The Relationship Between Molybdenum Oxidation State and Iodine Volatility in Nuclear Fuel

The relationship between molybdenum oxidation state and iodine volatility in nuclear fuel was investigated using high-temperature Knudsen cell-mass spectroscopy. It was observed that the ratio of the intensities of molecular iodine ions I₂⁺ and CsI⁺ in the Knudsen cell-mass spectroscopic experiments can be used to investigate the iodine volatility in fuel under different conditions. The experiments show that the iodine volatility is similar in systems consisting of CsI alone, CsI/UO₂, and CsI/UO₂/MoO_x (with molybdenum in oxidation states 0, 2, and 4). The iodine volatility is much higher, however, in CsI/UO₂/MoO₃ systems (with molybdenum in oxidation state = 6). The iodine volatility in the fuel increases significantly if oxidation of the molybdenum goes to the MoO₃ stage. The increase in the iodine volatility is caused by the formation of elemental iodine from cesium iodide. It is concluded from these measurements that the oxidation of the fuel to the UO_2.2 will substantially increase the volatilization of fission product iodine. An analysis of the literature data suggests that the enhanced iodine volatilization process may be initiated when the fuel is oxidized to UO_2.02.