Abstract: Uncertainty is an important indicator for the performance of monitoring devices. Analysis and calculation of uncertainty are necessary prerequisites for correct representation of seawater ¹³⁷Cs monitoring results. The main uncertainty causes of ¹³⁷Cs direct measuring with a seawater monitoring and alarming device were analyzed. Four seawater samples from Western Pacific were measured and the results were compared with the radiochemical analyses. The main causes of the device's uncertainty were the relative uncertainty for the gamma count rate of seawater sample, the gamma count rate and the volume of ¹³⁷Cs seawater standard source. The device was proved to give consistent results with radiochemical analysis within the range of uncertainty.

Key words: seawater radioactivity, ¹³⁷Cs, direct measurement, uncertainty analysis