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Isotopic exchange between hydrogen and deuterium in the process of permeating through Li0.17Pb0.83
Fusion Engineering and Design 85 (2010) 1225–1228
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Fusion Engineering and Design journal homepage: www.elsevier.com/locate/fusengdes
Isotopic exchange between hydrogen and deuterium in the process of permeating through Li0.17 Pb0.83 Y. Edao ∗ , H. Noguchi, S. Fukada Department of Advanced Energy Engineering Science, Interdisciplinary Graduate School of Engineering Sciences, Kyushu University, 6-10-1, Hakozaki, Higashi-ku, Fukuoka 812-8581, Japan
a r t i c l e
i n f o
Article history: Available online 29 March 2010 Keywords: Tritium Hydrogen isotopes Permeation Recovery Lithium lead
a b s t r a c t The permeation process of hydrogen isotopes through Li0.17 Pb0.83 has been investigated experimentally. We obtained the overall D permeation rates that take into consideration of the effects of the H–D isotopic exchange reaction on the surface and diffusion in the Li–Pb layer. It was proved that the rate-determining step was not the surface reaction but the diffusion in the Li–Pb bulk. We proposed a system of gas–liquid counter-current extraction using a packed column for continuous tritium recovery. The concentration profiles through the column and tritium extraction rates are calculated. It is estimated that the column height of 5 m is sufficient to achieve the tritium recovery ratio of 99.999%. This counter-current extraction method is easy to operate and efficient to recover tritium from Li–Pb. © 2010 Elsevier B.V. All rights reserved.
1. Introduction Blanket concepts using lithium–lead eutectic alloy (Li0.17 Pb0.83 ) are considered to constitute an effective blanket system for a fusion reactor. The Li–Pb blanket concept has the following advantages; a large tritium-breeding ratio (TBR) due to Pb working as an efficient neutron multiplier, easy tritium recovery because of low tritium solubility in LiPb, high thermal conductivity, comparatively lower reactivity with oxygen and water vapor, and possibility of constituting a simple blanket system. From the above reasons, some concrete Li–Pb blanket concepts are proposed by several countries for ITER TBM and for DEMO; helium-cooled lithium lead (HCLL) by EU and dual-coolant lithium lead (DCLL) by US [1]. In Japan, Li–Pb is also adopted as a conceptual design of a liquid wall self-cooled blanket in an inertial fusion reactor of KOYO-Fast [2]. On the other hand, there are some disadvantages of Li–Pb; i.e., risk of Po generation and a large circulating power caused by its high density and the magneto-hydrodynamic (MHD) pressure drop. A lot of problems to be solved have been left on the tritium issues of Li–Pb blanket. Therefore, we researched tritium behavior in Li–Pb for designing a reliable tritium recovery system to minimize not only tritium inventory in a blanket but also tritium leak to the outside of blanket or the environment. In relation with the present study, we experimentally determined the data of permeability, diffusivity and solubility of hydrogen isotopes in Li–Pb for designing a tri-
∗ Corresponding author. Tel.: +81 92 642 3785; fax: +81 92 642 3784. E-mail address: [email protected] (Y. Edao). 0920-3796/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.fusengdes.2010.03.010
tium recovery system and for evaluating tritium permeation rates through Li–Pb blanket system [3]. In this paper, we discuss tritium flow in a Li–Pb circulation loop and a method to recover tritium recovery method from the loop. In addition, we search the influence of the isotope exchange reaction between H and D on the Li–Pb surface in the overall permeation process. The isotopic exchange reaction between hydrogen isotopes was considered promising as one of the measures to enhance a tritium recovery rate from Li–Pb. 2. Tritium recovery from Li–Pb loop 2.1. Estimation of tritium in Li–Pb blanket We consider a circulation loop of liquid Li–Pb in a laser fusion blanket system composed of a target chamber, a tritium recovery apparatus set in the by-pass loop and a heat exchanger system. Fig. 1 schematically shows a diagram of the Li–Pb loop and tritium flow in a blanket system. A fusion reactor having 1 GW thermal power under steady-state operation, the tritium generation rate is estimated as 1.8 MCi/day (=180 g/day, 0.77 TBq/s). It is comparatively easy to recover tritium from Li–Pb because of its low solubility of hydrogen isotopes. However it causes a serious problem that tritium easily permeates through the blanket to the environment. The maximum permissible tritium leak rates should be less than 10 Ci/day (=370 GBq/day) when a fusion power plant is operated properly, according to a previous report [4]. This is because of protecting human bodies from internal radiation from tritium radio isotope. In addition, the tritium recovery
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Nomenclature L cLiPb cT k av z h KT pT2 Rg T G pt HL or HG NL or NG DL or DG
molar liquid Li–Pb flow rate per unit area [mol/m2 s] molar density of Li–Pb [mol/m3 ] tritium concentration in Li–Pb [mol/m3 ] mass-transfer coefficient [m/s] specific surface area of gas bubbles [m2 /m3 ] distance from the bottom of the column [m] height of the column [m] Sieverts’ constant of tritium in Li–Pb [mol/m3 Pa0.5 ] tritium partial pressure in gas phase [Pa] gas law constant [m3 Pa/mol K] temperature [K] molar He gas flow rate per unit area [mol/m2 s] total pressure [Pa] height per transfer unit of Li–Pb or He [m] number of transfer units of Li–Pb phase or He phase tritium diffusivity in Li–Pb or He [m2 /s]
Greek letters viscosity [kg/ms] density [mol/m3 ] Subscripts L Li–Pb side G He gas side T tritium atom T2 tritium molecule i interface in inlet out outlet
rate and the control of tritium leak rate were estimated. When the difference in Li-Pb temperature between the inlet and the outlet of the heat exchanger is set 200 ◦ C, the Li–Pb flow rate is calculated to 3.5 m3 /s. Then, the concentration difference of tritium in Li–Pb is 0.07 wppb, and the tritium partial pressure dif-
Fig. 2. A schematic diagram of a gas–liquid counter-current extraction tower using a packed column.
ference is approximately 2.4 × 10−9 Pa. The solubility of tritium is estimated by considering the isotope effect between our previous solubility data of H and D, KS = 2.73 × 10−7 exp(−4.21/Rg T) (H/LiPb)/Pa0.5 . When we assume a tubing material of F82H which thickness is 3 mm, temperature is 500 ◦ C, and tritium permeability is 6.93 × 10−11 mol/m s Pa0.5 [5], the tritium leakage rate through tubing where Li–Pb flows from a reactor to a tritium recovery apparatus is maintained to be lower than 10 Ci/day under the conditions of Li–Pb flow rate of 3.5 m3 /s and flow velocity of 1 m/s. The value of 10 Ci/day is decided for safety reason. If the rate of the overall tritium leak through tubes and walls is limited down to the allowed level, these necessary conditions demand the tritium recovery rate of higher than 99%. Therefore, tritium must be recovered under the severe conditions. 2.2. Tritium recovery method from Li–Pb We here consider the recovery of tritium generated in the liquid Li–Pb blanket by the reaction of 6 Li(n, ˛)T, we suggest a method of a counter-current extraction tower to recover tritium dissolved in Li–Pb. The method is based on a packed column where gas and liquid flow counter-currently, which is industrially used for extracting SOx or NOx from exhaust gas by water. The gas–liquid countercurrent extraction tower is utilized to extract tritium dissolved in Li–Pb by a packed column for tritium recovery. Fig. 2 shows a schematic diagram of the extraction tower. In order to extract tritium dissolved in liquid Li–Pb to gas phase, liquid Li–Pb including tritium is supplied from the top of the column and He is supplied from the bottom. Tritium diffuses from the liquid phase to the gas phase through the interface between the both. In the tower, the liquid phase contacts with the gas phase efficiently by packed materials such as raschig rings. The rate of gas absorption can be described by a mass-balance equation. The concentration profiles through the column and tritium extraction rates are calculated from the equation. When raschig rings are filled in an extraction tower to promote gas–liquid contact, the differential mass-balance equation in the height direction of tower under a steady-state condition is described by the following equation:
Fig. 1. Li–Pb blanket system.
L cLiPb
dcT = kL av (cT − cT,i )dz = kexc av (cT,i − KT
pT2,i )
Y. Edao et al. / Fusion Engineering and Design 85 (2010) 1225–1228
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Table 1 Conditions of tritium recovery by a packed column. Fusion thermal power Tritium generation rate Inlet temperature of Li17 Pb83 Outlet temperature Li17 Pb83 flow rate Tritium concentration of Li17 Pb83 Tritium leak to the environment Tritium recovery rate in total system Diffusivity of Tritium of Li17 Pb83 Sieverts’ constant of Li17 Pb83 Density of Li17 Pb83 Viscosity of Li17 Pb83 Viscosity of He Total pressure of gas Packed material He flow rate Diameter of packed column HL HG
=
1 GW 1.8 MCi/day 500 ◦ C 300 ◦ C 3.5 m3 /s 0.07 wppb 10 Ci/day 99.999% 2.96 × 10−9 m2 /s 4.73 × 10−7 1/Pa0.5 9.33 × 103 kg/m3 1.58 × 10−3 kg/ms 3.85 × 10−5 kg/ms 1.0 × 105 Pa 1 inch raschig ring 47 m3 /s 4m 0.46 m 0.0056 m
2kG av 2G dpT2 (pT2,i − pT2 )dz = Rg T pt
(1)
cT and pT2 of Eq. (1) are integrated to determine the column height as follows: L h = HL NL = cLiPb kL av = = HG NG =
cT,in
cT,out
GRg T kG av pt
dcT cT − cT,i
pT
dpT2
2
pT ,out 2
pT2,i − pT2
(2)
HTU (height per transfer unit) of gas side, HG , and HTU of liquid side, HL , for 1 in. raschig ring are given in a handbook [6]. The values of HG and HL in previous experiment are correlated to the following equation: HG = 3.07 HL =
1 430
G0.32 L0.51
2/3 G G DG
L 0.22 0.5 L L
L DL
(3) (4)
The values of parameters appeared in the above correlations are shown in Table 1. The isotope effect between H and T in the values of diffusivity and solubility were taken into consideration. The values of HG and HL were calculated using the experimental correlation for a H2 O–He gas absorption system, where the properties of Li–Pb are substituted in place of those of the H2 O–He system. HG and HL for are usually correlated by dimensionless parameters. The mass-transfer parameters such as viscosity and diffusivity in He–H2 O system are replaced by those of Li–Pb. This is because the dimensionless parameters are assumed to be valid for any combinations of gas–liquid systems. The largest defect of the counter-current extraction tower is that there is possibility in that flooding occurs under a high He flow rate so as to achieve the high recovery ratio. The flooding flow rate was determined using the previous correlation curve for the raschig ring packing [6]. The T concentration in the packed column was determined under no flooding conditions and operating condition where the outlet T concentration is 1 ppm in He purge gas. Fig. 3 shows the concentration profiles in the packed column and the ratio of permeated tritium amount to the total tritium one, and shows the correlation between the ratio of T permeation rate to the total T generation rate and Li–Pb flow rate. It was estimated that the column height of 5 m is sufficient to achieve the tritium recovery ratio of 99.999% in inlet Li–Pb. The estimation of the packed column is in the case of Li–Pb flow rate of 3.5 m3 /s and flow velocity of 1 m/s (the tube diameter = 2m). In that case, it was estimated that the ratio of the
Fig. 3. T concentration profiles in a packed column and the rate of permeation T to the total T amount.
permeation rate to the total T generation rate could be lower than 1/105 . The tritium leakage rate depends on Li–Pb flow rate in the by-pass loop to the tritium recovery system and the tube diameter. The tritium leak rate is high under the conditions of higher flow rate and larger diameter of the tube. This counter-current extraction method is easy to operate and efficient to recover tritium from Li–Pb. In addition, proper conditions of Li–Pb flow rates and tube diameter should be selected to design for the tritium recovery apparatus. 3. Isotopic exchange between H and D on surface of Li0.17 Pb0.83 3.1. Experiment of isotopic exchange between H and D on surface of Li–Pb In order to enhance the tritium recovery rate, we propose a process using isotopic exchange reaction between tritium and hydrogen added in the inert gas. In the present experiment, we examined how much the isotopic exchange reaction influenced the overall tritium flow from the Li–Pb bulk to an inert purge gas. Fig. 4 shows a schematic diagram of the experimental apparatus. The details of the experimental apparatus and method were described in our previous paper [3,7]. The different things are that H2 gas is used in place of Ar purge gas and permeated gas is D2 . The H2 gas was supplied to the downstream side by 5 cc/min, the D2 gas in the upstream side by a flow rate of 5 cc/min. D permeated through Li0.17 Pb0.83 , and the D2 and HD concentrations in H2 purge gas were measured by a gas chromatograph. A range of the experimental temperature was 400–700 ◦ C at the pressure of 105 Pa. Based on comparison with calculation, the effect of isotopic exchange reaction between H and D on surface of Li–Pb is investigated. 3.2. Results and discussions The experimental results of the concentration of D2 permeating through Li–Pb in H2 purge are shown in Fig. 5. It must be noticed that natural hydrogen gas includes deuterium atoms. The D atomic concentration in the natural H2 gas was 150 ppm, the background concentration was taken into consideration in the experimental results. Comparing the permeation rates in 1 cm and 2 cm of Li–Pb layer thickness, the rate of 1 cm is two times higher than that of 2 cm. It was found that the permeation rate is inverse propor-
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tion with the thickness. After D2 molecules dissociate into two D atoms on the lower surface of Li–Pb, they dissolve into it and diffuse through it. D atoms exchange immediately with H2 molecules on the upper surface, and they become the form of D2 or HD molecules. Finally they are purged by H2 . It was proved that the rate-determining step in the overall permeation process through Li–Pb was not isotopic exchange but diffusion. This is because the experimental data are well fitted by the analytical diffusion equation as seen in the solid line in the figure. Since the rate of isotopic exchange reaction among hydrogen isotopes is so faster than the diffusion rate in Li–Pb, there is no need to add D2 in actual process. Diffusion is the rate-determining step in the overall process. A packed column is proposing as a tritium recovery apparatus for the Li–Pb loop. Therefore, the effect of isotopic exchange reaction is considered small for the counter-current extraction tower for tritium recovery. 4. Conclusions
Fig. 4. A schematic diagram of the experimental apparatus.
A gas–liquid counter-current extraction column was proposed as one of the promising tritium recovery methods. Correlations between tritium recovery rate and a size of the packed column were presented. It was estimated that the column height of 5 m and the diameter of 4 m is necessary in order to achieve the tritium concentration drop of 1/105 in Li–Pb. The effect of isotopic exchange reaction is considered small for the overall tritium transfer process. The rate-determining step was diffusion in the tritium permeation process through Li–Pb. References
Fig. 5. Experimental results of deuterium concentration permeating through Li–Pb in H2 purge.
[1] S. Malang, A.R. Raffray, N.B. Morley, Fusion Eng. Des. 84 (2009) 2145–2157. [2] S. Fukada, Y. Edao, Y. Maeda, T. Norimatsu, Fusion Eng. Des. 83 (2008) 747–751. [3] Y. Edao, S. Fukada, S. Yamaguchi, Y. Maeda, K. Katayama, Fusion Sci. Technol. 56 (2009) 831–835. [4] J.S. Watson, et al., Fusion Technol. 12 (1987) 354–363. [5] E. Serra, A. Perujo, G. Benamati, J. Nucl. Mater. 245 (1997) 108–114. [6] R.H. Perry, D.W. Green, Perry’s Chemical Engineers’ Handbook, 8th ed., McGrawHill, New York, 2008, sec. 5–80. [7] Y. Maeda, Y. Edao, S. Yamaguchi, S. Fukada, Fusion Sci. Technol. 54 (2008) 131–134.
Contents lists available at ScienceDirect
Fusion Engineering and Design journal homepage: www.elsevier.com/locate/fusengdes
Isotopic exchange between hydrogen and deuterium in the process of permeating through Li0.17 Pb0.83 Y. Edao ∗ , H. Noguchi, S. Fukada Department of Advanced Energy Engineering Science, Interdisciplinary Graduate School of Engineering Sciences, Kyushu University, 6-10-1, Hakozaki, Higashi-ku, Fukuoka 812-8581, Japan
a r t i c l e
i n f o
Article history: Available online 29 March 2010 Keywords: Tritium Hydrogen isotopes Permeation Recovery Lithium lead
a b s t r a c t The permeation process of hydrogen isotopes through Li0.17 Pb0.83 has been investigated experimentally. We obtained the overall D permeation rates that take into consideration of the effects of the H–D isotopic exchange reaction on the surface and diffusion in the Li–Pb layer. It was proved that the rate-determining step was not the surface reaction but the diffusion in the Li–Pb bulk. We proposed a system of gas–liquid counter-current extraction using a packed column for continuous tritium recovery. The concentration profiles through the column and tritium extraction rates are calculated. It is estimated that the column height of 5 m is sufficient to achieve the tritium recovery ratio of 99.999%. This counter-current extraction method is easy to operate and efficient to recover tritium from Li–Pb. © 2010 Elsevier B.V. All rights reserved.
1. Introduction Blanket concepts using lithium–lead eutectic alloy (Li0.17 Pb0.83 ) are considered to constitute an effective blanket system for a fusion reactor. The Li–Pb blanket concept has the following advantages; a large tritium-breeding ratio (TBR) due to Pb working as an efficient neutron multiplier, easy tritium recovery because of low tritium solubility in LiPb, high thermal conductivity, comparatively lower reactivity with oxygen and water vapor, and possibility of constituting a simple blanket system. From the above reasons, some concrete Li–Pb blanket concepts are proposed by several countries for ITER TBM and for DEMO; helium-cooled lithium lead (HCLL) by EU and dual-coolant lithium lead (DCLL) by US [1]. In Japan, Li–Pb is also adopted as a conceptual design of a liquid wall self-cooled blanket in an inertial fusion reactor of KOYO-Fast [2]. On the other hand, there are some disadvantages of Li–Pb; i.e., risk of Po generation and a large circulating power caused by its high density and the magneto-hydrodynamic (MHD) pressure drop. A lot of problems to be solved have been left on the tritium issues of Li–Pb blanket. Therefore, we researched tritium behavior in Li–Pb for designing a reliable tritium recovery system to minimize not only tritium inventory in a blanket but also tritium leak to the outside of blanket or the environment. In relation with the present study, we experimentally determined the data of permeability, diffusivity and solubility of hydrogen isotopes in Li–Pb for designing a tri-
∗ Corresponding author. Tel.: +81 92 642 3785; fax: +81 92 642 3784. E-mail address: [email protected] (Y. Edao). 0920-3796/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.fusengdes.2010.03.010
tium recovery system and for evaluating tritium permeation rates through Li–Pb blanket system [3]. In this paper, we discuss tritium flow in a Li–Pb circulation loop and a method to recover tritium recovery method from the loop. In addition, we search the influence of the isotope exchange reaction between H and D on the Li–Pb surface in the overall permeation process. The isotopic exchange reaction between hydrogen isotopes was considered promising as one of the measures to enhance a tritium recovery rate from Li–Pb. 2. Tritium recovery from Li–Pb loop 2.1. Estimation of tritium in Li–Pb blanket We consider a circulation loop of liquid Li–Pb in a laser fusion blanket system composed of a target chamber, a tritium recovery apparatus set in the by-pass loop and a heat exchanger system. Fig. 1 schematically shows a diagram of the Li–Pb loop and tritium flow in a blanket system. A fusion reactor having 1 GW thermal power under steady-state operation, the tritium generation rate is estimated as 1.8 MCi/day (=180 g/day, 0.77 TBq/s). It is comparatively easy to recover tritium from Li–Pb because of its low solubility of hydrogen isotopes. However it causes a serious problem that tritium easily permeates through the blanket to the environment. The maximum permissible tritium leak rates should be less than 10 Ci/day (=370 GBq/day) when a fusion power plant is operated properly, according to a previous report [4]. This is because of protecting human bodies from internal radiation from tritium radio isotope. In addition, the tritium recovery
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Y. Edao et al. / Fusion Engineering and Design 85 (2010) 1225–1228
Nomenclature L cLiPb cT k av z h KT pT2 Rg T G pt HL or HG NL or NG DL or DG
molar liquid Li–Pb flow rate per unit area [mol/m2 s] molar density of Li–Pb [mol/m3 ] tritium concentration in Li–Pb [mol/m3 ] mass-transfer coefficient [m/s] specific surface area of gas bubbles [m2 /m3 ] distance from the bottom of the column [m] height of the column [m] Sieverts’ constant of tritium in Li–Pb [mol/m3 Pa0.5 ] tritium partial pressure in gas phase [Pa] gas law constant [m3 Pa/mol K] temperature [K] molar He gas flow rate per unit area [mol/m2 s] total pressure [Pa] height per transfer unit of Li–Pb or He [m] number of transfer units of Li–Pb phase or He phase tritium diffusivity in Li–Pb or He [m2 /s]
Greek letters viscosity [kg/ms] density [mol/m3 ] Subscripts L Li–Pb side G He gas side T tritium atom T2 tritium molecule i interface in inlet out outlet
rate and the control of tritium leak rate were estimated. When the difference in Li-Pb temperature between the inlet and the outlet of the heat exchanger is set 200 ◦ C, the Li–Pb flow rate is calculated to 3.5 m3 /s. Then, the concentration difference of tritium in Li–Pb is 0.07 wppb, and the tritium partial pressure dif-
Fig. 2. A schematic diagram of a gas–liquid counter-current extraction tower using a packed column.
ference is approximately 2.4 × 10−9 Pa. The solubility of tritium is estimated by considering the isotope effect between our previous solubility data of H and D, KS = 2.73 × 10−7 exp(−4.21/Rg T) (H/LiPb)/Pa0.5 . When we assume a tubing material of F82H which thickness is 3 mm, temperature is 500 ◦ C, and tritium permeability is 6.93 × 10−11 mol/m s Pa0.5 [5], the tritium leakage rate through tubing where Li–Pb flows from a reactor to a tritium recovery apparatus is maintained to be lower than 10 Ci/day under the conditions of Li–Pb flow rate of 3.5 m3 /s and flow velocity of 1 m/s. The value of 10 Ci/day is decided for safety reason. If the rate of the overall tritium leak through tubes and walls is limited down to the allowed level, these necessary conditions demand the tritium recovery rate of higher than 99%. Therefore, tritium must be recovered under the severe conditions. 2.2. Tritium recovery method from Li–Pb We here consider the recovery of tritium generated in the liquid Li–Pb blanket by the reaction of 6 Li(n, ˛)T, we suggest a method of a counter-current extraction tower to recover tritium dissolved in Li–Pb. The method is based on a packed column where gas and liquid flow counter-currently, which is industrially used for extracting SOx or NOx from exhaust gas by water. The gas–liquid countercurrent extraction tower is utilized to extract tritium dissolved in Li–Pb by a packed column for tritium recovery. Fig. 2 shows a schematic diagram of the extraction tower. In order to extract tritium dissolved in liquid Li–Pb to gas phase, liquid Li–Pb including tritium is supplied from the top of the column and He is supplied from the bottom. Tritium diffuses from the liquid phase to the gas phase through the interface between the both. In the tower, the liquid phase contacts with the gas phase efficiently by packed materials such as raschig rings. The rate of gas absorption can be described by a mass-balance equation. The concentration profiles through the column and tritium extraction rates are calculated from the equation. When raschig rings are filled in an extraction tower to promote gas–liquid contact, the differential mass-balance equation in the height direction of tower under a steady-state condition is described by the following equation:
Fig. 1. Li–Pb blanket system.
L cLiPb
dcT = kL av (cT − cT,i )dz = kexc av (cT,i − KT
pT2,i )
Y. Edao et al. / Fusion Engineering and Design 85 (2010) 1225–1228
1227
Table 1 Conditions of tritium recovery by a packed column. Fusion thermal power Tritium generation rate Inlet temperature of Li17 Pb83 Outlet temperature Li17 Pb83 flow rate Tritium concentration of Li17 Pb83 Tritium leak to the environment Tritium recovery rate in total system Diffusivity of Tritium of Li17 Pb83 Sieverts’ constant of Li17 Pb83 Density of Li17 Pb83 Viscosity of Li17 Pb83 Viscosity of He Total pressure of gas Packed material He flow rate Diameter of packed column HL HG
=
1 GW 1.8 MCi/day 500 ◦ C 300 ◦ C 3.5 m3 /s 0.07 wppb 10 Ci/day 99.999% 2.96 × 10−9 m2 /s 4.73 × 10−7 1/Pa0.5 9.33 × 103 kg/m3 1.58 × 10−3 kg/ms 3.85 × 10−5 kg/ms 1.0 × 105 Pa 1 inch raschig ring 47 m3 /s 4m 0.46 m 0.0056 m
2kG av 2G dpT2 (pT2,i − pT2 )dz = Rg T pt
(1)
cT and pT2 of Eq. (1) are integrated to determine the column height as follows: L h = HL NL = cLiPb kL av = = HG NG =
cT,in
cT,out
GRg T kG av pt
dcT cT − cT,i
pT
dpT2
2
pT ,out 2
pT2,i − pT2
(2)
HTU (height per transfer unit) of gas side, HG , and HTU of liquid side, HL , for 1 in. raschig ring are given in a handbook [6]. The values of HG and HL in previous experiment are correlated to the following equation: HG = 3.07 HL =
1 430
G0.32 L0.51
2/3 G G DG
L 0.22 0.5 L L
L DL
(3) (4)
The values of parameters appeared in the above correlations are shown in Table 1. The isotope effect between H and T in the values of diffusivity and solubility were taken into consideration. The values of HG and HL were calculated using the experimental correlation for a H2 O–He gas absorption system, where the properties of Li–Pb are substituted in place of those of the H2 O–He system. HG and HL for are usually correlated by dimensionless parameters. The mass-transfer parameters such as viscosity and diffusivity in He–H2 O system are replaced by those of Li–Pb. This is because the dimensionless parameters are assumed to be valid for any combinations of gas–liquid systems. The largest defect of the counter-current extraction tower is that there is possibility in that flooding occurs under a high He flow rate so as to achieve the high recovery ratio. The flooding flow rate was determined using the previous correlation curve for the raschig ring packing [6]. The T concentration in the packed column was determined under no flooding conditions and operating condition where the outlet T concentration is 1 ppm in He purge gas. Fig. 3 shows the concentration profiles in the packed column and the ratio of permeated tritium amount to the total tritium one, and shows the correlation between the ratio of T permeation rate to the total T generation rate and Li–Pb flow rate. It was estimated that the column height of 5 m is sufficient to achieve the tritium recovery ratio of 99.999% in inlet Li–Pb. The estimation of the packed column is in the case of Li–Pb flow rate of 3.5 m3 /s and flow velocity of 1 m/s (the tube diameter = 2m). In that case, it was estimated that the ratio of the
Fig. 3. T concentration profiles in a packed column and the rate of permeation T to the total T amount.
permeation rate to the total T generation rate could be lower than 1/105 . The tritium leakage rate depends on Li–Pb flow rate in the by-pass loop to the tritium recovery system and the tube diameter. The tritium leak rate is high under the conditions of higher flow rate and larger diameter of the tube. This counter-current extraction method is easy to operate and efficient to recover tritium from Li–Pb. In addition, proper conditions of Li–Pb flow rates and tube diameter should be selected to design for the tritium recovery apparatus. 3. Isotopic exchange between H and D on surface of Li0.17 Pb0.83 3.1. Experiment of isotopic exchange between H and D on surface of Li–Pb In order to enhance the tritium recovery rate, we propose a process using isotopic exchange reaction between tritium and hydrogen added in the inert gas. In the present experiment, we examined how much the isotopic exchange reaction influenced the overall tritium flow from the Li–Pb bulk to an inert purge gas. Fig. 4 shows a schematic diagram of the experimental apparatus. The details of the experimental apparatus and method were described in our previous paper [3,7]. The different things are that H2 gas is used in place of Ar purge gas and permeated gas is D2 . The H2 gas was supplied to the downstream side by 5 cc/min, the D2 gas in the upstream side by a flow rate of 5 cc/min. D permeated through Li0.17 Pb0.83 , and the D2 and HD concentrations in H2 purge gas were measured by a gas chromatograph. A range of the experimental temperature was 400–700 ◦ C at the pressure of 105 Pa. Based on comparison with calculation, the effect of isotopic exchange reaction between H and D on surface of Li–Pb is investigated. 3.2. Results and discussions The experimental results of the concentration of D2 permeating through Li–Pb in H2 purge are shown in Fig. 5. It must be noticed that natural hydrogen gas includes deuterium atoms. The D atomic concentration in the natural H2 gas was 150 ppm, the background concentration was taken into consideration in the experimental results. Comparing the permeation rates in 1 cm and 2 cm of Li–Pb layer thickness, the rate of 1 cm is two times higher than that of 2 cm. It was found that the permeation rate is inverse propor-
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Y. Edao et al. / Fusion Engineering and Design 85 (2010) 1225–1228
tion with the thickness. After D2 molecules dissociate into two D atoms on the lower surface of Li–Pb, they dissolve into it and diffuse through it. D atoms exchange immediately with H2 molecules on the upper surface, and they become the form of D2 or HD molecules. Finally they are purged by H2 . It was proved that the rate-determining step in the overall permeation process through Li–Pb was not isotopic exchange but diffusion. This is because the experimental data are well fitted by the analytical diffusion equation as seen in the solid line in the figure. Since the rate of isotopic exchange reaction among hydrogen isotopes is so faster than the diffusion rate in Li–Pb, there is no need to add D2 in actual process. Diffusion is the rate-determining step in the overall process. A packed column is proposing as a tritium recovery apparatus for the Li–Pb loop. Therefore, the effect of isotopic exchange reaction is considered small for the counter-current extraction tower for tritium recovery. 4. Conclusions
Fig. 4. A schematic diagram of the experimental apparatus.
A gas–liquid counter-current extraction column was proposed as one of the promising tritium recovery methods. Correlations between tritium recovery rate and a size of the packed column were presented. It was estimated that the column height of 5 m and the diameter of 4 m is necessary in order to achieve the tritium concentration drop of 1/105 in Li–Pb. The effect of isotopic exchange reaction is considered small for the overall tritium transfer process. The rate-determining step was diffusion in the tritium permeation process through Li–Pb. References
Fig. 5. Experimental results of deuterium concentration permeating through Li–Pb in H2 purge.
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