A concept of micro nuclear reactor with supercritical $CO_2$ (S-$CO_2$) Brayton cycle namely KAIST Micro Modular Reactor (MMR) with 12MWe power output is being developed to serve as a distributed power source for remote region. In order to successfully develop MMR, issues of bearing capacity for shaft levitation have to be resolved. In order to support a shaft for 12MWe system the magnetic bearing is suggested to be suitable instead of gas foil bearing which has lower levitation force. However, several previous studies on the magnetic bearing showed that instability issues exist under high speed operating with high pressure fluid conditions. Therefore, controlling magnetic bearing instability becomes an important issue for the realization of MMR
The purposes of this research is to analyze instability of the magnetic bearing under S-$CO_2$ condition and propose functional requirements of the magnetic bearing. The shaft with magnetic bearings and operating under high pressure fluid is analyzed. In addition, the analysis results can be used for predicting the instability. Based on the results, a magnetic bearing test rig is constructed. By comparing experimental data to the analysis results, the analysis model is verified.