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Are molecular solvents, aqueous biphasic systems and deep eutectic solvents meaningful categories for liquid–liquid extraction?

The precipitation of strontium in highly concentrated nitric acid is often used in standard radiochemical methods for 90Sr separation. In this work, the description of Sr behavior in such conditions was studied in the point of speciation simulation. Standard speciation method based on Phreeqc software was considered but two problems underlined the limits of this approach to study this process: inappropriate ionic strength correction model (for I > 10 M) and the particular dissociation of nitric acid. To overcome these issues, a multipurpose-fitting routine, Minuit, was used to fit chemical model parameters. A polynomial expression was also added to the model in order to take into account nitric acid dissociation. This alternative was proved successful to calculate the Sr precipitate in highly concentrated nitric acid. Comparison was made with the results obtained with Design of Experiment methodology. 

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Description of strontium precipitation in highly concentrated nitric acid

Abstract A need for method validation in radiochemical analyses of decommissioning waste is a challenging task due to lack of commercial reference materials. Participation in an intercomparison exercise is one way for a laboratory to assess their performance and validate their analysis results. A three-year project within the Nordic Nuclear Safety Research (NKS) community was initiated in order to carry out intercomparison exercises on difficult to measure (DTM) radionuclides in real decommissioning waste. Both Nordic and Non-Nordic laboratories participated. This paper reports the results from the final year of the project focusing on beta- and gamma emitter (i.e., easy to measure, ETM) analysis in spent ion exchange resin. The assigned values were derived from the participants’ results according to ISO 13528 standard and the performances were assessed using z scores. The results showed generally good performances for both DTMs and ETMs. 

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Intercomparison exercise on difficult to measure radionuclides in spent ion exchange resin

90Sr radiochemical method using nitric acid for Sr and Ca separation was optimized using design of experiments methodology. Nitric acid concentration and total volume of solution were determined as influencing factors applying Plackett–Burman design. These parameters were then optimized using Box–Behnken experimental design. An optimal nitric acid concentration of 63% and a total solution volume of 26 mL were found to achieve a Sr recovery of 93% with only 5% of Ca for a single precipitation. The developed procedure was applied on real radwaste samples: effluent, sludge and concrete. By comparison of the recovery and the 90Sr activity measurement with the results of the in-house standard method based on fuming nitric acid, the optimized method with concentrated nitric acid was validated. The fuming nitric acid can now be eliminated for 90Sr radioactive waste characterization.

Eric Destrel

Papers

Optimization of Sr-90 precipitation in nitric acid using design of experiments for radioactive waste characterization method