Extraction Properties of some Nitrogen-Containing Extractants and Determination of Nitrate Complex Formation for Lanthanides and Selected Actinides Using Solvent Extraction

Abstract: Partitioning and transmutation of spent nuclear fuel will reduce the amount of waste and its radiotoxicity in a long-term perspective. At the same time the energy content of the spent nuclear fuel can be used more efficiently by recycling of uranium and plutonium and by using energy from the transmutation reactor. To be able to efficiently transform the long-lived radiotoxic elements and avoid the production of more long-lived waste, it is necessary to separate some of the different elements. Solvent extraction is a useful technique for meeting the strict demands on purity and separation efficiency that must be fulfilled before the actual transmutation can occur.

Methodologies for investigating basic properties of some synergic systems were tested. The protonation constants of 2,2':6',2''-terpyridine were investigated using solvent extraction and spectrophotometry. The influence of pH, ionic strength and ionic media was studied using the synergic system of 2,6-bis-(benzoxazolyl)-4-dodecyloxylpyridine and 2-bromodecanoic acid. The nitrate complex formation of some actinides and the lanthanides was also investigated using the same system. In the case of berkelium, these constants have not been determined before. Finally, extraction properties within the lanthanide series and between lanthanides and actinides were studied. A clear tetrad effect was found within the lanthanide group.

A basic design was also made on a possible flow sheet for the solvent extraction system with 2,6-bis-(benzoxazolyl)-4-dodecyloxylpyridine and 2-bromodecanoic acid and a mixture of studied lanthanides and actinides. This demonstrates that in theory it is possible to reach the desired separation factors.

The influence of uncertainties in distribution ratios and stability constant determinations was investigated. The stability constants are usually assumed to be non-correlated, which is often not the case. Given a certain value of the first stability constant, it is necessary to reduce the uncertainty interval of the second one because of their correlation. This will affect the calculated speciation in many chemical systems.

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