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Role of NDT in condition based maintenance of nuclear power plant componentsL. M. Davies
LMD Consultancy, England
B. Gueorguiev, P. Trampus
International Atomic Energy Agency, Vienna
Nuclear power is an important contributor to the world's electricity needs. In 1999 it supplied roughly 17 % of global electricity generation, a percentage similar to that over the past decade. Nuclear power is, however, facing big challenges and as a result its share could decrease significantly in the decades ahead. Amongst other factors cost competitiveness of different generation sources is one of the major factors which relates not only to new plants but also for existing operating nuclear power plants (NPPs).
Revolutionary changes have taken place in the electricity industry in many countries: market liberalization and privatization processes have led to a highly competitive market environment. The well managed NPPs, with their low fuel costs and permanently declining operation and maintenance costs, are often among the least expensive base load power plants to operate. In a competitive regime this situation encourages but also forces NPP owners to reduce further their operation and maintenance costs but of course without compromising safety which is, as always, a paramount consideration.
The major portion of maintenance costs, which at 25-30 % is the highest charge among the elements of NPP operation and maintenance costs, are realized during planned maintenance and refuelling outage periods. Since the primary purpose of NPP maintenance is to allow nuclear operators to use all those features necessary for safe and reliable power production by keeping them available, adequate maintenance is essential from the point of view of reliability of components and equipment. From the very simple 'only fix it when it's broken' technique (~1950) through to the preventive maintenance (from early 1950s to end of 1970s), optimized versions of NPP maintenance have been evolved. One of these, where the component conditions determine the maintenance actions, is the predictive or condition based maintenance approach. The various maintenance types and their characteristics are shown in Fig 1.
|Fig 1: Types of Maintenance|
Using condition based maintenance, unnecessarily costly maintenance actions and associated maintenance error induced failures can be avoided. This can lead to the achievement of nearly all maintenance targets concerning safety, reliability and cost. Condition monitoring can be continuous or periodic, with on-line or off-line techniques, sampling and so on. NDT is the basic technique for both detecting and sizing large flaws and monitoring material degradation. Consequently, it is a fundamental element of NPP maintenance and its effectiveness influences the maintenance performance.
The International Atomic Energy Agency (IAEA) is an independent, intergovernmental, science and technology based organization, within the United Nations family and serves as global focal point for nuclear co-operation. According to its mandate, the IAEA assists its Member States (130 at present) in planning for and using nuclear science and technology for peaceful purposes, which includes the generation of electricity. It also develops nuclear safety standards, promotes achievement and maintenance of high levels of safety in all application of nuclear energy. In addition, the IAEA verifies that States comply with their commitments, under the Non-Proliferation Treaty and other agreements, in using the nuclear materials only for peaceful purposes.
The IAEA's programme regarding NDT and condition based maintenance are basically embedded into the systematic programme of its International Working Group on Life Management of Nuclear Power Plants (IWG-LMNPP). The IWG is composed of representatives from 26 countries and 2 international organizations. It has been providing the Director General of the IAEA with advise and opinion on the current and future activities related to life management of NPPs, and the Member States with information, recommendations and support for almost 25 years. The IWG-LMNPP activities consist of organizing specialists meetings, preparing technical publications on related topics and arranging international Co-ordinated Research Projects.
This paper summarizes the major issues of the IAEA's activity associated with the role of NDT in NPP maintenance in light of the conclusions of the following specialists meetings: 'NDE techniques capability demonstration and inspection qualification' (1), 'NDT methods for monitoring degradation' (2), and 'NPP condition monitoring and maintenance' (3).
Periodic in-service inspection (ISI) carried out during maintenance outages at NPPs deliver a basic input for assessing structural integrity of primary circuit components. Results of round-robin tests within various international research projects as well as the appearance of real in-service defects in pressurized components (e.g. cracks caused by stress corrosion or thermal fatigue) have demonstrated the need for improving ISI effectiveness. There are two ways to be pursued which should be integrated to strengthen each other's potential. The first is selection of those components where the mitigation action, as a result of a proper NDT, effectively reduces the risk of failure. This is called risk based or risk informed ISI. The other way to improve ISI effectiveness is a demonstration of the capability of ISI systems (NDT procedure, equipment and personnel) that the ISI objective can be achieved. The latter one is called inspection qualification or performance demonstration.
The assurance that flaw detection and sizing by NDT means is not obtained by chance requires inspection qualification. This demonstration process is based on information available, by physical reasoning and on the analysis of the system to identify essential variables. Further demonstration of capability and understanding of the approach is often made on special with simulated or 'real' defects test blocks which have been designed as a result of the technical justification and used for both open or blind trials. The NDT systems are tested to verify their capability on limit cases defined by the essential variables analysis.
|Fig 2: The qualification process and its relation with the qualification dossier|
The IAEA prepared and published a guideline for qualification of ISI systems for WWERs (4) within the framework of the IAEA's Extrabudgetary Programme on the Safety of WWER and RBMK Nuclear Power Plants. The guideline was produced in due recognition of the high safety significance of ISI at WWERs and also took into consideration requests and suggestions from several WWER operating countries. This document was developed keeping in mind the approaches and experiences in several WWER operating countries, from the USA and from the European Union and other Western European countries. It should be noted that these latter qualification approaches were different in a number of aspects as a result of the different industry and regulatory environments. In this respect, the IAEA qualification methodology was intended to be a pragmatic synthesis appropriate, in the short and medium term, to the specific circumstances of the various WWER operating countries. The qualification process, according to the guideline, and its relation with the qualification dossier is summarized in Figure 2.
Benefits deriving from ISI qualification having strong connection to NPP maintenance have the following features:
Normally, the assessment of material degradation (irradiation embrittlement, thermal ageing, fatigue, plastic deformation, residual and/or applied stress, stress corrosion cracking, creep, etc.) requires extensive destructive testing methods. Using NDT for determination of the material condition, forecasting the degradation provides a greater opportunity to reduce the amounts and costs of testing the materials and also contributes to the possible early warning of excessive component degradation. It will also enable plant operators and owners to optimize the long term operation of their NPPs. This optimization should certainly result in an increased cost competitiveness.
The monitoring of material degradation by NDT means considers
Different NDT methods and techniques have been developed for monitoring degradation. They include, among others:
The current situation on NDT monitoring material degradation rests mainly on experimental solutions. When a potential event occurs in any NPP, the approach is to reproduce the event experimentally in laboratory, conduct a parametric study of the conditions and establish a forecast of the occurrence over a number of years of experimentation, or based on some parameters supported by experiments. However, the situation in the future will be essentially based on diagnostic systems and degradation modelling.
As stated earlier, see Fig 1, condition based (preventive) maintenance is triggered by a predefined event which indicates the extent of degradation of component or equipment. The difference between time based (periodic) and condition based maintenance can be defined in terms of the way in which they aim to prevent functional failure:
A pre-requisite for condition based maintenance is the availability or development of non-destructive monitoring techniques. In-service monitoring consists of continuous or periodic tracking of key operating parameters which characterize the component/equipment condition. The goal is to detect degraded condition, and if possible, assess changes in them with a view to preventing failure.
Monitoring techniques include a continuous measure of temperature and pressure at critical areas of lifetime limiting components/equipment to use for calculation of accumulation process of fatigue damage. Vibration monitoring on rotating equipment both in primary and secondary circuits of the plant, acoustic loose part monitoring in the primary circuit, monitoring of growing cracks as detected by ISI using acoustic emission, leak detection by acoustic means or measuring humidity to provide input for an Leak-Before-Break (LBB) concept, infrared thermography, borescope inspection enabling monitoring changes in equipment behaviour, or corrosion (including erosion/corrosion phenomena) monitoring techniques can also be applied.
Condition based maintenance leads not only to a reduction of unnecessary preventive maintenance actions and in shifting some maintenance operations away from the outage period (on-line maintenance) but ensures that actions are performed when justified by component/equipment condition. Condition based maintenance also helps ensure closer relation between operations and maintenance departments as well as among different maintenance personnel of different background skills.
Before implementing condition based maintenance, acceptance of the changes by maintenance and operations personnel has to be sought and the impact of the change on industrial and contracting policy evaluated. A cultural change is required at NPPs to foster the joint development of comprehensive diagnostic systems by operations and maintenance departments to ensure that corrective maintenance tasks are minimized. There may be changes for maintenance and operations personnel in terms of reduced maintenance work load and in work patterns. Indeed there may be the need for specific department to discharge these activities in some plants. Condition based maintenance also entails changes within maintenance crafts as it necessitates in-depth understanding of phenomena to enable performance of component/equipment condition diagnosis, analysis and prognosis. Networks of expertise could be established to provide technical support to operators. There could be a need for establishing common and/or harmonized procedures for the selection of monitoring technique and equipment and for implementation and qualification to guarantee effectiveness and reliability.
The assessment of condition based maintenance effectiveness is common question on any new maintenance policy. Indicators of its effectiveness may be based on safety; component/equipment, structure and system reliability; cost effectiveness; personnel doses. It should be noted that experience in this area is not well documented. But, in some utilities it is a 'growth' area of activity.
According to its mandate, the IAEA is authorised to make provisions for materials, services, equipment and facilities to meet the needs of research on, and development and practical application of, atomic energy for peaceful purposes, including the production of electric power, with due consideration for the needs of developing areas of the world. The assistance is provided as a priority to the developing Member States of the IAEA through the Technical Co-operation (TC) programme. Various national and regional ISI/NDT related TC projects have been implemented during the last decade in recognition of the high priority of the issues.
Following publication the participating Member States further requested that the IAEA set up a Pilot Study to investigate the practical aspects of using the qualification guidelines. The Pilot Study, started in 1998 to provide WWER operating Member States with practical advice, became an integral part of the ongoing regional TC project on 'Advanced NDT of Primary Circuit Components of WWER NPPs' and was an important step to the general application of the guidelines for other components and NPPs. A key feature of the Pilot Study is that it is based upon a real component and brings together actual data and procedures (real inspection company and real regulatory environment) that ultimately may be used at the pilot plant. Taking into consideration the number of different WWER units as well as the transferability of the results to other components and other plants, the target application of the Pilot Study is the preparation of a technical justification for ultrasonic testing of a shell weld of a WWER-1000 reactor pressure vessel (unit 5 at Kozloduy NPP, Bulgaria). Implementation of the qualification guidelines by WWER operating countries would enable them to reach a common level of qualification related infrastructures, data bases, experience and expertise (5).
In this paper we have discussed the development of condition based maintenance of NPP. It is part of the spectrum of NPP inspection and recording activities to do with plant life management and is a 'growth area' which could encompass and include some of the existing inspection and record keeping activities. This subject continues to be developed.
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