Study on the Effective Diffusion Coefficient of Inert Gases in Porous Graphite Materials

Abstract The high-temperature gas-cooled reactor (HTGR), is globally recognized as one of the reactor types embodying the safety features of the fourth generation, with the advantages of safety, high thermal efficiency in power generation, a small emergency planning zone, and the ability to use high-temperature helium for hydrogen production. HTGRs utilize a significant amount of graphite materials, including fuel graphite matrices, graphite moderator material, and graphite reflector layers and so on. Fuel containing fissile materials produce inert gas fission products such as Kr and Xe, while isotopes like Ar-40 in the pores of graphite materials can become activated, forming inert gases such as Ar-41. The fuel element matrix graphite is the necessary pathway for these inert gas nuclides to enter the primary coolant circuit. However, current research rarely provides a comprehensive analysis of the diffusion behavior of inert gases in porous graphite materials. This study establishes a computational model and methodology for determining the effective diffusion coefficients of inert gas nuclides in porous material graphite. It quantitatively calculated the cumulative release fractions and release ratios of Ar, Kr, Xe, in both open and closed pores of the fuel element matrix graphite and elucidated the impact and patterns of factors such as pressure, temperature, and half-life on the diffusion behavior of inert gases in porous material graphite. This research provides valuable insights for future studies on the diffusion behavior of Ar, Kr, and Xe in graphite.