During reactor operation, the heavy water flowing through the pressure tubes reacts with their inside-surfaces forming a zirconium oxide film and releasing elemental deuterium. The loss of metal from this reaction is very small and does not limit the life of the pressure tube. However some of the released deuterium enters the pressure tube increasing the susceptibility of flaws in the pressure tube to crack initiation and growth by delayed hydride cracking (DHC) and potentially decreasing the fracture toughness if very high levels were eventually reached. Additional deuterium also enters the pressure tube end by crevice effects at the rolled joint.

Deuterium ingress in the body of the pressure tube is monitored using a tool that takes small samples from the inside surfaces of the pressure tubes in situ or by punching through-wall coupons from tubes removed from service. The resulting specimens are analysed for deuterium content. Pressure tube sampling campaigns (for hydrogen/deuterium concentration measurements) that have been completed at several reactors and results continue to show a low deuterium ingress rate relationship with time. Repeat scrapes however indicate that the ingress may be increasing with time and hence monitoring is required to confirm this trend.