Partitioning of iodine in offgas generated by dissolution of spent nuclear fuel

A considerable amount of research has been performed regarding the scrubbing of nitrogen oxides (NO_x) from offgas streams with water and dilute nitric acid solutions. Considerably less information concerning the partitioning of iodine between the liquid and vapor phases has been accumulated. The available information has been combined to evaluate the distribution of iodine in an NO_x offgas scrubbing system. The particular system of interest removes nitrogen oxides from the iodine-containing offgas generated by the dissolution of spent nuclear fuel.

A computer simulation of the scrubbing system was developed and used to evaluate the gas/liquid iodine distribution resulting from three alternative system flowsheets. In the first, the vapor from steam stripping of the NO_x scrubber bottoms (to volatilize iodine) is recycled to the scrubber. In the second, air sparging of the scrubber bottoms tank is added. In the third, the iodine-bearing vapor from the steam stripper is not recycled to the NO_x scrubber, but the bottoms tank sparge is retained.

It has been hypothesized that recycle of the iodine-bearing stripper offgas stream results in an accumulaton of iodine in the scrubber liquid. Both this hypothesis and the effect of the air sparge on NO_x removal efficiency and iodine partitioning have been evaluated.

The flowsheet simulations indicate that iodine does accumulate in the NO_x scrubber. The iodine level in the scrubber liquid was observed to be decreased by sparging the bottoms tank, thereby volatilizing iodine from solution. An additional slight decrease in system iodine retention was indicated when the iodine stripper offgas was not recycled to the NO_x scrubber. The difference in magnitude between the effects of air sparging and steam stripping is believed to be due to the positioning of the two process in the system. In all simulations air sparging preceded steam stripping, therefore the potential for iodine removal was greatest for air sparging.

With regard to NO_x scrubbing, the removal efficiencies of the scrubber were in excess of 90% for all three flowsheets.