Vibration of the pressure tubes caused by installation activities, such as rolling the pressure tube into the end fittings, commissioning and operation has been found to cause migration of some loose fitting spacers away from their design locations if the spacers are not sufficiently pinched between the pressure tube and the calandria tube. This displacement, if sufficient, allows the pressure tubes to sag into contact with their calandria tubes. If the hydrogen equivalent concentration at the point of contact is above a threshold value then hydride blisters can start to form.

Material property changes

Irradiation of the Zr-2.5 Nb pressure tube material causes hardening of the metal structure, an increase in the yield and tensile properties and a decrease in ductility and fracture toughness.