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-temperatur e strength are used for the Advanced Thermal Reactor Fugen (boiling-li ght-water-cooled heavy-water-moderated pressure-tube-type reactor) in Japan. In general, zirconium alloys are considered to be degraded in t he fracture toughness by picking up hydrogens from reactor coolant dur ing reactor operation. It is reported in CANDU (Canada Deuterium Urani um) reactors that the deuterium pickup rate of cold-worked (CW) Zircal oy-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) in to 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 tha n that of the hydrogen solubility limit at 285-degrees-C, to yield alm ost no degradation of the pressure tubes under the operational tempera ture. Furthermore, a qualitative equation was proposed for the hydroge n pickup of the pressure tube.