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Viscosity of molten lithium, thorium and beryllium fluorides mixtures
Journal of Nuclear Materials 419 (2011) 361–365
Contents lists available at ScienceDirect
Journal of Nuclear Materials journal homepage: www.elsevier.com/locate/jnucmat
Presented at the NuMat 2010 Conference, 4–7 October 2010, Karlsruhe
Viscosity of molten lithium, thorium and beryllium fluorides mixtures Alexander V. Merzlyakov, Victor V. Ignatiev ⇑, Sergei S. Abalin National Research Centre ‘‘Kurchatov Institute’’, Moscow, Russian Federation
a r t i c l e
i n f o
Article history: Available online 8 July 2011
a b s t r a c t Considering development of Molten Salt Fast Reactor (MSFR) concept, following Molten Salt fluorides mixtures have been chosen as an object for viscosity studies in this work (in mol%): 78LiF–22ThF4; 71LiF– 27ThF4–2BeF2 and 75LiF–20ThF4–5BeF2. Additionally, the effect of the 3 mol% CeF3 additives on viscosity of the molten 75LiF–20ThF4–5BeF2 (mol%) salt mixture has been investigated experimentally. The method of torsional oscillations of cylindrical crucible filled by molten fluorides mixture has been chosen for kinematic viscosity measurement at temperatures up to 800–850 °C. In temperature ranges, where melts behave as normal liquids, dependences on viscosity vs. temperature are received: m = A exp [B/T(K)], where m – kinematic viscosity, m2/s; T – temperature, K. The kinematic viscosity Rout mean squares (RMS) estimated in the assumption about dispersion homoscedasticity is (0.04–0.12) 106 (m2/s). Discrepancies left in the data of viscosity for molten mixtures of LiF, BeF2 and ThF4 received by different researchers need further investigations in this area to be continued. Ó 2011 Elsevier B.V. All rights reserved.
1. Introduction For all molten salt reactor (MSR) concepts, material selection is a very important issue. Selection of the fuel salt composition strongly depends on the specific design application. Particularly, fuel/blanket molten salt mixtures for Th-U MSFR breeder concepts based on homogeneous core without graphite moderator can have high enough concentration of a fertile material (up to 20 mol% or more) and melting temperatures >550 °C [1]. At the same time the concentration of fissile materials in starting fuel salt loading may reach 2–3 mol% for uranium tetrafluoride and 6–7 mol% for transuranium (TRU) elements trifluorides. In MSFR fuel/blanket salt operating temperatures are in the range from 650 °C up to 750 °C. The viscosity of the molten salt mixtures of lithium, beryllium, thorium and uranium fluorides was first experimentally studied in the US ORNL [2,3] (the declared error of measurements ±10%) and later in Russia [4] (the declared error of measurements ±2%). The comparison of the experimental and calculated data on viscosity and temperature range of measurements published by various researchers is given in Table 1. The analysis of certain experimental [2–4] and calculated results [5] lets draw some conclusions about the viscosity of the examined fuel salts: 1. The dependence of viscosity of all molten salt fluorides mixtures on the reverse absolute temperature is described with the sufficient degree of accuracy by the exponential equations. ⇑ Corresponding author. E-mail address: [email protected] (V.V. Ignatiev). 0022-3115/$ - see front matter Ó 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.jnucmat.2011.06.030
2. The viscosity increases with adding heavy metals tetrafluorides to binary molten salt mixtures containing fluorides of lithium as the second components. 3. Experimental data [4] on viscosity for the binary and ternary molten fluorides mixtures of lithium, thorium and beryllium containing ThF4 molar fraction >20 mol% are received at temperatures >700 °C. At 700–800 °C kinematic viscosity isotherms for the LiF–ThF4 molten salt mixture rise higher than the one for molten LiF–UF4 salt mixture [4]. Nevertheless, the data of experiments on kinematic viscosity of molten 80LiF–20UF4 salt mixture given in Janzs work [3] are also considerably higher (up to 100% at 720 °C), whereas at lower temperatures the difference will be essentially more than those measured by Desyatnik et al. [4] for the same system and for molten 80LiF–20ThF4 salt mixture (see Fig. 3). So, for the binary and ternary molten fluorides mixtures containing ThF4 molar fraction >20 mol%, there are shortage in the experimental results for temperature range from liquidus up to 700 °C and considerable controversies in the data published by different researchers that makes new measurements up-todate. The following molten salt mixtures of lithium, thorium and beryllium fluorides (in mol%) is the main issue singled out as an object for viscosity studies in this work: (A) 78LiF–22ThF4; (B) 71LiF–27ThF4–2BeF2 and (C) 75LiF–20ThF4–5BeF2 (mol%). According to phase diagrams the chosen salt mixtures melt at 565 °C (A), 560 °C (B) and 560 °C (C) respectively [7]. Besides the influence of the 3 mol% CeF3 additive on viscosity of the molten salt mixture (C) has experimentally been investigated. According to ORNL data of experiments [8] at 550 °C and 650 °C CeF3 solubility in molten
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Table 1 Viscosity of molten salt fluorides of lithium, beryllium and thorium [2–6]. Salt composition (mol%)
Viscosity Range of measurements (m) (m2/ (Dt) (°C) s 106) 600 °C
700 °C 800 °C
70LiF–30ThF4 71LiF–29ThF4 80LiF–20ThF4 80LiF–20UF4 78LiF–22ThF4 78LiF–22ThF4 78LiF–22ThF4
637–989 600–700 723–997 723–997 Estimation Estimation 625–846
– 4.66 – – – 2.30 –
1.94 2.93 – – 1.40 1.64 2.46
1.48 – 1.37 0.81 1.11 1.31 1.73
71LiF–27ThF4–2BeF2 75LiF–20ThF4–5BeF2 75LiF–20ThF4–5BeF2
593–805 Estimation 578–820
2.94 2.21 2.81
2.04 1.74 2,25
1.52 1.37 1.88
– 3,75 –
2.27 2.29 –
1.94 1.94 1.37
75LiF–20ThF4–5BeF2 with CeF3 693–842 73LiF–11ThF4–16BeF2 500–800 72LiF–18ThF4–10BeF2 730–1060
75LiF–20ThF4–5BeF2 respectively.
salt
mixture
are
2.14
Refs.
and
[4] [2] [4] [4] [6] [5] This work [5] This work [2] [4]
3.85 mol%
2. Purification of salt composition The initial salts taken for preparation of the eutectic mixtures were commercially made anhydrous powdered fluorides of lithium and beryllium, which were purified of possible traces of adsorbed moisture by treating them at a low pressure (1 Pa) while heating gradually from room temperature to 450 °C for 2–3 h. The chemical analysis of ThF4 available has revealed that this product contains up to 2.5 mass% of oxygen. Attempts to remove water at slow heating of the sample (fine powder) in vacuum up to 370 °C have shown that not more than 0.5% of mass are left. Further on dewatering was carried out by heating ThF4 and NH4HF2 mixture. First, the mixture of ThF4 and NH4HF2 was prepared with the molar ratio 1:1, and placed in a glass carbon crucible with a copper cover under which pure argon was supplied. This argon together with the products of the reaction was brought away through an outlet. The thermocouple under a copper cover was placed in a salt powder mixture. The temperature in the crucible was slowly risen till steam and aerosol start to go out of the outlet after which the temperature was fixed. The mixture was kept at this temperature until steam and aerosol stop to go out. Thus, the temperature rose up to 350 °C. The salt components received in this way were used for preparation of salt mixtures for the subsequent analysis of viscosity. 3. The method of measurement In this paper as well as in [4], the method of torsional oscillations of cylindrical crucible filled with molten salt mixture has been chosen for the viscosity measurement. This method is widely used for measurements of the viscosity of liquid metals and semiconductors. Most researches consider this method to be suitable and reliable. The merits of this method involve: the definition of absolute values of kinematic viscosity without preliminary calibration of the test section with other liquids; the measurement of viscosity in a wide range of its values with comparatively high accuracy; low sensitivity to side effects (a surface tension, formation of films etc.). The method is described in details in the ISTC # 1606 project report [9]. The scheme of experimental facility is presented in Fig. 1. Through a metal rod (2) the cylindrical crucible with molten salts (1) was suspended by an elastic metal string (3) in the
Fig. 1. The test section for kinematic viscosity measurement: 1 – crucible with molten salt, 2 – rod, 3 – string, 4 – mirror, 5 – thermostatic insert, 6 – heater, 7 – thermocouple, 8 – vessel, 9 – thermal insulation, 10 – He–Ne laser, 11 – window, 12 – scale.
thermostatic insertion (5) made of the material with high thermal conductivity. There was a heater outside the thermostatic insertion (6). The temperature was measured by the thermocouple (7) placed close to the crucible in the thermostatic insertion. Water cooled vessel (8) separated the heating zone by a thick layer of thermal insulation (9). A mirror (4) was installed on a core of the suspended system (2). The He–Ne laser beam (10) was directed through a window (11) and went on the mirror (4). The position of the reflected beam was pointed out on a scale (12). All experiments have been carried out under inert gas at the pressure that was a bit higher than the atmosphere one. The viscosity of several glycerin solutions in water was measured to check up the operability of the system and accuracy of data processing. The comparison of the data received by experiments with the ones given in manuals has proved that the method of measurements and processing of the data of experiments leads to receiving accurate results in the values of kinematic viscosity ranging from 106 up to 105 m2/s [9]. Carefully mixed salt composition was put into the stainless steel cylinder that had been made for measurements. The cylinder was suspended by an elastic string in the installation. Cleared helium filled the installation chamber. The thermostat and the cylinder were heated up to 250 °C. At this temperature the installation chamber was pumped out, the sample was heated in vacuum up to 450 °C and then was maintained at this temperature for 20– 30 min. Further on, the installation was again filled with helium, the one being heated at the rate of 2–3 °C per minute. The sample was heated up to 800 °C and maintained for about 6–8 h so that the salt mixture melted down and the system came to thermodynamic equilibrium. The decrement of attenuation and viscosity were measured at the cooling. 4. The results of experiments 4.1. 78LiF–22ThF4 system Dependences of attenuation decrement on the temperature in experiments with molten 78LiF–22ThF4 salt mixture (mol%) are given in Fig. 2. At the temperatures higher than 625 °C the system is characterized by the high attenuation decrement which decreases with the temperature increase. This phenomenon occurs in completely melted substances. At temperatures lower than
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At low temperatures the data received are closer to the data [2,3] rather than to the data [4], whereas high temperature range slightly exceeds the data of both the above mentioned works.
300
4.2. 71LiF–27ThF4–2BeF2 system 200
100
0 500
600
700
800
Temperature (oC)
900
Fig. 2. Temperature dependence of attenuation decrement for 78LiF–22ThF4 (mol%) melt.
625 °C the system is characterized by decrement which goes down with the temperature decrease that shows partial freezing of the melted substances. The dependence of attenuation decrement longs to low values at temperatures 550–570 °C that fully agrees with the phase diagram [7]. The experimental points received at temperatures higher than 625 °C were used to determine viscosity. The dependence of viscosity of the temperature is given in Fig. 3. The data of experiments of processing by the least square method leads to the following dependence of the viscosity logarithm on the temperature (K):
lnðm 106 ½m2 =sÞ ¼ 0:6830 þ 3689 ð1=T 0:9689E 3Þ The RMS of the kinematic viscosity logarithm estimated in the assumption of dispersions homoscedasticity is 0.019. The dependence of kinematic viscosity on the temperature ranging from 625 to 846 °C is presented as follows:
The temperature dependence of the attenuation decrement, received by viscosity measurements of molten 71LiF–27ThF4–2BeF2 salt mixture (mol%) is given in Fig. 4. As in the first case, salt melted substances show themselves as normal liquid at temperatures higher than 593 °C. At lower temperatures attenuation decrement goes down with the temperature decrease that testifies partial freezing of the melted substances. The experimental points received at temperatures higher than 593 °C were used to determine viscosity. The dependence of viscosity on the temperature for molten 71LiF–27ThF4–2BeF2 salt mixture (mol%) is given in Fig. 5. Processing of the data of experiments by the least square method leads to the following dependence of the viscosity logarithm on the temperature (K):
lnðm 106 ½m2 =sÞ ¼ 0:7302 þ 3093 ð1=T 1:033E 3Þ The RMS of the kinematic viscosity logarithm estimated in the assumption of dispersions homoscedasticity is 0.042.
500 400
Decrement *10 3
Decrement*10
3
400
300 200 100 0 500
m 106 ðm2 =sÞ ¼ 1:9798 expf3689 ð1=TðKÞ 0:9698E 3Þg The RMS of the kinematic viscosity estimated in the assumption of dispersions homoscedasticity is 0.042 106 (m2/s).
600
700
Temperature (oC)
800
Fig. 4. Temperature dependence of attenuation decrement for molten 71LiF– 27ThF4–2BeF2 salt mixture (mol%).
4
3
2
1
0
600
700
o
800
Viscosity*10 6 (m 2 /s)
Viscosity*106 (m2/s)
4
3
2
1
Temperature ( C) Current work 78LiF–22ThF4, experimental points Current work 78LiF–22ThF4, approximation 80LiF–20ThF4, Janz [3] 70LiF–30ThF4, Desyatnik [4] 80LiF–20ThF4, Desyatnik [4] 80LiF–20UF4, Desyatnik [4] 78LiF–22ThF4, Benes [6]
Fig. 3. Temperature dependence of kinematic viscosity for lithium, thorium and uranium fluorides melts (in mol%): (N) Current work 78LiF–22ThF4, experimental points. (—) Current work 78LiF–22ThF4, approximation. (– – –) 80LiF–20ThF4, Janz [3]. (– j – j –) 70LiF–30ThF4, Desyatnik et al. [4]. (- - - - - -) 80LiF–20ThF4, Desyatnik et al. [4]. (– j j j –) 80LiF–20UF4, Desyatnik et al. [4]. (– jj – jj –) 78LiF–22ThF4, Benes and Koning [6].
0 550
600
650
700
o
750
800
Temperature ( C) Current work 71LiF–27ThF4–2 BeF2, experimental points Current work 71LiF–27ThF4–2 BeF2, approximation 80LiF–20ThF4, Desyatnik [4] 70LiF–30ThF4, Desyatnik [4]
Fig. 5. Temperature dependence of kinematic viscosity for lithium, beryllium and thorium fluorides melts (mol%): (N) Current work 71LiF–27ThF4–2 BeF2, experimental points. (—) Current work 71LiF–27ThF4–2 BeF2, approximation. (– j – j –) 80LiF–20ThF4, Desyatnik et al. [4]. (- - - - - -) 70LiF–30ThF4, Desyatnik et al. [4].
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3
m 106 ðm2 =sÞ ¼ 2:075 expf3093 ð1=TðKÞ 1:033E 3Þg The RMS of the kinematic viscosity estimated in the assumption of dispersions homoscedasticity is 0.10 106 (m2/s). 4.3. 75LiF–20ThF4–5BeF2 system The dependence of the attenuation decrement, on the temperature resulting from the measuring of molten 75LiF–20ThF4–5BeF2 salt mixture (mol%) is given in Fig. 6. As in the first two cases, salt melted substances show themselves as normal liquids at temperatures higher than 578 °C. At lower temperatures attenuation decrement goes down with the temperature decrease that proves partial freezing of the sample. Freezing goes through two stages. At the temperatures ranging from 570 to 540 °C there is easy decrease of attenuation decrement when the temperature goes down, whereas at temperatures lower than 540 °C it sharply decreases. The experimental points received at temperatures higher than 578 °C were used to determine viscosity. The dependence of viscosity on the temperature of this molten salt mixture is given in Fig. 7. Processing of the data of experiments by the least square method leads to the following dependence of the viscosity logarithm on the temperature (K):
Viscosity*106 (m2 /s)
The dependence of kinematic viscosity on the temperatures ranging from 590 to 810 °C is presented as:
550
650
750
800
850
Current work 75LiF–20ThF4–5BeF2, experimental points Current work 75LiF–20ThF4–5BeF2, approximation 70LiF–30ThF4, Desyatnik [4] 80LiF–20ThF4, Desyatnik [4] Fig. 7. Temperature dependence of kinematic viscosity for lithium, beryllium and thorium fluorides melts (mol%): (N) Current work 75LiF–20ThF4–5BeF2, experimental points. (—) Current work 75LiF–20ThF4–5BeF2, approximation. – j – j –) 70LiF–30ThF4, Desyatnik et al. [4]. (- - - - - -) 80LiF–20ThF4, Desyatnik et al. [4].
400
300
200
100
0
500
600
700
800
900
Temperature (oC)
4.4. 75LiF–20ThF4–5BeF2 system with CeF3 addition
Fig. 8. Temperature dependence of attenuation decrement for molten 75LiF– 20ThF4–5BeF2 salt mixture with 3CeF3 addition (mol%).
3.2
Viscosity*10 6 (m2/s)
The experiments on measuring the viscosity of molten 75LiF– 20ThF4–5BeF2 salt mixture (mol%) having been conducted, when 7.4 g (3 mol%) of CeF3 were added into the same crucible. The new experiment was being carried out under the same conditions as it had been done without CeF3. The data on attenuation decrement and viscosity are presented in Figs. 8 and 9 respectively.
500 400 300 200
2.8
2.4
2
1.6 600
700
o
800
Temperature ( C)
100 0 500
700
o
The RMS of the kinematic viscosity estimated in the assumption of dispersions homoscedasticity is 0.12 106 (m2/s).
Decrement *10 3
600
Temperature ( C)
Decrement*10 3
m 106 ðm2 =sÞ ¼ 2:1905 expf1877 ð1=TðKÞ 1:013E 3Þg
1
0
lnðm 106 ½m2 =sÞ ¼ 0:7841 þ 1877 ð1=T 1:013E 3Þ The RMS of the kinematic viscosity logarithm estimated in the assumption of dispersions homoscedasticity is 0.058. The dependence of kinematic viscosity on the temperature ranging from 572 to 815 °C is presented as:
2
600
700
800
Temperature (oC) Fig. 6. Temperature dependence of attenuation decrement for molten 75LiF– 20ThF4–5BeF2 salt mixture (mol%).
75LiF–20ThF4–5BeF2 with 3CeF3 addition, experimental points 75LiF–20ThF4–5BeF2 with 3CeF3 addition, approximation 75LiF–20ThF4–5BeF2, approximation Fig. 9. Temperature dependence of kinematic viscosity for molten 75LiF–20ThF4– 5BeF2 salt mixture without and with 3CeF3 addition (mol%): (N) 75LiF–20ThF4– 5BeF2 with 3CeF3 addition, experimental points. (—) 75LiF–20ThF4–5BeF2 with 3CeF3 addition, approximation. (– – –) 75LiF–20ThF4–5BeF2, approximation.
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Viscosity*106 (m2 /s)
4
Table 2 Viscosity vs. temperature dependences for molten salt fluorides of lithium, beryllium and thorium (this work).
3
Salt composition (mol%)
Dt (°C)
m 106 (m2/s)
RMS 106 (m2/s)
78LiF–22ThF4
625– 846 593– 805 578– 820 693– 842
1.980 exp{3689⁄(1/ T(K)0.9698E3)} 2.075 exp{3093⁄(1/ T(K)1.033E3)} 2.1905 exp{1877⁄(1/ T(K)1.013E3)} 2.037 exp{1465⁄(1/ T(K)0.9627E3)}
0.042
2 71LiF–27ThF4–2BeF2 75LiF–20ThF4–5BeF2
1
75LiF–20ThF4–5BeF2 with CeF3
0.10 0.12 0.053
0 500
600
700
800
Temperature ( oC)
900
78LiF–22ThF4 71LiF–27ThF4–2BeF2 75LiF–20ThF4–5BeF2 75LiF–20ThF4–5BeF2 with 3CeF3 addition Fig. 10. Kinematic viscosity vs. temperature for the lithium, beryllium and thorium fluorides melts under study (mol%): (—) 78LiF–22ThF4. (– j – j –) 71LiF–27ThF4– 2BeF2. (– jj – jj –) 75LiF–20ThF4–5BeF2. (- - - - - -) 75LiF–20ThF4–5BeF2 with 3CeF3 addition.
As Fig. 8, presents the dependence of a decrement on the temperature at high temperatures is very smooth so it is very difficult to determine higher temperatures when the system starts to show itself as normal liquid. This temperature has been accepted to be 693 °C. At temperatures decreasing from 693 °C the decrement slowly goes down to approximately 570 °C that indicates freezing of a small part of the melted substances. At decreasing the temperature from 570 down to 542 °C an already substantial part of melted substances under consideration turns into the solid state. At temperatures lower than 542 °C the decrement sharply decreases so as it might be taken as freezing of the sample. The state of melted substances as common liquid may be examined only at temperatures higher than 693 °C. Processing of the data of experiments by the least square method gives the following dependence of the viscosity logarithm on temperature (K):
lnðm 106 ½m2 =sÞ ¼ 0:7117 þ 1647 ð1=T 0:9627E 3Þ The RMS of the kinematic viscosity logarithm estimated in the assumption of dispersions homoscedasticity is 0.026. The dependence of kinematic viscosity in the temperature ranging from 693 to 835 °C is presented as follows:
m 106 ðm2 =sÞ ¼ 2:037 expf1465 ð1=TðKÞ 0:9627E 3Þg The RMS of the kinematic viscosity estimated in the assumption of dispersions homoscedasticity is 0.053 106 (m2/s). For comparison the dependence of viscosity on the temperature for molten 75LiF–20ThF4–5BeF2 salt mixture (mol%) without CeF3 additive is given by a dotted line in Fig. 9. It is clear that practically CeF3 additive does not increase the viscosity of molten salt mixture under study, but sharply raises the liquidus temperature. Its increase at adding CeF3 had earlier been singled out by ORNL researchers [8]. Correlation curves of viscosity for all compositions analyzed in this paper are given in Fig. 10. Viscosity vs. temperature dependences for three molten salt fluorides mixture of lithium, beryllium and thorium received in this work are summarized in Table 2. For
comparison, temperature intervals where the viscosity of molten salt mixtures was measured in this study as well as the ones of other researchers are given in Table 1. 5. Conclusion 1. The viscosity of the three molten salt mixtures such as 78LiF– 22ThF4; 71LiF–27ThF4–2BeF2 and 75LiF–20ThF4–5BeF2 (mol%) has been measured at the temperature ranging from liquidus up to 800–850 °C by the method of torsional oscillations attenuation of the cylinder with the melt under study. 2. In temperature ranges where molten salt mixtures show themselves as normal liquids, the following dependences of viscosity on the temperature have been drawn out: m = A exp [B/(t + 273)], where m – kinematic viscosity, m2/s; t – temperature (°C). The kinematic viscosity RMS estimated in the assumption about dispersion homoscedasticity is (0.04–0.12) 106 (m2/s). 3. As a whole the received values of viscosity correlate well enough with the ones of ORNL researchers [1–3]. Our data agree with the data of experiments [4] for molten 71LiF–27ThF4– 2BeF2 salt mixture (mol%), but substantially exceed their kinematic viscosity for 78LiF–22ThF4 b 75LiF–20ThF4–5BeF2 ones. 4. The addition of 3 mol% CeF3 to molten 75LiF–20ThF4–5BeF2 sat mixture (mol%) substances considerably increases the liquidus temperature without particular effect on the value of kinematic viscosity. 5. Discrepancies left in the data of viscosity for molten mixtures of LiF, BeF2 and ThF4 received by different researchers need further investigations in this area to be continued.
References [1] C. Renault, S. Delpech, V. Ignatiev, et al., in: Proceedings of Seventh European Commission Conference on Euroatom Research and Training in Reactor Systems, Prague, Czech Republic, June 22–24, 2009. [2] S. Cantor, Density and Viscosity of Several Molten Fluoride Mixtures, Report ORNL/TM-4308, Oak Ridge, USA, 1973. [3] G.J. Janz, Molten Salts Handbook, Academic Press, NY, USA, 1967. [4] V.N. Desyatnik, A.I. Nechayev, Y.u.F. Chervinsky, Z. Prikladnoi Khim. 54 (10) (1981) 2310–2312. [5] V. Khokhlov, V. Ignatiev, V. Afonichkin, J. Fluorine Chem. 130 (1) (2009) 30–37. [6] O. Beneš, R.J.M. Konings, J. Fluorine Chem. 130 (1) (2009) 22–29. [7] R.E. Thoma, Phase Diagrams of Nuclear Reactor Materials, Report ORNL-2548, Oak Ridge, USA, 1959, p.42. [8] J.A. Fredricksen, L.O. Gilpatrick, C.J. Barton, Solubility of Cerium Trifluoride in Molten Mixtures of LiF, BeF2 and ThF4, Report ORNL-TM-2335, Oak Ridge, USA, 1969, 23p. [9] V. Ignatiev et al., ISTC#1606 Project Final Report, phase 1, International Scientific Technical Centre, Moscow, Russia, 2004.
Contents lists available at ScienceDirect
Journal of Nuclear Materials journal homepage: www.elsevier.com/locate/jnucmat
Presented at the NuMat 2010 Conference, 4–7 October 2010, Karlsruhe
Viscosity of molten lithium, thorium and beryllium fluorides mixtures Alexander V. Merzlyakov, Victor V. Ignatiev ⇑, Sergei S. Abalin National Research Centre ‘‘Kurchatov Institute’’, Moscow, Russian Federation
a r t i c l e
i n f o
Article history: Available online 8 July 2011
a b s t r a c t Considering development of Molten Salt Fast Reactor (MSFR) concept, following Molten Salt fluorides mixtures have been chosen as an object for viscosity studies in this work (in mol%): 78LiF–22ThF4; 71LiF– 27ThF4–2BeF2 and 75LiF–20ThF4–5BeF2. Additionally, the effect of the 3 mol% CeF3 additives on viscosity of the molten 75LiF–20ThF4–5BeF2 (mol%) salt mixture has been investigated experimentally. The method of torsional oscillations of cylindrical crucible filled by molten fluorides mixture has been chosen for kinematic viscosity measurement at temperatures up to 800–850 °C. In temperature ranges, where melts behave as normal liquids, dependences on viscosity vs. temperature are received: m = A exp [B/T(K)], where m – kinematic viscosity, m2/s; T – temperature, K. The kinematic viscosity Rout mean squares (RMS) estimated in the assumption about dispersion homoscedasticity is (0.04–0.12) 106 (m2/s). Discrepancies left in the data of viscosity for molten mixtures of LiF, BeF2 and ThF4 received by different researchers need further investigations in this area to be continued. Ó 2011 Elsevier B.V. All rights reserved.
1. Introduction For all molten salt reactor (MSR) concepts, material selection is a very important issue. Selection of the fuel salt composition strongly depends on the specific design application. Particularly, fuel/blanket molten salt mixtures for Th-U MSFR breeder concepts based on homogeneous core without graphite moderator can have high enough concentration of a fertile material (up to 20 mol% or more) and melting temperatures >550 °C [1]. At the same time the concentration of fissile materials in starting fuel salt loading may reach 2–3 mol% for uranium tetrafluoride and 6–7 mol% for transuranium (TRU) elements trifluorides. In MSFR fuel/blanket salt operating temperatures are in the range from 650 °C up to 750 °C. The viscosity of the molten salt mixtures of lithium, beryllium, thorium and uranium fluorides was first experimentally studied in the US ORNL [2,3] (the declared error of measurements ±10%) and later in Russia [4] (the declared error of measurements ±2%). The comparison of the experimental and calculated data on viscosity and temperature range of measurements published by various researchers is given in Table 1. The analysis of certain experimental [2–4] and calculated results [5] lets draw some conclusions about the viscosity of the examined fuel salts: 1. The dependence of viscosity of all molten salt fluorides mixtures on the reverse absolute temperature is described with the sufficient degree of accuracy by the exponential equations. ⇑ Corresponding author. E-mail address: [email protected] (V.V. Ignatiev). 0022-3115/$ - see front matter Ó 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.jnucmat.2011.06.030
2. The viscosity increases with adding heavy metals tetrafluorides to binary molten salt mixtures containing fluorides of lithium as the second components. 3. Experimental data [4] on viscosity for the binary and ternary molten fluorides mixtures of lithium, thorium and beryllium containing ThF4 molar fraction >20 mol% are received at temperatures >700 °C. At 700–800 °C kinematic viscosity isotherms for the LiF–ThF4 molten salt mixture rise higher than the one for molten LiF–UF4 salt mixture [4]. Nevertheless, the data of experiments on kinematic viscosity of molten 80LiF–20UF4 salt mixture given in Janzs work [3] are also considerably higher (up to 100% at 720 °C), whereas at lower temperatures the difference will be essentially more than those measured by Desyatnik et al. [4] for the same system and for molten 80LiF–20ThF4 salt mixture (see Fig. 3). So, for the binary and ternary molten fluorides mixtures containing ThF4 molar fraction >20 mol%, there are shortage in the experimental results for temperature range from liquidus up to 700 °C and considerable controversies in the data published by different researchers that makes new measurements up-todate. The following molten salt mixtures of lithium, thorium and beryllium fluorides (in mol%) is the main issue singled out as an object for viscosity studies in this work: (A) 78LiF–22ThF4; (B) 71LiF–27ThF4–2BeF2 and (C) 75LiF–20ThF4–5BeF2 (mol%). According to phase diagrams the chosen salt mixtures melt at 565 °C (A), 560 °C (B) and 560 °C (C) respectively [7]. Besides the influence of the 3 mol% CeF3 additive on viscosity of the molten salt mixture (C) has experimentally been investigated. According to ORNL data of experiments [8] at 550 °C and 650 °C CeF3 solubility in molten
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Table 1 Viscosity of molten salt fluorides of lithium, beryllium and thorium [2–6]. Salt composition (mol%)
Viscosity Range of measurements (m) (m2/ (Dt) (°C) s 106) 600 °C
700 °C 800 °C
70LiF–30ThF4 71LiF–29ThF4 80LiF–20ThF4 80LiF–20UF4 78LiF–22ThF4 78LiF–22ThF4 78LiF–22ThF4
637–989 600–700 723–997 723–997 Estimation Estimation 625–846
– 4.66 – – – 2.30 –
1.94 2.93 – – 1.40 1.64 2.46
1.48 – 1.37 0.81 1.11 1.31 1.73
71LiF–27ThF4–2BeF2 75LiF–20ThF4–5BeF2 75LiF–20ThF4–5BeF2
593–805 Estimation 578–820
2.94 2.21 2.81
2.04 1.74 2,25
1.52 1.37 1.88
– 3,75 –
2.27 2.29 –
1.94 1.94 1.37
75LiF–20ThF4–5BeF2 with CeF3 693–842 73LiF–11ThF4–16BeF2 500–800 72LiF–18ThF4–10BeF2 730–1060
75LiF–20ThF4–5BeF2 respectively.
salt
mixture
are
2.14
Refs.
and
[4] [2] [4] [4] [6] [5] This work [5] This work [2] [4]
3.85 mol%
2. Purification of salt composition The initial salts taken for preparation of the eutectic mixtures were commercially made anhydrous powdered fluorides of lithium and beryllium, which were purified of possible traces of adsorbed moisture by treating them at a low pressure (1 Pa) while heating gradually from room temperature to 450 °C for 2–3 h. The chemical analysis of ThF4 available has revealed that this product contains up to 2.5 mass% of oxygen. Attempts to remove water at slow heating of the sample (fine powder) in vacuum up to 370 °C have shown that not more than 0.5% of mass are left. Further on dewatering was carried out by heating ThF4 and NH4HF2 mixture. First, the mixture of ThF4 and NH4HF2 was prepared with the molar ratio 1:1, and placed in a glass carbon crucible with a copper cover under which pure argon was supplied. This argon together with the products of the reaction was brought away through an outlet. The thermocouple under a copper cover was placed in a salt powder mixture. The temperature in the crucible was slowly risen till steam and aerosol start to go out of the outlet after which the temperature was fixed. The mixture was kept at this temperature until steam and aerosol stop to go out. Thus, the temperature rose up to 350 °C. The salt components received in this way were used for preparation of salt mixtures for the subsequent analysis of viscosity. 3. The method of measurement In this paper as well as in [4], the method of torsional oscillations of cylindrical crucible filled with molten salt mixture has been chosen for the viscosity measurement. This method is widely used for measurements of the viscosity of liquid metals and semiconductors. Most researches consider this method to be suitable and reliable. The merits of this method involve: the definition of absolute values of kinematic viscosity without preliminary calibration of the test section with other liquids; the measurement of viscosity in a wide range of its values with comparatively high accuracy; low sensitivity to side effects (a surface tension, formation of films etc.). The method is described in details in the ISTC # 1606 project report [9]. The scheme of experimental facility is presented in Fig. 1. Through a metal rod (2) the cylindrical crucible with molten salts (1) was suspended by an elastic metal string (3) in the
Fig. 1. The test section for kinematic viscosity measurement: 1 – crucible with molten salt, 2 – rod, 3 – string, 4 – mirror, 5 – thermostatic insert, 6 – heater, 7 – thermocouple, 8 – vessel, 9 – thermal insulation, 10 – He–Ne laser, 11 – window, 12 – scale.
thermostatic insertion (5) made of the material with high thermal conductivity. There was a heater outside the thermostatic insertion (6). The temperature was measured by the thermocouple (7) placed close to the crucible in the thermostatic insertion. Water cooled vessel (8) separated the heating zone by a thick layer of thermal insulation (9). A mirror (4) was installed on a core of the suspended system (2). The He–Ne laser beam (10) was directed through a window (11) and went on the mirror (4). The position of the reflected beam was pointed out on a scale (12). All experiments have been carried out under inert gas at the pressure that was a bit higher than the atmosphere one. The viscosity of several glycerin solutions in water was measured to check up the operability of the system and accuracy of data processing. The comparison of the data received by experiments with the ones given in manuals has proved that the method of measurements and processing of the data of experiments leads to receiving accurate results in the values of kinematic viscosity ranging from 106 up to 105 m2/s [9]. Carefully mixed salt composition was put into the stainless steel cylinder that had been made for measurements. The cylinder was suspended by an elastic string in the installation. Cleared helium filled the installation chamber. The thermostat and the cylinder were heated up to 250 °C. At this temperature the installation chamber was pumped out, the sample was heated in vacuum up to 450 °C and then was maintained at this temperature for 20– 30 min. Further on, the installation was again filled with helium, the one being heated at the rate of 2–3 °C per minute. The sample was heated up to 800 °C and maintained for about 6–8 h so that the salt mixture melted down and the system came to thermodynamic equilibrium. The decrement of attenuation and viscosity were measured at the cooling. 4. The results of experiments 4.1. 78LiF–22ThF4 system Dependences of attenuation decrement on the temperature in experiments with molten 78LiF–22ThF4 salt mixture (mol%) are given in Fig. 2. At the temperatures higher than 625 °C the system is characterized by the high attenuation decrement which decreases with the temperature increase. This phenomenon occurs in completely melted substances. At temperatures lower than
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At low temperatures the data received are closer to the data [2,3] rather than to the data [4], whereas high temperature range slightly exceeds the data of both the above mentioned works.
300
4.2. 71LiF–27ThF4–2BeF2 system 200
100
0 500
600
700
800
Temperature (oC)
900
Fig. 2. Temperature dependence of attenuation decrement for 78LiF–22ThF4 (mol%) melt.
625 °C the system is characterized by decrement which goes down with the temperature decrease that shows partial freezing of the melted substances. The dependence of attenuation decrement longs to low values at temperatures 550–570 °C that fully agrees with the phase diagram [7]. The experimental points received at temperatures higher than 625 °C were used to determine viscosity. The dependence of viscosity of the temperature is given in Fig. 3. The data of experiments of processing by the least square method leads to the following dependence of the viscosity logarithm on the temperature (K):
lnðm 106 ½m2 =sÞ ¼ 0:6830 þ 3689 ð1=T 0:9689E 3Þ The RMS of the kinematic viscosity logarithm estimated in the assumption of dispersions homoscedasticity is 0.019. The dependence of kinematic viscosity on the temperature ranging from 625 to 846 °C is presented as follows:
The temperature dependence of the attenuation decrement, received by viscosity measurements of molten 71LiF–27ThF4–2BeF2 salt mixture (mol%) is given in Fig. 4. As in the first case, salt melted substances show themselves as normal liquid at temperatures higher than 593 °C. At lower temperatures attenuation decrement goes down with the temperature decrease that testifies partial freezing of the melted substances. The experimental points received at temperatures higher than 593 °C were used to determine viscosity. The dependence of viscosity on the temperature for molten 71LiF–27ThF4–2BeF2 salt mixture (mol%) is given in Fig. 5. Processing of the data of experiments by the least square method leads to the following dependence of the viscosity logarithm on the temperature (K):
lnðm 106 ½m2 =sÞ ¼ 0:7302 þ 3093 ð1=T 1:033E 3Þ The RMS of the kinematic viscosity logarithm estimated in the assumption of dispersions homoscedasticity is 0.042.
500 400
Decrement *10 3
Decrement*10
3
400
300 200 100 0 500
m 106 ðm2 =sÞ ¼ 1:9798 expf3689 ð1=TðKÞ 0:9698E 3Þg The RMS of the kinematic viscosity estimated in the assumption of dispersions homoscedasticity is 0.042 106 (m2/s).
600
700
Temperature (oC)
800
Fig. 4. Temperature dependence of attenuation decrement for molten 71LiF– 27ThF4–2BeF2 salt mixture (mol%).
4
3
2
1
0
600
700
o
800
Viscosity*10 6 (m 2 /s)
Viscosity*106 (m2/s)
4
3
2
1
Temperature ( C) Current work 78LiF–22ThF4, experimental points Current work 78LiF–22ThF4, approximation 80LiF–20ThF4, Janz [3] 70LiF–30ThF4, Desyatnik [4] 80LiF–20ThF4, Desyatnik [4] 80LiF–20UF4, Desyatnik [4] 78LiF–22ThF4, Benes [6]
Fig. 3. Temperature dependence of kinematic viscosity for lithium, thorium and uranium fluorides melts (in mol%): (N) Current work 78LiF–22ThF4, experimental points. (—) Current work 78LiF–22ThF4, approximation. (– – –) 80LiF–20ThF4, Janz [3]. (– j – j –) 70LiF–30ThF4, Desyatnik et al. [4]. (- - - - - -) 80LiF–20ThF4, Desyatnik et al. [4]. (– j j j –) 80LiF–20UF4, Desyatnik et al. [4]. (– jj – jj –) 78LiF–22ThF4, Benes and Koning [6].
0 550
600
650
700
o
750
800
Temperature ( C) Current work 71LiF–27ThF4–2 BeF2, experimental points Current work 71LiF–27ThF4–2 BeF2, approximation 80LiF–20ThF4, Desyatnik [4] 70LiF–30ThF4, Desyatnik [4]
Fig. 5. Temperature dependence of kinematic viscosity for lithium, beryllium and thorium fluorides melts (mol%): (N) Current work 71LiF–27ThF4–2 BeF2, experimental points. (—) Current work 71LiF–27ThF4–2 BeF2, approximation. (– j – j –) 80LiF–20ThF4, Desyatnik et al. [4]. (- - - - - -) 70LiF–30ThF4, Desyatnik et al. [4].
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3
m 106 ðm2 =sÞ ¼ 2:075 expf3093 ð1=TðKÞ 1:033E 3Þg The RMS of the kinematic viscosity estimated in the assumption of dispersions homoscedasticity is 0.10 106 (m2/s). 4.3. 75LiF–20ThF4–5BeF2 system The dependence of the attenuation decrement, on the temperature resulting from the measuring of molten 75LiF–20ThF4–5BeF2 salt mixture (mol%) is given in Fig. 6. As in the first two cases, salt melted substances show themselves as normal liquids at temperatures higher than 578 °C. At lower temperatures attenuation decrement goes down with the temperature decrease that proves partial freezing of the sample. Freezing goes through two stages. At the temperatures ranging from 570 to 540 °C there is easy decrease of attenuation decrement when the temperature goes down, whereas at temperatures lower than 540 °C it sharply decreases. The experimental points received at temperatures higher than 578 °C were used to determine viscosity. The dependence of viscosity on the temperature of this molten salt mixture is given in Fig. 7. Processing of the data of experiments by the least square method leads to the following dependence of the viscosity logarithm on the temperature (K):
Viscosity*106 (m2 /s)
The dependence of kinematic viscosity on the temperatures ranging from 590 to 810 °C is presented as:
550
650
750
800
850
Current work 75LiF–20ThF4–5BeF2, experimental points Current work 75LiF–20ThF4–5BeF2, approximation 70LiF–30ThF4, Desyatnik [4] 80LiF–20ThF4, Desyatnik [4] Fig. 7. Temperature dependence of kinematic viscosity for lithium, beryllium and thorium fluorides melts (mol%): (N) Current work 75LiF–20ThF4–5BeF2, experimental points. (—) Current work 75LiF–20ThF4–5BeF2, approximation. – j – j –) 70LiF–30ThF4, Desyatnik et al. [4]. (- - - - - -) 80LiF–20ThF4, Desyatnik et al. [4].
400
300
200
100
0
500
600
700
800
900
Temperature (oC)
4.4. 75LiF–20ThF4–5BeF2 system with CeF3 addition
Fig. 8. Temperature dependence of attenuation decrement for molten 75LiF– 20ThF4–5BeF2 salt mixture with 3CeF3 addition (mol%).
3.2
Viscosity*10 6 (m2/s)
The experiments on measuring the viscosity of molten 75LiF– 20ThF4–5BeF2 salt mixture (mol%) having been conducted, when 7.4 g (3 mol%) of CeF3 were added into the same crucible. The new experiment was being carried out under the same conditions as it had been done without CeF3. The data on attenuation decrement and viscosity are presented in Figs. 8 and 9 respectively.
500 400 300 200
2.8
2.4
2
1.6 600
700
o
800
Temperature ( C)
100 0 500
700
o
The RMS of the kinematic viscosity estimated in the assumption of dispersions homoscedasticity is 0.12 106 (m2/s).
Decrement *10 3
600
Temperature ( C)
Decrement*10 3
m 106 ðm2 =sÞ ¼ 2:1905 expf1877 ð1=TðKÞ 1:013E 3Þg
1
0
lnðm 106 ½m2 =sÞ ¼ 0:7841 þ 1877 ð1=T 1:013E 3Þ The RMS of the kinematic viscosity logarithm estimated in the assumption of dispersions homoscedasticity is 0.058. The dependence of kinematic viscosity on the temperature ranging from 572 to 815 °C is presented as:
2
600
700
800
Temperature (oC) Fig. 6. Temperature dependence of attenuation decrement for molten 75LiF– 20ThF4–5BeF2 salt mixture (mol%).
75LiF–20ThF4–5BeF2 with 3CeF3 addition, experimental points 75LiF–20ThF4–5BeF2 with 3CeF3 addition, approximation 75LiF–20ThF4–5BeF2, approximation Fig. 9. Temperature dependence of kinematic viscosity for molten 75LiF–20ThF4– 5BeF2 salt mixture without and with 3CeF3 addition (mol%): (N) 75LiF–20ThF4– 5BeF2 with 3CeF3 addition, experimental points. (—) 75LiF–20ThF4–5BeF2 with 3CeF3 addition, approximation. (– – –) 75LiF–20ThF4–5BeF2, approximation.
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Viscosity*106 (m2 /s)
4
Table 2 Viscosity vs. temperature dependences for molten salt fluorides of lithium, beryllium and thorium (this work).
3
Salt composition (mol%)
Dt (°C)
m 106 (m2/s)
RMS 106 (m2/s)
78LiF–22ThF4
625– 846 593– 805 578– 820 693– 842
1.980 exp{3689⁄(1/ T(K)0.9698E3)} 2.075 exp{3093⁄(1/ T(K)1.033E3)} 2.1905 exp{1877⁄(1/ T(K)1.013E3)} 2.037 exp{1465⁄(1/ T(K)0.9627E3)}
0.042
2 71LiF–27ThF4–2BeF2 75LiF–20ThF4–5BeF2
1
75LiF–20ThF4–5BeF2 with CeF3
0.10 0.12 0.053
0 500
600
700
800
Temperature ( oC)
900
78LiF–22ThF4 71LiF–27ThF4–2BeF2 75LiF–20ThF4–5BeF2 75LiF–20ThF4–5BeF2 with 3CeF3 addition Fig. 10. Kinematic viscosity vs. temperature for the lithium, beryllium and thorium fluorides melts under study (mol%): (—) 78LiF–22ThF4. (– j – j –) 71LiF–27ThF4– 2BeF2. (– jj – jj –) 75LiF–20ThF4–5BeF2. (- - - - - -) 75LiF–20ThF4–5BeF2 with 3CeF3 addition.
As Fig. 8, presents the dependence of a decrement on the temperature at high temperatures is very smooth so it is very difficult to determine higher temperatures when the system starts to show itself as normal liquid. This temperature has been accepted to be 693 °C. At temperatures decreasing from 693 °C the decrement slowly goes down to approximately 570 °C that indicates freezing of a small part of the melted substances. At decreasing the temperature from 570 down to 542 °C an already substantial part of melted substances under consideration turns into the solid state. At temperatures lower than 542 °C the decrement sharply decreases so as it might be taken as freezing of the sample. The state of melted substances as common liquid may be examined only at temperatures higher than 693 °C. Processing of the data of experiments by the least square method gives the following dependence of the viscosity logarithm on temperature (K):
lnðm 106 ½m2 =sÞ ¼ 0:7117 þ 1647 ð1=T 0:9627E 3Þ The RMS of the kinematic viscosity logarithm estimated in the assumption of dispersions homoscedasticity is 0.026. The dependence of kinematic viscosity in the temperature ranging from 693 to 835 °C is presented as follows:
m 106 ðm2 =sÞ ¼ 2:037 expf1465 ð1=TðKÞ 0:9627E 3Þg The RMS of the kinematic viscosity estimated in the assumption of dispersions homoscedasticity is 0.053 106 (m2/s). For comparison the dependence of viscosity on the temperature for molten 75LiF–20ThF4–5BeF2 salt mixture (mol%) without CeF3 additive is given by a dotted line in Fig. 9. It is clear that practically CeF3 additive does not increase the viscosity of molten salt mixture under study, but sharply raises the liquidus temperature. Its increase at adding CeF3 had earlier been singled out by ORNL researchers [8]. Correlation curves of viscosity for all compositions analyzed in this paper are given in Fig. 10. Viscosity vs. temperature dependences for three molten salt fluorides mixture of lithium, beryllium and thorium received in this work are summarized in Table 2. For
comparison, temperature intervals where the viscosity of molten salt mixtures was measured in this study as well as the ones of other researchers are given in Table 1. 5. Conclusion 1. The viscosity of the three molten salt mixtures such as 78LiF– 22ThF4; 71LiF–27ThF4–2BeF2 and 75LiF–20ThF4–5BeF2 (mol%) has been measured at the temperature ranging from liquidus up to 800–850 °C by the method of torsional oscillations attenuation of the cylinder with the melt under study. 2. In temperature ranges where molten salt mixtures show themselves as normal liquids, the following dependences of viscosity on the temperature have been drawn out: m = A exp [B/(t + 273)], where m – kinematic viscosity, m2/s; t – temperature (°C). The kinematic viscosity RMS estimated in the assumption about dispersion homoscedasticity is (0.04–0.12) 106 (m2/s). 3. As a whole the received values of viscosity correlate well enough with the ones of ORNL researchers [1–3]. Our data agree with the data of experiments [4] for molten 71LiF–27ThF4– 2BeF2 salt mixture (mol%), but substantially exceed their kinematic viscosity for 78LiF–22ThF4 b 75LiF–20ThF4–5BeF2 ones. 4. The addition of 3 mol% CeF3 to molten 75LiF–20ThF4–5BeF2 sat mixture (mol%) substances considerably increases the liquidus temperature without particular effect on the value of kinematic viscosity. 5. Discrepancies left in the data of viscosity for molten mixtures of LiF, BeF2 and ThF4 received by different researchers need further investigations in this area to be continued.
References [1] C. Renault, S. Delpech, V. Ignatiev, et al., in: Proceedings of Seventh European Commission Conference on Euroatom Research and Training in Reactor Systems, Prague, Czech Republic, June 22–24, 2009. [2] S. Cantor, Density and Viscosity of Several Molten Fluoride Mixtures, Report ORNL/TM-4308, Oak Ridge, USA, 1973. [3] G.J. Janz, Molten Salts Handbook, Academic Press, NY, USA, 1967. [4] V.N. Desyatnik, A.I. Nechayev, Y.u.F. Chervinsky, Z. Prikladnoi Khim. 54 (10) (1981) 2310–2312. [5] V. Khokhlov, V. Ignatiev, V. Afonichkin, J. Fluorine Chem. 130 (1) (2009) 30–37. [6] O. Beneš, R.J.M. Konings, J. Fluorine Chem. 130 (1) (2009) 22–29. [7] R.E. Thoma, Phase Diagrams of Nuclear Reactor Materials, Report ORNL-2548, Oak Ridge, USA, 1959, p.42. [8] J.A. Fredricksen, L.O. Gilpatrick, C.J. Barton, Solubility of Cerium Trifluoride in Molten Mixtures of LiF, BeF2 and ThF4, Report ORNL-TM-2335, Oak Ridge, USA, 1969, 23p. [9] V. Ignatiev et al., ISTC#1606 Project Final Report, phase 1, International Scientific Technical Centre, Moscow, Russia, 2004.