J Radioanal Nucl Chem (2012) 293:743–749 DOI 10.1007/s10967-012-1672-7
Effect of accelerators on thoria based nuclear fuels for rapid and quantitative pyrohydrolytic extraction of F2 and Cl2 and their simultaneous determination by ion chromatography Ashish Pandey • Anoop Kelkar • R. K. Singhal • Chetan Baghra • Amrit Prakash • Mohd. Afzal • J. P. Panakkal
Received: 16 December 2011 / Published online: 11 February 2012 Ó Akade´miai Kiado´, Budapest, Hungary 2012
Abstract Pyrohydrolysis is a fast, reliable and convenient method for the decomposition of solid refractory samples. Thoria based mixed oxide nuclear fuels requires more than 1,200 °C reaction temperature to lose its structural integrity so as to release the halides. In the present paper, we report WO3 accelerated pyrohydrolytic extraction technique for the separation of F- and Cl- from thoria based fuels along with the feasibility of using MoO3 and V2O5. The mechanism of extraction has been investigated in detail using X-ray diffraction and recovery studies. ThO2 along with its halides undergo high temperature solid state reaction with WO3 forming Th(WO4)2 and releasing the halides for their subsequent hydrolysis. The quantification was carried out by ion chromatography with suppressed ion conductivity detection. The average recoveries of the spiked samples for F- and Cl- were 93–99%. The method was successfully applied for simultaneous determination of F- and Cl- in thorium based nuclear fuel samples at 950 °C. Keywords Pyrohydrolysis Accelerators Ion chromatography
A. Pandey A. Kelkar C. Baghra A. Prakash Mohd. Afzal J. P. Panakkal (&) Advanced Fuel Fabrication Facility, Bhabha Atomic Research Centre, BARC Complex, Post Ghivali, Dist. Tarapur, Thane 401502, Maharashtra, India e-mail:
[email protected] R. K. Singhal Analytical Chemistry Division, BARC, Mumbai 400084, India
Introduction The sources of fluoride and chloride are mainly in the form of impurities from various chemicals used during processing of fuel materials. Impurities are introduced at the back end of nuclear fuel cycle during dissolution of spent fuel in HF/HNO3. Fluoride and chloride cause local depassivation of the oxide film on the internal surface of the clad tube leading to detrimental effect in the operating reactor environment [1]. Reactions of fluoride on constituent elements of various clad materials in the reactor environmental conditions are very complex. Its most reactive characteristics, initiates the chemical reaction and exposes the clad materials to reactor operating environment resulting in enhancement of various other chemical reactions. Therefore, depending upon the type of fuel, both Cland F- have stringent specifications limits [2]. Diffusion [3, 4], distillation [5] and pyrohydrolysis [6–8] are the three main techniques used for the separation of fluoride and chloride from diverse environmental matrices. Diffusion and distillation methods have their own limitations and are mainly used for liquids, bio-organic and environmental samples. Pyrohydrolysis technique has been routinely used in the nuclear industry as it is suitable for the ceramic nuclear fuels and materials [8–10]. It is fast, reliable and convenient for the decomposition of solid samples. Being most refractory among the oxides of desirable actinides, ThO2 requires more than 1,200 °C reaction temperature or higher pyrohydrolysis distillation time to lose its structural integrity to release the halides [9]. Higher temperature open tubular furnaces are not recommended inside the glove-box/fume-hood operations. Hence it is necessary to develop fast and low temperature pyrohydrolysis method for the quantitative extraction of F – and Cl- from ThO2 powder and (Th, 3.25%U) O2 nuclear fuel. Pyrohydrolysis
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temperature can be minimized by using proper accelerator. The oxides WO3, U3O8, MoO3, V2O5, and the salt Na2W2O7 have all been recommended as accelerators but the first two are most commonly used [6]. The accelerator assisted pyrohydrolytic extraction technique has been widely used for separating small amounts of fluoride occurring in glasses, nuclear samples and raw materials for their manufacture in the starting material or product of some industrial processes [11–13]. This paper describes the use of V2O5, MoO3 and WO3 as accelerators in pyrohydrolytic extraction for the separation of F- and Cl- from thoria based fuels and quantification by ion chromatography. The mechanism of accelerated extraction has been investigated by X-ray diffraction and the method was validated by recovery studies.
Experimental
A. Pandey et al.
45 min and subjected to pyrohydrolysis. ThO2 and sintered (Th,3.25%U)O2 powder were homogenized with metal oxide accelerators V2O5, MoO3 and WO3 powder in an agate mortar prior to pyrohydrolysis to see their effect on the recovery of F- and Cl- from ThO2 and (Th, 3.25%U)O2. During pyrohydrolysis high purity O2 carrier gas was bubbled through water at 100–150 mL/min flow rate and moist gas was allowed to pass through the sample at 950 °C temperature. Distillate was collected in a graduated polypropylene bottle containing 5 mL of H2O. The IC system was calibrated using a series of standard solutions and calibration plots were obtained. Five replicates of pyrohydrolysis distillate samples were injected and concentration of F- and Cl- were determined by using this calibration. Effects of various parameters like sample size, pyrohydrolysis distillate collection time, pyrohydrolysis temperature, amount and role of accelerators on the recovery of F- and Cl- from ThO2 powder and (Th,3.25%U)O2 were extensively studied.
Starting materials and reagents (Th,3.25%U)O2 pellets sintered at 1,450 °C in air and spectroscopically pure WO3, MoO3 andV2O5 were starting materials. Analytical grade Na2CO3, Na2CO3 were procured from fluka and all solutions were prepared using high purity de-ionized water of 18.2 MX cm. All solutions were filtered through 0.45 lm membrane filter and degassed before use. The Fluka made standard solutions of F- and Cl- were prepared by appropriate dilution of 1,000 mg/L. Instrumentation A modular ion chromatographic system was used for all the chromatographic analyses. The system consisted of a reciprocating pump, pulsation damper, separation center with two, six way sample injection port with 100 lL sample loop, thermo-stated ion conductivity detector, anion regenerant chemical suppressor and CO2 suppressor module. The experiments were carried out on PRP-X 100 (250 9 4 mm ID) anion column having polystyrene divinyl-benzene copolymer with surface aminated with trimethyl ammonium functional group with its guard column. The separation centre consisting of injectors, column(s), chemical and CO2 suppressor, conductivity cell of detector and waste reservoir were kept inside a fume-hood. Pyrohydrolysis of samples were performed in all Quartz apparatus installed in a glove-box. Modified pyrohydrolysis apparatus [10] was used for the extraction of F- and Clfrom ThO2 and (Th,U)O2 Procedure ThO2 and sintered (Th,3.25%U)O2 pellets were mechanically pulverized and homogenized in mortar and pestle for
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Results and discussion Optimization of chromatographic conditions for the separation of anions In the present study, PRP-X100 was explored for the determination of common anions using mixture of Na2CO3 ? NaHCO3 as mobile phase. Mobile phase composition was optimized for the baseline separation of common anions by varying it’s composition from 2.4 mM Na2CO3 ? 2.25 mM NaHCO3 to 4.0 mM Na2CO3 ? 3.75 mM NaHCO3. PRP-X100 column being high capacity resulted in excellent separation of fluoride from the solvent front and showed large resolution with chloride. Figures 1 and 2 shows the effect of Na2CO3 ? NaHCO3 concentration on the retention of F-, Cl-, NO2-, NO3-, HPO42-, and SO42- on PRP-X100 column. Retention time for SO42- ion decreases appreciably on increasing the mobile phase concentration. When the mobile phase concentration was greater than 4.0 mM Na2CO3 ? 3.75 mM NaHCO3 the ion exchange capacity of the strong cation exchange suppressor cartridge exhausted prior to elution of SO42ion and baseline drift was observed. Therefore mobile phase composition of 3.2 mM Na2CO3 ? 3.0 mM NaHCO3 was optimized for proposed analysis. Linearity and repeatability The optimized mobile phase composition 3.2 mM Na2CO3 ? 3.0 mM NaHCO3 was used to evaluate linearity and repeatability. The calibration plots of Cl- and Fwere obtained in the range of 0.01–2 ppm. Table 1 shows the calibration data for the analysis of Cl- and F-. The
Effect of accelerators on thoria based nuclear fuels
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Optimization of pyrohydrolysis conditions for quantitative recovery of fluoride and chloride
55 -
45
F Cl NO2
40
NO3
-
2-
HPO 4
35
SO 42-
30 25 20 15 10 5 1
2
3
mobile phase no.
Fig. 1 Effect of mobile phase concentration on the retention of common anions Mobile phase 1 2.4 mM Na2CO3 ? 2.25 mM NaHCO3, flow rate 1 mL/min, Mobile phase 2 3.2 mM Na2CO3 ? 3.0 mM NaHCO3, flow rate 1 mL/min, Mobile phase 3 4.0 mM Na2CO3 ? 3.75 mM NaHCO3; flow rate 1 mL/min
Concentration in ppm
1.5
Fluoride in ThO2
1.4
Chloride in ThO2
1.3
Fluoride in (Th,U)O2
1.2
Chloride in (Th,U)O2
In order to optimize the conditions for pyrohydrolysis conditions various experiments were carried out. Firstly, the effect of pyrohydrolysis distillate collection time on the recovery of F- and Cl- from ThO2 and sintered (Th,U)O2 powders was studied. It was optimised by varying it from 30 to 150 min at constant pyrohydrolysis temperature of 950 °C and condensate volume of 25 mL. Figure 2 shows that the values of F- and Cl- did not change appreciably. Hence distillate collection time of 30 min was used for the extraction of F- and Cl-. The effect of sample size on the recovery of F- and Clfrom ThO2 and sintered (Th,U)O2 powders was studied by varying the sample size from 0.2 to 0.6 g. The results are presented in Fig. 3. The F- and Cl- values did not change beyond 0.2 g. Hence sample size was fixed to 0.2 g for subsequent studies. The effect of pyrohydrolysis temperature was studied by varying it from 900 to 1,100 °C at an interval of 50 °C, keeping distillate collection time constant as 30 min. The results are presented in Fig. 4. It was reported [9] that the 1.7
1.1
Fluoride in ThO
1.6
1.0
Chloride in ThO
1.5 0.9
1.4
0.8
1.3
0.7 0.6 0.5 0.4 20
40
60
80
100
120
140
160
Concentration in ppm
Retention time (min)
50
Distillate collection time (min)
2 2
Fluoride in (Th,U)O
2
Chloride in (Th,U)O
1.2
2
1.1 1.0 0.9 0.8 0.7 0.6 0.5
Fig. 2 Effect of distillate collection time on the recovery of F- and Cl- from ThO2 and (Th,3.25%U)O2, temperature: 950 °C
0.4 0.3 0.2
system was linear over a wide range of concentration. Detection limit obtained was based on S/N = 3 for these ions. The coefficients of variation obtained were better than ±3.5% in ten replicates.
0.2
0.3
0.4
0.5
0.6
Sample size in gm
Fig. 3 Effect of sample size on the recovery of Cl- and F- from ThO2 and (Th,3.25%U)O2, pyrohydrolysis time: 30 min, temperature: 950 °C
Table 1 Calibration parameters and limits of detection (LOD) of Cl- and FIons
Range (mg L-1)
LODa (ng L-1)
Linearity (R2)
Slope
Intercept
%R.S.D. for given concentration level (conc. Level mg L-1)b
F-
0.01–2.0
10
0.9996
11794.5 ± 115.9
64.6 ± 58.2
2.5 (0.1)
Cl-
0.01–2.0
15
0.9997
43.1 ± 40.1
3.2 (0.1)
a
Calculated on the basis of S/N = 3
b
RSD data of ten replicates
9075 ± 79.8
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Fluoride in ThO2
1.6
Fluoride in ThO2
30
Chloride in ThO2
Fluoride in (Th,U)O2
1.4
Fluoride in (Th,U)O2
25
Chloride in (Th,U)O2
Concentration in ppm
Concentration in ppm
Chloride in ThO2
1.2
1.0
0.8
Chloride in (Th,U)O2 20
15
10
0.6
5 900
950
1000
1050
1100
Temperature (°C)
0 0.0
0.1
0.2
Fig. 4 Effect of temperature on the recovery F- and Cl- from ThO2 and (Th,3.25%U)O2; distillate collection time 30 min
Effect of accelerators for the recovery of F- and Clfrom ThO2 The effects of accelerator, amounts of accelerator, pyrohydrolysis temperature and distillation time in presence of accelerator were optimized. Vanadium penta-oxide (V2O5), Molybednum tri-oxide (MoO3) and tungsten oxide (WO3) have been employed as an accelerator to extract fluoride and chloride from ThO2 and (Th,U)O2 matrix. A strongly adhered glassy material was formed on the quartz sample boat after pyrohydrolysis in presence of V2O5. Reaction products of MoO3 and ThO2; (Th,U)O2 were highly volatile which in turn caused the choke up of the quartz reaction tube. Hence V2O5 and MoO3 were found unsuitable as accelerators for routine pyrohydrolysis operations. The above said problems were overcome by the use of refractory WO3 (m.p. = 1,473 °C). Therefore, WO3 was selected as an accelerator. Different compositions of ThO2 ? WO3 and (Th,3.25%U) O2 ? WO3 mixture were prepared from ThO2; sintered (Th,U)O2 powder and WO3. They were homogenized well in mortar and pestle for 15 min and samples were subjected to pyrohydrolysis at 950 °C for 30 min. Figure 5 reveals that there was significant increase of F- and Cl- with the increase in
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0.4
0.5
0.6
Fig. 5 Effect of amount of WO3 on the recovery of F- and Cl from ThO2 and (Th,3.25%U)O2; weight of sample 0.2 g, pyrohydrolysis temperature 950 °C, pyrohydrolysis time 30 min
WO3 and were almost constant when the WO3 C 0.4 g at 0.2 g sample size of ThO2 and (Th,3.25%U)O2. Hence ThO2:WO3 and (Th,3.25%U)O2:WO3 (1:2) ratio was selected for the other experiments. Temperature had pronounced effect on the recovery of F and Cl- from ThO2 and (Th,3.25%U)O2 in presence of WO3. Temperature was increased from 800 to 1,100 °C with the pyrohydrolysis time of 30 min. Figure 6 shows that the values of F- and Cl- increase in the temperature range of 800–950 °C, but their values were almost constant after 950 °C. This indicated the complete recovery of F-
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Fluoride in ThO2
30
Concentration in ppm
recovery of fluoride in ThO2 was complete in 2.0 h at 950–975 °C and 50–60 cc/min oxygen flow rate. But the present study revealed that fluoride and chloride recoveries were incomplete up to 2.5 h at 950 °C. Their values increase with the increase in the pyrohydrolysis temperature up to 1,100 °C in both Thoria and sintered (Th,3.25%U)O2 samples. This necessitated the need for accelerators for quantitative recoveries of F- and Cl- from ThO2 and (Th,3.25%U)O2.
0.3
WO3 (gm)
28
Chloride in ThO2
26
Fluoride in (Th,U)O 2
24
Chloride in (Th,U)O 2
22 20 18 16 14 12 10 8 6 4 2 800
850
900
950
1000
1050
1100
Temperature (°C)
Fig. 6 Effect of temperature on the recovery of fluoride and chloride from ThO2 and (Th,3.25%U)O2; ratio of sample : WO3 = 1:2(0.2:0.4 g), pyrohydrolysis time 30 min
Effect of accelerators on thoria based nuclear fuels 32
Quantitative extraction of F- and Cl- was confirmed by carrying out recovery studies. Known quantities of F- and Cl- were added as LaF3 and BiOCl in pre-pyrohydrolyzed ThO2 and (Th,3.25%U)O2 samples. These samples were mixed with WO3 in 1:2 weight ratio and pyrohydrolyzed. Table 3 shows the recoveries of F- and Cl- from ThO2 and (Th,3.25%U)O2 matrix. The average recoveries for Fwere 94.3 and 92.3% in ThO2 and (Th,3.25%U)O2. While the average recoveries of Cl- were 99.1 and 96.5% in ThO2 and (Th,3.25%U)O2. These recoveries were comparable to the recoveries of F-, 91–95% and Cl- 95–99% in pure LaF3 and BiOCl standards [10] and spiked radioactive liquid wastes [8]. A typical chromatogram of ThO2 and (Th, 3.25%U)O2 sample are also shown in Fig. 9.
Fluoride in ThO2
30
Chloride in ThO2
28
Fluoride in (Th,U)O2
26
Concentration in ppm
747
Chloride in (Th,U)O2
24 22 20 18 16 14 12 10 8 6 20
40
60
80
100
120
140
Distillation collection time in min
Fig. 7 Effect of pyrohydrolysis time on the recovery of F- and Clfrom ThO2 and (Th,3.25%U)O2; ratio of sample: WO3 = 1:2(0.2:0.4 g), pyrohydrolysis time 30 min
Mechanism of the effect of catalyst: The general reaction of pyrohydrolysis of halides can be expressed as MX2n + nH2 O ! MOn + 2nHX
Table 2 Optimized pyrohydrolysis conditions for complete extraction of F- and Cl- from ThO2 and sintered (Th,3.25%U)O2 Parameters
Optimum condition
Sample size (g)
0.2
Wt of WO3 (g)
0.4
Extraction time
30 min
Pyrohydrolysis temperature
950 °C
Oxygen flow rate (mL/min)
100–150
Volume of the condensed distillate (mL)
20
and Cl- at 950 °C in 30 min from ThO2 and (Th,3.25%U)O2. Effect of pyrohydrolysis time on the recovery of F- and Cl from ThO2 and (Th,3.25%U)O2 has been investigated in presence of WO3. The pyrohydrolysis time was increased from 30 to 130 min and results are depicted in Fig. 7. The values for fluoride and chloride remain constant with the increase in the time of pyrohydrolysis. Hence their recovery was complete in 30 min in presence of WO3. Optimized pyrohydrolysis parameters for ThO2 and (Th,U)O2 are given in Table 2. X-ray diffraction and recovery studies ThO2 being the major component in (Th,3.25%U)O2 sintered pellet. Hence pyrohydrolyzed samples of ThO2, ThF4, and ThCl4 and WO3 (1:2) mixture were subjected to X-ray diffraction study in order to examine the reaction products. The XRD patterns are presented in the Fig. 8a–c.
ð1Þ
Thermodynamic data reveals that at higher temperature equilibrium constant of pyrohydrolytic reaction favours the formation of hydrogen halide and metal oxide rather than the metal halide and water [6]. As described from our experiments ThO2 and (Th,3.25%U)O2 being refractory in nature, requires higher temperature and WO3 as a catalyst to achieve quantitative recovery of F- and Cl-. Hence mechanism was studied. Pyrohydrolyzed sample of ThO2 and WO3 (1:2) mixture was subjected to X-ray diffraction study. The XRD pattern shows that WO3 take part in the replacement reaction with ThO2 resulting in the formation of Th(WO)4 as shown in Fig. 8a. It can be expressed as follows: ThO2 ðsÞ + 2 WO3 ðsÞ ! ThðWO4 Þ2 ðsÞ
ð2Þ
During the formation of Th(WO4)2, its matrix decomposed and exposed to moist air simultaneously. Fluoride and chloride in ThO2 and (Th,3.25%U)O2 matrix may be present as halides of major as well as minor component such as Fe, Cr, Ca. The reaction of FeF3 or CrF3 with metal oxides such as MoO3 at higher temperature yields Fe2O3 or Cr2O3 with metal fluorides (MoF6 etc.) [14]. While the reaction of CaF2 and WO3 yields CaWO4 and volatile WF6 [15]. In order to make sure the detail mechanism of the effect of WO3 the pyrohydrolysis of ThF4 and ThCl4 in presence of WO3 (ThX4/WO3; 1:2) was carried out. Figure 8b and c shows the XRD pattern of their reaction residue, respectively. Peaks of Th(WO4)2 and ThO2 are observed in the pattern. Hence the reaction of ThX4 (X = F and Cl) with WO3 resulted in the formation of Th(WO4)2, ThO2 and WX6 by the following reaction.
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(b)
(a) 200 Th(WO4 )2 WO3 ThO2
Intensity (a.u.)
Intensity (a.u.)
150
100
50
0 10
20
30
40
50
2θ (degree)
60
2θ (degree)
Intensity (a.u.)
(c)
2θ (degree)
Fig. 8 XRD pattern of pyrohydrolyzed sample of a ThO2, b ThF4, c ThCl4 and WO3 (1:2) at 950 °C
Table 3 The results of F and Cl recovery from spiked ThO2 and (Th,3.25%U)O2 samples Spiked sample
F and Cl added (mg L-1)
F- recoveries in ThO2
F- recoveries in (Th,3.25%U)O2
Cl- recoveries in ThO2
Cl- recoveries in (Th,3.25%U)O2
Observed (mg L-1)
Observed (mg L-1)
Observed (mg L-1)
Recovery %
Observed (mg L-1)
Recovery %
Recovery %
Recovery %
1
25
23.9
95.6
23.7
94.8
24.9
99.6
24.1
96.4
2
50
47.4
94.8
45.5
91
51.1
101.2
47.9
95.8
3
100
92.6
92.6
92.8
92.8
96.5
96.5
97.2
97.2
Avg. recovery %
94.3
6ThX4 ðsÞþ10WO3 ðsÞ ! 3ThO2 ðsÞ + 3ThðWO4 Þ2 ðsÞ + 4WX6 ðgÞ
92.9
WX6 þ 3H2 O ! WO3 þ 6HX
96.5
ð4Þ
ð3Þ
Halides and oxyhalide species of W are very volatile. Thus WO3 works as a accelerator by forming Th(WO4)2 and volatile intermediate WX6 which in contact with moisture or water hydrolyzed to give HX.
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99.1
Conclusion A rapid method for simultaneous determination of fluorine and chlorine in ThO2 powder and sintered (Th,3.25%U) O2
Effect of accelerators on thoria based nuclear fuels
749
with ion chromatography after pyrohydrolysis was proposed for routine analysis. WO3 acts as an effective accelerator for the quantitative extraction of F- and Clfrom refractory ThO2 and sintered (Th,3.25%U) O2 at 950 °C by pyrohydrolysis .
(a) mV 1
100
2
80 60 40
5
20
Acknowledgment The authors are grateful to Dr. G. J. Prasad, Director, Nuclear Fuels Group, BARC for his keen interest in this work.
3 4
0
References
-20 ch1
-40 0
2
4
6
8 10 12 14 16 18 20 22 24 26 28 30 32 min
(b) mV
25 2
20
5 3 1
15 10
4
5 0 ch1 0
2
4
6
8
10 12 14 16 18 20 22 24 26 28 30 32 min
Fig. 9 Chromatogram of pyrohydrolysis distillate collected from a ThO2, b (Th,3.25%U)O2 sample Mobile phase 3.2 mM Na2CO3 ? 3.0 mM NaHCO3; Flow rate: 1 mL/min; fluoride (1), chloride (2), nitrite (3), orthophosphate (4), sulphate (5)
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