The behavior of U, Pu, and Eu radionuclides on heating of nuclear graphite in air was investigated by thermodynamic modeling. A complete thermodynamic analysis was preformed in the temperature range from 300 to 3600 K with the TERRA software package in order to establish the possible composition of the gas phase. Uranium in the temperature range from 300 to 2000 K is found to be in the form of condensed UO2(c), UOCl2(c), UOCl(c), CaUO4(c), whereas with an increase in temperature from 2000 to 3600 K, in the form of gaseous UCl4, UO3, UO2 and also in the form of ionized UO3–, UO2+. Plutonium at temperatures from 300 to 1900 K represents condensed PuCl3(c), PuOCl(c), Pu2O3(c), PuO2(c) and with an increase in temperature from 1900 to 3600 K, gaseous PuO2, PuO, and ionized PuO+. Europium in the temperature range from 300 to 2000 K is in the form of condensed EuCl2(c), EuCl3(c), EuOCl(c), Eu2O3(c), EuO(c), with an increase in temperature from 2000 to 3600 K in the form of gaseous EuO, Eu and ionized Eu+. The main reactions within the individual phases and between the condensed and gas phases were determined. Their equilibrium constants are calculated. © 2023, Pleiades Publishing, Ltd.