- Email: [email protected]
Mass-spectrometric study of PuCd2
Journal of Physics and Chemistry of Solids 66 (2005) 639–642 www.elsevier.com/locate/jpcs
Mass-spectrometric study of PuCd2 Kunihisa Nakajima*, Yasuo Arai, Toshiyuki Yamashita Department of Nuclear Energy System, Japan Atomic Energy Research Institute, 3607 Narita-cho, Oarai-machi, Higashi-ibaraki-gun, Ibaraki-ken 311-1394, Japan Accepted 24 June 2004
Abstract PuCd2 intermetallic compound was prepared by heating pure Pu and Cd metals at w950 K. Plutonium metal used in the preparation of PuCd2 was prepared by the tantalothermic reduction of Pu2C3 followed by selective evaporation. PuCd2 is found to be a prototype of CdI2 by means of powder X-ray diffractometry as in the cases of CeCd2 and LaCd2. Mass-spectrometric experiment was performed in the temperature range of 650–770 K. It is found that the vapor pressures of Cd over PuCd2CPu are three to five orders of magnitude lower than those over pure Cd in this temperature range. From these vapor pressures, Gibbs free energy of formation of PuCd2 is evaluated. q 2004 Elsevier Ltd. All rights reserved. Keywords: A. Intermetallic compounds; C. X-ray diffraction; D. Thermodynamic properties
1. Introduction
2. Experiments
A pyrochemical process for the metallurgical treatment of spent nuclear fuel has been developed as an advanced reprocessing of the nuclear fuel cycle [1,2]. This process consists of several steps. Uranium is recovered from the spent fuel by electrorefining in a molten salt at a solid cathode. Transuranics (TRUs) along with some uranium and rare-earth fission products are collected into a liquid cadmium cathode by electrorefining. The cadmium is separated from the TRU-containing product by distillation and TRUs are recovered as the residue. Knowing volatilization behavior of the Pu–Cd system, therefore, is essential in order to make the cadmium-distillation step feasible because plutonium is the major constituent of the residue. In the present study, Knudsen-effusion mass-spectrometric measurements of PuCd2, which is the Pu-richest compound among Pu–Cd intermetallic compounds, were carried out in order to determine the vapor pressures of Cd over PuCd2C Pu. Furthermore, the standard Gibbs free energy of formation of PuCd2 was evaluated from these vapor pressures.
2.1. Sample preparation
* Corresponding author. Tel.: C81 29 264 8422; fax: C81 29 264 8478. E-mail address: [email protected] (K. Nakajima). 0022-3697/$ - see front matter q 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.jpcs.2004.06.072
The Pu metal used for the preparation of PuCd2 was prepared in the following manner. At first plutonium sesquicarbide, Pu2C3, was prepared from PuO2 by the carbothermic reduction method [3]. The product was then mixed with Ta powder, compacted into disks, heated at about 1770 K and resulted in Pu metal and Ta carbides. The characteristics of the Ta powder purchased from Rare Metallic Co., Ltd. were the purity of 99.96% and 325 mesh. The Pu metal produced by the tantalothermic reduction was separated from the Ta carbides by selective evaporation under a pressure of 10K4 Pa and about 2000 K [4]. The Pu metal condensed on a W collector was then heated at about 600 K and stripped down by producing plutonium hydride powders. These powders were collected, pressed into disks and sintered at about 770 K in vacuo for the preparation of Pu metal. The Pu metal prepared in this way was identified by means of powder X-ray diffractometry. This XRD pattern is shown in Fig. 1. As shown in this figure, traces of Pu2O3 (bcc), PuC and W might exist as impurities. PuCd2 intermetallic compound was prepared by heating pure Pu and Cd metals at 950 K for about 15 min in an Y2O3
640
K. Nakajima et al. / Journal of Physics and Chemistry of Solids 66 (2005) 639–642
Fig. 1. X-ray diffraction pattern of sintered Pu metal prepared by tantalothermic reduction followed by selective evaporation and hydride recovery.
Knudsen cell with an orifice of 0.5 mmf in diameter. This Knudsen cell was also used in the mass-spectrometric experiment. The Cd reagent was obtained by filing a Cd ingot purchased from Rare Metallic Co. Ltd with the purity of 99.9999%. The diphasic PuCd2CPu sample after the massspectrometric measurement was stripped out of the Knudsen cell, filed and identified by means of powder X-ray diffractometry. This XRD pattern is shown in Fig. 2. PuCd2 was found to be almost the same CdI2 prototype structure as in the cases of LaCd2 and CeCd2 [5]. The broad peaks in this XRD pattern, however, make it difficult to be indexed on a hexagonal unit cell. This XRD pattern also indicates the existence of Cd as shown in Fig. 2. Further, it was found that the intensities of the Cd peaks increased with repetition of the X-ray diffraction measurements and that PuCd2 ultimately decomposed into PuO2 and Cd. So the appearance of Cd peaks in Fig. 2 is considered to be caused by the oxidation of the sample during the X-ray diffraction measurement. These sample preparations mentioned above were conducted in argon atmosphere glove boxes where the oxygen and moisture contents were kept less than 3 ppm.
Fig. 2. X-ray diffraction pattern of PuCd2Ca-Pu after the mass-spectrometric measurement.
the ionization efficiency curve of CdC. The sample weight is about 110 mg constituted of about 70 mg of Pu and about 40 mg of Cd. The Y2O3 Knudsen cell with a purity of 99.6% and a density of 99% theoretical density (T.D.) was used. The inner diameter and height of the Knudsen cell are f7.4X8H in mm. The lid with an effusion orifice of 0.5 mm in diameter is shaved convexly for matching with the cell. The Knudsen cell was put on a Ta cap perforated in the center and then put into a Ta holder. The sample temperature was measured with two sets of W/Re3-25 thermocouples inserted into upper and lower part of the Ta holder. These two thermocouple signals are allowed to control the upper and lower W spiral coil heaters, respectively. The upper temperature was, however, always about 10 K higher than the lower temperature although the upper heater was not charged with electricity. So the average value of the upper and lower temperatures was regarded as the sample temperature. The vacuum pressure in the chamber was kept below 10K3 Pa during the massspectrometric measurement.
2.2. Mass-spectrometric measurement Mass-spectrometric experiment was performed in the temperature range from 650 to 770 K. Quadrupole mass spectrometer (MEXM-1200 ABB EXTREL, USA) combined with an Y2O3 Knudsen cell was used. The impact electron energy of 28 eV was selected, at which the ion current intensities of Cd became the maximum according to
2.3. Determination of the absolute vapor pressure from the ion current by the modified integral method The absolute vapor pressures of Cd over diphasic PuCd2CPu are determined by the modified integral method [6]. This method requires the total lost amount of Cd and its ion currents at any time during the mass-spectrometric
K. Nakajima et al. / Journal of Physics and Chemistry of Solids 66 (2005) 639–642
measurement. Then, the total lost amount of Cd was obtained from the weight difference of the Knudsen cell containing the sample before and after the mass-spectrometric measurement. Now there exist many Cd isotopes, 106 Cd, 108Cd, 110Cd, 111Cd, 112Cd, 113Cd, 114Cd and 116Cd. So the isotopic effect must be considered when the modified integral method is applied in the conversion of the ion current into the absolute vapor pressure. The ion current, I(t), is given by the following relation IðtÞ Z c
ss css zðtÞ Z qffiffiffiffiffiffiffiffiffiffi zðtÞ; vðtÞ 8RTðtÞ
(1)
pM
where c is the factor for instrument geometry, s the relative ionization cross-section of the vaporizing element, s the relative detector sensitivity which includes the sensitivity of the mass filter and the efficiency of the electron multiplier, v the average velocity of the vaporizing atoms which obey Maxwell distribution of speeds, M the mass number of the vaporizing atoms, R the gas constant, T the absolute sample temperature, z the number of the vaporizing atoms passed per unit time and unit area. This number, z, can be also expressed by the following equation gpðtÞNA ffi; zðtÞ Z pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi 2pMRTðtÞ
(2)
where g is the isotopic abundant ratio for the vaporizing element, p the partial pressure of the vaporizing element and NA the Avogadro constant. I(t), v(t), z(t), p(t) and T(t) mean the time-dependent variables. In order that unknown factors, or c, s, s should not be required for the conversion of the measured ion currents into the absolute vapor pressures, the sample weight difference before and after the massspectrometric experiment, DW, is utilized in the modified integral method. That is to say, DW can be calculated based on the kinetic theory of gases from the following equation ð tend X DW Z Mi aL zi ðtÞ=NA dt; i
the isotopes, the following relation holds: sg Ii ðtÞ Z i i IðtÞ: sg
Consequently, the partial pressure, p(t), can be expressed from Eqs. (4) and (6) as the following equation: pffiffiffiffiffiffiffiffiffi 2pRDWIðtÞTðtÞ (7) pðtÞ Z Ð tend pffiffiffiffiffiffiffiffi P pffiffiffiffiffiffi : aL 0 IðtÞ TðtÞdt i gi Mi In this study, the ion currents for the vapor species of Cd, which was the most abundant ratio, were measured. Further the orifice area times the Clausing factor, or aL, is obtained from the mass-spectrometric measurement of pure Cd by comparing the experimental results determined by the modified integral method mentioned above to the reference data, pref. Precisely, the determined uncertain factor, aL, is the average values calculated by using the following equation pffiffiffiffiffiffiffiffiffi 2pRDWIðtÞTðtÞ Ð tend aLðtÞ Z (8) pffiffiffiffiffiffiffiffi P pffiffiffiffiffiffi ; pref ðTÞ 0 IðtÞ TðtÞdt i gi Mi 114
where pref is given by Kubaschewski [8].
3. Results and discussion The determined absolute vapor pressures of Cd over diphasic PuCd2CPu, p(Cd), are plotted in Fig. 3 and
0
Now, the following relation can be obtained from Eqs. (1) and (2): 4R IðtÞTðtÞ: cssgNA
(4)
This equation can be applied to any kind of isotopes. Since the factors of R, c, s and NA are independent of
(5)
Then, Eq. (3) can be expressed by using the ion current of only one kind of isotopes, or I(t) as the following equation: pffiffiffiffiffiffi ð t end pffiffiffiffiffiffiffiffi X pffiffiffiffiffiffi aL 8R DW Z pffiffiffiffi IðtÞ TðtÞdt gi Mi : (6) pcssgNA 0 i
where a is the effusing orifice area of the Knudsen cell and L the Clausing factor which is the fraction of atoms effusing through theP orifice among the vaporizing atoms entering the orifice [7]. i means the summation for all kinds of isotope. From Eq. (1), the above equation reduces to the following equation ð X aLpffiffiffiffiffiffiffiffiffiffiffi pffiffiffiffiffiffiffiffi 8RMi tend pffiffiffiffi Ii ðtÞ TðtÞdt: (3) DW Z pcssi NA 0 i
pðtÞ Z
641
Fig. 3. Vapor pressures of Cd(g) over PuCd2CPu.
642
K. Nakajima et al. / Journal of Physics and Chemistry of Solids 66 (2005) 639–642
expressed as the following equation:
4. Conclusion
Log pðCdÞ Z Kð14670G310Þ=T C ð19:32G0:44Þ:
PuCd2 intermetallic compound was prepared from Cd and Pu metals. Pu metal was also prepared by the tantalothermic reduction of Pu2C3 followed by selective evaporation. PuCd2 is found to be a prototype of CdI2 by means of powder X-ray diffractometry. It is found that the vapor pressures of Cd over PuCd2CPu are three to five orders of magnitude lower than those over pure Cd in the temperature range of 650–770 K. From these vapor pressures, Gibbs free energy of formation of PuCd2 is evaluated. The results obtained in this study will be useful for optimization of cadmium-distillation step in pyrochemical reprocessing.
It is found that the vapor pressures of Cd over diphasic PuCd2CPu are three to five orders of magnitude lower than those over Cd in the temperature range from 650 to 770 K. From these vapor pressures, Gibbs free energy of formation of PuCd2, DfG8 (PuCd2,s), is evaluated assuming that the following reaction holds: PuCd2 ðsÞ Z 2CdðgÞ C PuðsÞ: Then, DfG8 (PuCd2,s) can be calculated from the following equation Df G8ðPuCd2 ; sÞ ¼ 2RT ln pðCdÞ þ 2Df G8ðCd; gÞ assuming that both the activities of Pu metal and PuCd2 are unity. The values of DfG8 (PuCd2,s) obtained by using DfG8 (Cd,g) derived from pref [8] are expressed as the following equation: Df G8ðPuCd2 ; sÞðJ=molÞ ZKð355700G12000ÞCð349:3G16:7ÞT ð650 K770 KÞ: In Fig. 3 the vapor pressures over pure Cd calculated by using the corrected factor, or aL, derived from Eq. (8) are also plotted. It was found that the Clausing factor, or L, derived from Eq. (8) was higher than that calculated from the shape of the effusion orifice [7]. This result suggested some leaks other than the effusion orifice occurred. But the Clausing factor calibrated with the mass-spectrometric measurement of Cd is considered to be applicable for the determination of vapor pressure, for this apparent factor can cancel out the effect of such a leakage.
Acknowledgements The authors wish to express their thanks to Drs T. Iwamura and H. Nakajima for their interests in this study.
References [1] [2] [3] [4]
[5] [6] [7] [8]
A. Michael, et al., Nucl. Technol. 120 (1997) 232–242. A. Michael, et al., Nucl. Technol. 120 (1997) 243–253. Y. Suzuki, et al., J. Nucl. Mater. 101 (1981) 200–206. J.C. Spirlet, O. Vogt, in: A.J. Freeman, G.H. Lander (Eds.), Handbook on the Physics and Chemistry of the Actinides vol. 1, Elsevier, Amsterdam, 1984, p. 79. T.B. Massalski et al. (Ed.), Binary Alloy Phase Diagrams vol. 649, American Society for Metals, 1986, p. 662. M. Asano, et al., Bull. Chem. Soc. Jpn 45 (1972) 82–87. R.P. Iczkowski, J.L. Margrave, S.M. Robinson, J. Phys. Chem. 67 (1963) 229–233. E.L. Kubaschewski, C.B. Evans, Metallurgical Thermochemistry, fifth ed., Pergamon Press, New York, 1979. p. 360.
Mass-spectrometric study of PuCd2 Kunihisa Nakajima*, Yasuo Arai, Toshiyuki Yamashita Department of Nuclear Energy System, Japan Atomic Energy Research Institute, 3607 Narita-cho, Oarai-machi, Higashi-ibaraki-gun, Ibaraki-ken 311-1394, Japan Accepted 24 June 2004
Abstract PuCd2 intermetallic compound was prepared by heating pure Pu and Cd metals at w950 K. Plutonium metal used in the preparation of PuCd2 was prepared by the tantalothermic reduction of Pu2C3 followed by selective evaporation. PuCd2 is found to be a prototype of CdI2 by means of powder X-ray diffractometry as in the cases of CeCd2 and LaCd2. Mass-spectrometric experiment was performed in the temperature range of 650–770 K. It is found that the vapor pressures of Cd over PuCd2CPu are three to five orders of magnitude lower than those over pure Cd in this temperature range. From these vapor pressures, Gibbs free energy of formation of PuCd2 is evaluated. q 2004 Elsevier Ltd. All rights reserved. Keywords: A. Intermetallic compounds; C. X-ray diffraction; D. Thermodynamic properties
1. Introduction
2. Experiments
A pyrochemical process for the metallurgical treatment of spent nuclear fuel has been developed as an advanced reprocessing of the nuclear fuel cycle [1,2]. This process consists of several steps. Uranium is recovered from the spent fuel by electrorefining in a molten salt at a solid cathode. Transuranics (TRUs) along with some uranium and rare-earth fission products are collected into a liquid cadmium cathode by electrorefining. The cadmium is separated from the TRU-containing product by distillation and TRUs are recovered as the residue. Knowing volatilization behavior of the Pu–Cd system, therefore, is essential in order to make the cadmium-distillation step feasible because plutonium is the major constituent of the residue. In the present study, Knudsen-effusion mass-spectrometric measurements of PuCd2, which is the Pu-richest compound among Pu–Cd intermetallic compounds, were carried out in order to determine the vapor pressures of Cd over PuCd2C Pu. Furthermore, the standard Gibbs free energy of formation of PuCd2 was evaluated from these vapor pressures.
2.1. Sample preparation
* Corresponding author. Tel.: C81 29 264 8422; fax: C81 29 264 8478. E-mail address: [email protected] (K. Nakajima). 0022-3697/$ - see front matter q 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.jpcs.2004.06.072
The Pu metal used for the preparation of PuCd2 was prepared in the following manner. At first plutonium sesquicarbide, Pu2C3, was prepared from PuO2 by the carbothermic reduction method [3]. The product was then mixed with Ta powder, compacted into disks, heated at about 1770 K and resulted in Pu metal and Ta carbides. The characteristics of the Ta powder purchased from Rare Metallic Co., Ltd. were the purity of 99.96% and 325 mesh. The Pu metal produced by the tantalothermic reduction was separated from the Ta carbides by selective evaporation under a pressure of 10K4 Pa and about 2000 K [4]. The Pu metal condensed on a W collector was then heated at about 600 K and stripped down by producing plutonium hydride powders. These powders were collected, pressed into disks and sintered at about 770 K in vacuo for the preparation of Pu metal. The Pu metal prepared in this way was identified by means of powder X-ray diffractometry. This XRD pattern is shown in Fig. 1. As shown in this figure, traces of Pu2O3 (bcc), PuC and W might exist as impurities. PuCd2 intermetallic compound was prepared by heating pure Pu and Cd metals at 950 K for about 15 min in an Y2O3
640
K. Nakajima et al. / Journal of Physics and Chemistry of Solids 66 (2005) 639–642
Fig. 1. X-ray diffraction pattern of sintered Pu metal prepared by tantalothermic reduction followed by selective evaporation and hydride recovery.
Knudsen cell with an orifice of 0.5 mmf in diameter. This Knudsen cell was also used in the mass-spectrometric experiment. The Cd reagent was obtained by filing a Cd ingot purchased from Rare Metallic Co. Ltd with the purity of 99.9999%. The diphasic PuCd2CPu sample after the massspectrometric measurement was stripped out of the Knudsen cell, filed and identified by means of powder X-ray diffractometry. This XRD pattern is shown in Fig. 2. PuCd2 was found to be almost the same CdI2 prototype structure as in the cases of LaCd2 and CeCd2 [5]. The broad peaks in this XRD pattern, however, make it difficult to be indexed on a hexagonal unit cell. This XRD pattern also indicates the existence of Cd as shown in Fig. 2. Further, it was found that the intensities of the Cd peaks increased with repetition of the X-ray diffraction measurements and that PuCd2 ultimately decomposed into PuO2 and Cd. So the appearance of Cd peaks in Fig. 2 is considered to be caused by the oxidation of the sample during the X-ray diffraction measurement. These sample preparations mentioned above were conducted in argon atmosphere glove boxes where the oxygen and moisture contents were kept less than 3 ppm.
Fig. 2. X-ray diffraction pattern of PuCd2Ca-Pu after the mass-spectrometric measurement.
the ionization efficiency curve of CdC. The sample weight is about 110 mg constituted of about 70 mg of Pu and about 40 mg of Cd. The Y2O3 Knudsen cell with a purity of 99.6% and a density of 99% theoretical density (T.D.) was used. The inner diameter and height of the Knudsen cell are f7.4X8H in mm. The lid with an effusion orifice of 0.5 mm in diameter is shaved convexly for matching with the cell. The Knudsen cell was put on a Ta cap perforated in the center and then put into a Ta holder. The sample temperature was measured with two sets of W/Re3-25 thermocouples inserted into upper and lower part of the Ta holder. These two thermocouple signals are allowed to control the upper and lower W spiral coil heaters, respectively. The upper temperature was, however, always about 10 K higher than the lower temperature although the upper heater was not charged with electricity. So the average value of the upper and lower temperatures was regarded as the sample temperature. The vacuum pressure in the chamber was kept below 10K3 Pa during the massspectrometric measurement.
2.2. Mass-spectrometric measurement Mass-spectrometric experiment was performed in the temperature range from 650 to 770 K. Quadrupole mass spectrometer (MEXM-1200 ABB EXTREL, USA) combined with an Y2O3 Knudsen cell was used. The impact electron energy of 28 eV was selected, at which the ion current intensities of Cd became the maximum according to
2.3. Determination of the absolute vapor pressure from the ion current by the modified integral method The absolute vapor pressures of Cd over diphasic PuCd2CPu are determined by the modified integral method [6]. This method requires the total lost amount of Cd and its ion currents at any time during the mass-spectrometric
K. Nakajima et al. / Journal of Physics and Chemistry of Solids 66 (2005) 639–642
measurement. Then, the total lost amount of Cd was obtained from the weight difference of the Knudsen cell containing the sample before and after the mass-spectrometric measurement. Now there exist many Cd isotopes, 106 Cd, 108Cd, 110Cd, 111Cd, 112Cd, 113Cd, 114Cd and 116Cd. So the isotopic effect must be considered when the modified integral method is applied in the conversion of the ion current into the absolute vapor pressure. The ion current, I(t), is given by the following relation IðtÞ Z c
ss css zðtÞ Z qffiffiffiffiffiffiffiffiffiffi zðtÞ; vðtÞ 8RTðtÞ
(1)
pM
where c is the factor for instrument geometry, s the relative ionization cross-section of the vaporizing element, s the relative detector sensitivity which includes the sensitivity of the mass filter and the efficiency of the electron multiplier, v the average velocity of the vaporizing atoms which obey Maxwell distribution of speeds, M the mass number of the vaporizing atoms, R the gas constant, T the absolute sample temperature, z the number of the vaporizing atoms passed per unit time and unit area. This number, z, can be also expressed by the following equation gpðtÞNA ffi; zðtÞ Z pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi 2pMRTðtÞ
(2)
where g is the isotopic abundant ratio for the vaporizing element, p the partial pressure of the vaporizing element and NA the Avogadro constant. I(t), v(t), z(t), p(t) and T(t) mean the time-dependent variables. In order that unknown factors, or c, s, s should not be required for the conversion of the measured ion currents into the absolute vapor pressures, the sample weight difference before and after the massspectrometric experiment, DW, is utilized in the modified integral method. That is to say, DW can be calculated based on the kinetic theory of gases from the following equation ð tend X DW Z Mi aL zi ðtÞ=NA dt; i
the isotopes, the following relation holds: sg Ii ðtÞ Z i i IðtÞ: sg
Consequently, the partial pressure, p(t), can be expressed from Eqs. (4) and (6) as the following equation: pffiffiffiffiffiffiffiffiffi 2pRDWIðtÞTðtÞ (7) pðtÞ Z Ð tend pffiffiffiffiffiffiffiffi P pffiffiffiffiffiffi : aL 0 IðtÞ TðtÞdt i gi Mi In this study, the ion currents for the vapor species of Cd, which was the most abundant ratio, were measured. Further the orifice area times the Clausing factor, or aL, is obtained from the mass-spectrometric measurement of pure Cd by comparing the experimental results determined by the modified integral method mentioned above to the reference data, pref. Precisely, the determined uncertain factor, aL, is the average values calculated by using the following equation pffiffiffiffiffiffiffiffiffi 2pRDWIðtÞTðtÞ Ð tend aLðtÞ Z (8) pffiffiffiffiffiffiffiffi P pffiffiffiffiffiffi ; pref ðTÞ 0 IðtÞ TðtÞdt i gi Mi 114
where pref is given by Kubaschewski [8].
3. Results and discussion The determined absolute vapor pressures of Cd over diphasic PuCd2CPu, p(Cd), are plotted in Fig. 3 and
0
Now, the following relation can be obtained from Eqs. (1) and (2): 4R IðtÞTðtÞ: cssgNA
(4)
This equation can be applied to any kind of isotopes. Since the factors of R, c, s and NA are independent of
(5)
Then, Eq. (3) can be expressed by using the ion current of only one kind of isotopes, or I(t) as the following equation: pffiffiffiffiffiffi ð t end pffiffiffiffiffiffiffiffi X pffiffiffiffiffiffi aL 8R DW Z pffiffiffiffi IðtÞ TðtÞdt gi Mi : (6) pcssgNA 0 i
where a is the effusing orifice area of the Knudsen cell and L the Clausing factor which is the fraction of atoms effusing through theP orifice among the vaporizing atoms entering the orifice [7]. i means the summation for all kinds of isotope. From Eq. (1), the above equation reduces to the following equation ð X aLpffiffiffiffiffiffiffiffiffiffiffi pffiffiffiffiffiffiffiffi 8RMi tend pffiffiffiffi Ii ðtÞ TðtÞdt: (3) DW Z pcssi NA 0 i
pðtÞ Z
641
Fig. 3. Vapor pressures of Cd(g) over PuCd2CPu.
642
K. Nakajima et al. / Journal of Physics and Chemistry of Solids 66 (2005) 639–642
expressed as the following equation:
4. Conclusion
Log pðCdÞ Z Kð14670G310Þ=T C ð19:32G0:44Þ:
PuCd2 intermetallic compound was prepared from Cd and Pu metals. Pu metal was also prepared by the tantalothermic reduction of Pu2C3 followed by selective evaporation. PuCd2 is found to be a prototype of CdI2 by means of powder X-ray diffractometry. It is found that the vapor pressures of Cd over PuCd2CPu are three to five orders of magnitude lower than those over pure Cd in the temperature range of 650–770 K. From these vapor pressures, Gibbs free energy of formation of PuCd2 is evaluated. The results obtained in this study will be useful for optimization of cadmium-distillation step in pyrochemical reprocessing.
It is found that the vapor pressures of Cd over diphasic PuCd2CPu are three to five orders of magnitude lower than those over Cd in the temperature range from 650 to 770 K. From these vapor pressures, Gibbs free energy of formation of PuCd2, DfG8 (PuCd2,s), is evaluated assuming that the following reaction holds: PuCd2 ðsÞ Z 2CdðgÞ C PuðsÞ: Then, DfG8 (PuCd2,s) can be calculated from the following equation Df G8ðPuCd2 ; sÞ ¼ 2RT ln pðCdÞ þ 2Df G8ðCd; gÞ assuming that both the activities of Pu metal and PuCd2 are unity. The values of DfG8 (PuCd2,s) obtained by using DfG8 (Cd,g) derived from pref [8] are expressed as the following equation: Df G8ðPuCd2 ; sÞðJ=molÞ ZKð355700G12000ÞCð349:3G16:7ÞT ð650 K770 KÞ: In Fig. 3 the vapor pressures over pure Cd calculated by using the corrected factor, or aL, derived from Eq. (8) are also plotted. It was found that the Clausing factor, or L, derived from Eq. (8) was higher than that calculated from the shape of the effusion orifice [7]. This result suggested some leaks other than the effusion orifice occurred. But the Clausing factor calibrated with the mass-spectrometric measurement of Cd is considered to be applicable for the determination of vapor pressure, for this apparent factor can cancel out the effect of such a leakage.
Acknowledgements The authors wish to express their thanks to Drs T. Iwamura and H. Nakajima for their interests in this study.
References [1] [2] [3] [4]
[5] [6] [7] [8]
A. Michael, et al., Nucl. Technol. 120 (1997) 232–242. A. Michael, et al., Nucl. Technol. 120 (1997) 243–253. Y. Suzuki, et al., J. Nucl. Mater. 101 (1981) 200–206. J.C. Spirlet, O. Vogt, in: A.J. Freeman, G.H. Lander (Eds.), Handbook on the Physics and Chemistry of the Actinides vol. 1, Elsevier, Amsterdam, 1984, p. 79. T.B. Massalski et al. (Ed.), Binary Alloy Phase Diagrams vol. 649, American Society for Metals, 1986, p. 662. M. Asano, et al., Bull. Chem. Soc. Jpn 45 (1972) 82–87. R.P. Iczkowski, J.L. Margrave, S.M. Robinson, J. Phys. Chem. 67 (1963) 229–233. E.L. Kubaschewski, C.B. Evans, Metallurgical Thermochemistry, fifth ed., Pergamon Press, New York, 1979. p. 360.