Large-Scale Modeling of Damage and Failure of Nuclear Graphite Moderated Reactor

Abstract

The UK Advanced Gas-Cooled reactors (AGRs) have cores made of graphite bricks with dual functions: as structural elements of the core, providing space for and separating fuel and control rods; and as moderator of the nuclear reaction. Nuclear graphite is a quasi-brittle material, where the dominant mechanism for failure is cracking. While cracking of isolated bricks is expected due to operation-induced changes in graphite microstructure and stress fields, these could be tolerated as far as the overall structural function of the core is maintained. Assessment of the whole core behaviour has been previously done with whole scale models where bricks have been considered as rigid body elements connected by elastic-brittle springs. This approach does not allow for the realistic assessment of the stresses in the bricks and associated brick cracking. Reported here are results from an ongoing project, which addresses this shortcoming. The proposed model uses deformable bricks with appropriate interactions, allowing for physically realistic whole core analysis. The results are focused on the damage that a graphite moderated reactor develops during a life cycle, how this affects the behaviour of the whole core, and how changes in bricks' behaviour impacts the core integrity. The proposed methodology is a major step towards high-fidelity assessment of AGRs' fitness for service, required for supporting continuous safe operation and life-extension decisions.

A Computational Framework for Crack Propagation Along Contact Interfaces and Surfaces Under Load

“…Modelling of brittle crack propagation becomes more complicated in the presence of contact interaction. One such example, considered in the present work, is the crack propagation in irradiated graphite bricks in a nuclear reactor [34]. In this application, one of the main challenges is the resolution of the complex crack morphology in 3D bodies subjected to internal stresses arising from irradiation and material properties' heterogeneity.…”