Configuration of nuclear detecting system for gas-cooled micro reactor and its application in core monitoring

<bold>Background</bold>The gas-cooled micro reactor is one of advanced reactor types that can be used as movable intelligent micro nuclear power sources. The nuclear detecting has been a main challenge for its deployment due to the compact layout and the harsh service environment inside the core, hence only a small number of ex-core detectors can be deployed for core monitoring.<bold>Purpose</bold>This study aims to explore the design method of the nuclear detecting system for gas-cooled micro reactor and apply it to core monitoring.<bold>Methods</bold>Firstly, a three-dimensional integrated reactor model which incorporated the core, ex-core structures, and detectors was built using reactor Monte Carlo code (RMC). By referring to relevant standards, an arrangement method for the nuclear detecting system of micro reactor was developed. Then, design methodologies were proposed for neutron count rate, detector sensitivity, measured neutron component, and detector range. The feasibility of startup without an external neutron source for the micro reactor was analyzed, along with the layout scheme of the system with external source. Finally, the validity of these approaches was then verified through computational simulations of the first criticality experiment and the ex-core detector calibration experiment.<bold>Results</bold>The results show that it is feasible to startup without external neutron sources if ³He detectors with thermal neutron sensitivities of no less than 290 cm² and 980 cm² are used as in-core temporary detector and ex-core source range detector. The requirements for scenarios with long-time operation and few personnel on duty can be met by the detecting system including an Am-Be neutron source with strength as low as possible, ex-core boron coated neutron counting tubes with low sensitivity, ex-core gamma compensation ionization chambers with wide range. The Am-Be neutron source can only be arranged at the front of active core to ensure that the proportion of fission neutrons meets the requirement. By installing cadmium and polyethylene sleeve structures outside ex-core detectors to monitor high-energy neutrons, more accurate nuclear measurement signals can be obtained. Simulation results of the first critical experiment indicate that the number of extrapolated critical fuel assembly columns is consistent with the theoretical value, and the deviation of extrapolated critical position for a single control rod is only -2 cm, and the deviation of <italic>k</italic>_eff is within 6×10^-4. Simulation of ex-core detector calibration experiment show that the absolute deviation of the power level and the axial power offset are within 0.2% and 0.4%.<bold>Conclusions</bold>The configuration of nuclear detecting system is reasonable and feasible. Simulation results of this study can provide guidance for core monitoring of various advanced micro reactors, and promote the development of movable intelligent micro nuclear power source products.