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.