For short-lived components an expert panel approach was taken to optimising the preventive maintenance programme, see Figure 12. Current maintenance task lists were compared against external best practice and gaps identified.

111.2.2. LCM ANNUAL REVIEW

At most of the multi-unit CANDU NPPs, the life cycle plans and condition assessments for all critical equipment in the safe operating envelope (SOE) systems are reviewed annually. Input from system health reports and system health monitoring plans (SHMP), as shown in Figure 1, ensures that the condition assessments remain current.

Fig. 13. Multi-unit CANDU Annual Review of LCPs/CAs.

• Performed life assessments on most important critical components and structures

• Carried out plant wide condition assessment, involving major systems, components, structures and commodity groups

• The SSC ageing assessments have provided important inputs into decisions for the life extension investment on items such as: Reactor core pressure tubes and calandria tubes, plus all the feeders, turbine refurbishment, control computer (DCC) replacement, shutdown system improvements, turbine condenser retubing.

• HQ has embarked upon a pre-project refurbishment project for G2. Positive decisions are expected from Quebec and Canadian governments on the environmental impact evaluation (although it was noted that this evaluation did not recognize that nuclear production is free of greenhouse gases).

• Ongoing programme on maintenance planning improvements.

• Prognosis is good for extended service operation of an additional 120, 000 effective full power hours, after an 18 months refurbishment outage in 2010-11.

CANDU Owner’s Group (GOG)

• Manages joint research & development programmes on behalf of CANDU industry participants, including projects on ageing mechanisms; on fuel channels, steam generators and other components; chemistry, degradation and materials; fitness-for — service guidelines; inspection method development.

• Provides an information exchange programme to maximize use of information that will lead to improved safety and performance. This includes gathering of OPEX data and providing to its members, as well as organizing station support workshops.

• Manages projects on feeders and the pressure tube surveillance.

• Periodically issues OPEX reports focussed on specific issues or components.

• Investigating long term support for DCC replacement.

The generic term of SMP assessments is used to reflect a number of assessment techniques most commonly referred to as reliability centered maintenance (RCM), streamlined RCM (SRCM), and preventive maintenance optimization (PMO).

This systematic assessment, combined with input from the various specialists, provides elements for establishing the maintenance basis as described in INPO AP-913 “Equipment Reliability guideline” that is becoming widely used in the nuclear industry. The goals of these assessments are:

• To develop a documented technical basis for the overall maintenance strategy associated with each component considered. This provides the foundation for a programme that is adaptive throughout the life of the plant.

• To ensure that a sufficiently comprehensive maintenance strategy is applied to components to achieve the safety and operational goals of the plant.

These goals have a significant impact on the choice of assessment technique. Users need to assure themselves that the assessment process they choose will not result in insufficient maintenance being specified. There is a real possibility, through the application of various assumptions inherent in some techniques, to specify incomplete or ineffective maintenance. Similarly, regardless of the rigor inherent in a given technique, there is the potential to make non-conservative decisions while attempting to streamline or reduce the overall effort.

In addition, these techniques provide the opportunity to refine the maintenance strategy for each component in order that a plant might optimize the maintenance plan such that only the right maintenance is performed on the right component at the right time.

There are several methods or approaches to applying these techniques. The key is that the process be systematic and rigorous and address the goals noted above. Whether done by single assessors, teams led by facilitators, or expert panels, they need to always follow the same overall process. This includes documenting the results and providing sufficient background to understand the basis of the maintenance strategy to be implemented. This basis needs to include an understanding of the components functions within the context of the system and the systems function within the plant.

The application of SMP assessments does allow for plants to focus on areas of highest risk first, especially when working with limited resources, while meeting the goals above. The system screening provides a focus on more critical systems. Within the system assessments, further focus can be achieved through the inclusion of a criticality evaluation step, typically a feature of streamlined techniques and part of the INPO AP-913 process. The criticality evaluation can be simplified through focus on major components within a system. The criticality is based upon the failure effects attributed to the component.

Other simplifying techniques, such as applying a maintenance strategy, as developed for one component, to group of similar components, is not uncommon, but is a practice that needs to be carried out carefully. Even if the component for which the strategy has been developed is highly critical, the context of operation (e. g. system function) and environmental considerations need to be applicable to the entire group. Assurance is needed that the failure modes of interest are common across the entire group of components. These cautions derive

from the intent of meeting the goals given above. The following are some examples of CANDU 6 systems have been assessed using this technology:

• Four containment systems (dousing, containment isolation, airlocks, Class III local air coolers);

• Auxiliary and main feedwater and condensate systems;

• Class III standby and emergency power supply diesel generators and auxiliaries;

• Emergency core cooling;

• Instrument air;

• Moderator and auxiliaries;

• Main heat transport and auxiliaries; and

• Shutdown cooling systems.

The main differences between the steam generators (SGs) at the single-unit CANDUs (including most of Indian PHWRs) and the multi-unit CANDUs are due to the time when the NPPs were designed and built, as the later units were able to take advantage of experience gained during the early operation of the multi-unit plants. Some examples include design of steam generators, selection of tube material, design of secondary side supports, chemistry control, etc.

The Indian PHWRs where loose fit garter springs and Zircaloy-2 pressure tubes have been used are having both contacting as well as non-contacting pressure tubes in service. A comprehensive strategy evolved with the operating experience over a period of time, as described below in Fig. A. I.1 is being followed for taking safety related decisions with regard to both of them.

NPP life is determined by a wide range of factors that include reactor type, material selection, design, operation and maintenance practices, regulatory and political environment, economics, etc. The original design life is generally 30 to 40 years. However, the actual service life may be less (or more) based on the wide range of factors. PLiM goal of optimizing safe operation with economical competitive operation is relevant no matter how long the plant operates.

Introduction of PLiM as a method for managing the plant service life in a safe and economically optimized way has to consider the following provisions:

• General condition of the plant;

• Current practices for operation, maintenance, testing, surveillance and inspection;

• Data records, reports and SSC’s life-history or capability to obtain & retain this information (in the case of a young plant); and

• Knowledge of design basis (number of load-cycles, material properties, etc.)

For PLiM, additional information should be considered, such as:

• Safety issues, their ranking and scheduling of necessary measures;

• Technical issues affecting the operational performance and costs;

• Market conditions and the economic environment;

• Political and regulatory environment, and

• Economic targets.

Korean government noticed the rule of LTO requirements for the PWR power plants, Guideline of Technical Criteria for the Continued Operation of Reactors beyond Design Life in October 2005. HWR requirements are not declared but are under review on the basis of the same technical philosophy as the PWR approach. In order to fix HWR requirements for LTO, further discussion and communication is expected in Korean industry in near future.

Korean Industry expects that the HWR LTO rule will incorporate the experiences from PWR regulations and international HWR practices in the frame work of PSR. Application of lessons learned PWR regulatory experiences to HWR plants could be a strong point of Korean nuclear industry.

Experience to date has demonstrated the significant costs of being surprised by ageing. In the past, there have been situations in older plants where ageing effects have been so developed when first detected, that component replacement is the only realistic option. In newer plants, improved materials, fabrication and inspection have been deployed so that age related degradation should be slower, suggesting that early implementation of a proactive ageing management strategy will optimize plant ageing management actions, making component replacement unnecessary.

It should be recognised that even the rate of normal expected ageing may be controlled and reduced by taking timely and appropriate operational measures. For example, even though design fatigue criteria are met, optimizing operating (and/or testing) procedures may reduce service loads on some components, hence increasing margins and/or service life expectancy. Another example: careful chemistry monitoring, trending and result assessment, beyond just specifications compliance, may allow reduction of tubing corrosion and increase steam generator lifetime.

Ageing degradation of some SSCs can also be controlled and reduced by taking proactive measures during the design and fabrication stages.

One example would be to avoid exposure of critical heat exchangers to potential corrosion degradation from uncontrolled open loop cooling water. This is typically done by providing an intermediate, closed-loop de-mineralized cooling water system, in which water chemistry is carefully controlled and monitored.

Another example is qualification of critical component fabrication processes via use of pre­production samples, which are destructively examined. This is done to ensure that the intended design material conditions (such as low residual stress) are achieved in the as-built product.

Also, ageing experience from plant operations and life assessments of in-service equipment should be fed back into materials specifications of newer HWR NPP designs. This will assist in ensuring safe and economic operation to longer design lifetimes.

At the existing NPPs, it would be prudent to review current ageing management strategies employed for Long lived passive SSCs and major types of active components (such as motor operated valves) to determine potential advantages of using the proactive strategy. The review should take into account:

• Current condition of an SSC

• Importance of that SSC to achievement of plant safety and production goals

• Current understanding of SSC ageing, including the significant ageing mechanisms and effects, their modelling/predictability, and likely degradation sites based on both operating experience and research

• Current ageing management practices and available monitoring and mitigation methods

• Planned service life of the NPP

Elements of a proactive ageing management strategy include:

• Risk-informed selection of critical SSCs and of the sub-components of complex assemblies

• Systematic ageing assessments of critical SSCs

• Implementation of measures to detect degradation initiation shortly after it first occurs or important “stressors” to the degradation mechanism.

• Identify and understand controllable ageing “stressors” or parameters and implement this understanding into plant practices to minimize effects.

• Recording and reporting of important plant inspection, maintenance and operations information, for use in the systematic assessments.

• Regular monitoring of ageing knowledge (external, internal).

• Regular feedback of plant experience (and that of others) to updating of the AM programme

The effectiveness of ageing management can be significantly enhanced by focusing ageing management actions on those SSCs where the risk and potential benefit is the greatest. Risk informed techniques (largely qualitative ones) are used to optimize inspection, testing, or maintenance (which are elements of ageing management), however, their application to ageing management is continuing to evolve to quantitative processes.

Risk oriented techniques are based on the assumption of adequate operating experience and understanding of ageing in order to predict future behaviour and events. Consequently, there is a need to demonstrate (in particular to the regulators) the adequacy of current knowledge to identify future problem areas. In situations where there is insufficient knowledge with an associated risk of unexpected ageing phenomena and failures, this must be covered by appropriate defence-in-depth measures, including safety margins, inspection/monitoring, and engineered safeguards.

One example of the use of a risk based approach in ageing management is in the screening methodology applied to plant SCCs in order to identify and select those SSCs that require specific ageing management focus in a PLiM programme. Such an approach may also be applied directly to a specific, particularly complex component or structure in order to identify those sub-components that warrant assessment at the sub-component level. For instance, age management of tubing is almost certainly needed for steam generators. However, there are many other important sub-components in this equipment and this type of risk based screening approach is useful to identify those particular internal sub-components that require ageing management attention.

A second example is in the consideration of the various degradation mechanisms. In the detailed ageing assessments of each critical SSC, active and plausible degradation mechanisms are usually assessed in order to identify the measures to be taken. However, there is certainly different management risks associated with different degradation mechanisms. For instance, fretting wear in steam generator tubing might be considered a relatively low risk (as it can be detected reasonably easily, is usually relatively slow growth and can then be successfully managed by a proactive tube plugging programme), whereas stress corrosion cracking could be considered a much more difficult degradation to manage (difficult to detect and can be rapid grow to failure). A risk-based approach is useful to assess the various mechanisms and the ability to successfully manage the degradation that could result from particular types.

Implementation of the systematic ageing management process facilitates the selection of appropriate strategies, proactive or reactive, and coordination of relevant programme to minimise premature ageing.

Screening of systems, structures, and components important to PLiM and identification of critical SSCs for ageing assessment is very essential part of the PLiM feasibility project to concentrate the efforts and to properly allocate PLiM resources. They are usually derived from the safety-related, non-safety but can affect plant safety function, and power concerned SSCs. Power concerned criteria showing the importance of SCs in power generation regarding plant availability and other safety requirements are also applied in the screening process. After screening critical SCs, they are to be identified and prioritized to determine their relative importance in PLiM programme. Critical components were prioritized using eight attributes as shown in Table A. II.1.

Most of screened SCs would be long lived passive ones that are costly, technically difficult to resolve degradation, and limit the continued plant operation because of hard and expensive replacement or no precedent experiences. Other Long lived passive components discriminated from the PLiM and active ones of the plant that are relatively easy to replace or refurbish are maintained in preventive maintenance and PSR. It is necessary to develop a methodology to rank plant structures and system components according to their relative importance based on failure risk assessment. Once relative ranking exercise is completed, a threshold is established above which the components are considered critical for formal assessment. For such critical SSCs, life assessment studies should be performed an in-depth understanding of the degradation mechanisms and the development of an ageing management plan to address them. Factors applied to prioritize the CSSCs are as follows: effect of failure on public safety, effect of failure on plant environment, effect of failure on plant production capability, component failure and repair implications on worker safety, cost of replacement or repair, likelihood of failure, etc.

Typical CSSCs can be screened in CANDU PLiM through the above screening process are as follows: Fuel channels, feeder pipes, reactor assembly, steam generator, pressurizer, primary heat transfer system piping, primary system pipes, secondary system pipes, pumps, pressure vessels, reactor building, supports, turbine, cables, structures, embedded facilities. Primary system pipes, secondary system pipes, pumps, pressure vessels, supports, cables are grouped and re-categorized according to their materials, operating condition and other characteristics. A few representative components are selected among the group components and assessed of their ageing. Phase I study covers the representative components and structures to understand the ageing phenomena.

It is widely recognized that the success of a PLiM programme is very dependent upon the success in integrating the programme into the plant organization and its current activities and programmes.

To launch a PLiM programme (if this has not already been done), 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 (usually of 6 months to 2 years duration) 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.

In addition to the plant PLiM group, a successful PLiM programme requires effective interaction with other key plant staff and groups (during assessments, they need to provide the current operational history and plant programmes) and understanding by plant staff (typically plant staff perform a detailed review of the assessment reports). This helps the staff undertake “ownership” and eventually “Do” PLiM themselves. Typically, system and component engineers, maintenance and inspection staff, reliability groups and operators all have a role to play in PLiM.

One important aspect is that understanding of the ageing assessments brings significant benefits to PLiM implementation. While various degrees of involvement of plant staff in assessments and implementation are possible, the objective is to increase the understanding of the “whys”, in order that adequate decisions can be made of what changes to make in plant programmes (these are the “hows”).

Involvement of plant staff also helps identify the training needed for effective transfer of PLiM technology. For instance, training of plant staff in ageing degradation mechanisms and in assessment techniques is important to the transfer of PLiM technology to the plant to ensure effective implementation and “adaptation” to plant specific situations.

Effective PLiM implies and requires some additional effort by utility staff as the plant ages. Hence, plant staff involvement in the PLiM programme does imply some additional responsibilities, but there are various ways to split the effort involved and the roles to minimize disruption to other day-to-day duties. The intent is to tailor the added effort to the specific utility or plant organization, which involves plant specific decisions.

However, it has been recognized that before those decisions can be made, the experience of the PLiM Pilot Programme must be obtained. It should be noted that one of the recommended Pilot Project tasks relates to developing utility specific PLiM procedures. One of these is a plant specific station instruction (which is a high level plant policy publication) that spells out who has what role in the PLiM programme and the additional PLiM responsibilities of various groups.