Multiphysics simulation of moisture-graphite oxidation in MHTGR

Highlights: • Moisture-graphite oxidation of nuclear graphite was investigated numerically. • Simulations indicate significant differences in the graphite's burn-off values. • Moisture-graphite oxidation mainly occurs at the bottom three to four blocks in an MHTGR core. • The oxidation alone may not noticeably threaten the graphite integrity. - Abstract: A small amount of moisture on the ppm level could present in the primary helium coolant under normal operations of High-Temperature Gas-cooled Reactors (HTGRs). One potential safety issue related to HTGRs is the chronic moisture-graphite oxidation, which could affect the integrity of the fuel blocks and reflectors in the reactor core, and the support columns in the hot plenum. However, it is infeasible to perform chronic moisture-graphite oxidation tests under the prototypic high-pressure, high-temperature conditions over a long period of time that is comparable to the full service time, i.e., 36 months for Modular High Temperature Gas-cooled Reactor (MHTGR). As an alternate, accelerated moisture-graphite oxidation tests at the atmospheric pressure and high moisture concentrations have been performed in the literature. Through these accelerated tests, global reaction rate equations have been proposed, which makes it possible to study the moisture-graphite oxidation numerically. This paper is aimed to establish a multiphysics model that can evaluate the moisture-graphite oxidation under MHTGR normal operation condition. COMSOL Multiphysics was applied to couple the modeling of the fluid flow, heat and mass transfer, chemical reaction and material structural changes. To reduce the calculation time, the prototypic three-dimensional structures were simplified into a two-dimensional simulation domain. The performance of four nuclear grades of graphite (IG-110, 2114, PCEA, and NBG-17) over a 36-month service period was then investigated under the most likely prototypic condition of MHTGR. The simulation results indicate that most oxidation occurs in the three or four bottom fuel blocks due to their higher temperatures. In general, however, only a thin layer of about 1.5 mm into the graphite will be considerably oxidized even for those three or four bottom fuel blocks.