Separation of An(III) from PUREX raffinate as an innovative SANEX process based on a mixture of TODGA/TBP

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1 Lisbon, Portugal, 3 March 2 April 20 Separation of An(III) from PUREX raffinate as an innovative SAEX process based on a mixture of TDGA/TBP Michal Sypula, Andreas Wilden, Christian Schreinemachers, Giuseppe Modolo Institut für Energieforschung, Sicherheitsforschung und Reaktortechnik (IEF-6) Forschungszentrum Jülich GmbH, Jülich, Germany Tel: +49(0) , Fax: +49(0) m.sypula@fz-juelich.de Abstract Within the ACSEPT project, an innovative SAEX process based on TDGA/TBP for selective An(III) separation from PUREX raffinate was studied. xalic acid usually used for Zr complexation is considered a weak point. An investigation to substitute oxalic acid with a different masking agent was carried out. A new masking agent already studied in FZJ was applied and showed good complexation properties towards Zr and Pd. Re-investigation of the formula of the actinide stripping solution was also performed. Good separation of Ln over Am was obtained by means of DTPA and malic acid. Glycine appeared to be the strongest within the tested buffers. A. ITRDUCTI Several processes for recycling nuclear spent fuel have been designed and tested but only the PUREX process (U, Pu and p recovery) has been employed on an industrial scale. After the partitioning step, Pu and U can be transferred back to the nuclear reactors as MX fuel (mixed oxide). Recovered minor actinides can be converted into short-lived nuclides in advanced reactors (transmutation). Within the ACSEPT project, a new innovative SAEX process (i-saex) combining two other separate processes, namely DIAMEX and SAEX, was investigated. This process consists of the coextraction of Ln(III) and An(III) from the PUREX raffinate by a malonamide (e.g. DMDHEMA) or diglycolamide (e.g TDGA), selective stripping of An(III) by polyaminocarboxylic acid (complexing agent) and carboxylic acid (buffer), and the stripping of Ln(III). An(III) + F.P. An(III) + Ln(III) Ln(III) for stripping PUREX raffinate co-extraction of An(III) + Ln(III) selective stripping of An(III) F.P. An(III) i - SAEX F.P. fission products Figure. Innovative SAEX concept for separation of An(III) from PUREX raffinate,,, -tetraoctyl diglycolamide (TDGA) was considered a suitable extractant for co-extraction of An(III) + Ln(III). This ligand (Figure 2) possesses a very strong affinity towards An(III) and Ln(III) with distribution ratios over 300. Moreover, its high hydrolytic and radiolytic stability makes it an ideal extractant for nuclear purposes. evertheless, high loading of the organic solvent with metals causes formation of the third phase making it inapplicable in a continuous process. Modolo et al. studied the influence of tributylphosphate (TBP, Figure 2) on suppression of the third phase formation []. It has been found that 0.5 mol/l TBP added to the solvent acts as an organic phase modifier increasing the limiting organic concentration (LC) up to 0.02 mol/l of d []. CP FP7-EURATM 2267

2 Lisbon, Portugal, 3 March 2 April 20 a) H 7 C C C C H 7 C H 7 C H 7 b) C 4 H 9 C 4 H 9 P C 4 H 9 Figure 2. The chemical structures of the solvent components; a) TDGA, b) TBP Due to the very high extraction strength of TDGA, some of the fission products were co-extracted together with An(III) + Ln(III). This problem was overcome by applying oxalic acid and -(2- hydroxyethyl) ethylenediaminetriacetic acid (HEDTA) to suppress the extraction of Zr and Pd, respectively []. Two very successful counter-current tests (spiked and hot test) were carried out in FZJ and ITU [2,3]. This process was adopted by CEA for innovative SAEX by substituting the Ln+An stripping step with two new steps, namely An-stripping and Ln-stripping steps (Figure 3). During the co-extraction of Ln+An step, part of H 3 was extracted by TBP and further back-extracted, changing the ph of the An-stripping solution. Therefore, a buffer is needed. TDGA itself possesses a higher affinity towards Ln(III) than An(III). The separation between these two elements can be enhanced by means of a hydrophilic complexing agent i.e. HEDTA or DTPA (Figure 4). Solvent TDGA 0.2 M TBP 0.5 M in TPH Scrub 2 Extraction Scrubbing to stripping Raffinate Feed H M xalic acid HEDTA Scrub Loaded solvent An(III)Stripping Ln(III)Stripping Spent solvent An(III) Product Strip Selective An(III) Complexant Ln(III) Raffinate Strip Low conc. H 3 + buffer Figure 3. An exemplary flow-sheet of TDGA/TBP process for separation of An(III) from PUREX raffinate Cold and spiked counter-current tests were performed by CEA [4] using DTPA as the An-complexing agent and malonic acid to buffer the solution for actinide stripping. The weak point of this process is the use of oxalic acid in the extraction step. xalic acid is partly extracted by TBP in the extraction step and further back-extracted, changing the ph of the stripping solution (similar behaviour to nitric acid). Heres et al. already pointed out a very high sensitivity of the An(III) stripping step of the process towards ph. Furthermore, at a high oxalic acid concentration, a slow precipitation of lanthanide oxalates can appear. Therefore, we proposed substituting oxalic acid with different Zr masking agen. Based on our previous studies, we decided to use a new hydrophilic masking agent which proved to efficiently complex Zr and Pd in the aqueous phase preventing their co-extraction with Ln+An. We also re-investigated the formula of An-stripping solution to possibly increase the separation factor of lanthanides over actinides and improve stabilisation of the ph of the stripping solution. a) H H H H H b) H H H Figure 4. The chemical structures of a) DTPA, b) HEDTA CP FP7-EURATM 2267

3 Lisbon, Portugal, 3 March 2 April 20 B. RESULTS AD DISCUSSI B.. Substitution of oxalic acid and HEDTA with the new masking agent The extraction step was tested by contacting TDGA/TBP dissolved in TPH with a High Active Raffinate (HAR) simulate solution. Two parallel extractions were carried out: one with the new masking agent in the aqueous phase and one without. The results of this experiment (Table ) show that the new masking agent efficiently suppresses the extraction of Zr and Pd. A slight increase in Sr extraction can be noticed when the masking agent is used. This is probably due to the lower loading of the organic phase with Zr (7 mg/l in HAR solution). The same behaviour was observed in previous studies on complexation of Zr by oxalic acid []. Table : The content of the HAR solution and distribution ratios of its elements in the extraction step o masking agent ew masking agent Element Concentration [mg/l] D 24 Am, 52 Eu trace amounts >0 >0 Y 90 >0 60 La Ce Pr d Sm 49 >0 7 Eu 34 >0 4 Gd 5 >0 43 Zr Pd Sr Ru Mo Cd Rb Ba Sb Cu <0.0 Cr Cs 542 <0.0 <0.0 i 40 <0.0 <0.0 Rh 73 <0.0 <0.0 Sn <0.0 <0.0 Te 65 <0.0 <0.0 Ag 2 n.d. n.d. Al 2 n.d. n.d. Fe 900 n.d. n.d. a 600 n.d. n.d. Se n.d. n.d. H 3 3. mol/l * n.d. not determinated B.2. Re-investigation of the formula of the solution for actinides stripping Five buffers were chosen for these studies, two already tested by CEA: glycolic acid and citric acid; and three new ones, namely malic acid, lactic acid and glycine (Figure 5). CP FP7-EURATM 2267

4 Lisbon, Portugal, 3 March 2 April 20 a) H d) H H H b) H 2 e) H H c) H H H H H Figure 5. The chemical structures of a) malic acid, b) glycine, c) lactic acid, d) citric acid, e) glycolic acid To improve the separation of Ln from An, two hydrophilic complexing agents were tested, namely HEDTA and DTPA (Figure 4). To keep the Ln(III) in the organic phase while An(III) were stripped to the aqueous phase, a salting-out agent was applied, namely a 3. The results in Figure 5 show that malic acid and citric acid gave the highest separation factor of Eu/Am (>2) from all tested buffers when used with DTPA. Lactic acid also gave SF Eu/Am over but this buffer is not strong enough to prevent ph changes. Citric acid appeared to be the best choice concerning separation of Eu/Am and buffering properties. evertheless, it was rejected from the beginning as CEA pointed out the difficulties with its destruction before the An co-conversion step. The strongest buffer at tested ph=2 was glycine, although the separation factor was slightly below (when used with DTPA). A very interesting combination was glycine + HEDTA, giving a separation factor around 2 and the smallest ph change. In further studies, we focused only on a combination of DTPA with malic acid or glycine. H DTPA HEDTA DTPA HEDTA ph change no buffer malic citric glycolic lactic glycine 0.00 no buffer malic citric glycolic lactic glycine * ph change = ph initial - ph final Figure 5. The influence of different buffers and complexing agents on separation factor of Eu/Am and ph change in the stripping solution The influence of DTPA on the extraction of Am and Eu was also tested. The organic phase consisted of TDGA/TBP dissolved in TPH and loaded with An/Ln 2.5 solution of 0.5mol/L H 3. The aqueous phase of ph=2 contained DTPA, buffer (malic acid or glycine) and salting-out agent (a 3 ). The results showed a decrease in the distribution ratios for both tested elements and an increase in the separation factor of Eu/Am while increasing the complexant concentration (Figure 6). This expected behaviour can be explained by DTPA forming stronger complexes with An(III) than with Ln(III). Therefore, the extraction of Ln(III) by TDGA is preferential as it causes the Ln/An separation factor to increase. CP FP7-EURATM 2267

5 Lisbon, Portugal, 3 March 2 April Malic acid Am-24 Eu-52 SF Glycine Am-24 Eu-52 SF DTPA [mol/l] DTPA [mol/l] Figure 6. The influence of the complexant concentration on the extraction of 24 Am and 52 Eu The back-extraction of Am from the loaded organic phase using a stripping solution of different ph was tested to obtain the best separation conditions. The stripping solution consisted of DTPA, malic acid and a 3. Adjusted to ph=2 it gave the highest SF Eu/Am (Figure 7). The less extracted of all tested lanthanides at ph=2 was praseodymium, although its separation factor over Am was still high enough for efficient separation (SF Pr/Am =6.7). 0 Am-24 Eu-52 SF 4 00 Y La Ce Pr d Sm Eu Gd Am-24 Figure 7. The influence of the ph of the stripping solution on the back-extraction of 24 Am and Ln (DTPA + malic acid) The combination of DTPA, glycine and a 3 at the optimal ph for An stripping (ph=.9) gave lower SF Eu/Am than with malic acid. evertheless, the separation factor between samarium (the lowest distribution ratio within tested Ln) and Am reached. (Figure ) Am-24 Eu-52 SF Y La Ce Pr d Sm Eu Gd Am-24 Figure. The influence of the ph of the stripping solution on the back-extraction of 24 Am and Ln (DTPA + glycine) C. CCLUSI The TDGA/TBP process for An(III) separation from the PUREX raffinate studied within the ACSEPT project was successfully proven by two counter-current runs [4]. evertheless, the oxalic acid used in the extraction step was considered a weak point. Therefore, a new masking agent for Pd and Zr was CP FP7-EURATM 2267

6 Lisbon, Portugal, 3 March 2 April 20 proposed. We also re-investigated the formula of the An-stripping solution. DTPA appeared to be a better complexing agent for An(III) separation than HEDTA. evertheless, the latter gave a reasonably high separation factor of Eu/Am (>2) when used with glycine. Within the tested buffers, the best buffering properties at ph=2 were shown by glycine, although with malic acid a higher SF Eu/Am was obtained. The optimal formula of the An-stripping solution appears to be the mixture of DTPA + malic acid + a 3 at ph=2 or DTPA + glycine + a 3 at ph=.9. Aknowledgements Financial support for this research was provided by the European Commission (project ACSEPT Contract o. FP7-CP ) References. G. Modolo, H. Asp, C. Schreinemachers, H. Vijgen, Development of a TDGA based Process for partitioning of actinides from a PUREX raffinate Part I: Batch extraction optimization studies and stability tests, Solvent Extr. Ion Exch., 25, 6, (2007). 2. G. Modolo, H. Asp, H. Vijgen, R. Malmbeck, D. Magnusson, C. Sorel, Demonstration of a TDGA-Based Continuous Counter-Current Extraction Process for the Partitioning of Actinides from a Simulated PUREX Raffinate, Part II: Centrifugal Contactor Runs, Solvent Extr. Ion Exch., 26,, (200). 3. D. Magnusson, B. Christiansen, J.P. Glatz, R. Malmbeck, G. Modolo, D. Serrano-Purroy, C. Sorel, Demonstration of a TDGA-Based Continuous Counter-Current Extraction Process for the Partitioning of Actinides from a Simulated PUREX Raffinate, Part III: Centrifugal Contactor Run using Genuine Fuel Solution, Solvent Extr. Ion Exch., 27,, (2009). 4 X. Hérès, C. Sorel, M. Miguirditchian, B. Camès, C. Hill, I. Bisel, D. Espinoux, E. Catherine, B. Pascal, L. Brigitte, Results of recent counter-current tests on An(III)/Ln(III) separation using TDGA extractant, Proc. of the Global 2009, Paper 934, Paris, France (2009). CP FP7-EURATM 2267