Thermochemical and Experimental Considerations of NOx Composition and Iodine Species in the Dissolution of Spent PWR-Fuel Specimens.

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The NOx composition and iodine species in the dissolution of spent fuels are discussed on the basis of thermochemical calculations and experimental results. The influence of NOx sparging on the expulsion of iodine is also discussed. The dissolution of a spent PWR-fuel specimen (3 g) in 30 ml of 3.5 M HNO3 at 100°C is calculated to yield a concentration of 7×10-2 atm of NO2, which is 80% of the total NOx in the dissolver. This NO2 fraction is much higher than experimental values of 15% or less that were reported for dissolver off-gas cooled near to room temperature. The high NO2 fraction suppresses the formation of iodate (IO3-) in the dissolution. The calculations predict that IO3- is not formed in 3.5 M HNO3 at 100°C at an NO2 pressure ?? 3×10-2 atm (3 kPa ). Attempts to expel iodine from the fuel solution indicated that the main iodine species in the solution was colloidal iodine and not iodate (IO3-) which earlier workers postulated. The obtained experimental results are consistent with the thermochemical predictions. For the decomposition of the colloidal iodine in the expulsion process, NO2 sparging has a negative effect. This is because an increase in NO2 pressure promotes the formation of colloidal Agl.
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