The electronic exchange reaction between U4+ and [UO2Clx](x-2)- as elementary process for 235 U enrichment

The electronic exchange reaction: U(IV) + *U(VI) ↔ *U(IV) + U(VI) with a single stage separation coefficient of 1.0013, is utilised as elementary process for 235 U enrichment. From kinetic experimental data in solution, this reaction is second order with respect to the [U(IV)] and first order with respect to the [U(VI)]. The rate of reaction has a minus third order dependence on hydrogen ion concentration. If it is considered an exchange mechanism involving an intermediate ion: [O=U(OH)-O-U(OH)-OH]3+, the rate equation: ν = kK[U(IV)]2[UO22+] / (([H+] / Kh + 1)2[H+]2) is in agreement with the experiments. The rate constant of the electronic exchange, calculated for 29 - 150 deg C temperature range, increased with 7 orders of magnitude. The acceleration of the exchange reaction by large amounts of chloride ion and undissociated hydrogen chloride may be explained by the formation of an activated complex, which contains U(IV), U(VI), chloride ion and molecular hydrogen chloride. Both U(IV) and U(VI) would exist as chlorocomplexes in the presence of high concentrations of hydrochloric acid. The HCl concentration over 6M determined an increasing of the exchange rate with more than 2 orders of magnitude. For HCl 8M an activation energy E = 32.2 Kcal/mol was determined. In the presence of FeCl2 0.4 M in HCl 4M (as catalyst) an activation energy E = 17.6 Kcal/mol was determined and the exchange rate increased 16,000 times for a temperature range of 12.7 - 150.7 deg C. The molecular fractions of the uranylchlorocomplexes for HCl solutions of 0.1 - 10M concentrations range were calculated. The structure and distribution of the uranylchlorocomplexes are important when the following two aspects are considered. One is the effect of the chlorine bridge formed between U(IV) and U(VI) chlorocomplexes on the electronic exchange rate. The other is the effect of the dimensions of the ions and of the pores of the adsorbent (ion exchanger) on the diffusion coefficient in the adsorbent. (author)