Nuclear PLiM programme is usually consisted of three phases. In the first phase, a feasibility of the continued operation is evaluated to support top manager’s decision making whether continuing to operate the plant. Once the policy is determined to operate the plant beyond design life on the basis of the feasibility study, the second phase programme works out to evaluate detailed lives of SSCs and to establish ageing management programmes together with field walk downs, tests, diagnosis and ageing inspections [1]. After a regulator evaluates results of PSR containing second phase life assessment and endorses, the ageing management programmes are implemented to the field. This is the PLiM third phase that replaces aged components, install new performance monitoring systems, and change designs to improve obsolescent systems in the following outages as they are scheduled.

Plant safety, which is prior to technical activities of PLiM can be affected by the status of system operating performance that could be dependent on the structural integrity and degradations of structures and components (SCs) belong to systems. To solve this issue IAEA has recommended member states to implement PSR as a tool of ensuring a high level of safety throughout plant service life. Reviewing plant safety in every 10 years, PSR can deal with the cumulative effects of the plant ageing, modification, operating experience, and technology evolutions [2].

Fig. A. II.1. Schematic diagram of a CANDU PLiMfeasibility project.

In spite that all plant SSCs has to be considered in PSR, PLiM basically focuses on the long lived passive components that are relatively hard to replace and refurbish during normal operation. Therefore it can be said that the scope of PSR ageing management is wider but depth shallower than that of PLiM life assessment, which includes engineering evaluations,
such as quantitative time limited ageing analysis (TLAA), residual life estimations, field tests and examinations, diagnosis and monitoring, and ageing management programmes.

Short lived active components excluded from PLiM programme are scoped into the ageing management of PSR, and the engineering level of life evaluation is not complicate and deep as much as that of PLiM. PSR reviews the current physical status and records of maintenance and inspection done to the components in the past. Comparing the review results with current safety standards and practical experiences on and off-shore in terms of ageing and maintenance, utility revises the technical procedures and plans how to improve the system safety and slow down the degradation of SCs for the next 10 years. So the depth of PSR engineering evaluation becomes shallower but the scope wider than that of PLiM.

The general process of ageing assessment for the critical SSCs of PLiM feasibility is shown in Figure A. II.2. The ageing assessment starts with the selection of the critical SSCs among all the plant structures and components. The selection methodology is described in detail next chapter. All possible publications about design, manufacturing, operation, inspection and maintenance should be collected and kept in database. Based on the previous CANDU PLiM experiences and publications, and technical consultations of experts, degradation and ageing mechanisms of each SCCs are identified and evaluation methodologies cleared. The ageing phenomena can be recognized through reviewing plant data and history of operation, test, inspection and maintenance. Current ageing status of the screened CSSCs is evaluated with the design criteria by document review. The next is to find the evaluation methodology for the recognized ageing phenomena and/or develop the methodology, when necessary. Finally, remaining lives of CSSCs are evaluated and PLiM cost estimation and work plan for the detailed life evaluation re established.

Fig. A. II.2. Process of ageing assessment.