Suppression of Radioiodine Releases from a Radiochemical Separations Plant

In the Purex process used at the Savannah River Plant, the suppression of the release of radioiodine by complexing it with mercury was tested. Suppressing radioiodine release would be desirable if short-cooled fuel were inadvertently charged to the dissolver in the plant. When irradiated uranium reactor fuels are processed, radioiodine is normally evolved during dissolution with HNO₃, clarification, subsequent solvent extraction, and waste evaporation. Normally (without mercury), ∼ 50% of the radioiodine is evolved from the dissolver solution and efficiently sorbed in a silver nitrate bed, but the small amount evolved from the remainder of the process is released to the atmosphere through tall stacks. This release is suppressed by adding mercuric nitrate to the dissolver solution. With 0.04M Hg²⁺ in the dissolver solution and a gelatin clarifier, the radioiodine atmospheric release was reduced 55-fold. A material balance indicated that the mercury-iodine complex remained in the organic solvent where it was slowly removed by solvent scrubbers and subsequently transferred to a seepage basin. In another test, the presence of 5 × 10⁻⁴ M Hg²⁺ in the dissolver and the use of a MnO₂ clarifier reduced ¹³¹I atmospheric releases a total of 22-fold. In tests without Hg²⁺, MnO₂ clarification reduced ¹³¹I atmospheric releases 2.5-fold over the normal gelatin clarification. Mercurous nitrate (4 × 10⁻⁴M) was not more effective than mercuric nitrate (5 × 10⁻⁴M) in suppressing ¹³¹I evolution.