Theoretical Study On Reduction Mechanism Of Np(?)by Hydrazine Derivatives

The concept of an advanced nuclear fuel cycle has been proposed internationally in recent years in order to achieve sustainable development of nuclear energy.This concept advocates the recycling of uranium?U?and plutonium?Pu?in spent fuel,and also separates elements such as plutonium?Np?.However,the valence state of Np is variable and the separation reagents used must also ensure the yield of U and Pu,making it difficult to separate from organic phase.Hydrazine derivatives can reduce Np?VI?to Np?V?without reducing U and Pu,which will hopefully allow Np to be separated before entering U/Pu separation in order to greatly reduce the subsequent process and realize the advanced PUREX process.In this paper,the reduction reactions of[Np ^VIO₂?H₂O?₅]²⁺with three hydrazine derivatives?2-hydroxyethyl hydrazine,methyl hydrazine and formyl hydrazide?were systematically studied by using the scalar relativistic DFT calculation.The results of the study found that the reduction reactions of three hydrazine derivatives are divided into two stages.And the first stage is the same,the second stage proposes two reaction mechanisms based on the different products of the first stage,namely free radical ion mechanism and free radical mechanism.In order to compare the reduction ability of hydrogen atoms at different positions of the free radical ion RN₂H₃^+·?R=HOC₂H₄,CH₃,CHO?,We propose Pathway I and Pathway II for the free radical mechanism,and Pathway III for the free radical mechanism.At the B3LYP/ECP60MWB/6-31g*/SMD level of theory,the transition state obtained was scanned and verified by IRC.By calculating the Gibbs free energy of each complex in the reaction pathways,the potential energy surfaces were obtained.The nature of the reduction of[Np ^VIO₂?H₂O?₅]²⁺by three hydrazine derivatives was explained by means of quantum theory of atoms-in-molecules?QTAIM?,natural bond orbitals?NBOs?and electron localization function?ELF?:the redox reaction of[Np ^VIO₂?H₂O?₅]²⁺with three hydrazine derivatives is one-electron reduction reaction by the gradual transfer of hydrogen atom.According to the potential energy diagrams of the reduction of[Np ^VIO₂?H₂O?₅]²⁺with three hydrazine derivatives,it can be seen that the free radical mechanism is most likely to occur,and the trend of the reduction ability of[Np ^VIO₂?H₂O?₅]²⁺by three hydrazine derivatives is formyl hydrazine>2-hydroxyethyl hydrazine>methyl hydrazine.Due to the different sites of hydrazine derivatives in contact with O_ylatom,we have obtained two reaction pathways for the free radical ion mechanism,and the change trend of reduction ability of three hydrazine derivatives is 2-hydroxyethyl hydrazine>methyl hydrazine>formyl hydrazine,which is in good agreement with the experiment result.It is worthy to note that INT^IIIis the structure of azo derivatives.The results may provide theoretical basis for designing more efficient and promising reductants for the separation of Np/U and Np/Pu in spent fuel reprocessing.