Iodine is the major halogen, and it is a fission product or a decay product of its precursors, for example, Te. Iodine will form some iodide with other fission products (e.g. CsI) which can dissolve into the primary salt. Therefore, nearly all iodine isotopes remain in the salt in the form of I- instead of validation. 131I and 133I were found in MSRE exiting gas which came as a result of volatilization of Te precursors. Since element iodine is a vaporized material, it is one of the major radioactive source term concerns for other types of reactor such as Na-cooled reactor. However, iodine is in the form iodides that can remain in the salt for MSRs, therefore, the salt has a high retention capability of iodine. The iodides in the salt may have some impacts on the salt properties including physical and thermochemical properties. However, such data are very scarce and impacts on the salt properties are not well understood. The iodide in the salt may also lead to corrosion. The present study investigated iodide electrochemical behaviors in molten fluoride salts and iodine ion separation from the salt. The study tested two type of working electrode: Tungsten and graphite. Based on the thermodynamic calculations experimental measurments, the oxidation potential of W is more negative than that of iodine ion, therefore, W electrode cannot be applied as the anode for iodine separation, and then graphite electrode was selected. The electrochemical measurements using graphite working electrode indicates that there is a passive layer which complicates the behaviors of iodide ion by the blocking effect of the layer at the graphite working electrode. The blocking effects made the redox potential shift to a sufficiently positive value. In this case, the oxidation current actually decreases as the potential moves positive, and the process of the iodide ions penetrating through this film becomes rate-limiting. An increase in the concentration of iodide relieved the ‘blocking effect’ and yielded a higher current. The reason for this might be the little amount of replacement of passivation layer by the addition of sodium iodide. Moreover, both cyclic voltammetry and chronoamperometry test were employed to identify the characteristic of the separation of iodide ions. Finally, two pathways, the formation of either I2 gas or KI3 gas, were proposed for the separation mechanism of iodide ions from the system from both thermodynamic calculations and experimental demonstrations.

Iodide in fluoride salts: Its behaviors and iodine ion separation

2019 Fall National Meeting

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