Measurement of Rates of Delayed Hydride Cracking (DHC) in Zr-2.5Nb Alloys-An IAEA Coordinated Research Project

Values of DHC rates in Zr alloys are sensitive to measurement procedures. A standard method has been developed at the laboratories of AECL and evaluated in a round-robin involving ten IAEA member states. Two test materials were used-Zr-2.5Nb pressure tubes in the cold-worked (CANDU(TM)) and heat-treated (RBMK) conditions. Cracks were grown from fatigued starter cracks in the axial direction on the axial-radial plane of the original tubes. To obtain the maximum value of V-c, specimens were heated to dissolve all their hydrogen, then cooled at 1 to 3 degrees C/min to the test temperature before loading at 15 MPa root m. Although the start of cracking was detected by potential drop, the extent of cracking was measured directly on the crack faces. The values of incubation time to the start of cracking were highly variable but V-c was well behaved. The values of V-c were normally distributed with a range varying from a factor of 1.2 to 5.2. At 250 degrees C the mean value of V-c from 80 specimens of cold-worked material was 8.9(+/-1.12)x10(-8) m/s and from 41 specimens of heat-treated material the mean value of V-c was 3.3(+/-0.64) X 10(-8) m/s. Tests were also done at six other temperatures between 144 and 283 degrees C, using up to 22 specimens at each temperature. Both materials had an Arrhenius-type temperature dependence, V-c = A exp(-Q/RT). The use of strictly defined and coordinated experimental procedures gave a consistent set of V-c values, allowing effective comparison of results obtained in different national laboratories and resulting in good correlations between the DHC velocity values and differences in strength, crystallographic texture, and distribution of P-phase in the test materials.