ИСТИНА

This report focuses on the thermodynamic modeling of the combined effect of impurities of oxygen and carbon on the phase composition of the mixed nitride of uranium and plutonium at a temperature of 600 K, the burn up of 80 GW•d/t, as well as assessing the effects of the β--decay of the radio nuclides 89Sr, 90Sr, 99Mo, 140Ba on the kinetics of changes in the composition of the formed inclusions of the oxide and carbide phases of the spent fuel. At any method of storage and processing of SNF it is necessary to know its chemical and phase composition. To this end, the model calculations of changes in the chemical and phase composition of uranium-plutonium mononitride(U0.8Pu0.2)(N0.9475O0.02625C0.02625) containing 0.125 wt% C and 0.166 wt% O used the data of [1,2] for U0.8Pu0.2N. The calculation was performed using the ASTRA-4 software [3]. The algorithm of the ASTRA-4 program complex is based on a universal thermodynamic method for determining the equilibrium characteristics of arbitrary heterogeneous systems based on the principle of maximum entropy. The computational algorithm is limited by the following assumptions of the mathematical model: 1) the system is in a state of equilibrium; 2) the system is closed (i.e. without the exchange of matter with the environment); 3) surface effects at the phase boundaries are not taken into account; 4) the solubility of gases in condensed phases is absent; 5) at the presence of the gas phase is necessary; 6) the condensed matter form a one-component immiscible phase or are part of the ideal or regular solutions. The database of the program complex ASTRA-4 was supplemented by the thermodynamic characteristics of condensed matter: BaUO3, SrPuO3 , URu3C0.7, etc. Thermodynamic modeling has shown that the accumulation of fission products under irradiation of mixed nitride of uranium and plutonium with admixture of 0.125 wt% C and 0.166 wt% O leads to the formation of a multicomponent oxycarbonitride solid solution containing U, Pu, Am, Np, Zr, Y and lanthanides, as well as separate oxide BaUO3, SrPuO3, Nd2O3, carbide URu3C0.7, Mo2C, nitride U2N3 phases, and intermetallic compounds U(Rh, Pd)3. β− decay of metal radio nuclides in separate carbide and oxide phases of SNF leads to changes in their chemical compositions. For these compounds, the following chains of β-decay of the most important fission products in the composition of individual inclusions in spent nuclear fuel with a half-life of more than a day are important [4, 5]: 89Sr→(52 d) 89Y(stable); 90Sr →(28.5 y)90Y→(64 h) 90Zr(stable), 140Ba →(12.8 d))140La→(40 h)140Ce(stable), 99Mo→(65.9 h)99Tc →(2.1*105 y)99Ru(stable). The transformation of radio nuclides in the individual phases considered. Calculation of the kinetics of transformation of compounds of radio nuclides was performed according to the system ofequations β--decay to burn up 80 GW•d/t. the β− decay of metallic radio nuclides in a separate oxide and carbide phases in the spent fuel leads to changes in their chemical compositions. The calculated kinetics of the transformation of the individual phases of carbides and oxides: 99Mo2C→99Tc2C→299Ru+C, U103Ru3C0.7→ U103Rh3C0.7 , 89SrPuO3→ 89Y2Pu2O7.5, 2140BaUO3 → 140La2UO6 +U → 140Ce2UO6+U . It is shown that 99Mo2C is conversed to 99Tc2C in 470 hours. The initially formed 99Tc2C is carbon over-saturated [6]. The specific cell volume measured in the experiment [6] (15.75 °A3) was in excellent agreement with the predicted [7] specific cell volume of Tc6C, and it does not further increase with C content. So, Тс2С, initially formed through beta-decay of Mo in Mo2C, should release some excessive carbon and form stable cubic Tc6C [6,7]. The released C can react repeatedly with other FP metals with new carbide phase formation. U103Ru3C0.7 – into phase U103Rh3C0.7, and 140BaUO3 in 140Ce2UO6 for 90 days. It should be noted that the kinetics of β−-decay of metals in oxide and carbide phases is the same. However, in the carbide and oxide phases, it is possible to change not only the chemical but also the phase composition. Changes in the chemical composition of the phases and fuel structure over time lead to changes in the molar volume of the phases, stresses, and changes in the electrochemical properties of SNF [8]. Thus, the results indicate the need to take into account these transformations in the storage and processing of spent nuclear fuel. [1] Lyubimov D. Yu., Androsov, A. V., Bulatov G. S., Gedgovd K. N. // At. Energy. 2013. T. 114, vol. 4. p. 198-202. [2] Lyubimov D. Yu., Deryabin A. I., Bulatov G. S., Gedgovd K. N. // At. Energy. 2015. T. 118, vol. 1. p. 24-29. [3] Sinyarev G. B., Vatolin N. A., Trusov B. G., Moiseev G. K. The use of computers for thermodynamic calculations of metallurgical processes. M.: Science, 1982. 264 p. [4] Nikiforov A. S., Kulichenko V. V., Zhikharev M. I. Disposal of liquid radioactive waste. M.: Energoatomizdat, 1985. 184 p. [5] Gusev N., Dmitriev P. Radioactive chain: Handbook. M.: Energoatomizdat, 1988. 2-ed. 112 p. [6] German K.E., Peretrukhin V.F., Gedgovd K.N., Grigoriev M.S., Bulatov G.S. et. all. // Journal of Nuclear and Radiochemical Sciences, Vol. 6, No.3, pp. 211-214, 2005. [7]Wang Q., German K.E., Artem R. Oganov et all. / Explaining stability of transition metal carbides – and why TcC does not exist // RSC Advances, 2016, Iss. 6, pp. 16197-16202, http://dx.doi.org/10.1039/C5RA24656C. [8] Bulatov G. S., Gedgovd K. N., Maslennikov A.G., Lyubimov D.Yu. // Radiochemistry, 2017, V. 59, No.1, pp. 54–57.