Extensive analysis and studies of HWR steam generators have already been completed, including an IAEA TECDOC that covers CANDU steam generators [I.2]. Typically, a comprehensive PLiM Life Assessment or a Life Cycle Management plan specific to the individual plant’s steam generators is completed and factored into the in-service inspection and maintenance to ensure plant life attainment. These plans are updated periodically as part of the plant life management programme for this component.

A detailed and comprehensive life assessment of the steam generating equipment will include the pressure boundary, the external support structure, the tubing, and all the key internal sub­components. Tubing is a key sub-component. For CANDU-6 NPPs (and also for Indian PHWRs built after Madulas power station (MAPS)), the SGs tube are with Alloy 800M (M means “modified”) and have experienced relatively little SG tube corrosion to date. For instance, at the Wolsong NPP Unit 1 plant (that has 21 years of in-service experience), there are only 9 SG tubes plugged, none as a consequence of corrosion, 7 of these before in-service operation, out of the total population of over 14,000 tubes. For other CANDU-6 SGs and Indian PHWRs the situation is similar. Elsewhere, the record with Alloy 800 SG tubing is similar after more than 25 years’ in-service experience.

This excellent service record requires a rather novel approach to predicting future performance, such as the potential for tubing corrosion degradation.

The assessments involve a very thorough review of tubing corrosion mechanisms that can occur in nuclear steam generators. The knowledge from R&D studies of SG tubing corrosion behaviour in various chemistry environments has been a key element of this methodology.

• First, a detailed assessment is made of worldwide experience with Alloy 800M, and other steam generator tubing alloys. From this review, all the specific types of corrosion mechanisms, and the chemistry environments that have been instrumental in causing tubing corrosion degradation, are systematically identified and the key stressors assessed.

• Second, each of these tubing corrosion situations is evaluated for relevance to the particular plant’s steam generator design and operation. The tolerance of the Alloy 800 tubing to the presence of plausible aggressive secondary side impurities (such as lead, sulphides & chlorides) in various plausible ranges of chemistry conditions that might exist in steam generator crevices (such as in the tubesheet sludge pile or in tube-to — support gaps that have become blocked with deposits), is assessed. (note: it seems obvious that experts would perform this assessment).

• Additionally, other degradation mechanisms are reviewed for their impact on SG condition, operation, and future life. These include mechanisms related to thermal — hydraulics such as vibration and fretting, particularly with respect to the potential for fretting of the tubing against the U-bend support structures. Fouling, both of the primary and secondary sides of the SG can also significantly reduce operating efficiency, and the efficiency of the station output.

While SG tubing degradation is considered the largest single potential source of SG problems, this alone is not the only important factor in determining the prognosis for achieving 50-year life. Steam generator non-tube components also present a challenge in estimating current condition as well as future life. Hence, the HWR steam generator life assessment also considers the other components in a SG that could compromise life. As there are a very large number of individual components in a nuclear SG, these are grouped into the following categories:

• Primary side pressure boundary

• Secondary side pressure boundary

• External supports

• Primary side internals

• Secondary side internals

As with the tubing, detailed consideration is given to all potential degradation mechanisms, from world wide and other HWR experience. Next the potential for plausible degradation is assessed, given the HWR NPP’s design and operation.

Based on the detailed plant SG studies performed to date, the overall SG condition at several CANDU 6 plants appears to be good with no obvious compromise to attaining the design life. However, there is sufficient uncertainty over the condition of SG secondary side internals that the life extension assessment requires additional inspection and analysis. The conclusions and recommendations are focused on chemistry control, proactive inspections/monitoring programme, and periodic cleaning on both the primary and secondary sides of the steam generators.

Despite limited information on the condition of the secondary side internals, the overall current condition of steam generators is sufficiently good to attain design life subject to a continued programme of inspections, cleaning and chemistry control. Similarly, the prognosis for life extension is also good, provided a proactive age management strategy is adopted and implemented.