The initial dry fuel loading and the on-power refuelling of the horizontally oriented pressure tubes has caused minor scratching of the lower quadrant of the tubes by the fuel bundle bearing pads. The use of stainless steel shims during initial fuel loading, in recent years, has eliminated the scratching at this stage. Examination of removed tubes has shown that the scratches are rounded and shallow and tests have shown that they are unlikely to cause Delayed Hydride Cracking (DHC) initiation under reactor operating conditions.

The high flow rate of the coolant through the fuel bundles causes bundle vibration, which results in minor fretting of the tube wall by the bearing pads. In reactors with a 12-bundle fuel string (i. e. all CANDU 6 and Pickering units), experience from the examination of removed tubes and from the many periodic and in-service inspections performed to date, has shown that these fret marks are shallow and are not likely to initiate DHC. In reactors with a 13- bundle fuel string, fuel bundle bearing pad fretting in the inlet rolled joint area, particularly at the burnish mark, has resulted in deeper fret marks.

Debris can possibly come from material left in the Primary Heat Transport System during construction/installation, from in-service degradation of components, or from use of unfiltered make-up water to the PHTS. Debris, which becomes entrained in the coolant and then trapped in the fuel bundles or between the bundles and the tubes, can result in debris fretting damage of both the fuel sheaths and the pressure tubes. The fret marks in the pressure tubes can be deep and may require tube removal, although cracking or tube failure have not been observed. The occurrence of severe debris fretting in pressure tubes is of a low frequency and random thus it is not seen as a generic ageing mechanism.