Hydrogenation with resultant embrittlement of zirconium items of VVER or RBMK fuel assemblies (FA) may cause their failure during a long-term operation under the conditions of high operating loads. Depending on the conditions of in-pile operation of FA items the influence exerted by hydrogen on their performance is determined by either fracture toughness characteristics under a short-term action of substantial tensile forces, e.g., upon discharging a fuel assembly from a reactor, or by the velocity of a crack propagation in items upon delayed hydride cracking (DHC) under the action of stresses (residual manufacturing and operating) and absorbed hydrogen. In this work, hydrogen-effected DHC and fracture toughness of zirconium items of FA (fuel rod cladding, GT, CT) and the influence produced on them by factors such as alloy composition. hydrogenation level. temperature and direction of a defect evolution in an item (axial, tangential, radial) were investigated. Effect of hydrogen upon geometrical sizes of FA items was also considered. The results of the implemented investigations allow a more-grounded approach to estimate the serviceability and to choose zirconium alloys for FA items operating under complicated temperature and stress-strain conditions in hydrogen containing environments. (C) 2002 International Association for Hydrogen Energy. Published by Elsevier Science Ltd. All rights reserved.