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

Abstract

The NO_x composition and iodine species in the dissolution of spent fuels are discussed on the basis of thermochemical calculations and experimental results. The influence of N0_x sparging on the expulsion of iodine is also discussed. The dissolution of a spent PWR-fuel specimen (–3g) in 30 ml of 3.5_MHNO₃ at 100°C is calculated to yield a concentration of 7×10⁻²atm of N0₂, which is 80% of the total NO_X in the dissolver. This N0₂ 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 N0₂ 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 (3kPa). Attempts to expel iodine from the fuel solution indicated that the main iodine species in the solution was colloidal iodine and not iodate (I0₃ ⁻) 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.

KEYWORDS:

• radioiodine
• dissolution
• spent fuels
• PWR-type reactors
• nitrogen oxides
• chemical compositions
• thermochemical properties
• iodates
• colloidal iodine
• silver iodide
• palladium iodide
• fuel reprocessing