Computer-simulated thermodynamic behavior of radionuclides U, Cl, Pu, Be, Ni, Am when radioactive graphite is heated in a carbon dioxide atmosphere

Abstract

The article examines the computer-simulated thermodynamic behavior of radionuclides U, Cl, Pu, Be, Ni, and Am when radioactive graphite is heated in a carbon dioxide atmosphere over a wide temperature range. The aim of the work is to estimate the equilibrium composition of the gas and condensed phases during a probable beyond design basis accident or reactor graphite reprocessing. Since graphite is a prevalent material of both decommissioned and advanced reactors, the formation of such a high-temperature system is not excluded. Obtaining such information about the composition of its phases by experimental methods can be difficult for many reasons, which makes it preferable to use complete thermal analysis methods to study the equilibrium behavior of this system. The system thermodynamics in the equilibrium state was simulated by means of the TERRA computer software package involving the methodology of the model of ideal solutions for interaction products to estimate the equilibrium composition of the resulting phases. The calculations used sets of thermodynamic properties of individual substances from the IVTANTERMO and HSC databases. It has been established that, in the studied high-temperature system, radionuclides U, Cl, Pu, Be, Ni, and Am as well as graphite are present in the condensed solution only in lower or average temperature ranges and, as equilibrium temperatures further increase, they are able to turn to gas forms. The study made it possible to define a set of basic reactions involving the above-mentioned radionuclides, determine the temperature ranges of their implementation and calculate their equilibrium constants. Carbon dioxide in the upper region of equilibrium temperatures should not be regarded as inert with respect to graphite and especially radionuclides. © 2019 Obninsk Institute for Nuclear Power Engineering, National Research Nuclear University 'MEPhI'. All rights reserved.