KINETICS OF THE REACTION BETWEEN THORIUM AND WATER VAPOR

An examination of the reaction between thorium metal and water vapor has been made between the temperatures of 2000 to 600° C. and at water vapor pressures of 40, 70 and 100 mm. Hg. Analyses of the products have indicated that Th02 and H2 are the main species formed during the reaction with thor~um hydride as a possible side-reaction product. Two methods, gravimetric and manometric, were used successfully to determine the rates of reaction. During these rate studies three variables were examinedt time, temperature and pressure. The reaction data were found to obey the logarithmic rate law w = k log(l + o.45t), where w = weight of water reacted per unit area of thorium surface, t = time and k is the rate constant. Three temperature regions have been examined in which differentappearing oxides were observed: 200° to 275°, black; 3000 to 45oo, gray or white; and 450° to 550°, metallic. The rate constants were found to be first order in dependence on water vapor pressure. The Arrhenius equation ln k = -E/RT + C, where k = rate constant, E = activation energy, T = absolute temperature and R and C are constants, has been used to determine the activation energy. An average value of 6.44 t 0.75 kcal./mole was calculated for the activation energy of the thorium-water vapor reactions. Several experiments were carried out in order to determine the reaction mechanism, and it has been postulated that inward diffusion of water molecules or a species thereof is the rate-determining step of the reaction • . A value for the diffusion coefficient of water in Tho2 was estimated to be approximately lo-7 cm.2/sec. Other experiments have shown hydrogen, produced by the reaction, to have diffused into the metal at the higher temperatures and lower pressures, thus causing erroneous results for rate constant values at those temperatures. *This report is based on a Ph.D. thesis by Bruce E. Deal submitted June, 1955 to Iowa State College, Ames, Iowa. This work was done under contract with the Atomic Energy Commission.