Journal of Nuclear and Radiochemical Sciences, Vol. 9, No. 2, pp. 37-39, 2008

Influence of Boric Acid in the Extraction of Uranium (VI), Plutonium (IV) and Europium(III) from Fast Reactor Fuel Dissolver Solution
K.A. Venkatesan, V. Sukumaran, M.P. Antony, and T.G. Srinivasan*
Fuel Chemistry Division, Indira Gandhi Centre for Atomic Research, Kalpakkam 602 102, India.
Received: May 17, 2008; In Final Form: August 16, 2008
Addition of boric acid to dissolver solution has been proposed for minimizing the criticality concern associatedwith reprocessing of fast reactor fuel. Extraction of uranium (VI) and plutonium (IV) from the resultant nitric acid– boric acid medium by a solution of tri-n-butyl phosphate (TBP) in n-dodecane (n-DD) has been studied at 298 K.
The distribution ratios of uranium (VI) and plutonium (IV) were found to be comparable with those obtained in
the absence of boric acid. Extraction of europium (III) from nitric acid – boric acid by a solution of 1.2 M TBP–0.2
M n-octyl(phenyl)-N,N-diisobutylcarbamoylmethylphosphine oxide (CMPO) in n-DD was also studied. The solubility limit of boric acid as a function of nitric acid concentration was determined and it was found to decrease
from 61.5 g/L in 0.1 M HNO3 to 32.4 g/L in 4 M HNO3 at 303 K. Extraction of boric acid in 1.1 M TBP/n-DD andin a solution of 1.2 M TBP–0.2 M CMPO in n-DD was studied and it was found to increase with increase in theconcentration of boric acid above ~0.2 M. The study indicated that the interference of boric acid during the extraction of uranium (VI) plutonium (IV) by PUREX solvent and Eu(III) by TRUEX solvent was negligible and theadded boric acid could be quantitatively retained in aqueous phase provided the boric acid concentration was fixed
below 6 g/L (~0.1 M) in the feed solution.

1. Introduction
PUREX process is being adopted for closing the nuclear fuelcycle and it basically consists of solvent extraction of uranium
and plutonium by 1.1 M solution of tri-n-butyl phosphate (TBP)
in n-dodecane (n-DD) from 3 – 4 M HNO3 medium, which is
obtained after the dissolution of spent nuclear fuel in nitricacid.1 The spent nuclear fuel from fast reactor differs remarkably from that of thermal reactors in the composition of fissile
elements, yield and amount of fission products. These factors
tend to pose several complications such as criticality, thirdphase formation, chemical and radiolytic degradation 2-5 etc.
during solvent extraction and subsequent re-conversion processes.
The concern over critically arises due to handling of significantly large amounts of fissile element, 239Pu, in addition toother transplutonium elements, in aqueous feed solution during
solvent extraction stage. This concern can be minimized byadding strong neutron absorbers such as rare-earth metal ions(e.g. gadolinium), boric acid etc, to aqueous feed. Addition ofthese external reagents may, however, pose problems during
the management of the raffinate, namely high-level liquidwaste (HLLW) obtained after extraction. For example additionof trivalent rare-earth metal ion eventually increases the concentration of lanthanide ions in HLLW, which is likely to facilitate third phase formation during minor actinide partitioningby TRUEX process3-6 and also complicate lanthanide-actinidegroup separation.7 Nevertheless, the advantage of adding rareearth element, from the extraction point of view, is its wellknown chemistry and route of trivalent lanthanides in PUREX
process flow sheet. In contrary, boron is not formed as a fission product. Therefore, the extraction chemistry of boron,
routing of boric acid in PUREX process and influence of boric
acid on the extraction of target metals etc. are unknown.
However, the merit of using boric acid is that the added boron
forms a compatible glass matrix with high-level liquid waste*Corresponding author. E-mail: tgs@igcar.gov.in, Fax: +91-4427480065

along with other glass formers during immobilization ofHLLW, provided it is pushed quantitatively in to HLLW stream.
In view of these, the primary aim of the present investigation isto address these unknown issues of using boric acid during
reprocessing. In this context, the extraction of behavior of uranium (VI) and plutonium (IV) from nitric acid – boric acidmedium by a PUREX solvent (1.1 M TBP/n-DD) and Eu(III)by TRUEX solvent (1.2 M TBP–0.2 M CMPO/n-DD) was
studied and the distribution ratios are compared with thoseobtained in the absence of boric acid. Solubility of boric acidin nitric acid medium was determined and the quantity of boric
acid that can be added to the dissolver solution during reprocessing was recommended.
2. Experimental
Materials. All the chemicals and solvents used in the present study were of analytical grade. Tri-n-butylphosphate anduranyl nitrate were obtained from E.Merck, Mumbai. CMPO
was synthesized at National Chemical Laboratories, Pune,
India and used after purification.
Solubility of boric acid in nitric acid medium. The solubility of boric acid in nitric acid medium was determined byadding a known quantity of boric acid (5–10 mg) to a solutionof nitric acid (25 mL) of desired concentration taken in anequilibration tube. This tube was immersed in a constant temperature water bath and the mixture was stirred at a desired
temperature. Addition of boric acid (5–10 mg) was continued
until the solution under stirring leaves a permanent insoluble
residue in solution. The cumulative weight of boric acidobtained after the observation of first insoluble residue wastaken as the solubility limit of boric acid in nitric acid mediumand it was represented in the units of g/L. The solubility limit
thus obtained was plotted against temperature. Solubility ofboric acid was also determined in a solution containing uranium (50 g/L) in 4.0 M HNO3 medium.
Extraction of uranium (VI) by TBP/n-DD. All the extraction studies were carried out at 298 K by mixing equal volumeof organic and aqueous phases (3 mL each), unless otherwise© 2008 The Japan Society of Nuclear and Radiochemical Sciences
Published on Web 12/05/2008

38

Venkatesan

J. Nucl. Radiochem. Sci., Vol. 9, No. 2, 2008

TABLE 1: Distribution ratios of uranium (VI) in TBP/n-DD at various nitric acid concentrations in the presence andabsence of 0.1 M H3BO3 at 298 K
TBP concentration in n-DD
0.5 M

0.2 M

[HNO3]/M

Absence of
H3BO3
0.08
0.79
2.04
3.62
4.75
4.81

0.1
12
3
45Presence of
H3BO3
0.04
0.99
1.95
3.98
4.69
5.00

Absence of
H3BO3
0.19
2.67
6.49
11.30
13.47
15.56

TABLE 2: Comparison of distribution ratios of uranium(VI) at various nitric acid and boric acid concentrations at298 K[HNO3]/M
0.11230.2 M TBP/n-DD
0.1 M
0.5 M
H3BO3H3BO3

0.5 M TBP/n-DD
0.1 M
0.5 M
H3BO3
H3BO3

0.04
0.99
1.95
3.98

0.11
2.88
6.21
12.76

0.01
0.91
2.2
4.15

0.11
2.94
6.43
11.23

mentioned, in a 20 mL capacity test tube. These tubes wereclamped in test tube rotator that provides an upside down rotation of tubes at a speed of 50 rpm. Pre-equilibration of organic
phase was carried out with desired concentration of nitric acidas well as with boric acid – nitric acid solution. Extraction ofuranium (VI) at various nitric acid concentrations (0.1 M to 5.0M) was studied by mixing organic phase containing TBP solution in n-DD (varied, 0.2 M to 1.1 M) with aqueous phase containing desired concentration of nitric acid, boric acid (varied,0.1 M to 0.5 M), and uranyl nitrate (4.2 × 10 -4 M) spiked with233
U tracer in a glass tube. After two hours of equilibration, thedistribution ratio (D) of uranium was determined from theradioactivity measurement of 233U present in organic and aqueous phase and by using eq 1.
D=[233U] org[233U] aq(1)

A similar experiment was carried out for the extraction of239
Pu(IV) and (152+154) Eu(III) by using PUREX and TRUEX solvent respectively and the distribution ratios were determined.
Extraction isotherm of boric acid. The extraction of boric
acid by 1.1 M TBP/n-DD was studied by equilibrating 3 mL oforganic phase with 3 mL of aqueous phase containing various
concentrations of boric acid. The amount of boric acid remaining in aqueous and organic phases was determined by alkalimetric titration of boric acid in the presence of mannitol. Asimilar experiment was carried out using TRUEX solvent.
3. Results and Discussion
Extraction studies. The distribution ratios for the extraction of uranium (VI) from nitric acid medium by a solution of
TBP in n-DD are tabulated in Table 1, and the values are compared with those obtained in the presence of 0.1 M H3BO3 in
the feed solution. It is observed that the distribution ratios ofuranium in the presence and absence of boric acid are similar
and they are within the limits of acceptable variations/trend.
Increasing the concentration of boric acid to 0.5 M in the feed
also does not alter the distribution ratios to any significant

Presence ofH3BO3
0.11
2.88
6.21
12.76
13.76
16.30

1.1 M
Absence of
H3BO3
0.46
7.28
14.55
21.80
29.20
30.91

Presence of
H3BO3
0.30
6.60
14.05
21.30
29.07
33.20

TABLE 3: Distribution ratios of plutonium (IV) in 1.1 M
TBP/n-DD and Eu(III) in 1.2 M TBP-0.2 M CMPO/n-DD
at various nitric acid concentrations in the presence andabsence of 0.1 M H3BO3 at 298 K(152+154)
Eu(III)
1.2 M TBP-0.2 M
CMPO/n-DD[HNO3]/M
Absence of Presence of Absence of Presence of
H3BO3
H3BO3
H3BO3
H3BO3239Pu(IV) in
1.1 M TBP/n-DD

0.1
0.5
123
45-0.96
2.23
5.55
9.25
11.4
15.9

-0.91
2.21
5.67
9.91
12.0
15.1

0.51
--9.58
14.0
14.6
15.2
14.8

0.11
--8.95
12.6
13.2
13.2
14.5

extent as shown in Table 2. The boric acid of concentration 0.5
M (~31 g/L) was not completely soluble in 4 M HNO3 at 298 K.
Therefore the distribution ratio of uranium from 4 M HNO3
medium containing 0.5 M H3BO3 could not be generated. The
study indicates that the influence of boric acid in the extractionof uranium (VI) by TBP/n-DD is negligible.
The distribution ratios for the extraction of 239Pu(IV) in 1.1
M TBP/n-DD and (152+154) Eu(III) in 1.2 M TBP–0.2 M CMPO/
n-DD from HNO3–0.1 M H3BO3 medium are shown in Table 3.
It is observed that the distribution ratios of plutonium in thepresence and absence of boric acid are similar. However, amarginal decrease in the distribution ratio is observed for theextraction of europium (III) by TRUEX solvent.
Solubility of boric acid in nitric acid medium. Solubilityof boric acid in nitric acid medium is required for assessing thequantity of boric acid that can be added to the dissolver solution for minimizing the criticality concern. The solubility limit
profiles of boric acid in various concentrations of nitric acid atdifferent temperatures are shown in Figure 1. It is seen that thesolubility limit of boric acid decreases with increase in the concentration of nitric acid due to common ion effect (eq 2), andthe solubility increases linearly with increase of temperature.
→ B(OH) 4 + H+,B(OH)3 + H2O ←(pKa = 9.05 at 298 K, I =0.1 M with NaCl)–(2)

The solubility limit of boric acid in 3 M HNO3 at 303 K is 34.8g/L and it increases to 58.2 g/L at 318 K. At a constant temperature of 303 K, the solubility limit of 61.5 g/L in 0.1 M HNO3decreases to 32.4 g/L at 4 M HNO3. The solubility of boric
acid in a solution of 50 g/L uranium in 4.0 M HNO3 at 303 K isin different from the value of ~32 g/L observed in the absenceof uranyl nitrate.
Extraction of boric acid. The extraction isotherm of boric
acid by PUREX and TRUEX solvent is shown in Figure 2. It
is observed that extraction of boric acid by 1.1 M TBP/n-DD isInfluence of Boric Acid in the Extraction of Uranium (VI), Plutonium
104

800.050
0.045

3 mL of boric acid + 3 mL of organic phase
T = 298 K; Equilibration time = 3 hours

0.040
0.035

[H3BO3]org /M-188390.055

0.1 M HNO31.0 M HNO32.0 M HNO33.0 M HNO34.0 M HNO396

Solubility of boric acid/ g.L

J. Nucl. Radiochem. Sci., Vol. 9, No. 2, 2008

726456

0.030
0.025
0.020
0.015
0.010

480.005

400.000

32300-0.005302304306308

310

312

314316

318

320

0.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

[H3BO3]aq /M

Temperature/KFigure 1. Solubility limit of boric acid in nitric acid medium at various temperatures.

Figure 2. Boric acid extraction isotherm by PUREX and TRUEX
solvent at 298 K.

negligible when the aqueous phase concentration of boric acidis below 0.2 M and significant extraction is observed whenboric acid concentration in aqueous phase exceeds 0.3 M. It isalso observed from Figure 2 that the extraction of boric acid by
TRUEX solvent is insignificant when the concentration ofboric acid is 0.1 M and thereafter the extraction increases withincrease in the concentration of boric acid in aqueous phase.
Since boric acid is a weak acid, the extraction could be attributed to the formation of TBP–H3BO3 complex in organic phasesimilar to nitric acid extraction8 by TBP. In PUREX process,the feed concentration of nitric acid is fixed between 3 and 4
M. In such medium, the solubility limit of boric acid is ~0.4 M(=25 g/L) at 298 K (from Figure 1) and the extraction of boric
acid by 1.1 M TBP/n-DD from 0.4 M H3BO3 medium is ~0.01
M as indicated in Figure 2. However, in the presence of 3 – 4
M HNO3, the dissociation of boric acid (eq 2) is suppressedsignificantly. Therefore, it is unlikely that proton from boric
acid gets associated with TBP and forms a complex, TBP–H 3BO3, in preference to strong acid–TBP complex, namelyT BP – H NO 3 , i n o r g a n ic p h a s e i n PU R E X p r o c e s s.
Nevertheless, it is desirable to fix the boric acid concentration
at 0.1 M in aqueous phase to ensure no extraction of boric acidin organic phase and to minimize further the dissociation ofboric acid. Under such conditions, the mechanism of uranium(VI) extraction by 1.1 M TBP/n-DD could be forced in to awell-established mechanism of extraction, shown in eq 3
involving two molecules of TBP and nitrate ion in the formation of neutral uranyl nitrate–TBP complex, [UO 2 (NO 3 ) 2(TBP) 2], in organic phase. The likely participation of borate
ion in the extraction step, shown in eq 4, could thus be excludedby the employment of low concentration of boric acid (0.1 M),
where the suppression of dissociation of boric acid is prevailingin 3 – 4 M HNO3 medium. The similarity in the distribution
ratios of uranium (VI) and plutonium (IV) in the presence andabsence of boric acid also supports that these metals are notextracted as uranium (VI) –borate or plutonium (IV) –borate
complexes. Analysis of boron in the aqueous phase by ICPAES after stripping of uranium from organic phase also showedthat the concentration of boron in it was negligible. In view ofthis, it is desirable to fix the boric acid concentration at 0.1 M
in aqueous phase to ensure no extraction of boric acid inorganic phase, to eliminate the contamination of boron in U/
Pu/Ln product stream and to retain the added boric acid inaqueous phase after extraction.

[UO2(NO3)2(TBP)2] org
UO22+ + 2 TBP

–(3)

–

2 NO3 + 2 H2BO3[UO2(H2BO3)2(TBP)2] org(4)

4. Conclusion
The solubility of boric acid, at 303 K, in 4 M HNO3 was 31g/L (~0.5 M) and therefore the maximum concentration ofboric acid that can be maintained in PUREX process feed was
0.5 M at 303 K. The extraction behavior of uranium (VI) andplutonium (IV) from the resultant nitric acid – boric acidmedium in 1.1 M TBP/n-DD was comparable with the behaviorobserved in the absence of boric acid. Extraction of boric acidby 1.1 M TBP/n-DD was negligible when the aqueous phase
concentration of boric acid was lower than 0.2 M and significant extraction was observed above 0.3 M. However, extractionof boric acid by TRUEX solvent was significant above 0.1 M inboric acid. Therefore, it is desirable to fix the concentration ofboric acid in PUREX feed to 0.1 M (= 6 g/L) to take the advantage of eliminating borate contamination in U/Pu/Ln productstream and route all the added boric acid to HLLW.
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