Determination of the critical iodine concentration for stress corrosion cracking failure of Zircaloy-4 tubing between 500 and 900°C

Abstract

Iodine can influence the burst strain and time-to-failure of Zircaloy-4 tubing up to about 800°C. Low-ductility failure of zircaloy specimens containing iodine is characterized by little cladding deformation compared to reference specimens without iodine. However, the failure strain is significantly reduced only after a critical concentration of iodine within the tube specimens has been exceeded. Therefore, the critical iodine concentration resulting in stress corrosion cracking failure of as-received zircaloy tubing has been determined in the temperature range 500 to 900°C in out-of-pile creep rupture tests (times-to-failure <900 s). The test results show that the critical iodine concentration depends strongly on temperature, varying between 0.01 mg/cm3 at 500°C and 50 mg/cm3 at 900°C. The inserted iodine reacts completely with the zircaloy cladding to form gaseous ZrI4 and various condensed ZrIx, phases. Gaseous ZrI4 is probably responsible for crack initiation and crack propagation.