AGEING MANAGEMENT PROGRAMME FOR REACTOR TRIGA PUSPATI (RTP)

Transcription

1 TECHNICAL NOTE AGEING MANAGEMENT PROGRAMME FOR REACTOR TRIGA PUSPATI (RTP) Mohd Fazli Zakaria, TonnyLanyau, MohamadFauziSaad, Ahmad Razali Ismail Malaysian Nuclear Agency, Bangi, Kajang, Malaysia ABSTRACT Ageing management is one of the important safety factors to be implemented proactively throughout the lifetime of a research reactor. In order to continue the safe operation of a research reactor from the reactor safety standpoint, a systematic and methodical approach should be taken. In practice, ageing management programme is accomplished by integrating the existing programmes including maintenance, periodic testing and inspection and periodic safety reviews. Such approach will be a good platform for the reactor operation and maintenance group to utilize data from existing maintenance programme to be incorporate into the ageing maintenance system. This paper will describe the ageing management programme for Reactor TRIGA PUSPATI (RTP. ABSTRAK Pengurusan penuaan ada satu faktor-faktor keselamatan penting dilaksanakan proaktif sepanjang hayat satu reaktor penyelidikan. Supaya meneruskan operasi selamat satu reaktor penyelidikan dari sudut pandangan keselamatan reaktor, satu pendekatan yang sistematik dan sistematik patut diambil. Pada praktiknya, program pengurusan penuaan dicapai dengan menyepadukan program sedia ada termasuk penyelenggaraan, ujian berkala dan pemeriksaan dan ulasan keselamatan berkala. Pendekatan sedemikian adalah satu platform baik untuk operasi reaktor dan kumpulan penyelenggaraan menggunakan data dari program penyelenggaraan wujud memasukkan ke dalam sistem penyenggaraan penuaan. Kertas ini akan menghuraikan program pengurusan penuaan untuk Reactor TRIGA PUSPATI (RTP). Keywords: Ageing management, periodic maintenance, research reactor licensing INTRODUCTION Since the first research reactor in 1942, over 670 research reactors have been constructed worldwide and of these, approximately 280 are currently in operation. About 70% of the operating reactors are over 20 years old with many over 30 years old. The present reactor PUSPATI TRIGA Reactor (RTP), a TRIGA MARK II, was supplied and constructed by General Atomic Co Ltd. San Diego, California, United State of America and built in It came into operation in 1982 and reached the first criticality on 28 June 1982 at nominal power of 1000 kw. The design, operation and utilization philosophy of nuclear research reactor is basically different from that of nuclear power reactors. This is because the purpose of a research reactor is utilization of neutrons, i.e. the use of the reactor as an experiment itself or the use of separate experiments during the operation of the reactor. Utilization may also lead to frequent modifications of the reactor. These differences require that separate criteria be established for power and research reactors even though many age related degradation mechanisms are similar. 94

2 BACKGROUND The objectives of RTP Ageing Management programme as follows: i. State the ageing problem and explain its relationship to the safety of research reactors. ii. Review ageing mechanisms for an understanding of their behavior and influence on reactor components and systems. iii. Provide guidelines to assist research reactor operators in detection and assessment of the effects of ageing. iv. Provide information which may be used for evaluating the safety of continued operation of an ageing research reactor. v. Recommend preventive and corrective measures to mitigate the effects of ageing, and give guidance on the decision-making process leading to a project for repairing, refurbishing and/or replacing a research reactor. SAFETY REQUIREMENTS FOR RESEARCH REACTORS The overall safety objective for a research reactor is to protect individuals, society and the environment by establishing and maintaining an effective defense against radiological hazards. In order to achieve this objective, a number of safety principles and requirements are utilized in the design stage and these as well as additional measures are utilized during reactor operation. These are defense in depth, reliability, safety analysis, quality assurance and regulatory supervision, including the review and assessment of the relevant associated safety documentation prepared by the operator of the facility. Ageing and defence in depth Defence in depth is usually achieved by a system of multiple barriers. The integrity of such a system can be impaired by failure of one or more of these barriers. The ageing process may lead to an increase in the probability of failure of a barrier component and ultimately to the failure of the barrier. Ageing and reliability The ageing of components may lead to an increase in failures of these components and a decrease in the availability of the reactor. In most instances, through the use of redundancy in the safety related systems, the safety of the system will not be compromised. Ageing and safety related documentation Along with components and materials, safety related documentation may also age in the sense that over time it may become out of date or even obsolete. In addition, research reactors are frequently modified to incorporate new experimental facilities and the modifications require updating of documentation. Ageing and advances in technology and safety requirements During the lifetime of a research reactor, technological advances will occur resulting in the introduction of new components and techniques. This will lead to difficulties with spare parts. Advances in safety concepts may require changes in hardware or software and may interfere with routine operation of the reactor. AGEING MANAGEMENT Ageing is defined as the continuous time dependent degradation of materials due to normal service conditions, which include normal operation and transient conditions. In a research reactor facility, the effects of such degradation may result in the reduction or the loss of the performance of components, systems and structures. Safety and utilization of the facility may be affected unless corrective measures are taken. 95

3 Service conditions Service conditions which contribute to the ageing act through chemical and physical processes that affect material properties or functional capabilities as follows: a) Stress and/or strain b) Temperature c) Environmental factors such as radiation, high humidity or the presence of chemically active liquids or gases (before or during operation) d) Service wear and corrosion, including changes in the dimensions and/or the relative position of individual parts of assemblies e) Excessive testing f) Inadequate design, improper installation or maintenance. In addition to these service conditions, there are conditions not related to chemical or physical processes which can lead to obsolescence and affect reactor safety. These are:. Technology changes. Safety requirements changes. Obsolete documentation. Inadequate design. Improper maintenance. Degradation of materials The main effect of ageing is degradation of materials. This degradation may be:. A change in physical properties (e.g. electrical conductivity). Irradiation embrittlement. Thermal embrittlement. Creep. Fatigue. Corrosion, including corrosion erosion and corrosion assisted cracking. Wear (e.g. fretting) and wear assisted cracking (e.g. fretting fatigue). The probability of a component, system or structure failure resulting from ageing degradation normally increases with the time of exposure to service condition unless countermeasures are taken. The objective of the management of ageing is to determine and apply these countermeasures. The management of ageing includes activities such as protection, repair, refurbishment or replacement, which are similar to other activities carried out at a reactor facility during routine maintenance or when a modification project takes place. However, it is important to distinguish between these different activities, because the management of ageing requires the use of methodology which will detect and evaluate deficiencies produced by the service conditions and will lead to the application of countermeasures for prevention and mitigation of the deficiencies. AGEING MANAGEMENT METHODOLOGY The basic causes of ageing phenomena are frequently service conditions which support the actuation of the particular ageing mechanism leading to these effects. 96

4 In brief it can be said that, SERVICE CONDITIONS + AGEING MECHANISMS = UNDESIRABLE EFFECTS OR FAILURES The following subsections present the methodology of the ageing management. Selection of structures, systems and components (SSCs) The selection and categorization of SSCs susceptible to potential ageing processes were performed during the design process. Equipment and materials were selected to minimize ageing effects. The selection process includes: - A recognition of the particular service conditions (e.g. pressure, temperature, radiation fluence, chemical environment) - The importance to safety - Materials of construction (e.g. carbon steel, stainless steel) - Required operation mode - Testing requirements - Maintenance requirements - Expected service life including an estimate of preservice use, and - The ease of replacement. Categorization of SSCs The categorization of components, structures and systems susceptible to ageing is based on factors such as importance to safety, repairability or replaceability. Categorization of components is as follows: Category I: Equipment of primary importance, not redundant, not easily repairable or replaceable (reactor tank, primary coolant system piping) Category II: Equipment of primary importance, but redundant or can be easily inspected or repaired (e.g. electric power supply, control rods) Category III: Equipment not primarily important but not easily inspectable or repairable (e.g. primary water purification system) Category IV: Other equipment (e.g. auxiliary diesel generators) Surveillance programme The ageing surveillance programme is essentially a long term programme. The programme is established as early as possible and continues throughout the operating life of the reactor. Based on design, manufacturer specifications, operating experience and judgement, the surveillance programme established are consistent with the equipment selection, categorization process and equipment qualification previously discussed. The surveillance programme also take advantage of the existing preventive maintenance and periodic testing programmes. It consists of the following: a) Maintenance b) Inspection and visual examination 97

5 c) Monitoring d) Testing e) Performance tests Detection and assessment of ageing effects Since ageing may affect the overall safety of the research reactor facility, there is a need to detect and assess the effect of aged components on safety. The detection of ageing effects is timely established in the framework of the ageing management activities and based on information from the design, maintenance and periodic testing of components and systems. Actual failures or incidents are factored into this programme, which also include an estimate of remaining service life for the components. In addition, this programme include the selection and categorization of equipment susceptible to ageing, the surveillance and testing programme, the management for data collection and for the further evaluation of the ageing effects. Evaluation of ageing effects As operation of the reactor facility progresses, data on ageing will become available. Periodic assessment of this data can yield information which can be used to evaluate the effectiveness of the programme. Modifications to the programme should be made based on experience. Experts may be utilized to supplement in-house capabilities especially in the applications of specialized inspection techniques, and in the areas of interpretation of inspection and test results. Once the consequences of an ageing related problem affecting a system or component have been identified, a final assessment of the situation regarding continuation of operation is made. Prevention and mitigation The prevention and mitigation of the effects of ageing accomplished by: (a) Appropriate provisions made during the design of the reactor; (b) Surveillance and testing programmes to assess degradation of components and systems; (c) A preventive maintenance programme; (d) Periodic evaluation of operating experience; (e) Optimization of operating conditions; and (f) Repairing, replacing or refurbishing of components. Periodic evaluation of operational experience is performed, including periodic review and analysis of operation and maintenance records and reports. This is to ensure that all data which has been collected will be taken into account during analysis of the safety state of the facility. Also the operation and maintenance procedures will be modified to compensate for changes due to ageing. SUMMARY Ageing management of SSCs should be implemented proactively throughout the lifetime of a RR project. This also applies to refurbishment and modernization projects. In practice, ageing management programme is accomplished by integrating existing programmes, including maintenance, periodic testing and inspection and periodic safety reviews, as well as applying good operational practices, and incorporating lessons learned from operating experience. Refurbishment and modernization projects should not be limited to a pure replacement of SSCs. They should also seek for safety improvements to comply with the up-to-date safety requirements, including the IAEA Safety Standards. 98

6 REFERENCES INTERNATIONAL ATOMIC ENERGY AGENCY, A Systematic Programme for Ageing Management for Research Reactors, Sydney, INTERNATIONAL ATOMIC ENERGY AGENCY, Maintenance, Periodic Testing and Inspection, Vienna, Program PengurusanPenuaan di RTP 2011, Nuklear Malaysia/L/2011/16 RTP Safety Analysis Report 2008, NUKLEARMALAYSIA/L/2008/34(S) 99