Coupled neutronics and thermal-hydraulics transient simulation of a gas-cooled reactor in the aircraft nuclear propulsion system

Aircraft nuclear propulsion (ANP) has many historical explorations, but also features with high power density and high operating temperature. These challenges lead to numerical investigations for the transient behaviors of gas-cooled reactors to be used for ANP system. This prismatic-block type reactor is featured by a large number of internal discrete cooling holes with high aspect ratio. When using conjugate heat transfer (CHT) model in CFD analyses for the entire solid core of such type of reactors, the refined mesh resolution near the interface between the fuel matrix and the flow channels leads to high computational costs. Therefore, we proposed a new porous heat transfer (PHT) model with Newton's law of cooling to predict three-dimensional distributions of temper-ature in the solid and fluid domain of the reactor core. The verification of the PHT model is conducted in the presented study firstly, and shows that the accuracy of this model is deeply influenced by the prediction of convective heat transfer coefficients. Sequentially, a neutronics (N) and thermal-hydraulics (TH) transient coupling code is developed in the open-source platform OpenFOAM. The point kinetic model is employed to evaluate the dependency of heat power on the time. Doppler broadening effect is considered in the coupling process by delivering fuel average temperatures. Then coupled N/TH simulations are carried out to analyze the gas-cooled reactor during the start-up process and the blockage accident. The obtained results show that the wall temperature is always lower than the design limit even in a rapid start-up process, indicating that the reactor is safe at extreme operations. Moreover, simulation of the blockage accident shows that if several flow channels are totally occupied by debris, the fuel temperature increment is less than 300 K, and the position of the peak fuel temperature transforms from the core center to the zone near the blockage.