Analysis of the combined effects of ageing is necessary to ensure that the unit is operating within the original design envelope, to demonstrate that there has been no deterioration of the operational or safety margins, and to ensure mitigation methods are effective in managing ageing. A specific example of the integrated safety/performance assessment part of the PLiM programme is the effort to predict and manage performance of the HTS, as plants age.

When the “first generation” CANDU 6 stations were originally designed, the need for a detailed thermal hydraulic predictive modeling capability for the CANDU Heat Transport System (HTS) was recognized. A number of ageing mechanisms were anticipated and margins were provided to cater to the in-service ageing degradation that would occur. Accurate predictions of HTS thermal and hydraulic parameters were recognized as an important capability. Hence predictive codes were not only developed but also extensively validated with both commissioning and operational data from the early CANDU 6 experience. This programme of code development and refinement for prediction of HTS ageing behaviour has continued throughout the operational period. In parallel, the supporting R&D programme developed tools to predict the thermal and hydraulic behaviour of deposits that accumulate on various surfaces in the HTS, including the primary and secondary sides of the steam generators. This effort provided additional important data and modeling parameters that were subsequently incorporated into the prediction codes. Also, a specialized eddy current interpretation method has been used to measure the extent and distribution of SG tube primary side fouling.

The result of the integrated programme is an enhanced HTS ageing predictive capability and proven mitigation techniques. The refined codes can both provide an accurate reflection of the current HTS condition of the plant and also be used as an important aid to plant management to predict the benefit of ageing mitigation techniques. An example is an assessment of the effectiveness of a cleaning process, prior to that technique being applied at the plant. Primary side mechanical cleaning at one HWR NPP restored ~+5% of core flow and decreased the reactor inlet header temperature (RIHT) by ~ 3°C.

These values were very close to the improvements predicted prior to the cleaning, using the refined HTS performance codes. Recent experience, including that mentioned, is with a mechanical method for cleaning the inside diameter of the Steam Generator tubes during a shutdown. The system employs robotic manipulators to visit several tubes at one time. A suitable grit material is propelled through the tubes cleaning the oxide from the ID surfaces by abrasion. By carefully controlling a number of parameters (including application time), the amount of oxide removed and overall cleaning efficiency can be optimized.

Hence, there are two important uses of these refined codes. First, they can be used to accurately reflect the current HTS condition of the plant for operational/safety margin assessment. Second, they can also be used as an important PLiM technique (to assist operators, maintainers and plant technical staff) to predict the benefit of ageing mitigation processes and plan when the mitigation implementation is needed.