Modular high temperature gas cooled reactor HTR-PM demonstration plant, designed by INET, Tsinghua University, is being built in Shidaowan, Shandong province, China. HTR-PM adopts pebble bed concept. Like other HTR, monitoring the core power distribution of HTR-PM is very important but challenging, as there is no in-core neutron detectors. There are some proposals to reconstruct the power distribution using the readings of ex-core neutron detectors. One method is harmonic synthesis method. In HTR, the neutron detectors are arranging symmetrically; and the power distribution of the core is also symmetrical. So in the before, only one set of detectors were used to reconstruct the power distribution, for considering the readings of different sets were almost same. In this paper, all readings of the neutron detectors were used, and new results were calculated and evaluated. It is showed that using more detector readings has no help to the accuracy of the harmonic synthesis method but improve the robustness of the method obvious. Using more detectors could improve both the robustness and the accuracy to the harmonics’ coefficients polynomial expansion method.

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