Journal of Nuclear and Radiochemical Sciences, Vol. 12, No.1, pp. 1-4, 2012

ArticlesBehavior of Fission Products in Sulfide Reprocessing Process
Akira Kirishima,* Yuuki Amano and Nobuaki Sato
Institute of Multidisciplinary Research for Advanced Materials, Tohoku University, 1-1 Katahira, 2-chome,Aoba-ku, Sendai 980-8577, Japan
Received: March 30, 2012; In Final Form: June 19, 2012
For the recovery of nuclear materials from spent fuel with more effective and convenient methods comparing withconventional process, the sulfurization and dissolution behavior of fission products, such as rare-earths, alkali,alkalline-earth and platinum group elements were studied. The sulfurization experiment was carried out using
tracer doped U3O8. The samples were reacted with CS2 at temperatures from 573 to 773 K for 1 hour followed bydissolution with 1M nitric acid solution for 1 hour at 323 K. The dissolution ratio for each element was obtainedby α- and γ-ray spectrometry. The alkali and alkaline-earth elements show higher dissolution ratios as well astrivalent lanthanide elements. On the other hand, U, Zr, Ce, and Ru showed lower dissolution ratios. Theseresults were in good agreement with those expected from the thermodynamic consideration.

1. Introduction
For the recovery of nuclear materials from spent fuel withmore simple and cost effective process comparing with theconventional wet process, the sulfide reprocessing process has
been proposed and studied.1-2 In this process, nuclear materials
are recovered as the oxides by a selective sulfurization of FPs
and selective dissolution of the sulfurized FPs. The points ofthis process are, 1) sulfurization treatment at relatively low
temperature, 2) reduction of uranium oxide by CS2, 3) less consumption of chemical reactants, 4) recovery of the fuel materials as oxides. In our previous studies,2-4 the phase analyses ofU, Nd and Eu after the sulfurization treatment by CS2 wereconducted using XRD and thermogravimetric analysis wherethe starting materials were U3 O8 , Nd 2 O3 and Eu 2 O3 respectively. The results indicated that CS2 reduced U3O8 to UO2 andsulfurized Nd2O3 and Eu2O3 to NdS2 and Eu3S4, respectively inthe range of sulfurization temperature 573 to 773 K. Thisresult reflects a unique and useful chemical property of CS2 ,that is, CS2 works as a reducing agent for U3O8 while it works
as a sulfurizing agent for trivalent lanthanide. After the sulfurization treatment, trivalent rare-earths were separated fromthe uranium matrix by the dissolution with a dilute nitric acidsolution, while UO2 was recovered as a solid phase by utilizing
the low solubility of UO 2 for the dilute acid solution. The
behavior of minor actinides as well as Pu was reported in theprevious works.4, 5 In this paper, therefore, the sulfurizationand dissolution behavior of fission products, such as rareearths, alkali, alkaline-earth and platinum group elements wasinvestigated using the tracer doped U3O8 with the help of thermodynamic consideration.
2. Experimental
2.1. Sample Preparation. The U3O8 was obtained by theoxidation of U metal turnings in air at 1073 K. Then the tracer
doped U3O8 was synthesized as follows. A nitric acid solutionof FP tracers, i.e., 152Eu, 141Ce, 95Zr and 103Ru were mixed withuranyl nitrate solution. Then, it was co-precipitated as a tracer
doped ammonium di-uranate (ADU) by adding adequate
amount of ammonia solution. This ADU was calcined to U3O8in air at 1273 K for 8 hours. The nitric acid solution containing
137Cs and 85Sr tracers was then added to the above mentioned
U3O8 powder and calcined in air again at 1073 K for 8 hours toavoid the volatilization of 137Cs. The chemical abundances ofCs, Sr, Eu, Ce, Zr and Ru in the tracer doped U3O8 sample werein trace levels which were chemically ignorable. Analyticalgrade of CS2 having the boiling point 46-47 °C and maximum
water content of 0.02% (Wako Pure Chemicals Co., Ltd.) andArgon gas of 99.99% purity (Nippon Sanso Co., Ltd.) wereused as received.
2.2. Sulfurization. The sulfurization experiments were carried out using a quartz apparatus shown in Figure 1. After theweighed sample was placed on the quartz boat in a quartz tube,K type thermocouple
FurnaceQuartz tube
Gas inlet

×Gas outlet
Quartz boatSample
Figure 1. Schematic drawing of sulfurization apparatus.

*Corresponding author. E-mail: kiri@tagen.tohoku.ac.jp© 2012 The Japan Society of Nuclear and Radiochemical Sciences
Published on Web 07/30/2012

KirishimaJ. Nucl. Radiochem. Sci., Vol. 12, No. 1, 2012

TABLE 1: Half life and γ-ray energy of the FP tracers

Tracer

137

85

152

Half life

30.17(y)64.9(d)Analyzedenergy (keV)

662

514

Cs

Sr

95

103

13.33 32.50
(y)
(d)

64.0
(d)

39.35
(d)

122

757

497

Eu

141

Ce

145

Zr

Ru

10000

counts / 80000 sec.

the inside of the reaction system was evacuated by a rotary
pump followed by the refilling with argon. Then the temperature of the furnace was raised to an intended temperature at aheating rate of 10 °C/min. At the intended temperature, themixed gas of CS2 with Ar, which was obtained by bubbling Ar
gas through a bubbler containing liquid CS2 , was introduced
into the quartz tube by a flow rate of 5 ml/min of CS2 and 30
ml/min of Ar. The flow rate of Ar gas was controlled using adigital mass flow meter (Kofloc Model DPM-2A). The sulfurization time was set to 1h, hence, the temperature and mixed
gas flow rate were kept constant during the sulfurization for 1h. After that, the supply of the mixed gas was stopped, andthen the inside of the quartz tube was flashed and cooled down
by Ar gas.
2.3. Dissolution. After the sulfurization treatment, the RI
tracer doped U3O8 sample was immersed into 100 ml of 1 M
nitric acid solution and shaken for 1 hour at 323 K by an air
bath shaker. Then, the solution and residue were separated bysuction filtration to determine the dissolution ratio of U, Eu,Ce, Zr, Ru, Cs and Sr to the acid solution by α- and γ-ray spectrometry.
2.4. α- and γ-ray spectrometry. For the determination ofthe dissolution ratio of each element, α- and γ-ray spectrometry
was applied by a surface semiconductor detector (Canberra,model-7401) and a coaxial Ge semiconductor detector(Camberra, model-GC1318), respectively. The isotopes usedfor analysis were 238U, 152Eu, 141Ce, 95Zr, 103Ru, 137Cs and 85Sr.
The half life and γ-ray energy of tracers used for the calculation of the distribution ratio between solution fraction and residue fraction are given in Table 1. As an example, the γ-rayspectrum of the sample is shown in Figure 2. For the quantitative α spectrometry of U, the samarium co-precipitation
method 6 was employed in this study, where a natural radionuclide 147Sm is used as the inner standard for the counting efficiency calculation. The detail of the α spectrometry isdescribed in our previous work.5 For the analysis of U, the αpeak of 238U at 4.20 MeV in the obtained spectrum was used.
The dissolution ratio (D R) of U, Eu, Ce, Zr, Ru, Cs and Sr isderived by the following equation.

1000

100100200

300

400500

600

700

800

Energy (keV)

Figure 2. The γ-ray spectrum of the tracer doped U3O8.

10080DR (%)

260CsSr40EuU

200

Activity of tracer in solution (cps)
DR =Activity of tracer in solution and residue (cps)(1)

3. Results and Discussions.
3.1. Behavior of rare earth oxides. In order to know thebehavior of rare earth oxides in the U3O8 matrix, the effect ofsulfurized temperature on D R was investigated with dissolution
time of 1 hour and nitric acid concentration of 1.0 M. The relation between sulfurization temperature and D R of Eu wasshown in Figure 3. The horizontal axis and vertical axis show
the sulfurization temperature and D R, respectively. The valuesof D R for Cs, Sr, Eu and U are shown as u, ¢ , p, and »,respectively. It can be seen that the D R of Eu is around 40 % attemperatures lower than 623 K, while Eu becomes soluble
when the sulfurization temperature is higher than 623 K. The
trivalent rare earth oxides are expected to show similar behavior to that of Eu, nevertheless, the heavy rare earth oxides aresupposed to need higher sulfurization temperature to makethem soluble than light ones by the sulfurization treatment. On
the other hand, D R of Ce was obtained as 6.2 % at sulfurization
temperature 673 K as shown in Table 2. Since Ce forms stable
tetravalent oxide CeO2, this result suggests that tetravalent rareearth oxides are less soluble even after the sulfurization treatment. As seen in Figure 3, D R of U is less than 5.5 % in theinvestigated temperature range, indicating that the insoluble U
could be separated from rare earth oxides by the selective sulfurization and the acid dissolution procedures. Although the523

623
723
Sulfurization Temp. (K)823

Figure 3. Dependence of DR for Cs(u), Sr(¢), Eu (p) and U(») onsulfurization temperature. Dissolution Time: 1.0 hour with 1.0 M
HNO3.
TABLE 2: Dissolution ratios of Ce, Zr and Ru at sulfurization
temperature 673 K

ElementCe

ZrRu

D R (%)

6.2

5.2

1.1

acid concentration, the volume of the acid solution, the solution
temperature and the shaking time were fixed in the dissolution
treatment in this paper, the D R of each elements are expected todepend on those conditions. The optimum dissolution condition will be sought in the future study. Figure 4 shows theGibbs free energy for the reactions of UO2, Eu2O3 and SrO withCS2 . According to our precious study, 2 U3 O 8 seems to be
reduced to UO2 by the reaction of CS2 first. As can be seen inFigure 4, the ∆G 0 values for the sulfurization of UO2 are positive showing that UO2 does not react with CS2. On the otherhand, the ∆G 0 values for Eu2O3 are very negative. These thermodynamic tendencies agreed with the experimental results,i.e., Eu2O3 was sulfurized by CS2 and forming soluble Eu2O2SHighly Time-Resolved Measurements of Airborne Radionuclides

J. Nucl. Radiochem. Sci., Vol. 12, No. 1, 212

100

1 /2O2=SO2

SrSO4log P(O2) / atm0(2)0�� / kJ mol-10

S2 +-200

C+S2 =CS220(1)

-100

3-20
C+O2=CO2

SrO-40

(3)
-300

-400

400

600

800

Temperature / K

1000

SrS-60(1)UO 2+CS2=US2+CO2(2)Eu2O3+CS2=Eu2O2S+COS(3)2SrO+CS2=2SrS+CO2

Sr-80
-80

1200

Figure 4. Gibbs free energy for the reactions of UO2, Eu2O3 and SrOwith CS2.

-60-40-200log P(S2) / atm

20Figure 5. Stability diagram for the Sr-O2-S2 system at 773 K.

1/ 2S+O
2 =SOZr(SO4)220log P(O2) / atmand Eu3S4 while U3O8 was reduced to insoluble UO2.
3.2. Behavior of alkali and alkaline earth oxides. To
investigate the behavior of alkali and alkaline earth oxides inthe sulfide reprocessing process, the sulfurization and aciddissolution was examined for Cs and Sr. The D R of Cs and Sr
were shown in Figure 3. In both cases, their D R values aremuch higher than that of U at the observed temperature range.
Then the D R values increases more than 80 % at 773 K, clearly
showing that Cs and Sr dissolve into nitric acid after thesulfurization treatment. From the ∆G 0 values for thesulfurization reaction of SrO with CS2 as given in Figure 4,
SrO is thermodynamically easily sulfurized by CS2 to SrS.
Unfortunately, the ∆G 0 values for the suflurization of Cs oxide
and other alkaline metal oxides could not be obtained for thelack of thermodymic data. The thermodynamic stability of thecompounds in the Sr-O2 -S2 system was discussed by using apotential diagram at 773 K constructed using the DATABASE
MALT for Windows.7 The result of the calculation is shown inFigure 5, where the SO2, CS2 and CO2 of 1 atm are given as thedotted lines. The stable sulfide and oxide are indicated to beSrS and SrO, and the SrSO4 phase appears at higher log P(O2)and log P(S2) region. When SrO react with 1 atm CS2 gas atSO2 pressure 1 atm, formation of SrS proceed along the dottedline. The cross point of the log P(S2 ) and log P(O 2 ) lines
locates in the SrS region. From these results, alkaline earthoxides are supposed to be easily sulfurized by CS 2 andseparated from UO2 by the acid dissolution.
3.3. Behavior of refractory metal oxides. Next, thebehavior of refractory metal oxides was discussed. The D R
values of Zr and Ru sulfurized at 673 K for 1 hour anddissolved by 1 M HNO3 at 323 K were given in Table 2. The
D R of Zr was 5.2 %, which was close to those of U and Ce.
The thermodynamic stability of the compounds in the Zr-O2-S2
system at 673 K was discussed with using a potential diagram
shown in Figure 6. In this figure, the dotted lines indicate 1atmof SO2 , CS2 and CO2 . The potential diagram shows that thestable sulfide and oxide of Zr are ZrS2 and ZrO2 , then, theZr(SO 4 ) 2 phase appears at higher log P(O 2 ) and log P(S2 )region. The crossing point of the log P(S2) and log P(O2) lines
locates in the ZrO2 region, suggesting that the formation ofZrS2 seems not to proceed along the dotted line when ZrO2reacts with 1 atm CS2 gas at SO2 pressure of 1 atm. Due to thishigh stability of ZrO2 , the D R value of Zr became similar toC+S2 =CS220-20
C+O2=CO2-40

ZrO2ZrS2-60-80-80

Zr-60-40-20log P(S2) / atm

020Figure 6. Stability diagram for the Zr-O2-S2 system at 673 K.

those of U and Ce. From these results, refractory oxides arestable under CS2 atmosphere at lower sulfurization temperature,and so they will be recovered with U and Ce as their oxides.
The development of a separation method of Zr and Ce form U
in the proposed sulfide process are one of the future subjects tobe investigated. Besides, those elements could be fabricated as
low decontamination factor UO2 fuel without separation fromU, since Ce and Zr can stably exist with UO 2 as the inertmatrix.
3.4. Behaivor of metal component. In a spent fuel, themetallic phase which consists of Mo and platinum group elements exists. This phase is chemically stable and remains as
the insoluble residue during wet reprocessing process. Thereaction between these metal components and CS2 was discussed by using the thermodynamic data. Figure 7 shows theGibbs free energy for the reactions of Pd, Rh, Ru and Mo with
CS2. It can be seen that the ∆G 0 values for Mo, Rh and Ru arevery negative in the above temperature range. In the case ofRh, the Rh2S3 was taken into account. It should be noted thatthe ∆G 0 values for the sulfurization of Pd changes from nega-

4Kirishima

J. Nucl. Radiochem. Sci., Vol. 12, No. 1, 2012

fide reprocessing process, the sulfurization and the dissolution
treatments were carried out using the tracer doped U3O8 sample. The sample was reacted with CS2 at temperatures from573 to 773 K for 1 hour followed by the dissolution into 1M
nitric acid solution. The alkali and alkaline-earth elements
showed higher dissolution ratios as well as rare-earths excepting Ce that kept stable and insoluble tetravalent dioxide state.
On the other hand, U, Zr, Ce and Ru showed lower dissolution
ratio. These results were in good agreement with those
expected from the thermodynamic consideration. These results
suggest the separation of fission products by the sulfide process.

100-100(4)-200(5)0�� / kJ mol-10(6)
-300

-400

(4)Pd+CS2=PdS2+C(5)4/3Rh+CS2=2/3Rh2S3=C(6)Ru+CS2=RuS2+C(7)Mo+CS2=M0S2+C

(7)

400

600

800

Temperature / K

1000

Acknowledgment. A part of this study is the result of“Basic Research on the Spent Fuel Reprocessing Process bySulfurization”carried out under the Strategic Promotion
Program for Basic Nuclear Research by the Ministry ofEducation, Culture, Sports, Science and Technology of Japan(MEXT).

1200

Figure 7. Gibbs free energy for the reactions of Pd, Rh, Ru and Mowith CS2.

tive to positive at 1100 K, meaning that the decomposition toPd and S seems to occur over this temperature. According theexperimental results given in Table 2, the D R value of Ru is
about 1 %, which is smaller than those of U, Zr and Ce. Thismay be caused by the insoluble property of RuS2. From theseresults, the metal component is supposed to remain in the residue as sulfide after the sulfurization and the acid dissolution.
Potential separation ideas of the platinum group elements fromU are, 1) selective dissolution of platinum group to a molten
metal, such as Cu, and 2) platinum group phase separation byutilizing the difference of specific gravities between platinumgroup and U oxide powders. It should be studied in the futurework.
4. Conclusion
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