Study On The Stability Of Uranium(Ⅳ) In Nitric Acid, Perchloric Acid And Organic Phase

With the development of nuclear energy, the reprocessing of spent nuclear fuel has aroused more and wide attention. As the unique commercial mature process of spent nuclear fuel reprocessing, the Purex process will remain predominant in the foreseeable future. In the Purex process, the separation of plutonium from uranium is achieved by using reductants which can quickly and quantitatively reduce Pu(IV) to Pu(III).Up to now the reductant U(IV)-N2H4 is still used widely for separation of Pu from U in the reprocessing plants of spent nuclear fuel. Some salt-free organic reductants such as derivatives of aldehyde, hydrazine, hydroxylamine have not been used in the reprocessing plants of spent nuclear fuel yet. But U(IV) is not stable in nitric acid and 30% TBP-kerosene solution. The oxidation of U(IV) has been studied at home and abroad. But some of results are discrepancy. In our research work the stability of U(IV) in nitric acid , perchloric acid and 30% TBP-kerosene solution is studied respectively. The effects of some parameters such as concentrations of U(IV), acid, nitrous acid, hydrazine, and temperature, illumination on the stability of U(IV) are examined. The experimental results are as follows:1. The stability of U(IV) in perchloric acid is studied.(l)The reaction rate equation of oxidation of U(IV) by nitrous acid is expressed as below:-d[U(IV)]/dt=k[U(IV)][HN02]0.4[HC104]0.8 The rate constant (k) equals 0. 113 + 0. 015(mol/L)0.4h-1 at 22C.(2)The reaction rate equation of oxidation of U(IV) without nitrous acid is described as below:-d[U(IV)]/dt=k[U(IV)]/[HCl04]0.84 The rate constant (k) equals (1. 70 0. 03) 10-2(mol/L)0.84d-1 at 30C .The stabilityof U(IV) decreases obviously with rising temperature. The apparent activation energy of this reaction is found to be 22. 57 4. 75kJ/mol.2. The stability of U(IV) in nitric acid is studied, the reaction rate equation of oxidation of U(IV) is described as follows:-d[U(IV)]/dt=k[U(IV)]/[HN03]1.5[N2H4]0.5([HN03]<4mol/L, [N2H4] 0. lmol/L)The rate constant (k) equals (1. 20 + 0. 03) X 10"2(mol/L)2 d ' at 30C. The stability of U(IV) decreases with rising temperature. The apparent activation energy of this reaction is found to be 16. 74 4. 09kJ/mol.3. The kinetics of the reaction between U(IV) and Tc(VII) in 30% TBP-kerosene solution is studied, the reaction rate equation may be expressed as below:-d[U(IV)]/dt=k[u(IV)][Tc(VII)]1.5The rate constant (k) equals (8. 11 0. 40) 103(mol/L)-1.5min-1 at 25 C .The oxidation rate of U(IV) by Tc(VII) in 30% TBP-kerosene solution is very quick. The reaction rate constant decreases obviously with decreasing temperature.Comparing the influence of Tc(VII), nitrous acid, and other factors on the stability of U(IV), the influence of Tc(VII) on the stability of U(IV) is the strongest.