3.1. SCREENING SSCS

Those SSCs 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.

The first steps in screening the SSCs are to customize the generic PLiM screening methodology for use at a specific plant. Standard criteria are safety, production, environmental impacts, worker safety and cost. A risk-based approach can be applied to develop the plant specific criteria, developing quantitative weighting measures, and then applying the plant specific screening process to the plant SCC list, to rank their importance. The resulting list of prioritised SSCs (sometimes known as the CSSCs — the Critical Systems, Structures and Components) will allow them to be included in the PLiM programme. Also the implied “residual” risk of not including other SSCs will be identified. The rigour of the ageing assessment, in terms of resources required and depth of evaluation will be determined by the SSC priority.

In situations where parts of a PLiM programme are already underway without undertaking the up-front SSC screening, a systematic procedure for SSC screening should be used to verify that there are no gaps in identification of the remaining SSC assessments to be performed. The SSC prioritisation also assists utilities in identifying less critical SSCs. Figure 8 shows a typical level of assessment versus SSC criticality as determined by screening.

For the most critical components and structures that provide mainly passive functions and are subject to long term ageing degradation mechanisms, life assessments are performed. Some of these key steps in the life assessment process and interfaces between the various groups are shown diagrammatically in Fig. 9.

Typical CSSC life assessment methodology is based upon IAEA methodology, as detailed in [4]. A particularly important part of the process is to understand and assess the importance of all the ageing degradation mechanisms that can impact on the functions of the SSC. Another important aspect is to tailor the generic methodology, including the diagnostic and assessment methods and techniques, to the specific technology and characteristics of the SCC under consideration. Some typical examples and component specific considerations are given in the Appendices.

Many components in the early HWR plants had a very good service record with little or no significant degradation history to date. However, this excellent in-service experience (and hence lack of degradation data) provides a unique challenge for the CSSC life assessments within the PLiM programme. In performing the systematic and detailed assessments, a key activity is diagnosis of the operational history for ageing indicators, as well as a thorough understanding of applicable degradation behaviour.

With little degradation data from the plant, the challenge is to provide a reasonably comprehensive and detailed assessment of ageing effects for the next 20 to 30 years of operation. To meet this challenge, a thorough understanding of the applicable degradation mechanisms and the associated —“stressors”— is used. This understanding derives from research and development programmes, integrated with knowledge from relevant field data of other plants.

An in-depth understanding of the plant operational history and the current plant programmes related to ageing are both key inputs to the life assessment process. Involvement of utility staff in the process is encouraged. Developing an efficient and effective team selected among key utility staff (such as ageing management experts, system engineers, component engineers, reliability engineers and maintenance personnel) contributes significantly to success and value of the PLiM assessment programme.

Fig. 9. Life assessment process and interfaces.