ANHYDROUS LIQUID AMMONIA AND IODINE–PROGRESS REPORT ON AN IMPROVED PROCESS FOR TREATMENT OF SODIUM IN FERMI-1 BLANKET FUEL

摘要

Sodium-bonded spent nuclear fuel(SBSNF)elements pose a potential hazard regarding eventual disposal in a geologic repository because sodium is pyrophoric and water-reactive.An improved process is needed that can extract the sodium from SBSNF elements and render the sodium into a nonhazardous chemical form that is not subject to the Resource Conservation and Recovery Act (RCRA). rnGlobal Technologies,Inc.(GTI)has successfully demonstrated such a process through sodium dissolution in ammonia and conversion to nonhazardous sodium iodide on unirradiated sodium- bonded Fermi-1 blanket fuel elements.Argonne National Laboratories-West(ANL-W)provided the fuel element and cut it between the 35.6-cm slugs of depleted uranium fuel meat to avoid freeing alpha contamination.A secondary objective was to demonstrate that the ammonia and resulting process solutions do not attack the depleted uranium in the fuel.This objective was accomplished.Treated Fermi-1 blanket fuel segments were returned to ANL-W for disposal as non-RCRA low-level radioactive waste,without metallic sodium or loose radioactive contamination from uranium corrosion.The products of the improved GTI process are solid, unregulated sodium iodide and the low-level radioactive depleted metallic fuel elements. rnA diffusion-only process with sodium and ammonia had been previously demonstrated by Commodore Advanced Sciences.An in-house experiment verifying the diffusion-based process on Fermi-1 segments was not successful.The diffusion-based process required access for liquid ammonia to continue dissolving sodium in the narrow annulus between the stainless steel cladding and slugs of depleted uranium.Progress of the dissolution front was inhibited by precipitation of insoluble sodium amide(NaNH_2)in the annulus.The study described herein overcame the deficiencies noted in the Commodore process by utilizing two enhancements.It was found that pressure cycling—gradually raising the reaction vessel pressure and then abruptly dropping it—forced loose sodium amide from the annulus,which improved ammonia access to the clad metallic sodium.Second,early addition of the elemental iodine was found to convert the resultant sodium amide from a dense,adherent form to a porous,powdery form.The powdery form of sodium amide was much more amenable to removal from the annulus by pressure cycling.