Ageing management of the pressure tubes requires a strategy that effectively addresses all ageing mechanisms so that the core remains fit-for-service. The two key aspects of the strategy are: 1) appropriate inspections involving measurement of axial elongation, radial expansion, and sag of the tubes as well as volumetric inspection for flaws and monitoring of the deuterium concentration; and 2), material surveillance to confirm the acceptability of the fracture toughness and delayed hydride cracking characteristics of the material as it ages.

Dimensional changes

During reactor operation, the conditions of temperature, stress and neutron flux change the dimensions of the pressure tubes. Irradiation-induced and thermally induced deformation of fuel channel components will, in the absence of other mechanisms, eventually establish fuel channel life. The following inter-related dimensional changes occur in pressure tubes during normal reactor operation:

• Axial elongation

• Diametral expansion

• Wall thinning

• Sag

Pressure tube axial elongation due to irradiation can require remedial action, and, in the extreme, become a tube life limiting factor if the bearing length provided by the design is not sufficient to accommodate the projected axial elongation for the design life. The difference in axial elongation rates between neighbouring channels is also monitored to ensure that interference between feeders or problems with fuelling machine access does not occur.

Elongation of all fuel channels can either be measured using the fuelling machine or measured periodically during planned outages using specialized gauging tools. These inspections provide information on the elongation rate of each individual channel as well as providing data to determine the variability in the creep and growth properties of the tubes. Current understanding of irradiation induced axial elongation indicates that elongation rates may be slightly non-linear. Therefore continued frequent monitoring is required to determine when the channels will go off bearing and to identify which channels are affected and will require remedial action. If the channels are predicted to go off bearing before the design life is reached, the following actions can be implemented:

• Shifting the channels to recover any available bearing travel on the current fixed end

• Defuelling a small number of channels

• Replacing a small number of channels

• Demonstrating that off bearing operation is acceptable