Material model for stress analysis of nuclear graphite component subject to irradiation

Abstract

Nuclear graphite is used as a solid moderator in many fission nuclear reactors such as gas-cooled reactors and molten salt reactors. The role of graphite is to slow down neutrons emitted by fission, thus enhance their probabilities of interacting to produce further fission reactions. Graphite also acts as a major structural component in nuclear reactors, providing channels for the coolant and control rods. Graphite undergoes significant changes in dimensions and material properties due to the effects of irradiation damages. These irradiation-induced changes can lead to build up of significant stresses and deformation in the graphite components. Throughout reactor life it is essential that the graphite core structure remains sufficiently strong and undistorted in order to maintain fuel cooling, permit loading and unloading of the fuel and allow the necessary movements of control rods, in both normal and fault conditions. To perform structural integrity assessments, the definition of the constitutive equation relating stress and strain in the irradiated graphite material is required. Apart from the elastic and thermal strain, graphite in nuclear reactors also experiences additional strains due to fast neutron irradiation. Irradiation creep and irradiation induced dimensional changes are two most important of these irradiation strains. Nuclear graphite component structural integrity assessments are usually carried out using the finite element method. To model the dimensional change, irradiation creep and material properties a complex material model is required. In this paper a simple mathematical material model for isotropic graphite under irradiation condition is derived here. The model largely relies on empirical graphite property data obtained from material test reactor experiments. The model can be used to assess the nuclear graphite component structural integrity throughout the life of reactors, taking account of increasing irradiation fast neutron fluence and operating temperature. Also the model is a useful tool for designing a suitable graphite component for a nuclear reactor.