Modeling of the aqueous phase uranium analysis method of the nuclear fuel reprocessing process by Time Resolution Laser Spectro-fluorimetry

This document presents the development of a chemical analysis technique allowing the measurement of low concentrations of uranium VI in the nuclear fuel reprocessing process, using a time-resolved laser spectro-fluorimetry device. Although such a technique leads to quite exceptional measurement sensitivities (∼10^-8 M), the physical process involved is particularly affected by the nitric medium, the concentration of which plays an essential role on the optical density of solutions and on the complexation of the uranyl ion. A method is proposed for correcting the effects of prefilters for optical densities extending from 0 to 4 at the emission wavelength of the laser source. Analytically, this correction increases the uncertainty on the measurement by 5 pc. The concentration of nitrate ions results in a deformation of the fluorescence spectrum and an increase in its intensity. The first effect has led to model the spectroscopy of uranyl species, and by extension, to determine with great precision the concentration of free nitrate ions in solution. Analytically this value allows to correct for variations in fluorescence yield to within 5 pc in the nitric concentration range extending from 0 to 5 M. Applied to real solutions of the reprocessing process, the technique turns out to be very competitive compared to the usual technique of liquid chromatography. It is particularly attractive in the case of analysis of traces of uranium in a medium heavily loaded with plutonium (ratio 1/50000). (author)