The use of a calixarene-crown ether in the removal of cesium from alkaline nitrate nuclear waste is a satisfying tale of fundamental chemistry advancing through applied research and development, scale-up, and deployment at full scale.1,2 A 30+ year odyssey, this endeavor—still in the making toward treating 34 million gallons of nuclear waste at the Savannah River Site in the USA—teaches hard lessons in adapting laboratory chemistry to the rigors of real-world application. Here is a recitation of some of the many questions encountered and lessons learned in bringing one solvent extraction (SX) process to fruition.
Chemistry
Flowsheet and engineering
Plant and system
General lessons
Every application is unique in the challenges that must be overcome. The Caustic-Side Solvent Extraction process (CSSX)1 and the Next-Generation CSSX process (NG-CSSX)2 were designed to remove cesium from alkaline waste containing 5–7 M sodium salts generated from decades of reprocessing spent nuclear fuel for weapons production. Approximately 34 Mgal (130,000 m3) such Cold War waste was produced at the Savannah River Site in the USA through the 1980s. Cesium-137 constitutes the primary soluble radionuclide in the waste and must by regulations be removed with a decontamination factor (DF) of >40,000 (i.e., >99.9975% removal).3 Calixarene-crown-6 ethers reported by European researchers provide the needed extraordinary separation factors of >10,000 for Cs vs Na.4 These molecules possess an elegant aromatic pocket of the perfect size for Cs+ ion, but they cannot function without a diluent modifier, as they have poor solubility and weak extraction strength by themselves in aliphatic diluents. Making them function adequately required designing an alcohol modifier with a fluorinated substituent to boost its hydrogen-bond-donor ability.5 The structure of the modifier had to be crafted to avoid third-phase formation, to resist hydrolytic degradation, and to yield degradation products that could be washed out. Together, the extractant-modifier combination proved robust to variations in waste composition.6 For the process to provide a DF of >40,000 means that stripping must be as efficient as extraction, which proved to be a perplexing problem.7,8 A strong exothermic temperature dependence was exploited for efficient stripping at elevated temperature. An organic base called a “suppressor” was included as an important stripping aid. Scrubbing was found to be essential to remove co-extracted impurities and adjust the solvent for stripping.6 The resulting expensive solvents exhibit good performance in centrifugal contactors and can also be recovered using passive coalescers on the raffinate and strip-effluent streams.9 Upstream filtration has limited throughput overall, and problems with deposits in the contactors and coalescers continue to require intervention.10,11 Despite the experience with such problems, the CSSX process and the powerful Next-Generation CSSX process deliver 15-fold or better concentrated streams of nearly pure cesium salt for vitrification.