Some pressure tubes in reactors moderated by heavy water have been made from heat-treated (HT) Zr-2.5Nb. One such tube was removed from the NPD nuclear reactor after 20 years of operation. An extensive program was carried out jointly by AECL and PNC to evaluate the condition and properties of this pressure tube. The investigations include irradiation creep, tensile, corrosion, delayed hydride cracking (DHC), fatigue, and fracture properties.
Results show that: (1) the in-reactor elongation rate is much lower and the transverse strain rates are slightly larger than in cold-worked (CW) Zr-2.5Nb tubes; (2) the tensile properties, hydrogen pickup, threshold stress intensity factor for DHC initiation, DHC velocity, and fatigue crack growth rates were similar to those of the CW Zr-2.5Nb material; (3) the fracture toughness of this tube, as measured by curved compact toughness specimens and burst tests, is slightly higher than the CW tubes. The results were also compared with other heat-treated Zr-2.5Nb materials irradiated in the Fugen reactor.
The tube was in excellent condition when removed from the reactor and would have been satisfactory for further service.
The prototype advanced thermal reactor, Fugen (165MWe), is a heavy-water-moderated, boiling-light-water-cooled, pressure tube type reactor. A Zr-2.5 Nb alloy, in the heat-treated condition to improve its high-temperature strength, is used for the pressure tubes in the Fugen. Pressure tube surveillance specimens were assembled in the inside of the special fuel assemblies from the initial stage of the operation and were exposed to irradiation. The post-irradiation examinations of the first and second surveillance specimens were performed. The tensile strength increased by 30% when fast neutron fluence was >2 × 1025 n/m2. The decreasing rate of the fracture toughness was large over the initial period of the irradiation and became smaller when fast neutron fluence was >2 × 1025 n/m2. The corrosion rate was measured as -1 μm/year and the hydrogen pickup rate was measured as 1 ppm/year. Based on the results, a predictive capability for each respective property for 30 years was established, which demonstrated the integrity of the pressure tube for 30 years.
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