The tendency towards equalization of the 234U/238U isotopic activity ratios in fractions of U(IV) and U(VI) during thermal annealing of metamict polycrase

Abstract

The fate of the immobilized actinides of the spent nuclear fuel in mineral-like matrices can be predicted by studying the behavior of uranium isotopes as representatives of the actinides in natural analogues. This work deals with the thermochemical behavior of uranium isotopes, which is accompanied by a redox process within the 25–800 °C temperature range in the metamict natural analogue of polycrase. The pattern of the changes in the relative contents of the U(IV) and U(VI) fractions at various temperatures was determined by a radiochemical analysis. The accuracy of the U(IV) and U(VI) values using the method of the chemical shift of the energy on the Lα1 X-ray emission line was verified. The changes in the 234U/238U activity ratios of U(IV) and U(VI) fractions from room temperature to 800 °C revealed that 234U and 238U chose distinct thermochemical behaviors. It was discovered that 234U within the range from room temperature to 600 °C participated in redox processes to a greater proportion than 238U, and in the 700–800 °C temperature range, 238U participated in redox processes to a greater proportion than 234U. In the original polycrase, AR 234U(IV)/238U(IV) = 1.003 ± 0.010 and AR 234U(VI)/238U(VI) = 0.944 ± 0.005 were obtained. After heating the polycrase at 400 °C, AR 234U(IV)/238U(IV) value increased to 1.022 ± 0.010; AR 234U(VI)/238U(VI) value decreased to 0.928 ± 0.007. Currently, we do not understand where the different chemical behavior of the two isotopes of an element comes from. It is hardly surprising that at 800 °C, the values of AR (234U/238U) in both valence forms of U(IV) and U(VI) equalize and acquire the total initial value AR (Σ) = 0.963 ± 0.004. It is remains unclear to us at the moment what unknown laws and mechanisms these isotopes follow. The above uncertainties were not addressed in this work; it is hoped that mysteries of nature will be more explained in future research.