Improved Harmonics Synthesis Method and its application in reconstructing power distribution of HTR-PM

•This reconstruction method is developed based on the harmonic synthesis method.•The new method reconstructs power distribution using more proper harmonics than in the harmonic synthesis method.•Better accuracy is achieved by the improved method in the normal operation status of HTR-PM. It is difficult to arrange neutron detectors in the active zone of high temperature reactor. Therefore, the power distribution of HTR-PM (High-Temperature-Reactor Pebble-bed Module) could be monitored mainly by ex-core neutron detectors. Harmonics Synthesis Method (HSM) has been developed to monitor the power distribution of HTR-PM, and could reconstruct the power distribution with the limited neutron detectors in HTR-PM. However, it was found that the error of existing HSM might be relatively large in some conditions. In this work, the performance of HSM has been analyzed furtherly. It shows that the use of improper harmonics to reconstruct power distributions is the main reason which causes the large error. Because HSM uses fixed harmonics on all core status, the harmonics used in HSM hardly suit to all core status. Improved Harmonic Synthesis Method (IHSM) is proposed in this work to improve the accuracy of power distribution reconstruction. In IHSM, harmonics are selected based on core status to make sure the harmonics are appropriate for the reconstruction. Then, the reconstruction results of IHSM are presented and compared with the results of the original HSM. The results show that the performance of improved method is better than that of the original method on normal operation status of HTR-PM.