Neutron irradiation of the Zr-2.5Nb pressure tube material results in an increase in yield and tensile strengths and a decrease in ductility and fracture toughness. The velocity of DHC also is increased. The extent of these changes varies along the length of the tube, from inlet to outlet. The mechanical property changes due to irradiation damage saturate relatively early in reactor operating life, usually after about 1 to 5 years of reactor operation. After saturation the rate of change is slow.

To manage this ageing behaviour, the material surveillance programme under CAN/CSA N285.4-94 verifies that the tubes in reactor are responding the same way as that predicted from an extensive fracture toughness database (from both full-sized burst tests and small compact toughness (CT) specimens made from ex-service pressure tube material or from material irradiated in test reactors). The most recent surveillance results from removed pressure tubes, support the expectations of fitness for service to the design life from a fracture toughness perspective.

The main differences between pressure tubes at the single unit CANDUs and the multi-unit CANDUs or in some of the earlier Indian PHWRs are due to the time when the NPPs were designed and built. The later units were able to take advantage of experience gained during the early operation of the multi-unit plants. Some examples include selection of pressure tube material, design and number of pressure tube-to-calandria tube spacers, design of rolled joints and designing for axial elongation of pressure tubes. Country wise reports given in Appendices identify details.(to be given by members in Appendices)