Thesis location: Radioelement Transfer Research Laboratory (LETR) - Cadarache (13)
Start: October 2021
Research Master's degree in theoretical chemistry or physical chemistry.
Age limit: 26 years old unless otherwise stated.
Several experimental and modelling works have made it possible to advance the understanding and modelling of the mechanisms leading to the formation of gaseous iodine in the primary circuit. Thus, H. Hijazi has shown by theoretical chemistry that CsI and AgI can lead to the formation of gaseous I2 from HO° radicals resulting from the radiolysis of water vapour and that the formation of iodine is not thermodynamically favourable in the absence of oxidant. D. Obada's experimental work has shown that, under air, the reaction between aerosols deposited of CsI/AgI and stainless steel type surfaces leads to the formation of caesium chromate and volatile iodine and that water vapour tends to limit the formation of gaseous iodine without, however, preventing it. This illustrates the fact that, without the presence of water vapour, other mechanisms for the formation of iodine gas exist and that, moreover, water vapour tends rather to limit this formation. D. Obada also illustrated the influence of boron on the formation of volatile iodine in the primary circuit. From a theoretical point of view, H. Hu illustrated the mechanisms of adsorption of AgI, CdI2, RuO4 and RuO2 on iron oxide and chromium oxide surfaces, which can lead to the release of gaseous iodine and ruthenium. However, other mechanisms not involving the surface of the primary circuit have also been identified by D. Obada. Indeed, under the effect of temperature, CsI aerosols deposited on a gold surface (chemically inert) partially decompose into volatile iodine (nearly 20% in associated tests). This indicates the existence of volatile iodine formation mechanisms taking place on the surface of the aerosol itself but without the participation of the support on which the aerosols are deposited. Experimental results suggest chemical mechanisms involving gaseous CsI/AgI and/or CsI/AgI in the form of aerosols (before deposition on a surface) and certain components of the gas phase, notably NOx and boron species, but no theoretical studies have been conducted on this subject. The objective of this thesis is therefore to complete the modelling work on the behaviour of iodine in the primary circuit. To do this, the steps are to continue the identification of the chemical reactions leading to the formation of gaseous iodine in the primary circuit and to quantify the associated energy barriers by theoretical chemistry for the most relevant reactions. Secondly, the integration in ASTEC/SOPHAEROS of the most relevant reaction kinetics will make it possible to determine which ones could, in fine, contribute to the formation of volatile iodine by completing the SOPHAEROS/Circuit reaction mechanism and then comparing it with the results of the CHIP, CHIP+, ESTER and PHEBUS tests. Finally, lessons can be drawn from it with regard to reactor applications.