Most plants place considerable emphasis on controlling the operational chemistry of the secondary side so that it is consistently within acceptable levels. In the steam generator, tubing life is directly related to local chemistry conditions at the tube secondary side surfaces. In the crevices at tube supports and in the tube sheet sludge pile region, successful long life requires maintaining the crevice chemistry within ranges that minimize tubing corrosion. Subtle changes that can significantly affect tubing life may result from variations in feedwater impurity. If these impurity fluctuations are within normal bulk water specifications, their potential to increase tubing corrosion damage, particularly in secondary side SG crevices, could go unnoticed until extensive damage becomes evident.

To assess crevice conditions for corrosion damage potential with given operational chemistry parameters, plant staff need knowledge of the local chemistry in the steam generator tube bundle crevices, in addition to the bulk chemistry of the water surrounding the tubing. In the past this was a rather difficult and time intensive task that could only be done by chemistry and corrosion experts not usually found at the plants. However, recently, tools have been developed to provide a nuclear SG crevice chemistry prediction that can be used by the plant operator on-line. The effects of impurity ingress to the secondary side water, on local crevice chemistry and fouling in the steam generator are identified and where of concern, flagged.

This type of on-line monitoring and prediction system gives the plant operator an important life management tool for maintaining good steam generator health and for attaining long life, by providing early indication of any change in chemistry parameters that could result in damage to the SG tubing. Ready on-line access by the operators to current and past chemistry conditions, including chemistry predictions in the critical crevice regions of the SG, enables appropriate responses while on-line (such as diagnosis of any change in corrosion susceptibility) and planning of future shutdown maintenance actions (such as inspections to verify local condition, and the need for cleaning specific areas).

Sliver sample scraping tool (SSST) has been developed for obtaining in situ scrape samples from the pressure tubes of an operating PHWR. The SSST incorporates mainly the scraping tool and hydraulic, pneumatic and mechanical sub-systems. This technique is used remotely to obtain metal samples from a desired axial location from the bore of the pressure tube at 12’O clock position. Initially an oxide layer of about 100 pm thickness is removed from the surface of pressure tube, which is followed by removal of a metal sample.

Two different versions of the tool, namely SSST-I and SSST-II are operable in dry condition of the pressure tube. Hence, de-fuelling, isolation and draining of the channel is required as pre-requisites for obtaining sliver samples from the tube. In order to eliminate these pre­requisites and to obtain samples from wet channels, additional features have been added in SSST-II, which makes the tool operable by the fuelling machine and in un-defuelled channel. Arrangements were made for keeping tool-bits at 12 O’ Clock position and avoiding falling of sample in the magazine of the fuelling machine. The modified tool was designated as WEt Scraping Tool (WEST-I). The average weight of metal sample removed using WEST is 90 mg and the time required for obtaining each sample is approximately 20 minutes.

Section 1 introduces the background, definition of terminology and related IAEA publications and Section 2 discusses the current trend of PLiM observed in NPPs to date and an overview of PLiM programmes and considerations. This includes key objectives of such programmes, regulatory considerations, an overall integrated approach, organizational and technology infrastructure considerations, importance of effective plant data management and finally, human issues related to ageing, and finally integration of PLiM with economic planning. Section 3 discusses general approach to HWR PLiM, including the key PLiM processes, life assessment for critical structures and components, conditions assessment of structures and components and obsolesces. It also gives regulatory consideration both for design life and for

long term operation. Section 4 presents conclusions and recommendations. In the appendices, technical aspects are described on component specific technology considerations for ageing assessment, example of a proactive ageing management programme, and Ontario Power Generation (OPG) experiences on common systems for multi units. In addition, country reports of Canada, India, the Republic of Korea, Romania and Argentina are attached as annexes.

LTO requires careful planning and scoping and several HWR utilities have already started the detailed planning of a plant LTO programme. The HWR utility would normally initiate a detailed LTO study at their particular plant many years before the end of design life in order to optimise effectiveness and cost, and to maximize asset value. The end product of this study is a business case that compares the costs of LTO for their NPP with the costs of the alternatives.

Typically, for HWRs, LTO involves a plant refurbishment, which includes pressure tube, calandria tube and partial or complete feeder replacement. Planning of these replacements is part of the LTO study work. Planning is also started for the environmental, safety and licensing issues that would need to be addressed to ensure safe and economical future operation of the unit. In addition to these studies, a systematic review of the plant SSCs is carried out to determine what other equipment refurbishment or replacement will be required due to ageing or obsolescence.

A key part of an LTO programme is to utilize the outcomes of the PLiM ageing assessments and implementation (Phase 1 and 2) in enhancing current plant programmes for extended operation. For instance, the life assessment work on the concrete containment has led to an enhanced inspection and monitoring programme at one HWR NPP. With knowledge from the containment condition assessment programme at the decommissioned Gentilly 1 plant, a detailed containment ageing management programme (including the monitoring instrumentation) for LTO was developed. The detailed life assessment work lays the foundation for the plant inspection, maintenance and operational programme enhancements to extend the life of critical equipment.

In general, ageing assessments provide the primary inputs for determining the work necessary for LTO, and for planning the optimized surveillance, maintenance, and operations programmes to achieve the utility’s targets for safety, reliability and production capacity during its extended life. Significant progress in Phase 1 and 2 of the PLiM programme provides the HWR utility with important in-depth assessments (and often with promising life prognosis) for key SSCs. These outcomes are important inputs into utility decisions to embark upon LTO.

The following list of actual recommendations was identified during one LTO study and gives an idea of the potential scope for an HWR LTO project:

• Station control computers — The control computers are obsolete. The original equipment supplier stopped making this equipment many years ago. In order to complete an additional 25 to 30 years of life, some action is needed to replace this equipment in view of the potential for declining reliability and difficulty in getting spare parts.

• Programmable digital comparators (PDCs) — The PDCs are used in the reactor shutdown systems. The issue here is quite similar to the control computers and replacement will be required to complete the refurbished life.

• Main generator — The windings of the generator have a limited life insufficient to last throughout the refurbished plant life. Several options need to be studied including replacement with a new generator and rewinding of the existing generator. Similarly, many of the generator auxiliaries will need to be addressed to ensure reliable operation after refurbishment.

• Safety improvements — In parallel with the plant physical assessment, work has been done to establish what design changes may be required to minimize regulatory concerns with future plant operation. A number of changes to improve reliability and functionality of systems and components are being studied in more depth to establish what safety benefits would come about if such modifications were made.

• Reactor component analysis — Recommendations have been made to analyse component parts of the reactor to deal with ageing issues. Parts of the calandria are to be analysed to better establish material ductility limits. Moderator nozzles are to be analysed for potential fatigue issues. Analysis is to be done to establish more clearly the source of the reactor vault leak.

• Reactor component inspection — The analyses discussed above will be supplemented by inspections during the refurbishment outage. This outage will provide a unique opportunity for these inspections, as the reactor will be without fuel, fuel channels and moderator during part of the outage.

• Balance-of-plant (BOP) components — While most of the BOP components are much easier to inspect and replace than those in the nuclear steam plant (NSP), some further investigation is underway on those BOP components whose ageing are important to long life of the NSP.

For this particular plant, the steam generators were deemed to be in good condition and have a good prognosis for 50-year life provided the detailed recommendations from the LA were implemented. Typically, additional inspections are planned on steam generators in order to gain more confidence, such as in the assessment of secondary side internals.

Some piping replacement may be necessary to address flow accelerated corrosion (FAC) issues. The ageing assessment studies for other critical systems, structures and components were completed and the results factored into the LTO programme as appropriate.

In the HWR nuclear steam plant, there are a number of large shell-and-tube heat exchangers (HXs). Typically, a comprehensive PLiM life assessment specific to the individual plant’s large HXs is completed and factored into the in-service inspection and maintenance to ensure plant life attainment. The detailed and comprehensive life assessment of each selected HX includes the pressure boundary, the external support structure, the tubing, and all the key internal sub-components. While heat exchangers are relatively complex components that can be subject to a variety of degradation mechanisms on their various sub-components, worldwide experience has demonstrated that it is corrosion of the tubing that is the greatest life threat.

In open-loop cooling water circuits, outside surface corrosion has occurred on a number of tubing alloys. This degradation is often wide spread (affects a lot of tubes) and has occurred rapidly (difficult to manage by inspection and plugging). It can cause tube leaks, which often leads to forced outages. In some cases, HXs have had to be replaced.

In contrast, the HWR experience with several tubing alloys on closed loop de-mineralized cooling water systems has been excellent. To date, there has been no detectable in-service corrosion degradation of tubing in the large heat exchangers inside the reactor building (RB), for HWR plants with closed loop de-mineralized cooling water flowing on the shell side. This favourable experience reinforces plant design with closed loop de-mineralized cooling water of large HXs in the RB, as it provides well controlled chemical conditions on the outside surface of HX tubing. Rigorous operational chemistry control of the closed loop de­mineralized system has also been an important contributor.

Given this excellent record and the long life prognosis due to absence of any tubing corrosion concerns, the life assessments on these critical heat exchangers give detailed consideration of other types of tubing degradation and also to the life capability of other sub-components of the heat exchangers. As there are many individual parts, a risk assessment screening was performed to assist in identifying those sub-components that warrant further detailed life assessment. Subsequently, the HX life assessment approach considered the potential of each of the top 10 most significant historical degradation mechanisms if we refer the “top 10”, we should include a list of them on each of the important HX sub-components. In this way, the generic PLiM programme life assessment methodology was specifically tailored to this component type, to ensure a systematic and comprehensive process was followed that covered the entire equipment. The outcome of this work was positive life prognosis for life extension of the critical nuclear heat exchangers in the plants assessed.

Another important outcome of the process was detailed understanding of ageing potential in each of the individual HXs considered. There are different equipment design details in these HXs and different primary side systems involved. This detailed knowledge is being used to “optimize” the plant programmes that provide important age management data of this equipment. An example is the inspection programme for these HXs. Since the de-mineralized cooling water plant design provides assurance against widespread tubing corrosion problems, a large and frequent tubing inspection programme is not required. But the detailed assessment of what other types of degradation might occur in future and where it might occur in each of the various HXs enables the life assessment to provide important input into the “optimized” HX inspection programme for life extension. The life assessment enables specific areas and regions of the HX to be selected for special attention and hence a more focused inspection programme, — “targeted ”— to areas that are sensitive to potential age degradation, can also be a positive outcome of the PLiM work.

As part of up gradation a supplementary control room was introduced in RAPS & MAPS so that whenever the main control room become inaccessible followings functions can be carried out.

• shut down of the reactor

• monitor critical plant parameters like PHT system pressure & temperature, reactor power, moderator level etc.

Civil structures

The following mechanisms of ageing related degradation / ageing effect have been identified for buildings and structures. (Ref. AERB /SG /O-14)

• Concrete structures Leaching and efflorescence

• Abrasion, erosion, cavitation

• Chemical attack

• Fatigue, vibration caused by equipment etc.

• Carbonation

• Settlement

• Cracking and spalling

• Reinforcing steel Corrosion

• Pre-stressing steel Corrosion

• Loss of pre-stressing force

The CNSC has recognized that the current level of ageing management effort may need to be further augmented in order to ensure plant safety as Canadian NPPs continue to age. This will require strengthening the role of proactive ageing management utilizing a systematic ageing management process.

As a result, the CNSC has undertaken the development of a regulatory standard on fundamental aspects of NPP maintenance programmes, which emphasizes the important role of proactive maintenance strategies. In addition, the CNSC has commenced the development of a regulatory document outlining the key components of ageing management programmes, recognizing that these are often integrated with economic factors into an overall PLiM strategy.

The design of fuel channels has taken into consideration the following factors related to pressure tube diametral expansion and wall thinning due to creep and growth:

• Stress

• Creep ductility

• Flow by-pass

• Spacer nip up (no gap between the pressure tube, calandria tube and spacer)

Diametral expansion occurs mainly by irradiation creep. For operating reactors, stress analyses to address strength requirements have been performed for operation of pressure tubes to 5% diameter increase and 0.368 mm wall thinning. Based on data from in-reactor experiments 5% is considered to be a very conservative limit with respect to creep rupture and creep ductility.

Results from diameter measurements from several plants suggest that the fastest creeping pressure tubes are experiencing an upperbound diametral expansion rate of about 0.2% per 7000 EFPH. Based on this upper bound rate, the following is predicted for the fastest creeping pressure tubes:

• Nip-up will occur before design life

• Diametral strain of 5% will be reached before design life.

However, several plants have lower diametral strain rates because of the lower operating temperature and flux. Data from these reactors currently suggest that the maximum diametral strain will not exceed 5% during the design life.

It is also recognized that the measured pressure tube diametral expansion rates will result in flow by-pass and a reduction in margins on cooling capability for the fuel. Because the operation of a unit depends upon the prediction of the maximum pressure tube diameter in the core, there may be a need to obtain additional data on high power channels to more precisely determine the distribution of diameters in each unit and to identify the fast creeping pressure tubes such that remedial action can be taken, if required.

Life management strategies therefore have been developed to evaluate the need for increased inspections. As the units age, these inspections enable the variability in diametral expansion to be more precisely quantified, in order to address the following:

• Strength and creep ductility requirements for operation with diametral strains greater 5%.

• Coolant flow bypass around the fuel bundles for diametral strains greater than 5%.

• Operating in a “nipped-up” condition (i. e. with no gap between the pressure tube, calandria tube, and spacer).

Development of the multi-unit CANDU life cycle management programme began in the early 1990s with the nuclear power life assurance (NPLA) programme. This programme included a small number of critical structures and components, with replacement cost or time greater than $100M or 6 months. The table below shows the initial NPLA components for the Pickering A station [Ш.1].

The multi-unit CANDU plants were required to respond to the CNSC Generic Action Item on continuing plant safety as components age (see section 2.2). As part of this response, components were classified into short lived and long lived components in order to select the appropriate ageing management programme [III. 1]. Long lived components were subject to ageing assessments; short lived components were considered to be addressed by regular plant maintenance programmes. For short lived components, the relevant plant programmes were reviewed for effectiveness to address ageing, under the auspices of the preventive maintenance optimization programme.

Starting in 1997, the multi-unit CANDU plants developed additional processes to address ageing of other important long lived components that were not included in the NPLA programme. Condition assessments were prepared for components selected on the basis of past incapability, plant concern, or external reports of component ageing. Condition assessments were prepared for about 50 additional components at each of Pickering, Darlington and Bruce. Procedures were also written for system and plant condition assessments.

• The overall PLiM programme is known as the Integrated Ageing Management Programme (IAMP).

• In addition to IAMP activities, a Fleet Life Cycle Management Plan has been prepared to provide integration of station ageing management activities and is updated periodically.

• Detailed ageing assessments for high importance, long life items (steam generators, fuel channels, feeders, and other reactor components) have been performed. These are usually called life cycle management (LCM) plans, and they are regularly updated and fed into station and fleet asset management and business plans. These define the required inspection and maintenance activities during station outages.

• Condition assessments have been performed at the component, system and plant level to varying degrees at OPG stations. Depending on the facility, the current focus ranges from extending the scope of condition assessments to completing assessments for the components/systems initially identified as high importance.

• Condition assessments are inputs to business planning and asset management processes, as well as to system and component health programmes.

• Replaceable and short life components are actively managed by predictive and preventive maintenance programmes.

• OPG is rapidly developing LCM at its plants — it is considered central to long term business success. Effort is concentrated on completion and improvement of LCM plans for all critical systems, structures and components.

• The importance of closer linkages between LCM and both business plans and the R&D programme, is recognized.

• Pickering U1 and 4 have been refurbished & restarted. A decision has been made not to refurbish U2 and U3. These Units are being placed into safe storage.