[en] The technique of low temperature neutron irradiation combined with isotopic exchange separation technique has been applied in the determination of mercury in aqueous samples. The kinetics of the isotopic exchange reaction has been studied for various sample volumes. The effect of the flux perturbation caused by aqueous samples has been investigated for samples of various size and geometry in a central position in a well moderated heavy water reactor. The effect has been studied both theoretically and experimentally. The 'Thermos' code has been used in the calculations