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Cesium and iodine, which are formed during a fission process in a nuclear reactor, are considered as major fission products responsible for the environmental burden in case of a nuclear accident. From the safety point of view, it is thus important to understand their release mechanism when overheating of the reactor core occurs. This work presents an experimental investigation of the behaviour of caesium iodide and caesium fluoride in fluoride based molten salt reactor fuel during high temperature events. It has been demonstrated that CsF will be retained in the fuel salt and thus its volatility will be significantly reduced, while CsI will not dissolve in the fluoride-based fuel matrix and will thus remain more volatile. The influence of the presence of CsI and CsF on the melting behaviour of the fuel has been investigated using calorimetry, revealing their negligible effects.

Knudsen effusion mass spectrometry (KEMS) has been used to investigate the vapour pressure over the molten LiF–ThF4 salt and determine the thermodynamic activity of LiF and ThF4 in the liquid solution. As part of the study, the vaporization of pure LiF and pure ThF4 was examined and the results were compared with the literature values finding a good agreement. Next, the vapour pressure of the LixTh1−xF4−3x liquid solution was investigated by measuring four samples having different compositions (XLiF ∼ 0.2, 0.4, 0.6, 0.8 mol%). In order to determine the thermodynamic activities, the vapour pressure of LiF and ThF4 species over the liquid solution, as calculated from our results, were compared with the vapour pressure over the pure LiF(l) and pure ThF4(l) systems. A strong deviation from the Raoult's law was observed, more evident in case of LiF species, in agreement with the predictions by our thermodynamic model.