The NPP containing RBMK-1500 type reactor is in operation in Lithuania. The RBMK-1500 is graphite moderated, boiling water, multi-channel reactor. Zr-2.5% Nb alloys are used as a constructional material for manufacturing claddings of both fuel assemblies and fuel channels (FC). Fuel channels of RBMK-1500 reactors are the major structural elements of the reactor core that have to meet strict requirements in terms of operational reliability. Therefore it is necessary to evaluate the influence of ageing mechanism on mechanical properties of zirconium alloys during operation of the reactor. Hydrogen absorption by zirconium alloy during corrosion process is the ageing mechanism of Zr-2.5% Nb fuel channel. When hydrogen concentration in fuel channel exceeds solubility limit, formation of hydrides under certain conditions can reduce resistance to brittle fracture and cause initiation and development of hydride cracks. Therefore the evaluation of the resistance to brittle fracture of zirconium alloy is important. The objective of this paper is modelling and assessment of the influence of hydrogen to the stress intensity factor of Zr - 2.5% Nb alloy using finite element method. The stress intensity factor is a parameter, which is used to estimate a material resistance to brittle fracture. Modelling of the stress intensity factor was performed using finite element method. The influence of the hydrogen concentration to stress intensity factor was evaluated. The prognosis results and experimental data are in close agreement. It was demonstrated that applied methodology for modelling of the influence of hydrogen on the stress intensity factor can be used.

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