Experimental and Numerical Investigation of Graphite-Steam Oxidation for HTGRs

Abstract

Nuclear graphite is proposed for use in High-temperature Gas-cooled Reactor (HTGR) designs as the neutron moderator, reflector, and core structural material. During the normal operation of an HTGR, a small amount of moisture can exist in the primary helium circuit even with a helium purification system included to remove any impurities present in the primary coolant. In addition, a large amount of moisture can quickly enter the primary side during a steam ingress accident for those HTGRs that feature a steam Rankine cycle as the power conversion unit. The moisture can react with nuclear graphite in high-temperature environments, which degrades mechanical strength of graphite. Therefore, it is necessary to investigate the graphite-steam oxidation phenomena in detail to facilitate future HTGR licensing and deployment. In this research, the oxidation behavior of nuclear graphite IG-110 by steam was investigated under various temperature, moisture concentration, and hydrogen partial pressure conditions. A graphite-steam oxidation test facility was constructed to obtain high-resolution experimental data. The reaction environment was jointly controlled by a tube furnace, a peristaltic pump, and gas mass controllers. The concentrations of production gases CO and CO2 were measured online by a gas chromatography, which were then used to derive the oxidation rates. A total of 141qualified data points of the kinetic oxidation rates were collected at temperatures 850 to 1100 °C with steam partial pressure up to 20 kPa and hydrogen partial pressure varied from 0 to 3 kPa. Boltzmann-enhanced Langmuir-Hinshelwood (BLH) reaction rate equation was obtained through multivariable optimization. The overall mean relative difference between the predicted oxidation rate and the experimental data is 24%, with the maximum difference being 55%. In addition, experiments were performed to investigate the effect of mass loss on graphite oxidation rate. It was believed the graphite-moisture reaction expands the existing micro pores in graphite and opens those originally isolated pores, both resulting in an increase of active surface area. In the experiment, the graphite mass loss fraction was found to have a more prominent effect on increasing the oxidation rate at lower temperatures. Furthermore, a multiphysics model was developed for graphite-steam oxidation. The numerical model couples all important physical processes, including the kinetic chemical reaction, multi-species transport, free and porous flow, heat transfer, and microporous structure evolution. The multiphysics model was validated against our experimental data. Our comparisons show that the numerical model can well simulate the apparent oxidation rate and accurately predict the post-oxidation density distribution. The validated model was then applied to the prototypic MHTGR design for normal operating conditions. The chronic graphite-moisture oxidation during a full MHTGR service period of 36-months was simulated. The simulation indicates that at the end of the 36-month operation, the maximum local graphite mass loss can reach to about 85%. However, the oxidation is well confined within a thin layer of about 0.5 mm thickness into the graphite surface. Therefore, chronic graphite-moisture oxidation will not significantly decrease the mechanical strength of graphite, nor jeopardize the integrity of graphite fuel blocks in MHTGR during its normal operation.