MODELLING AND SIMULATION OF NUCLEAR GRAPHITE USING XFEM

Abstract

Graphite plays a major role in nuclear industries (as moderator, reflector and structural material) and in automobiles (as bearings and other self-lubrication components). Extreme - heating and irradiation in the nuclear reactor leads to change in the properties such as thermal conductivity, elastic modulus, strength, microstructure, which in turn leads to the development of internal stresses in the graphite structure. The main challenge in the study of graphite is that the pattern of the crack induced by the internal stresses is random and the failure is catastrophic. Therefore, there is a need to study the mechanical and thermal properties of nuclear graphite for the integrity and safety of the nuclear reactor. In the present work, nuclear graphite has been modelled as a composite material of harder and stronger inclusions embedded in soft tar like material having a significant porosity. A Representative Volume Approach (RVE) is adopted in which microstructural features of Nuclear Graphite are modelled explicitly at lower length scale and the response so obtained is homogenized to predict the macroscopic behaviour. Pores and inclusions will be randomly modelled within the RVE. This RVE would be analysed using XFEM to obtain the equivalent properties of the graphite. For evaluating the strength, the whole test specimen will be modelled using XFEM. The failure portion of the specimen would be modelled with full heterogeneity, reflecting the actual microstructure obtained from SEM/TEM/Tomography data. The rest of the specimen will be modelled as homogeneous material with equivalent properties obtained using RVE. With help of Weibull and Normal distributions, the mean value of failure strength will be calculated. The numerical values obtained by this approach are found in good agreement with the experimental results.