THERMODYNAMICS OF HIGH-TEMPERATURE

A method for performing a thermodynamic analysis of the high-temperature nuclear fuel using the ASTA computer program is substantiated. Calculations of the chemical composition and pressure of the gas phase of the ternary systems U‐O‐C and Pu‐O‐C are performed. The results obtained are compared with existing experimental data and theoretical studies performed by other authors. The results show that the entropy factor apparently plays an appreciable role in the thermodynamics of the systems studied. A comparative analysis of micropellets with uranium and plutonium fuel is performed. An estimate of the diffusion kinetics of the chemical interaction in micropellets is given as substantiation of the application of the methods of equilibrium thermodynamics for calculating the chemical and phase composition of the nuclear fuel. A promising direction for nuclear power is the development of high-temperature nuclear reactors with gas or fused-salt coolant and fuel based on micropellets. Micropellets [1‐5] are fuel kernels several hundreds of microns in diameter, coated with a multilayer cladding. Their main feature is the possibility of providing high fuel burnup up to 80% at working temperatures up to 1873 K. The actual range of operability of micropellets depends on many factors ‐ the structure and geometric parameters of micropellets, the chemical composition of the kernel, the initial free volume in the micropellet, and other factors. Thermodynamic calculations make it possible to estimate the composition of condensed and gaseous components in micropellets and the internal pressure and to predict the possible mechanisms of chemical interaction of various components resulting in the destruction of the micropellet cladding and rupture. In turn, this makes it possible to give a predictive assessment of the range of operability of micropellets and to optimize the construction. In the USA the thermodynamic calculations are performed using the SOLGASMIX-PV computer program [6]. In the present work the computational investigations were performed using the ASTRA program [7], which was developed for calculating the equilibrium parameters of multicomponent heterogeneous thermodynamic systems. A large amount of experimental data has now been accumulated and many computational-theoretical studies of the thermodynamics of the fuel systems U‐O‐C and Pu‐O‐C have been performed. Analysis of the data obtained by different authors shows that in the many cases a large variance is noted in the values, which is only weakly explainable by the accuracy of the experiments and the computational studies. The purpose of the present work is to study the chemical composition of fuel systems and to substantiate a method for calculating the thermodynamic parameters of high-temperature nuclear fuel, using the ASTRA program. The following basic problems must be solved for this: • modify the IVTANTERMO [8] database of theoretical properties of individual substances for the fuel systems U‐O‐C and Pu‐O‐C; • develop computational models and perform test calculations of these systems; • compare the results obtained with existing experimental data and theoretical investigations performed by other authors;