Study of multi-pebble oxidation process in high-temperature gas-cooled reactor

Abstract

The oxidation of the matrix graphite due to air-ingress accidents creates a risk of the exposure of TRISO fuel particles and local re-criticality for high temperature gas-cooled reactors. In this paper, a reaction kinetics model of graphite pebble oxidation is established, and then oxidation of the graphite pebbles is simulated based on computational fluid dynamics with a dynamic mesh method to study graphite shape evolution during the air-ingress process. As the flow rate increases during air-ingress accidents, the instability of the flow among the pebble bed is enhanced and changes the geometric shape of graphite significantly. Since oxidation is weak in the recirculation region of the pebble bed and strong in the impact region, the graphite pebbles tend to form a structure with a narrow front and wide tail. Most of the nonuniform oxidation factors of the graphite pebbles ranges from 2.3 to 5.1, and the nonuniformity decreases with increasing flow velocity and becomes greater with increasing temperature.