Abstract <p>The article presents the results of thermodynamic modeling of the behavior of Am, Pu, Cs radionuclides when radioactive graphite is heated with an atmospheric atmosphere. Using the TERRA software package, thermodynamic analysis was performed in the temperature range from 300 to 3600 K in order to determine the possible composition of the ionized, gaseous and condensed phases. It has been established that americium in the temperature range from 300 to 2200 K is in the form of condensed AmO_2(k). When the temperature rises from 2200 to 3600 K, it is in the form of gaseous Am and condensed Am₂O_3(k). Plutonium in the range of temperatures from 700 to 1200 K is in the form of condensed PuCl_3(k), PuOCl_(k) with an increase in temperature from 1900 to 3600 K in the form of gaseous PuO₂, PuO in the form of condensed PuO_2(k), Pu₂O_3(k) and ionized PuO⁺. Caesium at temperatures from 600 to 900 K is in the form of condensed CsCl_(k) and gaseous CsCl as the temperature rises from 1300 to 1700 K in the form of ionized PuO⁺. The main reactions within individual phases and between condensed, gaseous and ionized phases are determined in this work. Calculations of the equilibrium constants of their reactions have been carried out.</p>