Implementing a comprehensive and systematic PLiM programme involves systematic assessment technologies, detailed understanding of degradation mechanisms and of system/component design and supporting tools, procedures, and methods. For HWR plants, this programme has been developed and advanced via extensive development and implementation experience at various utilities over the past decade.

Many lessons have been learned on effective interfaces between the various disciplines and the organizations that will be involved in performing PLiM work. Efforts continue to improve and update this knowledge base, as experience continues to grow with PLiM at various plants. In particular efforts are underway to organize the large amount of ageing-related data in an easily accessible fashion for current PLiM efforts but to retain the knowledge for future use in PLiM programmes.

HWR NPP owner/operators and the design support organizations should work closely together on PLiM. Detailed and close collaboration at one plant can lead to on-going improvements, which can be applied to PLiM programmes at other plants or to better assessments within the same plant. There are many potential benefits of this collaboration to both parties.

For instance, design organizations can either undertake complete ageing assessments or support the utility ageing assessments; via direct experience in design, procurement, construction, commissioning and operations feedback from other HWR plants. Also a successful PLiM programme will utilize results from an active R&D support programme that focuses on plant ageing mechanisms, surveillance methodologies, mitigation methods, and improved inspection technologies. Another valuable input can come from the OEMs (Original Equipment Manufacturer) who often have the most detailed information on a particular SSC. A mature PLiM programme takes an industry approach and utilizes the best expertise from various organizations.

Neutron irradiation of the Zr-2.5Nb pressure tube material results in an increase in yield and tensile strengths and a decrease in ductility and fracture toughness. The velocity of DHC also is increased. The extent of these changes varies along the length of the tube, from inlet to outlet. The mechanical property changes due to irradiation damage saturate relatively early in reactor operating life, usually after about 1 to 5 years of reactor operation. After saturation the rate of change is slow.

To manage this ageing behaviour, the material surveillance programme under CAN/CSA N285.4-94 verifies that the tubes in reactor are responding the same way as that predicted from an extensive fracture toughness database (from both full-sized burst tests and small compact toughness (CT) specimens made from ex-service pressure tube material or from material irradiated in test reactors). The most recent surveillance results from removed pressure tubes, support the expectations of fitness for service to the design life from a fracture toughness perspective.

The main differences between pressure tubes at the single unit CANDUs and the multi-unit CANDUs or in some of the earlier Indian PHWRs are due to the time when the NPPs were designed and built. The later units were able to take advantage of experience gained during the early operation of the multi-unit plants. Some examples include selection of pressure tube material, design and number of pressure tube-to-calandria tube spacers, design of rolled joints and designing for axial elongation of pressure tubes. Country wise reports given in Appendices identify details.(to be given by members in Appendices)

Differences between PWR and HWR nuclear piping are primarily due to the horizontal CANDU fuel channel design, which requires more extensive piping runs. The feeders are also a design feature specific to HWRs. IAEA TECDOC [III.1.20] addresses ageing of primary piping in PWRs.

Process water in all the’ multi-unit CANDUs is lake water, not seawater as in most of the single-unit HWRs, which leads to different ageing concerns. Degradation mechanisms for single-unit HWRs, generally apply to this section as well.

Transient monitoring programmes were developed as part of the NPLA programme in the 1990s to address thermal fatigue due to process transients in the multi-unit CANDUs. Other factors to include (to be added by OPG and Bruce Power) are listed below:

• Carbon steel — Chromium content

• Feeders — FAC susceptibility

• LBB concept

• Underground piping

The design life of a nuclear power plant (NPP) does not necessarily equate with the physical or technological end of life (EOL) in terms of its ability to fulfill safety and electricity production requirements. Systems, structures and components (SSCs) in a NPP is subjected to a variety of chemical, mechanical and physical conditions during operation. Stressors lead changes with time in the SSC materials, which are caused and driven by the effects of corrosion, varying loads, flow conditions, temperature and neutron irradiation. It is quite feasible that many NPPs will be able to operate for times in excess of their nominal design lives, provided appropriate and proven ageing management actions are implemented in a timely manner.

To launch a PLiM programme the utility or plant should establish a PLiM group at the plant to develop a detailed plan specific to the utility organization. Then a PLiM pilot project is typically undertaken. The scope of the PLiM pilot project can vary but should include PLiM planning and some pilot ageing assessments, so that the utility PLiM team can gain significant level of experience and learning that will enable them to plan, perform and implement the full comprehensive PLiM programme. Also, procedures developed early in the PLiM pilot project should be updated with the experience gained in application from the pilot ageing assessments.

Those systems, structures and components that are to be included in a PLiM programme are usually identified by a systematic screening process, which prioritizes the systems based on their importance to achievement of plant goals, such as nuclear safety, environmental safety, and production reliability. In addition, structures and components whose failure would result in a major replacement cost or in a significant loss of production capability are also typically considered.

Effective plant practices in monitoring, surveillance, maintenance, and operations are the primary means of managing ageing. From general experience with HWR PLiM programmes, it can usually be expected that the PLiM assessment programme will lead to modifications and enhancements, but not likely replacements, of the existing plant programmes that address ageing. However, a successful PLiM programme will provide assurance that current plant programmes are modified to be effective in managing ageing. This requires a structured and managed approach to the implementation process. The overall objective is to optimize plant programmes for ageing management, both for the remaining design life period and for the plant extended operation to come.

Long term operation can be considered as other of a fully integrated PLiM programme. LTO requires careful planning and scoping. The HWR utility would normally initiate a detailed LTO study at their particular plant (with an objective to extend plant operation another 20 to 30 years) many years before the end of design life. In fact, ideally, a station should implement this from initial startup for optimal effectiveness and cost, and to maximize asset value. The end product of this study is a business case that compares the costs of refurbishing their NPP with costs of alternate means of generation.

Atomic Energy Commission is working in a programme of life management of Embalse nuclear power plant, with view to their safe operation and to be able to for long term operation. (LTO). This programme is based on the studies of some specifics selected critical components. In some of them is carried out an evaluation of their condition assessment (CA), like it is the case of the system of feeding water and of the moderator’s system. Also, this is carrying out in the bombs of the control of pressure and inventory system.

Within this programme is also carried out the life evaluation (LE) the steam generators, the moderator’s exchanges, the system of feeding water bombs system and the feeders. All this is related with a detailed study of the effects of erosion corrosion in the secondary system for which a computational programme has been elaborated that it has been corroborated with obtained data obtained from the power plant. The programme includes also pressure tube and the calandria tube.

For the realization of these activities there is being integrated a group of professionals that they can dominate in a future with ease the topic of life management of facilities. This is a multidisciplinary group that will allow carrying out the task and used typical procedure for CANDU reactors, where the history of each component is continued during the operation as well as possible deviations in its construction and the influence on the degradation mechanism.

In time, plant components become outdated and they cannot be adequately maintained without compensatory actions to mitigate the effects of their obsolescence. From this perspective when key components from process systems are obsolete, the system health is at high risk of deterioration. A systematic obsolescence mitigation programme comprises 3 phases:

• Preliminary identification of equipment obsolescence

• Obsolescence assessment and resolution (evaluation, identification of alternatives, definition of implementation strategies and resolution)

• Develop a replacement programme and prioritize upgrade/replacement solutions.

Many NPPs are currently operating using programmemable electronic systems and equipment. Future NPPs and retrofit projects will also use these types of devices, which are the state of science and technology solution for I&C. The life cycle of such equipment has to be considered, taking into account the specific characteristics of electronic information technology (IT), and not be limited to the aspect of ageing of components (hardware).

Analog and digital electronics used to convert the sensor signals should be included in the management of I&C ageing. This equipment has not, in the past, been the subject of ageing concerns because they are normally located in instrument cabinets in the easily accessible and environmentally benign areas of the plant and consequently age very slowly. However, obsolescence of this equipment is important, especially when it relates to LTO.

Obsolescence is more of a problem with this equipment than ageing, because electronic components and digital systems are frequently upgraded by manufacturers, and older equipment is no longer available. Consequently, in the focus of ageing management for such systems, it is necessary to ensure that the required functions are met independently of the I&C technology applied.

Thus, the ageing of programmable electronic devices is to be considered within the concept of LTO, and maintenance of equipment, considering both hardware and software facets, and the human/organizational associated consequences.

Extensive analysis and studies of HWR piping systems (including supports) have already been completed, including an IAEA TECDOC (which is in process of being issued) that covers some CANDU primary piping considerations. Typically, comprehensive PLiM Life Assessments specific to the individual plant are completed and then factored into the in­service inspection and maintenance to ensure plant life attainment. These plans are updated periodically as part of the overall plant life management programme.

For instance, at several HWR NPPs, life assessments of the piping and supports have been completed. For piping systems, due to the large number of lines and supports to be covered, combined with the need to provide recommendations directly to the plant for their age management activities, a particular focus was made to tailor the generic life assessment methodology to these requirements. For instance, one technique was to use piping and support design and structural qualification specialists, as well as plant piping specialists and supplemented by system design and fluid chemistry/materials specialists, working in a team to perform the assessments. This team developed the detailed assessment approach for this type of equipment.

An example is preliminary fatigue assessment of the piping in many of these systems. First, an evaluation of the number and types of stress cycles that the piping system had experienced was performed. It quickly became evident that the plant had experienced a very low number of the key thermal cycles versus the design basis expectation for a HWR plant of its age. Hence, it was decided early on to concentrate the assessment effort on other sources of potential degradation (versus starting quantitative fatigue usage evaluation, which could be done later).

It was also recognized that a key potential stressor for fatigue assessment was piping support condition. If the piping support design intent was not met (for instance, by support material degradation or modifications), then this could be a very significant factor. Procedures were developed to focus the fatigue assessment effort on piping support condition via walkdowns by the piping qualification specialists, by assessment of the plant hanger/support inspection programme and the results of the work done by the plant on support configurations. This approach proved to be a practical and cost effective approach to screen and assess the preliminary life assessment of piping thermal and mechanical fatigue.

Various other plausible piping degradation mechanisms were also considered in the piping system life assessments, using a variety of other assessment techniques. In general, it was concluded that piping and supports in the systems assessed are in good condition and are expected to generally perform well in plant extended operation given good chemistry control. Some local portions of the piping in certain systems are prone to degradation from FAC and in certain cases, have led to its replacement usually with higher chromium alloy material. Piping supports that are located in the open are more subject to environmental deterioration and hence should be further monitored (and sometimes refurbished) for proper performance.

Feeder pipes in the heat transport system need to be subjected to a more rigorous assessment process, given the recent field experience where wall thinning of some portions of the feeders, and cracking of feeders at one station, have been reported. An industry methodology and feeder-specific fitness-for-service guidelines for the degradation types that have been experienced on outlet feeders are now used by all Canadian CANDU stations. Wall thickness inspection and monitoring programmes are underway and mitigation strategies for older plants are under development. Most feeder pipes will meet their design life.

A limited number of outlet feeder bends and/or welds may require replacement before pressure tube replacement. The techniques and procedures for feeder replacement have been developed, and have been recently used successfully at a CANDU 6 plant. The feeder repair and replacement process is now routine and feeders on the reactor face can be replaced with little difficulty during an outage. It is clear that repair of feeders due to ageing has now proven to be an effective and economical Age Management technique.

For life extension and as part of the refurbishment and large scale fuel channel replacement, at least a portion of all feeders will be replaced to meet the extended life. A more proactive approach may be to replace the entire feeder at refurbishment, thus reducing inspection and maintenance that could be associated with leaving in portions of the original feeders. The material used for replaced feeders could be improved to have desired chromium content). This is an economic decision for the utility to make. A number of programmes are underway to address the details of the feeder engineering but the mechanical properties of removed ”aged” feeder pipes have already been measured to determine if there has been any effect of ageing (early indications are that, as expected, ageing effects on mechanical properties are not significant). This information will be useful to the life qualification of feeder pipes for extended plant operation.

IAEA-TECDOC-1361 [I.3] addresses ageing of primary piping in PWRs. Differences between PWR and HWR nuclear piping are primarily due to the horizontal CANDU fuel channel design, which requires more extensive piping runs. The feeders are also a design feature specific to HWRs. Other factors to include are listed below:

• LBB concept

• Underground piping

The SGs and other HXs (such as Moderator HXs, S/D HXs, Bleed coolers etc.) undergo periodic Eddy current Testing of tubes as a part of ISI. Steam generators of RAPS and MAPS are hair pin HXs type and are not amenable to ISI. These HXs use monel tubes. SG tube

failure took place in five HXs of MAPS. The cause after dismantling was found to be under deposit pitting near tube sheet. However similar HXs of RAPS when inspected neither did nor show any build up of deposit and tube degradation. By improving the chemical treatment /condensate polishing and blow down practice in MAPS (where condenser is sea water cooled) the failures were arrested. However, all hairpin HXS of these SGs are being replaced in the time slot for pressure tube replacement.

SGs from NAPS onwards are mushroom type and thus more amenable for In-service Inspection and use alloy 800 tubes. The indigenously developed automated robotic system is used for this purpose. These remotely controlled fully automatic devices are capable of traversing the probe head parallel to the tube sheet to the next selected tube, pushing the eddy current probe into the full length of the tube and recording information as the probe is subsequently retracted. This would reduce the time required for In-service Inspection of SGs as well as men-rem consumption. No generic degradation in SG tubes has been observed so far in any SG. However few SG tubes were found leaky due to damage caused by foreign material.

License renewal (LR) in the USA is based on the pre-requisite that ageing management of active components and systems is adequately addressed by the maintenance rule (MR) [26] requirements (10 CFR Part 50.65) and other established regulatory processes. This assumption is validated by the nuclear regulatory commission (NRC)’s regulatory oversight of the current licensing basis (CLB), which includes regulatory oversight to ensure implementation of continuous performance monitoring of active system functions in accordance with the MR, on-going compliance with operation technical specifications and regular updating of the so-called final safety analysis report (FSAR).

LR provides NPPs with the regulatory option to continue to operate beyond the 40-year term of the original licence, whilst the final decision to continue operation will depend on economic analyses of individual NPPs. Obviously, if the plant becomes uneconomical to operate, it may be shutdown and decommissioned at any time.

LR focuses primarily on the following three areas:

• Integrated plant assessment to evaluate the AM of passive, long lived SSCs, to ensure that they can support continued safe plant operation beyond the 40-year term of the original operating license and remain within the safety evaluation and requirements;

• Assessment of time-limited ageing analyses (TLAA) (e. g. fatigue, neutron embrittlement, environmental qualification analysis) to address the additional twenty years of operation; and

• Environmental impact assessment of the additional twenty years of operation.

The primary bases for determining the adequacy of passive SSC ageing management are operating experience, research results, and material sciences. Considerable documentation of operating experience is available in published reports, such as NRC regulatory guides, generic ageing lessons learned reports (e. g. NUREG-1801, [27]), and industry reports (e. g. NEI 95­10, [28]). NPPs must have at least 20 years of operating experience to demonstrate the adequacy of existing AMPs prior to submitting an application for LR.

The LR process typically takes about 4 to 5 years to complete. The utility takes about 2 years to do the engineering and environmental assessment work needed to prepare an application, and the NRC takes about 22 months (range of 17 to 30 months based on experience so far) to review the application and prepare a safety evaluation report (SER) and environmental impact statement. The overall cost of the LR process is $10 to $20 million (including utility costs and regulatory review fees) over this 4 to 5 year period. Fig.3 shows the review process of LR application.

This publication provides an overview of the various PLiM methodologies, technologies and

processes for HWRs. Implementation of a systematic and comprehensive PLiM programme,

such as that outlined in this report, goes a long way towards meeting the overall goal of HWR

owners/operator to successfully achieve design life and LTO.

• PLiM programmes should integrate and improve current plant maintenance, surveillance and inspection programmes as these programmes are the primary means for managing ageing processes.

• PLiM programmes serve to aid in the development of sound technical and economic bases for the attainment of design life and preserve the option for LTO. PLiM will facilitate conditions where LTO becomes a realistic option in terms of safety and economics.

• PLiM programmes may be generally planned using experience from NPPs worldwide, but plant-specific PLiM programmes are required. While there are elements of HWR PLiM that are generic, it should be recognized that each NPP is unique and hence, this uniqueness of plant history and its ageing-related programmes need to be considered in detail.

• Effective PLiM programme is aided by complete and accurate documentation on important SSCs. This includes information on SSC design, materials, treatments, manufacturers and modifications. Comprehensive documentation allows operators and regulators to follow the progress of ageing and the effectiveness of any mitigating actions in NPPs.

• Data keeping of a complete set of baseline inspection/testing data for critical SSC’s is an essential prerequisite to allow trend analysis and prediction of critical SCC’s remaining service life for an effective PLiM programme.

• PLiM programmes are dependent on the availability of qualified, well-trained NPP personnel with a questioning and interactive attitude.

• Operating experience is an important element of effective NPP’s PLiM strategy.

• Historical inspection, testing and operating data related to critical SSC’s shall be available for a long period of time because such data are used as input for life assessment, condition assessment and LTO studies. HWR owner/operators must monitor and respond to rapid changes of software and hardware technology in order to maintain availability of essential data for PLiM and LTO Programmes.

• Knowledge management is essential for safe and economical operation of NPPs, especially so for LTO.

• R&D is essential in such an evolving sector as NPP ageing research. It is necessary to continuously investigate the ageing phenomena and mitigating measures by enhancing the evaluation technology and inspection techniques, as well as collecting actual plant data and knowledge obtained from R&D results. Regulators and operators must be aware of R&D results concerning ageing of SSCs, and their impact on safety and economic issues.

• For new designs, PLiM technology should be considered during the design phase, and carried on through construction, commissioning and operation.

• Management systems envelops all elements of plant performance, safety culture and PLiM programme. Degradation in the management infrastructure will thus impact the total business plan.

• Proactive and continually improving trend in safety performance of the plant, upgrading and maintenance of human resources is essential for PLiM and LTO.

• PLiM programmes hould be linked to the station business plan.

• Each HWR owner/operators should implement a systematic and comprehensive PLiM programme at the earliest possible time in the life of the plant, to provide on-going assurance that ageing effects are adequately addressed and traceable. Tighter exchange of information on PLiM methodology, OPEX and practices is beneficial to LWR and HWR owners/operators

• Periodically, review the programme in light of the progress achieved or assessment made and experience gained. As and when necessary, refine/revise the plan.

• PSR should be used as an efficient tool for PLiM programme for LTO.

• All necessary documentation should be validated to confirm the current plant configuration. Documentation management is an important task within PLiM. Some utilities have a configuration management service dedicated to this activity in their organization.

• It is important to also recognize that the reliability of secondary side SSCs will become important as NPPs operate for PLiM. Although such SSCs may be adequately managed for ageing effects (e. g. replacement), their contribution to overall costs have to be considered in the business case for LTO. Extensive replacement and refurbishment tasks may become common place in the future.

• Operators should obtain regulatory feedback on their PLiM programmes in order to ensure safety issues have been duly considered.

• A good practice for transferring technical excellence is to involve NPP employees during major replacement projects within PLiM under the leadership of experienced personnel. Young employees will thus be motivated to acquire essential knowledge through participation.

• The HWR owners/operators of the new plants should consider the scope of commissioning and inaugural inspection programmes and extend it as required to meet the PLiM programme prerequisites.

This section gives comments on the application of each of the elements of the systematic ageing management process, shown in Figure 12.

II.2.1. UNDERSTANDING SSC AGEING

Adequate understanding of SSC ageing is the basis of the systematic ageing management process and the key to successful proactive ageing management. The level of understanding of SSC ageing depends to a large extent on the degree of technical/scientific understanding of relevant ageing mechanisms and the quality and quantity of relevant data from operating experience. Understanding ageing enables predicting future SSC ageing degradation, and the predictability enables optimizing and coordinating SSC operation/use, inspection and maintenance.

In practice, predictability requires modelling of the relevant ageing mechanism and SSC degradation, and measuring the progress of SSC degradation “along the curve” (i. e. condition monitoring).

Irradiation embrittlement is an ageing mechanism leading to changes in bulk material properties that has been successfully modelled; its future effects can be accurately predicted

and measured. Fretting wear is also a reasonably predictable degradation mechanism; at least in sense that it can be easily detected with current inspection technology and, with repeat inspections, the growth rate can quite confidently be determined. Hence it is a relatively low risk degradation mechanism to manage, provided one is looking for it. The predictability of thermal degradation of polymers (used in cable insulation and seals), which also produce changes in bulk material properties, is also generally adequate.

On the other hand, the predictability of stress corrosion cracking or high cycle fatigue, which produce changes in material surfaces or interfaces (loss of material or formation of small cracks), is generally low. The resulting uncertainty has caused significant NPP unavailability and O&M costs.

Operating experience has revealed SSC degradation and failures caused by previously unrecognized ageing mechanisms. So, in addition to improving the understanding and predictability of known ageing phenomena, there is a need to provide for early detection of new ageing mechanisms.