Fracture property changes with oxidation and irradiation in nuclear graphites

Abstract

In British gas-cooled nuclear reactors, fast fracture of graphite moderator bricks may happen near the end of the reactor life because fast neutron irradiation induces internal stresses and radiolytic oxidation weakens the moderator bricks. A finite element analysis method, developed by British Energy Generation Ltd, based on a maximum stress criterion predicts the time of first failure of a moderator brick pessimistically due to the assumption that the stress-strain curve of the moderator graphite becomes linear when it is irradiated by fast neutrons. As an alternative, this work sets out to investigate, for the first time, crack growth resistance curves, R and K_R-curves, for UK moderator graphites. Crack growth resistance curves are not affected by changes in non-linearity in stress-strain curves, as the 'plastic' component of the fracture is taken into account during the measurement of R or K_R. The shapes of the crack growth resistance curves consist of an increase from crack initiation to a plateau followed by a decrease for R-curves and an increase for K_R-Curves as crack termination is approached. The shapes of the crack growth resistance curves are determined by energy dissipation mechanisms that take place ahead of the crack tip (microcracking defining a process zone) and at the back of the crack tip (crack bridging and friction defining the friction zone). Frictions and crack bridging, identified by scanning electron microscopy, are responsible for the initial increase in R and K_R, whereas microcracking is responsible for the decrease in R at crack termination. Stress and energy-based mechanical properties are reduced exponentially as a function of the increasing oxidation, with thermal oxidation being more destructive than radiolytic oxidation. After oxidation, crack growth resistance curves are shifted downwards by an exponential factor depending on the degree of oxidation, but the general shape of R-curves remains the same with an increase followed by a plateau and a decrease towards crack termination. For IM1-24 graphite, pinning of glissile dislocations in the basal planes induced by fast neutron irradiation increases the value of stress-based parameters, whereas it reduces energy-based parameters. A new model predicting the shape of crack growth resistance curves for oxidised and irradiated IM1-24 graphite was developed and applied to the case of a reactor brick. A methodology for using the model to predict the time of failure of reactor bricks was also proposed. (author)SIGLEAvailable from British Library Document Supply Centre-DSC:DXN042301 / BLDSC - British Library Document Supply CentreGBUnited Kingdo