Abstract
The graphite bricks of the UK carbon dioxide gas cooled nuclear reactors are subjected to neutron irradiation and radiolytic oxidation during operation which will affect thermal and mechanical material properties and may lead to structural failure. In this paper, an empirical equation is obtained and used to represent the reduction in the thermal conductivity as a result of temperature and neutron dose. A 2D finite element thermal analysis was carried out using Abaqus to obtain temperature distribution across the graphite brick. Although thermal conductivity could be reduced by up to 75% under certain conditions of dose and temperature, analysis has shown that it has no significant effect on the temperature distribution. It was found that the temperature distribution within the graphite brick is non-radial, different from the steady state temperature distribution used in the previous studies [1,2]. To investigate the significance of this non-radial temperature distribution on the failure of graphite bricks, a subsequent mechanical analysis was also carried out with the nodal temperature information obtained from the thermal analysis. To predict the formation of cracks within the brick and the subsequent propagation, a linear traction–separation cohesive model in conjunction with the extended finite element method (XFEM) is used. Compared to the analysis with steady state radial temperature distribution, the crack initiation time for the model with non-radial temperature distribution is delayed by almost one year in service, and the maximum crack length is also shorter by around 20%.
Highlights
Nowadays, in the UK, the gas-cooled graphite moderated type dominates over 90 % of the nuclear reactor [3]
Without using an atomistic physical approach for calculating of the variation of thermal conductivity for the graphite such as the one proposed in [16], the thermal conductivity dependence on dose and temperature can be deducted from experimental data curves or include empirical terms obtained from the experiments using Material Test Reactor (MTR)
The empirical formula of varying thermal conductivity of common irradiated graphite form, Gilsocarbon, due to the change in temperature and dose has been developed. This formula is developed by using the experimental thermal conductivity of irradiated graphite from the literature
Summary
In the UK, the gas-cooled graphite moderated type dominates over 90 % of the nuclear reactor [3]. Graphite is exposed to neutron radiation and temperature gradients that could cause irradiation and radiolytic oxidation These occurrences will affect material properties such as weight loss, porosity changes and thermal conductivity which can potentially lead to material and structural failure [5]. For significant nuclear irradiated graphite grades e.g. Gilsocarbon, Kelly [11] proposed a formula as shown in Eq (2), to calculate the thermal conductivity It includes the terms, f and Sk, obtained empirically as a function of dose and temperature [12]. Without using an atomistic physical approach for calculating of the variation of thermal conductivity for the graphite such as the one proposed in [16], the thermal conductivity dependence on dose and temperature can be deducted from experimental data curves or include empirical terms obtained from the experiments using Material Test Reactor (MTR). To illustrate the accuracy of the calculated thermal resistivity using Eq (5), the values are plotted against experimental results as shown in Figure 2 at 100 ̊C, 300 ̊C and 600 ̊C
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