Investigation of conditions for extraction of radionuclides from salt-rich solutions

Laboratory studies of the possibility of simultaneous recovery of long-lived radionuclides from HLW in the framework of a single process have resulted

in the development of a flowsheet for waste treatment on the basis of the CCD, CMPO and PEG in diluent FS-13 extraction system. Since HLW, as a rule, has a complicated chemical composition, the influence of different HLW components on the extraction of long-lived radionuclides needs to be investigated to create an effective technology.

In particular, owing to the rather low concentration of CMPO in the extraction mixture, its extraction properties with regard to trivalent TPE and REE depend strongly on the concentration and state of such extract­able impurities such as zirconium and iron, which may compete with TPE and REE. Tables 9.10 and 9.11 show the effect of zirconium and iron on the distribution coefficients of europium from 1 M HNO3, using an extract­ant of 0.08 M CCD + 0.024 M CMPO + 1.4% PEG-400 in FS-13.

Tables 9.10 and 9.11 show that zirconium and iron exert a suppressing effect on europium extraction. Increasing the aqueous solution acidity to 1.5 M reduces the iron effect drastically, and thus special precautions should be taken to suppress zirconium extraction.

The extraction of hindering impurities and thus their effect can be sup­pressed with the use of different complexones, such as fluorine-ion or citric acid. As citric acid deteriorates the properties of cement (in the case of cementing LLW-raffinate) after the extraction of radionuclides, preference was given to fluorine-ion. Table 9.12 shows the effect of fluorine-ion on europium extraction at various concentrations of zirconium.

The data given in Table 9.12 illustrate the possibility of establishing condi­tions for the application of fluorine-ion to suppress zirconium extraction. As to the iron effect, the table shows that, even at the molar ratio 1 : 1 between iron and fluorine-ion, iron extraction is effectively suppressed.

244 Advanced separation techniques for nuclear fuel reprocessing Variant 3

• stripping of Cs, Sr, An and REE by a solution containing 1 M guanidine carbonate and 20 g/l DTPA.

On the basis of experimental results concerning the processes of extrac­tion and stripping, the flowsheets using the UNEX-extractant have been developed which involve the fraction or combined separation of long-lived radionuclides from HLW.