Peter E. J. Flewitt

Synthetic polygranular graphites have a wide range of current and potential applications. Indeed, some are being considered as candidate moderator materials for the next generation of nuclear power plants, which are designed to operate at temperatures as high as 1000 °C. Detailed experimental work has been performed to investigate the electrical resistivity and mechanical behaviour of a synthetic graphite over a range of temperatures. An electrothermomechanical testing rig has been employed to conduct tensile tests to evaluate elastic modulus and fracture strength over the temperature range from room temperature to 700 °C for a near-isotropic synthetic polygranular graphite. A series of mm length scale ‘dogbone’ geometry specimens, containing varying levels of porosity between 8% and 18%, were used for testing. Acquired data revealed an average of 30% gain in material stiffness, occurring at temperatures above 400 °C. Tensile strength decreased linearly with increasing porosity at higher temperatures (700 °C). The accumulated percentage reduction of resistivity during the transition from room temperature to 700 °C reduced linearly with increasing amounts of porosity. The resistivity and mechanical property measurements are discussed, with particular attention given to the porosity of the synthetic graphite.

The current generation of advanced gas-cooled nuclear reactors (AGRs) in the United Kingdom are approaching the end of their intended operating periods. Whilst awaiting the commissioning of new plant, there is a desire to extend the operational lifetime of the AGR fleet to maintain electricity supply to the UK grid. One of the factors determining lifetime extension of the AGRs is

The UK is entering a new era for civil nuclear power which involves extending the operating life of existing plant, building new plant and designing plant for future fission reactors (Gen IV). The latter two will be required to operate for periods which present challenges for the present materials and methodologies. This paper reviews these potential challenges in the context of the current understanding of materials performance and existing assessment methodologies. In addition, it provides an example of an unexpected creep-related challenge arising from long term operation of Type 316H boiler tubes.

Commercial graphites are used for a wide range of applications. For example, Gilsocarbon graphite is used within the reactor core of Advanced Gas Cooled Reactors (UK) as a moderator. In service, the mechanical properties of the graphite are changed as a result of neutron irradiation induced defects and porosity arising from radiolytic oxidation. In this paper, we discuss measurements undertaken of mechanical properties at the micro-length-scale for virgin and irradiated material. These data provide the necessary inputs to an experimentally-informed model that predicts the deformation and fracture properties of Gilsocarbon graphite at the centimetrelength-scale. The results provide an improved understanding of how the mechanical properties and fracture characteristics of this type of graphite change as a result of exposure to the service environment.