Reason Analysis for Unsatisfactory of Accuracy and Improvement of Accuracy of On-line Boron Meter in a Nuclear Power Plant

In order to find out the reason for the unsatisfactory of measurement accuracy of the online type boron meter (BCMS) supplied by AREVA NP GmbH, the scheme to improve the BCMS measurement accuracy was studied. According to the working principle of BCMS, the factors affecting the measurement accuracy of BCMS were analyzed in terms of the neutron measurement and total boron concentration calculation, and the measurement data of BCMS is statistically analyzed; the offline type boron meter(OFBM) with measurement accuracy higher than that of BCMS was selected as the comparison object, and the comparison of the measurement accuracy between BCMS and OFBM was made under the same statistical error analysis condition; the online type boron meter (ONBM) developed by Nuclear Power Institute of China was selected as the reference, and the quantitative analysis of the effect of the measured pipe size on the BCMS neutron count rate was made. The results show that: the unsatisfactory of measurement accuracy existed  both in the manufacture factory test and in the nuclear power plant operation of BCMS, which  was caused by the extremely low value of the cumulative neutron count rate measured by its neutron measurement device within a fixed counting time; and the loss neutron by polyethylene exists between the neutron source and the neutron detector of BCMS. A modification scheme to improve the BCMS measurement accuracy was proposed, i.e., to cut the polyethylene shielding layer and replace with the higher thermal neutron sensitivity detector .