Volatilization iodine conversion

The chemical forms of gaseous iodine in the containment atmosphere that are of interest are molecular iodine and organic iodides whether these species reached the containment as a result of iodine release from the fuel, partitioning of volatile iodine species from water or as a result of combustion processes. Another volatile form of iodine, HI, is very hygroscopic and will rapidly dissociate in contact with water to form solvated T. There can be some conversion of molecular iodine, hig), into organic iodides, RI, in the radiation field of the containment atmosphere ... 

Most analytical techniques require sample decomposition, which is a delicate problem in iodine determination, because elementary iodine and some of its compounds, such as HI and CH3CI, are highly volatile. This problem can be circumvented in two ways. First is the conversion of all iodine species into elementary iodine (I2) by distillation or combustion, with subsequent trapping of the analyte for further processing. Secondly, on the contrary, is the conversion of all volatile species to nonvolatile ones, such as iodide or iodate. 

Iodine in water can be present in a number of chemical forms including the familiar iodide ion, r, the volatile form, l2(aq), and the highly oxidised iodate ion, lOs, as well as a munber of transient forms with oxidation states intermediate between those of the more familiar forms. Even under normal conditions, there is some tendency for interconversions among these forms, but the conversion processes can be glacially slow under normal conditions. A radiation field can greatly accelerate the formation of various chemical forms of iodine in water. Products of water radiolysis can oxidise the iodide ion to form molecular iodine following a reaction scheme that is summarily described by ... 

The extent and strength of iodine retention on charcoal depends on various parameters, with one of them being the type of charcoal applied, in particular when higher temperatures have to be taken into consideration. Charcoals made from coconut shells generally show the best retention properties, probably due to their comparatively high potassium content which raises the pH, resulting in the conversion of adsorbed iodine species to non-volatile iodide. 

At the comparatively low temperatures prevailing in the containment, Csl (as well as the dissolved I" ion) is not expected to react with surfaces, either with those immersed in the sump water or with the surfaces exposed to the containment atmosphere. However, elemental I2 may undergo reactions resulting in a conversion of this volatile species to non-volatile I. Since the fraction of iodine which originally enters the containment from the primary system as I2 is very small, such reactions... 

Summing up, it can be expected that chemical reactions of elemental iodine I2 with the surfaces in the containment, in particular those occurring in the liquid phase, will effect a conversion to non-volatile I , thus effectively counteracting the radiolytic oxidation of 1 to form I2. 

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