Historical evolution

By the end of the 1980’s programmes were in place related to ageing, however a more comprehensive and systematic ageing management strategy was needed. As a result, in 1990, Canadian Nuclear Safety Commission (CNSC) staff requested licensees to demonstrate that:

• Potentially detrimental changes in the plant condition are being identified and dealt with before challenging the defence-in-depth philosophy;

• Ageing related programmes are being effectively integrated to result in a disciplined overall review of safety;

• Steady state and dynamic analyses are, and will remain, valid;

• Review of component degradation mechanisms is being conducted;

• Reliability assessments remain valid in light of operating experience; and

• Planned maintenance programmes are adequate to ensure the safe operation of the plant.

The IAEA guidelines were accepted as an appropriate framework for such a programme. As a result of the above request, the Canadian nuclear industry put systematic ageing management programmes in place that were based on the IAEA guidelines. The specific processes and procedures developed in support for the ageing management plan varied from plant to plant, though a summary of the general approach is presented below.

Using the guidance provided by the IAEA publications, utilities undertook efforts to identify gaps in their operating policies and procedures with regards to the ageing management of critical components. Initially discussions focused on the selection of critical components. Generally, economically “critical” components were incorporated as well as the safety critical ones into an overall plant life management programme. The CNSC accepted either approach provided the safety critical components are sufficiently addressed.

Programmes were developed that considered the known degradation mechanisms of the selected components. Industry also considered operating experience to ensure that all mechanisms that had previously caused failures were addressed. The programmes already in place to deal with known degradation mechanisms were evaluated to determine their effectiveness.

Coincident with the above activities, generic procedures for evaluating component and system ageing were developed, often in conjunction with the plant designer. Along with these, condition assessments of the major plant components were and are being performed. These assessments evaluated the feasibility, from a safety standpoint, of continued use of the components.