The behavior of Cl, Ca, Be, Ni, and Cs radionuclides in the course of heating of radioactive graphite in steam was studied by thermodynamic modeling. Complete thermodynamic analysis at temperatures in the interval from 373 to 3273 K was performed using the TERRA program with the aim of determining the possible composition of the gas phase. The modeling shows that chlorine occurs in the form of gaseous HCl in the temperature interval from 873 to 1973 K and in the form of gaseous HCl and Cl at temperatures from 1973 to 3273 K. Calcium in the temperature range from 1973 to 3273 K occurs in the form of gaseous Ca(OH) 2 , Ca, and CaOH and ionized CaOH + and Ca + . Beryllium in the temperature interval from 1873 to 3273 K occurs in the form of gaseous Be(OH) 2 , BeOH, Be, and BeOH. Nickel at temperatures from 1673 to 2173 K occurs in the form of gaseous NiH, Ni(OH) 2 , NiOH, and Ni, and at temperatures from 2173 to 3273 K, in the form of gaseous NiH, NiOH, and Ni. Cesium at temperatures from 973 to 1473 K occurs in the form of gaseous CsCl and CsOH and of ionized Cs + , and at temperatures from 1473 to 3273 K, in the form of ionized Cs + . The main reactions within separate phases and between the condensed and gas phases were determined, and their equilibrium constants were calculated.