Due to its obsolescence, there was a shortage of spare parts for the existing system and this situation could only worsen. Additionally, there were limits to the existing system's ability to gather data online and monitor system performance.

The Action Plan

Once the analysis was completed, the team evaluated the data and began to map out alternative programs for the operation and maintenance of the EHC system, taking into account the economic impact of each alternative. Based on the results of this exercise, an action plan, deemed the most effective at the best overall cost, was developed into a formal proposal and presented to the plant.

The action plan called for replacing the existing analog EHC system with a redundant digital control system that would reduce the single-failure susceptibility, allow on-line maintenance, provide more thorough monitoring and trending of system performance, and potentially eliminate some existing EHC system components.

It was estimated that this course of action would save the utility approximately $15 million (15.05 million Euros) in operation and maintenance costs over the remaining life of the plant. This example illustrates only one small segment of plant operation but the impact of LCM cannot be disputed.

COMSY: An Innovative Software Tool for Plant Life Management

To focus maintenance activities on those safety and availability-relevant components where potential degradation exists, Framatome ANP has developed a software tool called COMSY, currently used in 21 power plants worldwide. The tool provides the capability to update and log new data from a plant on a regular basis and to store and make this data available for analysis of all issues relating to aging and plant life management by utilizing a virtual plant data model. Sophisticated documentation functions serve to compile data on materials and design, integrity criteria, operating conditions and examination results.

Integrated analytical functions identify existing degradation potentials and provide accurate service life predictions for individual components and structures. Using this information, suitable remedies and precaution measures can be introduced and the maintenance and inspection activities can be adjusted to minimize costs and optimize the availability of the plant.

COMSY closed loop process