Influence of Iodine on the Strain and Rupture Behavior of Zircaloy-4 Cladding Tubes at High Temperatures

Abstract

The stress corrosion cracking (SCC) behavior of short tubular Zircaloy-4 specimens by the action of iodine (initial iodine concentration: 0.1 to 10 mg/cm3) was investigated out-of-pile between 600 and 1100°C under inert gas conditions. The Zircaloy cladding tube was used as-received and with an inner preliminary oxidation. Moreover, the influence of the uranium oxide (UO2) oxygen potential on the SCC behavior was studied. The burst and creep-rupture tests (time-to-rupture ≤ 15 min) clearly show that the deformation behavior of Zircaloy cladding tubes below 850°C is heavily influenced by the presence of iodine. A failure of Zircaloy tubes takes place that is characterized by little deformation as compared to reference specimens without iodine. With decreasing temperature, the burst strain is greatly reduced. Moreover, in the isothermal, isobaric experiments, the time-to-failure of the iodine containing specimens is markedly shorter as compared to the iodine-free reference specimens. Internal preoxidation of the cladding tube or UO2 in the specimens exert an additional influence on the mechanical properties of Zircaloy. Scanning electron microscope examinations of the rupture surfaces of the cladding tubes show that in the presence of iodine and at burst temperatures below 850°C the cracks in the cladding material are mainly intergranular followed by ductile residual rupture. By contrast, iodine-free specimens are exclusively subject to ductile failure.