A Scoping Study of the Chemical Behavior of Cesium Iodide in the Presence of Boron in the Condensed Phase (650°C and 400°C) Under Primary Circuit Conditions

This work is a contribution toward understanding the chemical reactions on the primary circuit surfaces involving gaseous iodine release during a severe nuclear reactor accident. Cesium iodide was used as a nonradioactive precursor material in order to highlight the effects of carrier gas composition (Ar/H₂O, Ar/H₂O/H₂, and Ar/Air), temperature, the initial cesium/iodine (Cs/I) molar ratio by adding cesium hydroxide, and the presence of boron on the molar composition of the deposited iodine compound and on the release of gaseous iodine from the deposit. The results from the experiments involving only cesium iodide as a precursor revealed a slight decomposition of cesium iodide and a release of gaseous iodine. Furthermore, the measured gaseous iodine mass decreased with the addition of hydrogen to the carrier gas at 650°C. At 400°C, the amount of released material (aerosol and gas) was decreased. However, whereas at 650°C the sampled iodine existed mainly as aerosols, the mass concentration recorded from the experiment at 400°C indicated a predominance of gaseous iodine. When the initial Cs/I molar ratio was significantly greater than unity (1.5 < Cs/I < 4.5), the mass of produced gaseous iodine was barely detectable, suggesting a reaction between cesium hydroxide and the gaseous iodine released from cesium iodide decomposition. In the presence of boron, the transport of gaseous iodine was increased as a result of the formation of glassy cesium borate in the evaporation crucible. The presence of steam and its quantity were shown to have an enhancing influence on the cesium borate formation and on the release of gaseous iodine.