Extensive analysis and studies of fuel channels have already been completed, including a TECDOC on HWR pressure tubes [I.1]. Comprehensive PLiM for pressure tubes are established for inspection and maintenance to ensure plant life attainment. These plans are updated periodically (typically every 3-4 years) as part of the plant life management programme for this component.

Under normal operating conditions the pressure tubes are exposed to an operating environment of high temperature (250 to 315o C), high internal pressure (9 to 11 MPa) and high flow rate D2O coolant. The tubes also experience a fast neutron flux of up to 3.5 x 10 n. m.’ s’ . These conditions result in the following ageing mechanisms being experienced by the tubes.

Creep and growth

Thermal creep, irradiation creep and irradiation growth, resulting from the above operating conditions, cause axial elongation, diametral expansion and wall thinning of the pressure tubes. In addition, since the fuel channels are horizontally oriented, the previous factors, along with the weight of the fuel and D2O coolant, also result in creep sag of the channel.

Corrosion

The internal surfaces of the pressure tube and the stainless steel end fitting are exposed to and corroded by the slightly alkaline (pH10) D2O coolant. A fraction of the deuterium released by the corrosion process is absorbed and retained by the pressure tube. Lithium Hydroxide (LiOH), used to control pH in the Primary Heat Transport System (PHTS), can concentrate under the fuel bearing pads due to local boiling effects. This concentration of LiOH under some fuel bundle bearing pads, mainly in the outlet half of the fuel channel, has resulted in crevice corrosion in some tubes. Examination of removed tubes has shown that the pits are wide and very rounded. These are not considered to be sites for the initiation of Delayed Hydride Cracking (DHC).