Nuclear power plant ageing and life extension: Safety ... - Laka.org

Nuclear power & safetyNuclear power plant ageingand life extension:Safety aspectsAn overview of issues and the IAEA's symposiumby Stanislav Novak and Milan PodestExperience with large fossil-fired electrical generatingunits, as well as in all process industries, shows thatplants begin to deteriorate with age after approximately10 years of operation. Similar phenomena will prevailfor nuclear plants, and it is reasonable to postulate thattheir availability will be affected, as will their safety, ifappropriate measures are not taken.It is evident that the average age of power reactors inthe IAEA's Member States is increasing. (See accompanyinggraphs.) By 2000, more than 50 nuclear plantswill have been providing electricity for 25 years orlonger. Most nuclear power plants have operating lifetimesof between 20 and 40 years.Ageing is defined as a continuing time-dependentdegradation of material due to service conditions,including normal operation and transient conditions. It iscommon experience that over long periods of time, thereis a gradual change in the properties of materials. Thesechanges can affect the capability of engineered components,systems, or structures to perform their requiredfunction. Not all changes are deleterious, but it is commonlyobserved that ageing processes normally involvea gradual reduction in performance capability.All materials in a nuclear power plant can suffer fromageing and can partially or totally lose their designedfunction. Ageing is not only of concern for active components(for which the probability of malfunctionincreases with time) but also for passive ones, since thesafety margin is being reduced towards the lowest allowablelevel.Effects of plant ageingThe main ageing effects of concern are changes inphysical properties (e.g., electric conductivity); irradiationembrittlement; thermal embrittlement; creep;fatigue; corrosion (including erosion and crackingassisted by corrosion); wear (e.g., fretting and crackingassisted by wear, such as fretting fatigue).The term "ageing" thus represents the cumulativechanges over time that may occur within a component orMessrs. Novak and Podest are staff members in the Department ofNuclear Power and Safety.structure because of one or more of these factors. Fromthis perspective, it is clear that this is a complex processthat begins as soon as a component or structure isproduced and continues throughout its service life.Age distribution of nuclear power plants0 5 10 15 20 25 30 35Years of operationProjected numbers of nuclear power reactorsreaching 30 years of operation 1990-2000.7060» 50I 40O| 30z20101II1I I Ii1990 91 92 93 94 95 96 97 98 99 2000Year-{Source: IAEA Power Reactor Information System.)IAEA BULLETIN, 4/1987 31

Nuclear power & safetyAgeing is certainly a significant factor in determiningthe limits of nuclear plant lifetime or life extensions. Nonuclear plant, including those still under construction orbeing mothballed, should be considered immune from itseffects.The rate of ageing depends strongly on both theservice conditions and the material sensitivity to thoseconditions. Therefore, consideration about ageing muststart in the design phase by selecting adequate materialsand it should be continued throughout its complete lifecycle.Although nuclear plant ageing could have an impacton the efficiency of electric power generation, safetycould be affected as well — if degradation of key componentsor structures is not detected before loss of functionalcapability, and if timely corrective action is nottaken. What must be understood is how the ageingprocess may change the likelihood of component failures(and therefore reduce safety margins), and how agedegradation can cause such events to be initiated.Concerns about ageing of equipment stem from thefact that failure can occur simultaneously (or effectivelyso) in redundant safety systems. Redundancy (coupledwith diversity) is the principal means of guarding againstthe consequences of random failures of equipment andproviding assurance that at least one complete chain ofsafety systems is functional at all times during plantoperation. The required protection would not beprovided if equipment ageing degrades the functionalcapability to the point where the increase in stress levelsassociated with a design basis event could cause simultaneousfailure of redundant systems (or their failurewithin a critical interval of time).Monitoring and detectionOperating organizations have been using differentprogrammes or methods to prevent, detect, correct, andmitigate failures of systems and components from anycause including the effects of ageing degradation. Thesemethods include preventive maintenance programmes,significant event reporting systems, and periodicreviews of plant performance. They have been undercontinuous development (based on lessons learned andnew knowledge), and overall they are quite effective inthe detection and mitigation of ageing effects.In the past, ageing was considered just one of manypossible causes of component failures. Ageing researchwas primarily in response to experienced operationalproblems or failures. However, in recent years, theindustry, regulatory bodies, and international organizationshave recognized that as the average age of nuclearplants increases, an enhancement of existingprogrammes with new technology would lead to a moresystematic and pro-active approach. As a result, a numberof programmes or projects have been initiated bydifferent Member States to understand ageing and thenew methods available to manage its effects. This has32led to an increased effort in the review and developmentof monitoring, testing, and inspection methods to ensuretimely detection of ageing degradation.Coping with the ageing process of nuclear powerplants requires a systematic approach in analysing thesephenomena. Experimental and theoretical methodsshould be developed to evaluate its impact on plantperformance. Effective methods of inspection, surveillance,and monitoring should be implemented duringoperation to evaluate the "qualified life" of the components,systems, and structures. This should be the basisfor their timely and effective maintenance, repair, andreplacement, with particular attention to systems andcomponents important to safety.International symposiumJust as the proper management of plant ageing isdrawing increasing interest, so are the economic aspectsof extending the lifetimes of nuclear power plants. TheIAEA gives attention to both topics in its programmes.Recently, the Agency organized the InternationalSymposium on Safety Aspects of Ageing and Maintenanceof Nuclear Power Plants, which was held inVienna, from 29 June to 3 July 1987.* It was the firstone organized by the IAEA on this subject and it wastherefore directed at a broad spectrum of participants —technical and managerial staff engaged in nuclear powerplant operation, regulatory body staff, consulting andarchitect-engineering organizations, vendor technicaland management staff, and nuclear power plant technicaland managerial staff involved in maintenance activities.It was attended by 140 participants from 30 countriesand three international organizations. Important topicsaddressed at the symposium are highlighted in the followingparagraphs.National effortsIn some countries, several programmes have beenstarted that encompass studies aimed at achieving anunderstanding of nuclear plant ageing, its potentialeffects on safety, and methods for its detection and mitigation.As an example, the US Nuclear RegulatoryCommission (NRC) has a programme called NuclearPlant Ageing Research (NPAR). It involves (1) identificationand selection of components whose ageing has ahigh impact on safety performance; (2) review ofdesign-basis safety margins, qualification testing, operatingexperience, experts' opinions, development ofmethods for surveillance, inspection, monitoring, andmaintenance; (3) engineering studies including verificationof inspection, surveillance, monitoring, and maintenancemethods, in-situ examinations, collection of datafrom operating equipment, post-service examinations,tests of naturally-aged equipment, and cost-benefitanalyses. The programme really shows the complexityof the ageing question.* Proceedings have been published by the IAEA.IAEA BULLETIN, 4/1987

Nuclear power & safetyThe symposium further addressed the approach toageing problems of other regulatory bodies. Importanttools that were mentioned include programmes forperiodical assessment based on broad operational datacollection; programmes for preventive maintenance;continuous monitoring of nuclear power plant components;feedback from incident analyses; individual andcomponent qualification; personnel education and training;external control and quality assurance. Proceduresare now under development to quantify the risk fromdata related to ageing and component failure.The collection and evaluation of operational data islikewise receiving more attention. The advantage of acomputerized data collecting system was described thatincorporates results of surveillance, testing, and maintenanceprogrammes. It further collects data on allmodes of operation which afterwards enables evaluationof the life of critical components. The role of operationalfeedback in developing test specifications for electricaland other equipment was also described.There are several other tools that are being employedand under further development to understand ageing.These include probabilistic models of ageing conditions,the use of plant components from decommissionednuclear units for ageing evaluation, and the use ofmethods that "artificially age" critical components todetermine their useful lifetimes.Steps can be taken to handle ageing phenomenaduring nuclear plant operation as well. Among thoseaddressed at the symposium were the use of technicaldiagnostic equipment to trace system degradation and toestimate remaining lifetimes; water-chemistry monitoringto prevent corrosion; and the monitoring of radiationdamage in the pressure vessel.Plant life extensionNuclear power plant life extension (technicallyknown by the acronym Nuplex) was discussed at thesymposium from several perspectives. Plant life extensionprogrammes in France, Japan, and the USA weredescribed. Even though different types of nuclear plantsare involved, these programmes are similar. They allinclude a database of necessary operational information;performance of research and development; and informationabout the availability of suitable devices for replacementor repair of critical equipment. In the USA, aNuplex Steering Committee was established whose mainrole is to advocate the timely establishment of a licenserenewal process by the NRC, to sponsor necessaryregulatory studies, and to speak for nuclear utilities onproposed NRC policy and rules on license renewal.Regarding the technological aspects of plant lifeextension, papers at the symposium were largelyoperations-oriented. The residual life assessment ofmajor components for pressurized-water reactors(PWRs) was described by the Idaho National EngineeringLaboratory, USA. The use of probabilistic riskassessment (PRA) techniques for performance monitoringto enhance the operational safety of nuclear powerplants was presented by Sandia National Laboratories,USA. Technological aspects were further addressed in apaper reviewing the symposium on plant life extensionorganized in February 1987 by the Nuclear EnergyAgency of the Organization for Economic Co-operationand Development, in co-operation with the IAEA.In Japan, research has been done to determine theessential life of the reactor pressure vessel, and symposiumparticipants were informed that results have beenused to start development of engineering procedures toevaluate ageing. As a useful aid for meeting regulatoryrequirements, a paper from France described theaccounting system for transients in the country'sstandardized PWRs.The symposium also featured three panel discussions.At the first one, some issues concerning the ageing ofactive and passive components were presented. It wasemphasized that the contribution to risk from the ageingof active components must be carefully considered. Thedatabase systems should be improved to collect moredetailed data on failures, repairs, and maintenance.Among other points that were stressed: Ageing is notonly an issue concerning the future, but one that concernsall operating nuclear plants. Current methods oftesting, monitoring, and maintenance are not adequatefor coping with the ageing issue. Quantification of theageing impact is very important to assure the safe operationof nuclear plants and to supply proven data to theregulatory body. Prevention rather than correctiveaction should be used to manage ageing, and activitiesshould concentrate on the most sensitive parts of nuclearpower plants.At the second panel session, emphasis was placed onan important operator activity, namely the systematicidentification of ageing degradation for components andparts important to safety and reliability. Mechanismsmust be understood to track the level of degradation andin such a way as to measure a certain deterioration. Anadequate maintenance programme based on detailed dataanalyses (with the purpose of replacing items beforetheir failure) was regarded as particularly important formanaging the phenomena of ageing at operating nuclearpower plants.The third panel concentrated on the IAEA's role ininternational co-operation on this subject. The Agency'swork in promoting dialogue and the exchange of informationand experience was especially noted, as was itsrole in disseminating information through documents,guides, and other publications. Overall, an active rolefor the Agency was recommended, in view of the factthat a broad spectrum of technical, economic, andregulatory aspects still have to be learned beforedecisions can be taken in regard to the ageing and lifeextension of nuclear power plants.The IAEA is now preparing a report for completionin 1988 that will take into account the results of symposiain 1987 on plant life extension and on plant ageingand maintenance.IAEA BULLETIN, 4/1987 33