Historical perspective and current trends

Interest in the separation of fission-product cesium and strontium dates back to the early days of reprocessing over half a century ago. This interest has been reflected in a large body of scientific research and process devel­opment that is growing unabated to this day. Motivation has been threefold: advanced reprocessing, waste management, and industrial applications.25,26 Separations of 137Cs and 90Sr being considered for advanced reprocessing today are driven by the expected increase in repository capacity upon reducing the heat load that would otherwise result from inclusion of these two fission products.27-29 Given the large cost of adding unit operations to a reprocessing plant, however, separation may not be preferable to decay — storage,30 at least using currently available separation technologies. Nevertheless, research has continued to the point of demonstration and even multimegacurie separations have been done,31 thereby establishing a baseline of mature technology options for future consideration. Major use of 137Cs and 90Sr separations technology occurs in the area of waste manage — ment.32-37 Wastes include a great variety of stored radioactive materials but are dominated in volume by >108 gal of legacy defense reprocessing wastes stored in the United States and Russia. Strategies generally aim to concen­trate radionuclides into a compact volume for disposal with concomitant high decontamination of the bulk volume of the waste stream. Environmental risk is thereby reduced, and cost savings accrue by reducing the volume of high-activity waste destined for expensive storage facilities. Radiation sources containing 137Cs or 90Sr have been in demand for multiple industrial uses dating back to the 1950s, motivating considerable research and mega­curie recovery operations.25,26,37’38 Historically, legacy wastes, such as the separated radiocesium and radiostrontium currently stored at Hanford, have been viewed as a resource in this regard.

The status of technology development to separate cesium and strontium from both acidic and alkaline solutions was reviewed comprehensively approximately two decades ago,25,26 describing the considerable progress that had been made in previous decades and laying out the needs and challenges for future processing. A need was singled out therein for new solvent-extraction technologies for cesium and strontium removal from acidic streams. The potential of crown ethers, recognized in the radiochemi­cal community since the 1970s,39-42 had not yet led to a practical system, and promising dicarbollide chemistry. Although promising dicarbollide chemis­try had gained the attention of Czech researchers as early as 1974,43 it had not yet emerged from dependence on toxic diluents like nitrobenzene. Several ion-exchange and precipitation systems had been in use on the acid side, but all were seen as having drawbacks,25 such as difficult recovery of the radionuclide from the reagent, low capacity, or instability. A number of separations on the alkaline side, pertaining mainly to cesium, were available or being implemented, such as the In-Tank Precipitation (ITP) process using tetraphenylborate for cesium removal from tank waste at the Savannah River Site (SRS),44 phenol-formaldehyde resin functionalized with sulfonic acid groups (Duolite ARC-359), zeolites, and several solvent-extraction systems.25,26,33 Strontium, largely insoluble as hydroxide or carbonate salts or as co-precipitated with other metals, is not as important as cesium for separation on the alkaline side, but monosodium titanate had been demonstrated to be effective in sorbing traces from tank waste.44 Megacurie recovery of strontium from acidified tank sludge had been carried out previously.25