In the HWR nuclear steam plant, there are a number of large shell-and-tube heat exchangers (HXs). Typically, a comprehensive PLiM life assessment specific to the individual plant’s large HXs is completed and factored into the in-service inspection and maintenance to ensure plant life attainment. The detailed and comprehensive life assessment of each selected HX includes the pressure boundary, the external support structure, the tubing, and all the key internal sub-components. While heat exchangers are relatively complex components that can be subject to a variety of degradation mechanisms on their various sub-components, worldwide experience has demonstrated that it is corrosion of the tubing that is the greatest life threat.

In open-loop cooling water circuits, outside surface corrosion has occurred on a number of tubing alloys. This degradation is often wide spread (affects a lot of tubes) and has occurred rapidly (difficult to manage by inspection and plugging). It can cause tube leaks, which often leads to forced outages. In some cases, HXs have had to be replaced.

In contrast, the HWR experience with several tubing alloys on closed loop de-mineralized cooling water systems has been excellent. To date, there has been no detectable in-service corrosion degradation of tubing in the large heat exchangers inside the reactor building (RB), for HWR plants with closed loop de-mineralized cooling water flowing on the shell side. This favourable experience reinforces plant design with closed loop de-mineralized cooling water of large HXs in the RB, as it provides well controlled chemical conditions on the outside surface of HX tubing. Rigorous operational chemistry control of the closed loop de­mineralized system has also been an important contributor.

Given this excellent record and the long life prognosis due to absence of any tubing corrosion concerns, the life assessments on these critical heat exchangers give detailed consideration of other types of tubing degradation and also to the life capability of other sub-components of the heat exchangers. As there are many individual parts, a risk assessment screening was performed to assist in identifying those sub-components that warrant further detailed life assessment. Subsequently, the HX life assessment approach considered the potential of each of the top 10 most significant historical degradation mechanisms if we refer the “top 10”, we should include a list of them on each of the important HX sub-components. In this way, the generic PLiM programme life assessment methodology was specifically tailored to this component type, to ensure a systematic and comprehensive process was followed that covered the entire equipment. The outcome of this work was positive life prognosis for life extension of the critical nuclear heat exchangers in the plants assessed.

Another important outcome of the process was detailed understanding of ageing potential in each of the individual HXs considered. There are different equipment design details in these HXs and different primary side systems involved. This detailed knowledge is being used to “optimize” the plant programmes that provide important age management data of this equipment. An example is the inspection programme for these HXs. Since the de-mineralized cooling water plant design provides assurance against widespread tubing corrosion problems, a large and frequent tubing inspection programme is not required. But the detailed assessment of what other types of degradation might occur in future and where it might occur in each of the various HXs enables the life assessment to provide important input into the “optimized” HX inspection programme for life extension. The life assessment enables specific areas and regions of the HX to be selected for special attention and hence a more focused inspection programme, — “targeted ”— to areas that are sensitive to potential age degradation, can also be a positive outcome of the PLiM work.