HYDROGEN PICKUP AND DEGRADATION OF HEAT-TREATED ZR-2.5 WT-PERCENT-NB PRESSURE TUBE

Heat-treat (HT) Zr-2.5 wt%Nb pressure tubes which have high-temperature strength are used for the Advanced Thermal Reactor Fugen (boiling-light-water-cooled heavy-water-moderated pressure-tube-type reactor) in Japan. In general, zirconium alloys are considered to be degraded in the fracture toughness by picking up hydrogens from reactor coolant during reactor operation. It is reported in CANDU (Canada Deuterium Uranium) reactors that the deuterium pickup rate of cold-worked (CW) Zircaloy-2 pressure tubes is very high but that the deuterium pickup rate of CW Zr-2.5 wt% Nb pressure tubes is very low. Taking these results and those of Fugen pressure tube surveillance specimens (present work) into account, the hydrogen pickup for HT Zr-2.5 wt%Nb pressure tubes was estimated as about 44 ppm/30 years, which is very small and lower than that of the hydrogen solubility limit at 285-degrees-C, to yield almost no degradation of the pressure tubes under the operational temperature. Furthermore, a qualitative equation was proposed for the hydrogen pickup of the pressure tube.