Extensive analysis and studies of HWR piping systems (including supports) have already been completed, including an IAEA TECDOC (which is in process of being issued) that covers some CANDU primary piping considerations. Typically, comprehensive PLiM Life Assessments specific to the individual plant are completed and then factored into the in­service inspection and maintenance to ensure plant life attainment. These plans are updated periodically as part of the overall plant life management programme.

For instance, at several HWR NPPs, life assessments of the piping and supports have been completed. For piping systems, due to the large number of lines and supports to be covered, combined with the need to provide recommendations directly to the plant for their age management activities, a particular focus was made to tailor the generic life assessment methodology to these requirements. For instance, one technique was to use piping and support design and structural qualification specialists, as well as plant piping specialists and supplemented by system design and fluid chemistry/materials specialists, working in a team to perform the assessments. This team developed the detailed assessment approach for this type of equipment.

An example is preliminary fatigue assessment of the piping in many of these systems. First, an evaluation of the number and types of stress cycles that the piping system had experienced was performed. It quickly became evident that the plant had experienced a very low number of the key thermal cycles versus the design basis expectation for a HWR plant of its age. Hence, it was decided early on to concentrate the assessment effort on other sources of potential degradation (versus starting quantitative fatigue usage evaluation, which could be done later).

It was also recognized that a key potential stressor for fatigue assessment was piping support condition. If the piping support design intent was not met (for instance, by support material degradation or modifications), then this could be a very significant factor. Procedures were developed to focus the fatigue assessment effort on piping support condition via walkdowns by the piping qualification specialists, by assessment of the plant hanger/support inspection programme and the results of the work done by the plant on support configurations. This approach proved to be a practical and cost effective approach to screen and assess the preliminary life assessment of piping thermal and mechanical fatigue.

Various other plausible piping degradation mechanisms were also considered in the piping system life assessments, using a variety of other assessment techniques. In general, it was concluded that piping and supports in the systems assessed are in good condition and are expected to generally perform well in plant extended operation given good chemistry control. Some local portions of the piping in certain systems are prone to degradation from FAC and in certain cases, have led to its replacement usually with higher chromium alloy material. Piping supports that are located in the open are more subject to environmental deterioration and hence should be further monitored (and sometimes refurbished) for proper performance.

Feeder pipes in the heat transport system need to be subjected to a more rigorous assessment process, given the recent field experience where wall thinning of some portions of the feeders, and cracking of feeders at one station, have been reported. An industry methodology and feeder-specific fitness-for-service guidelines for the degradation types that have been experienced on outlet feeders are now used by all Canadian CANDU stations. Wall thickness inspection and monitoring programmes are underway and mitigation strategies for older plants are under development. Most feeder pipes will meet their design life.

A limited number of outlet feeder bends and/or welds may require replacement before pressure tube replacement. The techniques and procedures for feeder replacement have been developed, and have been recently used successfully at a CANDU 6 plant. The feeder repair and replacement process is now routine and feeders on the reactor face can be replaced with little difficulty during an outage. It is clear that repair of feeders due to ageing has now proven to be an effective and economical Age Management technique.

For life extension and as part of the refurbishment and large scale fuel channel replacement, at least a portion of all feeders will be replaced to meet the extended life. A more proactive approach may be to replace the entire feeder at refurbishment, thus reducing inspection and maintenance that could be associated with leaving in portions of the original feeders. The material used for replaced feeders could be improved to have desired chromium content). This is an economic decision for the utility to make. A number of programmes are underway to address the details of the feeder engineering but the mechanical properties of removed ”aged” feeder pipes have already been measured to determine if there has been any effect of ageing (early indications are that, as expected, ageing effects on mechanical properties are not significant). This information will be useful to the life qualification of feeder pipes for extended plant operation.

IAEA-TECDOC-1361 [I.3] addresses ageing of primary piping in PWRs. Differences between PWR and HWR nuclear piping are primarily due to the horizontal CANDU fuel channel design, which requires more extensive piping runs. The feeders are also a design feature specific to HWRs. Other factors to include are listed below:

• LBB concept

• Underground piping