| With the development of nuclear energy, more attention has been given to the reprocessing of spent nuclear fuel. As the only commercially matured process of spent nuclear fuel reprocessing, Purex process will remain predominant in the foreseeable future. In Purex process, the separation of uranium/plutonium is achieved based on quick and quantitative reduction of Pu(IV) to Pu(IH). Reductants, therefore, play an important role in the process. To reduce Pu(IV) ions, various chemical reagents can be utilized, such as hydroxylamine, U(IV) and Fe(II) nitrates mixed with hydrazine, etc. But there are several disadvantages to use these reductants in Purex process, such as formation of iron(III) nitrates and ammonium that results in a higher salt content in high level liquid waste, instability of U(TV) in organic phase, potential explosion due to formation of azides as well as slow reduction rate of Pu(TV) with hydroxylamine. Therefore, continued efforts have been paid to explore the new highly efficient saltfree reductants for Pu(TV). Among which, derivatives of hydroxylamine are capable of quick reducing Pu(FV) into Pu(IH).N, N-Dimethylhydroxylamine(DMHAN) is a type of derivative of hydroxylamine whose two H atoms are both substituted by methyl groups. Preliminary experimental results show that DMHAN not only reduces Pu (IV) ion in a very rapid kinetic rate, but also stabilizes Pu(III) in acidic solution. Thus, It is expected that the Purex process would be improved greatly through using DMHAN as a reductant.In order to study the kinetics of the reaction of Pu(IV) by DMHAN thus providing basis for the design an improved process of spent nuclear fuel reprocessing, a systematic investigation has been performed as described below:(1) N,N-dimethylhydroxylamine was prepared through pyrolysis of tert-amine oxide. With improvement of the oxidant and reaction steps, the aimed compound was obtained in high yield and high purity.(2) The kinetics and mechanism of redox reaction between DMHAN and vanadium(V)in nitric acid medium have been studied by spectrophotometry at 25.2癈. By investigating the influence of the concentration of DMHAN and acidity on the reaction rate, the rate equation was derived as -d[V(V)]/dt=V(V)] [DMHAN] [H+] and the rate constant, k. was found to be 119? (mol/L^-min'1. Meanwhile, according to the spectra of ESR of intermediate in reaction, following reactions could be regarded as the rate controlling step:A possible mechanism was suggested on the basis of the path of single-electron transfer.(3) The kinetics of plutonium(IV) reduction by DMHAN has been studied in aqueous nitric acid media spectrophotometrically. The empirical initial rate expression for the reaction was found to be -d[Pu(IV)]/dt = Ao[Pu(IV)] [DMHAN]1 18/[H+]2 22 and the completerate equation was given out as 8.12[Pu(III)] + 95.9[Pu(IV)] ^and 14.5, the rate constants were determined to be O.Stl.TOnol/L)104^'1 and A^103(mol/L)' ^-s"1, The effect of some factors such as concentration of Fe3+, UC>22+, ion strength and temperature on the reaction was also investigated and a reaction mechanism was discussed as well.(4) The reaction between DMHAN and nitrous acid was investigated in the media of HC1O4 and HNO3 respectively. It was found that in the media of HC1O4, DMHAN reacted with nitrous acid obeying the following rate equation:-d[HNO2]/dt(mol/L-min'1H12.80)[HNO2][DMHAN], here T=18., u=2.0mol/L. In the solution of nitric acid, the reaction of DHMAN with HNOj could induce the oxidation of DMHAN by HNOs. By gas-chromatogram methanol was found to be produced in the reaction.(5) In order to predict the distribution of U, Pu and nitric acid in the Purex process, a computer program, MDCEX1.0, for simulating the distribution of the solutes was developed. The mathematical model is based on quisa-Newton arithmetic. The distribution profiles of 1A were calculated respectively and the calculated results were in good agreement with the experimental results. A conclusion could be drawn that t...
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