Thermogravimetric study of the reaction of uranium oxides with fluorine

Thermogravimetric study of the reaction of uranium oxides with fluorine was conducted by TG-DTA method using anti-corrosion type differential thermobalance. When UO₂ was heated from R.T. to 500 °C in 5% F₂/He atmosphere, the weight increase appeared at ca. 250 °C with an exothermic peak. Then the weight decreased slightly with a small exothermic peak followed by the complete volatilization with a large exothermic peak at ca. 350 °C. At a flow rate of 15, 30, 60 ml min^-1, there seemed to be no significant change for the fluorination of UO₂. With the different heating rates of 1, 2, 5 and 10 °C min^-1, the fluorination peak shifted to higher temperature with increasing heating rates. For the comparison with thermogravimetric results, phase analysis by XRD method was conducted for the products obtained at different temperatures. At 260 °C, the product was UO₂ with a small amount of the intermediate compound, UO₂F. The amount of this compound increased with increasing temperature up to 320 °C. Then another phase of UO₂F₂ appeared at 340 °C but it was immediately fluorinated to the volatile fluoride. When U₃O₈ was used as a starting material, it was found that the steep weight decrease peak appeared at ca. 350 °C and the uranium volatilized completely. This result suggests that fluorination of U₃O₈ occurs at this temperature forming UF₆. Uranium trioxide showed the similar fluorination behavior to that of U₃O₈.