Pressure tubes cracking due to DHC mechanism

Zr-2.5wt%Nb alloy, used in fabrication CANDU and RMBK pressure tubes, fulfils the requirements of a material to be used under specific thermal, mechanical, irradiation and corrosion environment conditions in a nuclear reactor. Despite these advantages, the structural integrity of this assembly can be affected under certain conditions (stress, temperature, hydrogen concentration above the terminal limit of solubility), the crack initiation and propagation process being the mechanism responsible of this behaviour. During their operation the pressure tubes are susceptible to a stable cracking process referred to as Delayed Hydride Cracking (DHC). This phenomenon is one of the most important factors responsible for the degradation of these reactor components. The hydrogen concentration and the stress distribution are the parameters affecting this mechanism, leading to an embrittlement effect of the material, to a loss in the ductility and in the fracture toughness. Therefore, the structural integrity and the in-service lifetime are affected. The pressure tubes fabricated from zirconium alloy occlude exothermically hydrogen during manufacture and during operational service. When the concentration in solution exceeds the TLS (Terminal Limit of Solubility), the excess hydrogen precipitates as platelets of hydride. These hydrides are generally brittle in nature and therefore they cause a structural embrittlement, a loss of ductility and fracture toughness. Under certain thermo-mechanical conditions, the hydrides oriented after the fabrication process in the circumferential direction, tend to reorient. This phenomenon is responsible for a time-dependent failure through the hydride zone and it will arrest in the ductile zirconium matrix. This paper presents the experimental results obtained as a part of the IAEA Co-ordinated Research Project on 'Hydrogen and Hydride Induced Degradation of the Mechanical and Physical Properties of Zirconium-based Alloys', included like a Research Contract 'Crack measurements on Zr - 2.5%Nb alloys' between the IAEA and the Institute for Nuclear Research, Pitesti, Romania. The studies have been performed on CANDU and RMBK pressure tubes to put into the evidence the susceptibility to DHC of the two type of tubes. (authors)