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.