A Hodgkins

Fracture behaviour of radiolytically oxidised reactor core graphites: a view

Abstract This paper provides a view on the fracture behaviour of polygranular graphites, used to moderate gas cooled nuclear reactors. Graphite is often cited as a classic example of a brittle material because failure, in tension, is associated with small strains. However, attempts to characterise the fracture behaviour of graphite by linear elastic fracture mechanics methods have been largely unsuccessful. Observations of graphite fracture show that elastic strain energy may be dissipated by the formation of distributed microcracks, and their formation may be responsible for non-linearity in the rising load–displacement curve. Progressive softening behaviour may also be observed in some specimens after the peak load. This type of load–displacement behaviour is a characteristic of quasi-brittle materials. Radiolytic oxidation increases the proportion of porosity within reactor core graphite so that the microstructure becomes increasingly skeletal. Consideration is given to the fracture of radiolytically oxidised graphite to support an argument for quasi-brittle behaviour.

Understanding fracture behaviour of PGA reactor core graphite: perspective

Abstract Magnox reactors are cooled by carbon dioxide gas. The pile grade A (PGA) graphite moderator bricks in the reactor core loose mass and become more porous during service due to the radiolytic oxidation caused by energy deposition, mainly gamma radiation. In addition, neutron irradiation brings about strengthening by irradiation hardening and dimensional change. In this perspective, experimental data related to the attendant microstructural changes and the associated initiation and propagation of cracks within the graphite are revisited. These results are compared with the predictions of multiscale finite element modelling based upon an idealised microstructure. The discussion considers the quasi-brittle characteristics of the PGA graphite over a range of service exposure conditions.

Computer Modelling of Crack Propagation in Porous Reactor Core Graphite

This study aims to develop computer models, with a microstructure representative of the PGA graphite, to contribute to the understanding of the relationship between the amount of porosity, the load-displacement behaviour and crack propagation. The project is in two linked parts, the first provides a model of the porous graphite which is then introduced into a lattice type finite element model to provide the load-displacement and crack propagation predictions. Microstructures consisting of matrix and pores with added aligned filler particles, typical of needle coke, were studied. The purpose was to isolate the effect of filler particles on fracture strength and the fracture path. In the paper crack paths and fracture mechanisms are discussed for different amounts of porosity and various filler particle arrangements.

Fracture Process Zones in Polygranular Graphite in Bending

The formation of fracture process zones in polygranular reactor core moderator graphites subjected to four-point bending has been investigated. The three-dimensional digital image correlation technique has been combined with resistance strain gauge measurements to evaluate, both the localised and the global displacements during testing. The non-linear load-displacement characteristics prior to peak load are correlated with the localised displacements which can extend up to ~3mm (process zone) from the tensile surface of the specimen. At peak load a macro-crack propagates rapidly along an irregular path controlled by the direction of the applied tensile load and the microstructure of the graphite. These cracks arrest prior to complete separation of the specimen. Localised tensile process zones extend for distances of up to ~3mm ahead of the tips of these cracks.

Quasi-Brittle Fracture Concepts to Improve Structural Integrity Assessments of High Oxidation Weight Loss Graphite Components

Quasi-brittle fracture behaviour is characterised by the shape of the load-displacement curve and the associated mechanisms. The curves commonly exhibit non-linearity in the rising part followed by a progressive loss of load carrying capacity post peak. The behaviour of quasi-brittle materials is subject to wide variations, in the load-displacement response, both tensile and compressive, which are associated with stochastic and deterministic size effects, component geometry, load regime and load rate. The behaviour is observed in materials with brittle, heterogeneous, porous microstructures such as concretes, porous ceramics, reactor core graphites and many geological materials. Non-linearity up to the peak load is associated with the formation of distributed micro-cracks that accommodate strain and modify the ability of the material to store strain energy. In this paper, we explore how experimental observations from a range of quasi-brittle materials may help to provide improved understanding of how the microstructure can influence the fracture behaviour of reactor core moderator graphite by modifying the elastic strain energy storage capacity. This is briefly discussed with respect to the impact on the structural integrity of irradiated reactor core graphites.