ИСТИНА

The nuclear fuel reprocessing is based on PUREX process. Usually 30% TBP in hydrocarbon diluents is applied. Then the composition of the organic phase can be expressed as H2O-HNO3-UO2(NO3)2-Pu(NO3)4-TBP-diluent. But the plutonium concentration is low and thus the system can be considered as a five-component one. This system has been considered early [1]. Modeling of the system includes few points: 1. The application of mole fractions and volume ones and rational activity coefficients, 2. The calculation of concentration of free (unbounded with solvates) water. 3. The methods of activity coefficient calculation. 4. The methods to estimate errors during the calculation of various coefficients. Gibbs-Duhem equation is used to calculate dependence of component “i” activity on concentration of other components xidlnai +∑ xjdlnaj = 0 (1) xidlnfi +∑ xjdlnfj = 0 (2) The dodecane is commonly used as a diluent. The main system is usually divided into sub-systems and every sub-system is calculated through thermodynamic activities. The model of H2O-HNO3-TBP-dodecane was given in [2]. Activities of nitric acid and uranyl nitrate were calculated in [3-5]. In order to calculate the system H2O-HNO3-UO2(NO3)2-TBP-dodecane it is necessary to determine a formation constant of di-solvate UO2(NO3)2·2TBP and its error. 1. Ochkin, A., Gladilov, D., Nekhaevskiy, S. Procedia Chemistry 7, 315-321 (2012). 2. Ochkin, A., Gladilov, D., Nekhaevskiy, S.Theoretical Foundations of Chemical Engineering, 2015, Vol. 49, No. 5, pp. 649–655. 3. Davis W., Lawson P.S., de Bruin H.J., Mrochek J. J. Physical. Chem. 1965. V. 69. P.1904. 4. Yu Yang-Xin, Zhang Qing-Yin, Gao Guang-Hua. J. Radioanal. Nuclear Chem. 2001. V.245. P. 581. 5. Ochkin A., Merkushkin A., Nekhaevskiy S., Tiupina E. Radiochemistry, 2016, Vol. 58, No. 3, pp. 280–286.