Abstract
The graphite oxidation of fuel element has obtained high attention in air ingress accident analysis of high temperature gas-cooled reactor (HTR). The shape function, defined as the relationship between the maximum and the average of the oxidation, is an important factor to estimate the consequence of the accident. There are no detailed studies on the shape function currently except two experiments several decades ago. With the development of computer technology, CFD method is used in the numerical experiment about graphite oxidation in pebble bed of HTR in this paper. Structured packed beds are used in the calculation instead of random packed beds. The result shows the nonuniform distribution of oxidation on the sphere surface and the shape function in the condition of air ingress accident. Furthermore, the sensitive factors of shape function, such as temperature and Re number, are discussed in detail and the relationship between the shape function and sensitive factors is explained. According to the results in this paper, the shape function ranges from 1.05 to 4.7 under the condition of temperature varying from 600°C to 1200°C and Re varying from 16 to 1600.
Highlights
Pebble-bed reactor is a typical design of high temperature gas-cooled reactor (HTR), including HTR-MODULE, PBMR, and HTR-PM
Spherical fuels are used in these HTRs and were packed in the core randomly
Porous model is built in these codes to simplify the complex phenomenon in the pebble bed and shows a good agreement with the experiments in terms of flow and heat transfer
Summary
Pebble-bed reactor is a typical design of high temperature gas-cooled reactor (HTR), including HTR-MODULE, PBMR, and HTR-PM. In case of HTR-PM, there are 420,000 fuel elements with a diameter of 6 cm in the equilibrium core. During the air ingress accident, which is a specific beyond design basis accident for HTRs, air will enter the reactor core and react with the graphite of fuel elements. Porous model is built in these codes to simplify the complex phenomenon in the pebble bed and shows a good agreement with the experiments in terms of flow and heat transfer. The oxidation result in porous model represents an average value in a mesh or on one fuel pellet. The so-called shape function is defined as the relation between the maximum and the average burn-off of a pebble. The shape function measured in this experiment is about 9 under lower flow rate [2]. There is no further research for the nonuniform oxidation after that
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