210 ANALYTICA CHIMICA ACTA

THE SEPARATION OF FISSION PRODUCTS BY ELECTROPHORETIC FOCUSSING OF IONS

M. I’AUWELS+, R. GIJBELS++ AND J. HOSTIS

lmtitutc foot Nzdea~ Sciemes, Glued University, Ghent (Belgium)

(Rcccivcd April xr)th, I~GG)

The quantitative determination of fission products in nuclear fuels is important for the control of the uranium “burn-up” and also during the reprocessing of fuel elements. For the control of the burn-up it is often desirable to determine several fission products, whereas during the reprocessing, the decontamination factor from the long-lived fission products is required. In principle, 2 different approaches can be used : non-destructive y-spectrometric determination of the fission products from the complex y-spectrum or chemical separation of the individual species. Both procedures have a number of inherent shortco,mings. Owing to the poor resolution of sodium iodide-thallium scintillators the hccuracy of file non-destructive procedure is often unsatisfactory and pure p-emitters cannot be determined, whereas the chemical separations are time-consuming, because of the complex nature of the fission product mixture. Different authors have investigated fast separation procedures by chromato- graphic or electrophoretic techniques 1-a. In comparison with electrofocussing of ions (E.F.I.) they are however time-consuming and do notsucceedin separatingcompletely all the long-lived fission products from each other. SCHUMACHER~ attempted to separate a number of fission products, namely lanthanum, strontium and barium, by E.F.I. The same technique was used by SHINAGAWA AND KISO”, but they did not achieve a quantitative separation of all the fission products. As E.F.I. is a fast quantitative separation technique, it seemed worthwhile to apply it to a separation of all the most important long-lived fission products, i.e. ooSr+ooY, OlY, ~Gr+o~Nb, 103+10~Ru, 13’Cs and 14lCe + lWZe, as these nuclides make up the main activity after a cooling period of 3 months or more. In this paper zirconium-niobium and ruthenium are not considered, as their behaviour is complex and will be dealt with subsequently.,

Pvincifiles of E.F.I. SCHUMACHER has described in detail the principles of E.F.I. separationso-10.

The technique is mainly based on paper electrophoresis of cations in a gradient of a complcxing agent. This is achieved by using a ligand in the cathode compartment and a comples destroyer, e.g. an acid, in the anode compartment. After spotting ?he solution in the center of a paper strip, the anoclic and cathodic solutions are allowed to diffuse towards the applied zone and an electric voltage of several hundred volts is applied. After a few minutes sharp focusses are obtained, whose locations depend upon the shape of the complex gradient and the stability constants of the complexes. As the

* l3ursal 1.I.IC.W. *+ Research fellow I.I.K.W.

Anal. Clrim. Acta, 3d (1966) 210-214

SEPARATION OF FISSION PRODUCTS 211

current is of the order of IO mA, cooling is required. This is achieved by immersing the paper strip in carbon tetrachloride, stirred by means of a magnetic stirrer. A diagram of the apparatus is given in Fig. I.

Choice of ex$erimental conditions In the long-lived fission products mixture, 2 rare earths occur, namely Y

and Ce. As SCHUMACHER 11 has already described a separation of rare earths, using nitrilotriacetic acid (NTA) as cathodic electrolyte, this ligand seemed an obvious

f anode

05M HCl

(pkQ3)

I-

applied zone

cathode =

0.08M(NHq)2 HNTA +Q06M NaAc

Fig. I. App;rrat11s for E.F. 1.

CCL, km -

- applied zone I .

, : *ir O”Y’ u4Ce =Nb

106R,

Fig. 2. Autoradiogram of scparatcd ions.

choice. To this ligand, used as a 0.08 M solution of the diammonium salt, 0.06 M sodium acetate was added to achieve a sharper separation of strontium from cesium. Hydrochloric acid (0.5 M) was chosen as the anodic electrolyte. 1t would of course be more practical to use nitric acid, as fuel elements are usually dissolved in this acid. Nitric acid however causes difficulties in the focussing of cerium, probably owing to oxidation of cerium(II1) to cerium(IV) and gives rise to doublefocusses of this element. Thus after removal of the nitric acid, the residue is dissolved in hydrochloric acid, taken to dryness again and redissolved in 0.08 M NTA adjusted to PH 7 with ammonia solution. A lower PH appears to give higher cross-contaminations of the fission products especially in the separation of cerium from yttrium. It also appeared that, in the presence of uranium, carrier-free fission products are not focussed very sharply and stay in the applied zone together with the broad uranium focus. This interference can be avoided by an adequate addition of carriers, namely IO pug per element in 25 ,ul

of applied solution. The carrier solution is added before the removal of the nitric acid.

Anal. Claim. Ada, 36 (x966) 210-214

2x2 M. PAUWELS, R. GIJBELS, J. HOSTE

If 500 V is applied during 6-10 min a quantitative separation is obtained. A typical autoradiogram of a Y -Ce-Sr-Cs separation is given in Fig. 2. It should be noted that under these conditions Zr-Nb-Ru give rise to a single focus at the left, i.e. more anodic than Y, the cross-contamination being negligible as appears from Fig. 3.

Nbo5 lcm

Appllecl zone __________

Fig. 3. Cross-contamination of focusscs.

To identify the focusses, the paper strip was cut into strips of I mm and counted in a well-type sodium iodide detector. Strontium-go and yttrium-go, wllich are pure &emitters were counted with an end-window (2 mg/cms) G.M.-tube. Scanning the paper strips under a G.M.-counter equipped with a lead slit did not give satis- factory results as broadening occurs, due to the high energy y-emission of some fission products.

EXPERIMENTAL

Procedure for E.F.I.

Add to ca. I pc of a nitric acid solution of a fuel element, containing no more than 200 mg of uranium, I mgeach of the following carriers: Zr, Nb, Ru, Y, Ce, Sr and Cs. Zirconium and niobium are added as aqueous solutions of the NTA complexes, the others as the chlorides. Add IO ml of 12 M hydrochloric acid and take to dryness on a hot plate. Dissolve the residue in IO ml of 12 M hydrochloric acid, take again to dry- ness and treat the residue with 20 ml of a 0.08 M solution of the diammonium salt of NTA. Adjust to PH 7 with ammonia solution. Boil for I h under reflux and dilute to 25 ml. Spot 25 ,ul on a paper strip, Whatman no. I, 22 x 2 cm. Place the ends of the strip in the cathode and anode compartments and immerse the central part in the carbon tetrachloride cell. Allow the anode and cathode solutions to diffuse to the applied zone. Apply the 500-v potential to the platinum electrodes during 6-10 min. Stir the carbon tetrachloride solution by means of the magnetic stirrer to provide more efficient cooling. Remove the paper strip and dry under a warm air blower. The detection of the focusses is done by autoradiography or by counting.

Anal. C/rim. Acta. 3G (1966) 210-214

SEPARATION OF FISSION PRODUCTS 213

Autovadiogvaphy Place the paper strip on Gevaert Scientia film Structure X and expose for 4 to

12 h. Develop in Gevaert 251 for 4 min.

Counting Cut the paper strip into pieces of 1-3 mm width. Place the pieces into counting

vials for counting the y-emitters or mount on planchets for G.M.-counting in the case of pure p-emitters. Purity checks of the focusses were done by y-spectrometry with a 4oo-channel analyser.

DISCUSSION

As can be seen from Figs. z and 3, clearly defined focusscs are obtained of the fission products Cs, Sr, Ce, Y and the mixture Zr-Nb-Ru. The uranium can be locat- ed visually as a rather broad zone of up to I cm situated between the focusses of Y and Zr +Nb + Ru. As also appears from Figs. 2 and 3, the distances between the focusses arerespectivelyZr+Nb+Ru-Y: I cm; Y-Ce: r-1.5 cm; Ce-Sr: 3.5 cm; Sr-Cs: 1.5

cm. The purity of the different focusscs was checked by y-spectrometry, referring to the pure nuclides. The results are given in Table I.

TABLE I

CROSS-CONTAMINATION OF TII15 POCUSSES

-.---_-.___-___-__-_-.~_____ _I__--

i~ocrts AVlrclide “/I Fotrmd Stut~dnrd deviafion

Zr f Nb + J<u 144& lO0Rl.l “e%r -I- "6Nb

Y- ‘4JCc ‘OaRll “a%r -t ObNb

cc l44&

lo”Ru “a%r -/- O*Nb

4.2 2.2

99.2 2.5

98.4 I . A{ 4.7 1.8

x.3 x.8 0.0 0.G

91.1 2.3 -0.5 1.8

I.5 o-7

l Cross-contaminations mcasurcd nftcr decay of QOY.

It appears from Table I that the cross-contaminations are practically within the computed standard deviations, except for cerium where a slight cross-contami- nation occurs in the Zr + Nb + Ru and yttrium focusses.

This work is part of the research sponsored by the “Tnteruniversitair Tnstituut voor Kemwetenschappen”.

SUMMARY

Electrophoretic focussing of ions was applied to the separation of the long-lived fission products Zr, Nb, Ru, Y, Ce, Sr and Cs. With hydrochloric acid and nitrilotri- acetic acid as the anodic and cathodic electrolytes respectively, a quantitative separation could be obtained, but Zr + Nb + Ru was left as one focus. Detection of the nuclides was by autoradiography or by y- and /I-counting.

Anal. Chim. Ada, 36 (x966) 2x0-2x4

2x4 M. PAUWELS, R. GIJBELS, J. HOSTE

Une mbthocle de focalisation electrophoretique d’ions est proposee pour la s&paration de produits de fission de longues pCriodes: Zr, Nb, Ru, Y, Ce, Sr et Cs. On peut obtenir une separation quantitative avec l’acidc chlorhydrique et l’acide nitrilo- triacdtique comme Clectrolytes respectivement anodique et cathodique (A l’exception de Zr+Nb+ Ru). La d&ection des nuclides se fait par autoradiographie ou par comptage y et /3.

ZUSAMMENI:ASSUNC

Es wurde die elelctrophoretische Fokussierung von Ionen bei der Trennung folgender langlebiger Spaltprodukte angewandt: Zr, Nb, Ru, Y, Ce, Sr und Cs. Mit SalzsSure und Nitrilotriessigsiiurc als anodischer bzw. kathodischer Elektrolyt gelang es, tine quantitative Trennung zu erhalten, jedoch blieben Zr+Nb+Ru als ein Fokus zurtick. Der Nachweis der Nuklide geschah durch Autoradiographie oder durch y- und /?-Zihlung.

REFERENCES

I

2

3

i! 6

z 9

IO II

A. GRAND-CLEMENT, %. JAKOVAC,~~. LEDERXR AND E. PLUCIIET, hoc. I~dwtr. Symp. Micvo- clrenristry, Rivmiqhant. 1958. D. IMRISOVA AND V. KNOI~LOCH, Collection Czech. Clrem. Commun., 28 (1963) 331, S. KAWAMURA, Japutr Analyst, II (8) (1gG2) 814. E. LAZZARINI AND I?. GERATOPOULOS, Metaltw&a Italia~ra, 54 (7) (1902) 300. S. I<. SHUKLA AND M. LEDERIZR, ,/. Cbonralog.. g (2) (xgG2) 255. F. V. ZIMAKOV, A. G. BYKOV AND J. A. USACHLWA, Soviet Uwiou Teclrnicd ScientiJic Cotaference on tire Applicatio~a of Radioactiva and Stable Isotopes, Moscow, 1957. E. SCHUMACHER AND 1-f. J. STRDII~, Ifelv. Chim. Acta, 41 (1958) 824. M. SEIINAGAWA AND Y. KISO, J@as /lmzlyst, IO (1961) grz. B. SCHUMACIIER, Helv. Cirim. Act%. 40 (x957) 221, 2322. E. SCHUMACIIER AND f-1. J. STRIZW, 8felv. Chim. /1ct~, 40 (fg57) 228, 234. E. SCHUMACI~ER AND W. FRIEDLI, Nelv. Chim. Acta, 43 (1900) 1706; 44 (1961) 1829.

Anal. Chim. Actu, 36 (1966) 210-214