Future space missions and deep-sea explorations will require small/micro nuclear reactors (kWe~MWe) for power generation. Compared with conventional energy systems such as storage batteries and fossil energy, nuclear reactors are featured higher energy intensity, higher reliability, and longer lifetime. According to the coolant, the candidate small/micro nuclear reactors include the heat pipe cooled reactor, liquid metal cooled reactor, and gas-cooled reactor, most of which are still in the conceptual design stage with numerical studies and experimental research. These emerging reactors have an entirely different core structure and working principle from the existing light water reactors, which has led to an increasing need for updated simulation methods and experimental studies.