Operation with reduced number of operators is considered as an important operational mode to improve economics of Nuclear Power Plant (NPP) in the future and it will be more important for the small modular reactors. To control NPPs efficiently and safely with a reduced number of operators, an intelligent operation system can be very useful. Moreover, human error will have a larger consequence than conventional operational mode and therefore there is a strong need to reduce the human error. Thus, to decrease the probability of human error occurrence, the development of autonomous operation system with certain level of intelligence is inevitable. The prediction capability of system’s behavior as well as understanding the root cause for the current situation should be an essential function of the designed autonomous operation system. The authors propose to use the current system thermal-hydraulics(STH) code platform to develop such intelligence. As a preliminary study, the authors will use the STH code to track the accident initiator of the large nuclear power plant and SMR and discuss how this method can be useful for NPP behavior prediction.