How Oconee plans to reach 60

The business of nuclear power has never been simple, but the effort required to deal with the complexities has been more than offset by the value added through the operation of these plants. This has naturally led to Duke Power’s decision to apply to extend the licences of the three Oconee units which expire in 2013 and 2014.

IMPORTANCE OF LIFE CYCLE MANAGEMENT

The driving force behind Life Cycle Management (LCM) at Oconee is the desire to predict and address the challenges that the plant will face in the future. Many of these challenges, if not appropriately addressed, can significantly limit or even end the life of the station.

The potentially life limiting issues for nuclear power plants come from a variety of sources but can be grouped into five categories of concern: technical; regulatory; environmental; political; and economic. The key aspects of each group of challenges are listed in the panel.

For Oconee Nuclear Station, each of these areas has been and continues to be analysed in order to understand how our future business may be affected by these concerns and risks. We performed these reviews with increasing levels of detail. Our initial reviews in the early 1990s showed us the economic viability of continued operation, while our more recent analyses have focused on plant level details in preparation for plant licence renewal.

TECHNICAL CHALLENGES: A SYSTEMATIC APPROACH

In an effort to effectively think, plan, and act on specific technical LCM concerns, a systematic approach was adopted at Oconee. Engineering assessments investigated applicable ageing and regulatory issues to determine the future plant equipment replacements and improvements needed for optimum operation within the regulatory environment. A long-range plan was instituted to capture all of the issues so that each could be addressed with other issues in mind. The need for a solution to a particular problem related to obsolescence or ageing can be prioritised relative to all other issues. The particular issues at Oconee Nuclear Station are component technology obsolescence, lack of spare parts and material ageing.

Obsolescence of component technology

Obsolete component technology can cripple the operation of a nuclear plant and a priority at Oconee is to upgrade components to avoid the crippling effect of obsolescence. Many large components have been replaced recently, particularly computer-related components. As obsolescence becomes an issue for a particular component or component type, engineering studies are performed to determine feasible replacement options. Management has taken an active role in authorising and allocating money for replacement of these components.

Oconee Nuclear Station recently completed a replacement of obsolete computer technology at each unit. One example was the Operator Aid Computer (OAC), which performs plant calculations, event alarming and extensive equipment condition monitoring for the operator. The operation of the OAC is not required by regulation but is a vital part of the operator’s collective unit control and monitoring system. Loss of the OAC function increases operator work for Technical Specification surveillance and may result in reduced unit capacity or even loss of the ability to run the unit. The OAC was procured in the late 1960s and was connected in tandem with computers that were added in the 1980s. Spare parts for the computer had become unavailable or obsolete, and the cost of custom manufactured parts had become prohibitive. Hardware maintenance and software programming expertise had become unavailable within Duke Power and in the rest of the industry. It had become increasingly impossible to find someone with experience on these computers to provide maintenance or programming support. The computers now have been replaced on all three Oconee units at a cost of $27 million. The expected life of the replacement OAC systems is ten years due to the advancing technology and expertise required to maintain them. The replacement systems have been specified so that individual components of the systems can be replaced as older technology becomes obsolete, without requiring replacement of the entire OAC system.

This example demonstrates that the issue of obsolescence of component technology will continue to challenge Oconee Nuclear Station.

Lack of spare parts

Duke Power’s Commercial Grade Programme is dedicated to managing the spare parts challenge at its nuclear power plants. Procurement Engineering personnel are responsible for the programme implementation as well as for performing the analysis required to dedicate commercial components for nuclear applications that require quality assurance (QA).

Due to the age of Oconee, many of the original equipment suppliers, particularly those of electronic equipment, are no longer in business or simply no longer support their previous product lines. Often it is not until a procurement engineer is reordering a part that he learns the product is unavailable. This is a growing problem that is difficult to assess for the future because it is occurring with greater frequency as time passes.

When the ability to obtain spare parts for a given component is discovered, interim solutions vary. In some cases, Duke has bought all the items that the manufacturer has in stock. In other cases, we have worked with the Babcock & Wilcox Owners’ Group (BWOG) to procure parts from other plants. These “fixes” only resolve the spare parts issue momentarily. The time is used to determine suitable replacements, because eventually these components will have to be replaced.

An Oconee-unique challenge lies in the fact that a separate hydro-electric facility (as opposed to diesel generators) provides emergency power to the nuclear station. Many parts for the hydro station components are not available under a vendor’s QA programme. For example, none of the replacement parts for the Westinghouse voltage regulator are available under the Westinghouse QA programme and in many cases are no longer manufactured by Westinghouse. There are a few critical spare parts in inventory, but the time has arrived to decide on an alternative before operation of the nuclear station is affected. Duke probably will be forced to replace the regulator in the next few years.

Parts obsolescence requires a dedication from the site in the form of an allocation of engineering resources and a budget for replacement modifications. Based on the experiences at Oconee, parts obsolescence will be more and more of a reality, especially if the operating licence is extended for an additional twenty years. This is an issue that must be considered when analysing and planning for future operation.

Material ageing

It was recognised early in the Oconee Licence Renewal process that planning for licence renewal would position the facility for extended operation, and be beneficial in optimising operation for the remainder of the facility’s currently licensed plant life. Further, conducting this systematic technical analysis required for licence renewal would provide important technical information in a more sophisticated and well-documented manner that would prove valuable in managing the life of plant hardware as related to material ageing.

The development and processing of the Oconee Licence Renewal application is one end point of the more detailed approach to Oconee LCM. The main purpose of the Licence Renewal Application is to demonstrate to the US Nuclear Regulatory Commission (NRC) that Oconee can be operated in a safe manner for twenty years beyond the existing forty-year licensed life. This approach, embodied in the principles of licence renewal, is that the regulatory process today is adequate to ensure that the licensing basis of all currently operating plants provides and maintains an acceptable level of safety, with the possible exception of the detrimental effects of ageing on the functionality of certain plant systems, structures, and components (SCCs) in the period of extended operation.

To prepare the Licence Renewal Application for Oconee Nuclear Station, the Licence Renewal Project Team followed a systematic process to assess material ageing in the plant. Age-related degradation of active components are fairly easily detected; normal pump operation or performance testing will indicate a declining performance trend, or motor-operated valve stroke time testing will indicate declining motor performance. The licence renewal approach is designed to focus on the passive equipment in the plant, ie equipment without moving parts whose gradual degradation will not be monitored through normal operation or performance testing. By following the approach laid out by the Licence Renewal Rule, the Licence Renewal Project Team has mechanistically determined what ageing effects can cause loss of function of these components in the plant and what programmes and activities manage material ageing in the plant. For the purposes of operating Oconee Nuclear Station to the end of its current licence term, the plant management has been armed with much insight into the kinds of activities required to manage the material ageing challenge. Areas of particular concern include ageing of structures and structural components, mechanical system components, and electrical components.

For those components designated by the USNRC for licence renewal review, a comprehensive academic exercise was performed to determine the ageing effects that are applicable to those components, and how the impacts of ageing will manifest themselves on this important hardware. If an ageing effect is determined to degrade a component beyond the component’s ability to perform its intended function, it must be managed so that the component can continue to perform its intended function through the period of extended operation. Note that it is not necessary that the ageing effect be precluded; however, there must be reasonable assurance that a programme is in place that will detect the degradation and take appropriate corrective action to prevent loss of component function due to the effects of ageing. Overall, it is necessary to establish reasonable assurance that the component will operate as intended for the remainder of plant life by demonstrating that the ageing effects pertinent to that component are addressed within existing programmes or that new or amended programmes will be available to provide this management.

Integrated Plant Assessment

The following sections provide some examples of technical issues that resulted from the integrated plant assessment performed for Oconee.

Structures

One of the issues in the structural area for the extended period of operation was loss of prestress over time in the containment post-tensioning system. Loss of prestress is due to material strain occurring under constant stress. Loss of prestress over time is accounted for in the design of the post-tensioning system.

By assuming an appropriate initial tensile loading stress and using appropriate prestress loss parameters, the magnitude of the design losses and the final effective prestress at the end of 40 years were calculated at the time of initial licensing. Extending the analysis to 60 years was crucial in determining whether the post-tensioning system would remain capable of performing its intended function as installed for an additional 20 years, need to be re-tensioned, or replaced.

The analysis (as described in Oconee Selected Licensee Commitment and developed using the guidance of Regulatory Guide 1.35) determined that the post-tensioning system would be capable of performing its intended function for the extended period of operation.

Mechanical system components

One issue identified with the mechanical hardware in the plant while analysing thermal fatigue effects of piping designed to USAS B31.1 was excessive thermal cycling of one section of piping. Piping designed to this Code is allowed 7000 thermal cycles over its lifetime, unless a stress range reduction factor is applied to the analysis. Extrapolating from past performance, it was determined that the pressuriser sample line would exceed this number of cycles during the current licence period. When the analysis was performed with the stress range reduction factor applied, modifications were required to bring the piping into Code compliance. These modifications have been completed on all three Oconee units.

Another issue in the mechanical area for the period of extended operation is ageing of the external surfaces of buried mechanical components. Depending on the material of construction, buried mechanical components are susceptible to loss of material due to corrosion and selective leaching, and cracking due to stress corrosion cracking. This ageing occurs in these buried components when the protective coating degrades to the point of allowing soil and ground water to contact the material surface.

During plant construction, buried mechanical components were externally coated. Twenty-five years of operating experience shows only two local failures, neither of which impacted system function. This good operating experience indicates that the coating is in good shape. However, no direct evidence exists that shows the actual condition of the coating. Directly determining the condition of the external coating requires excavation, which is costly and increases the likelihood of damage to the coating that accelerates its degradation and the onset of material ageing. However, the condition of the external coating can be determined indirectly by inspecting a portion of the interior surfaces of these buried components.

Every five years, the interior surfaces of a large portion of the buried mechanical components are inspected. Approximately 460 000 ft2 (about 43 000 m2) of interior surface are directly inspected for signs of degradation from either the interior or exterior environment. The two local failures that have occurred were from the external environment with one being identified by the internal inspection. Through periodic inspection of the interior surfaces, an increase in the number of local failures from the external environment would be an indication of general degradation of the coating that can be corrected prior to impacting the system function. This periodic inspection will ensure that the system will be capable of performing its intended function for the period of extended operation.

Another key mechanical area is the Reactor Coolant System (RCS). Ageing management of the RCS components was addressed for Oconee primarily through the BWOG. The results of the BWOG efforts were then integrated into the Oconee licence renewal application. Ageing issues that were addressed ranged from those that are well understood to emerging issues that are still under research. Management of these issues range from analytical solutions to plant maintenance programmes. Issues such as reactor vessel embrittlement and the potential cracking of reactor vessel Alloy 600 parts were addressed. Emerging issues on the reactor internals bolting were investigated and a new baseline programme introduced to begin further characterisation of ageing of these internals parts. Also, steam generator ageing management issues were reviewed and reliance has been placed on the steam generator tubing inspection programme that has been implemented at Oconee over a number of years. Each of these examples point out the range of ageing management issues covered in this significant mechanical system.

Electrical components

In recent years, electrical cables in nuclear power plants have been seen as one of the greatest challenges for operating a plant beyond the originally licensed 40 years. Management at some plants has even expressed the idea that a decision on applying for an extended operating licence would depend on the outcome of a plant cable evaluation. At the same time, no complete plant evaluation of electrical cables had been performed at a nuclear plant. This was the setting when planning began for the electrical cables evaluation for licence renewal at Oconee Nuclear Station.

The technical challenges were to determine which cables were within the scope of the NRC’s licence renewal rule (10 CFR 54.4) and how to adequately evaluate ageing of these cables for the twenty-year extended period of operation. At the time, no clear path to a resolution of these challenges was evident.

When first looking at these challenges, the general “think” was to perform the electrical cable reviews by starting with the plant mechanical systems. The general outline of that process is to first identify mechanical systems that are within the scope of the licence renewal rule. Following this, identify the electrical equipment that support the mechanical system intended functions and the cables that connect the electrical equipment.

As a demonstration of this process, a single component, known to be within licence renewal scope, was chosen (the motor-driven emergency feedwater pump). A review of the electrical connection diagrams for power and control of the pump identified more than twenty separate electrical cables. The specific licence renewal intended functions of the pump were identified, as were the electrical equipment and cable intended functions. These cables were studied to determine their construction and primary insulating material. The route of each cable running between each piece of electrical equipment involved was mapped and the plant areas the cables travelled through were identified. Next, the service environment in each of these areas was determined as was the applicable self-heating temperature rise. Once the service conditions of each cable and its primary insulating material were identified, a determination could be made of the cable’s expected life.

During this demonstration, there was a realisation that the electrical cables in the demonstration are routed in the same areas as many other cables that are probably within licence renewal scope. For these other in-scope cables, the same process would need to be followed. If bounding values for cable materials, service environments and self-heating temperature rise could be determined, individual parameters for each cable would not need to be identified. In addition, by using a bounding approach, it may not be necessary to specifically identify all the cables that are within licence renewal scope and may only be necessary to ensure that the bounding values include the characteristics of all in-scope cables. Identifying specific and individual functions for in-scope cables is also a major task. Therefore, coupled with the bounding cable review is the idea that there must be reasonable assurance that all cable functions need to be maintained, which in essence includes a bounding set of functions in the review.

The use of this bounding review approach rather than a specific components identification approach, led to numerous economies during the review process. These economies in process made it possible to overcome the technical challenges and complete the evaluation of electrical cables for the Oconee Nuclear Station licence renewal application. This activity represents enormous success not only for Oconee Nuclear Station, but for the industry as a whole.

CONTINUING WORK

The challenges that face ageing nuclear power plants are expansive in nature and require systematic solutions. At Oconee Nuclear Station, we are facing these technical challenges in a structured and systematic manner. Licence renewal planning for Oconee has stimulated our establishing a larger LCM perspective for the station.

The Oconee Licence Renewal Project Team continues to work closely with other industry groups to develop technical processes and common solutions to ageing management issues associated with licence renewal. These groups include the B&W Owner’s Group, the Westinghouse Owner’s Group, Electric Power Research Institute, the Nuclear Energy Institute, and the Institute of Electrical and Electronics Engineers.

The several examples illustrated here demonstrate that solutions to the technical challenges that face Oconee can be resolved and the issues managed. As long as the continued financial analyses show that it is economically viable to continue to do so, these issues will not limit the life of Oconee Nuclear Station.