Fracture toughness and delayed hydride cracking of Zr-alloys for Candu and RBMK pressure tubes

Fracture toughness and resistance to delayed hydride cracking (DHC) are important characteristics that govern the fail-safe operation of CANDU and RBMK pressure tubes. To upgrade the materials of those tubes comprehensive information is needed on the influence effected by the chemical composition, structure-phase state, texture and strength of tubes on fracture toughness and resistance to DHC. For this purpose, using unified procedures applied world-wide comparative investigations of fracture toughness and DHC velocity were carried out using unirradiated Zr-2.5Nb and Zr-1.2Sn-1Nb-0.35Fe pressure tubes of the CANDU and RBMK types that had different compositions, structure-phase states, textures and strengths. The tests were implemented at 250 C on compact tensile specimens 17 mm wide having axial fatigue cracks. Before testing for DHC the specimens were hydrided to the hydrogen contents of 60-100 ppm. It follows from the tests that the more radial is the texture type the higher is the fracture toughness of a pressure tube. As the value of the ratio between fractions of normals to basal planes in the radial and tangential directions fR/fT increases from 0.57 to 1.29 fracture toughness grows according to the law close to the parabolic one and changes most intensively with fR/fT increased to 0.7. The DHC velocity is primarily defined by the pressure tube material strength. The highest DHC velocity is demonstrated by standard CANDU pressure tubes having the highest strength properties. The DHC velocity of RBMK pressure tubes subjected to standard processing may be more than an order lower. For a hydride crack to initiate in those tubes a much longer incubation period and higher initial stress intensity factor are needed. The dependence of DHC velocity on zirconium alloy yield stress in the direction of normal to hydride cracking plane obeys the linear regularity. Fractographic observations established quantitative relations of fracture toughness parameters and DHC velocity to structural elements of fractures which broadens the understanding of the mechanisms inherent in tough fracture and delayed hydride cracking of Zr-alloys. (authors)