There are two primary types of in-service damage; inlet rolled joint fuel bundle bearing pad fretting and debris fretting. Inlet rolled joint fretting affects only the 13 fuel bundle channel design used in the Bruce and Darlington reactors. With this design there is interaction between the pressure tube at the inlet rolled joint burnish mark and the fuel bundle bearing pads. Inspections have generated sufficient information to characterize the severity and distribution of the inlet fretting in these units. The combination of research data and data obtained from removed tubes make it possible to disposition all tubes with such flaws for continued service.

Inadequate cleanup after construction and commissioning and isolated operational incidents have led to debris entering the Heat Transport System. Debris, which is carried around the circuit by the coolant and then trapped in the fuel bundles, or between the bundles and the tubes, can result in wear of both the fuel sheaths and the pressure tubes. In many reactors, full length volumetric inspections indicate that the level of debris fretting is unit dependent. Because of the random nature of this fretting mechanism, it is difficult to predict the location and severity of potential fretting. To ensure that debris fretting that may exist in a particular reactor core will not result in an unacceptably high probability of tubes being susceptible to DHC, the following programmes are used to complement the limited volumetric inspections.

• Deuterium monitoring programme. This is important because debris frets in the body of the tube are not considered to be an integrity concern if the hydrogen equivalent concentrations in the pressure tubes remain below the terminal solid solubility limit at normal operating temperatures at the flaw tip.

• Additional volumetric inspections to assess the distribution of debris fret geometries in a core.

• Probabilistic core assessments to establish the probability of initiating DHC from this mechanism.

• Pressure — Temperature limits to avoid full pressurization of the pressure tubes at conditions when DHC can occur, i. e., when the hydrogen concentration exceeds the terminal solid solubility limit and to ensure that there are adequate margins against fracture at all operating temperatures.

• Fuel failure monitoring. Debris can cause wear of the fuel sheaths and can be an indicator of wear of the pressure tube.