Journal of The Electrochemical Society, 157 共9兲 E135-E139 共2010兲
E135
0013-4651/2010/157共9兲/E135/5/$28.00 © The Electrochemical Society
Solubility of Li2O in Molten LiCl–MClx (M = Na, K, Cs, Ca,
Sr, or Ba) Binary Systems
Yoshiharu Sakamuraz
Central Research Institute of Electric Power Industry, Komae, Tokyo 201-8511, Japan
The solubility of oxide ions in LiCl-rich LiCl–MClx 共M = Na, K, Cs, Ca, Sr, or Ba兲 melts was measured in the temperature range
723–923 K as part of the development of an electrolytic reduction process for spent nuclear fuel because the solubility of oxide
ions strongly affects the rate of oxide reduction. With the exception of the LiCl–CaCl2 system, the precipitate was Li2O under an
oversaturated condition. The solubility of Li2O in molten LiCl at 923 K was determined to be 11.64 ⫾ 0.04 mol %. The
solubility significantly decreased when alkali metal chlorides 共NaCl, KCl, and CsCl兲 were added to LiCl and slightly decreased
when BaCl2 was added. In contrast, the addition of SrCl2 increased the Li2O solubility. Empirical formulas for the relationship
between Li2O solubility and salt composition were derived. The logarithm of Li2O solubility plotted against the reciprocal
temperature gave an approximately linear relationship in the LiCl–KCl system. The addition of CaCl2 to LiCl saturated with Li2O
gave a CaO precipitate.
© 2010 The Electrochemical Society. 关DOI: 10.1149/1.3456631兴 All rights reserved.
Manuscript submitted March 30, 2010; revised manuscript received May 25, 2010. Published July 16, 2010.
The metal fuel cycle, involving metal fuel fast breeder reactors
共FBRs兲, pyrochemical reprocessing, and fuel fabrication by injection
casting, is a promising option for next-generation nuclear systems
that satisfy the requirements of economic benefit, environmental
safety, and high proliferation resistance.1-3 To supply metal fuel materials to FBRs, the oxide fuels of light water reactors must be
reduced to metals. Apparently, an electrolytic reduction technique
has recently been developed for this purpose.4-16
In the electrolytic reduction process, the cathode and anode reactions are described as follows
Cathode:AnO2 + 4e− → An + 2 O2−共salt兲
关1兴
Anode:2 O2−共salt兲 → O2 + 4e−
关2兴
where An denotes actinides such as uranium and plutonium. At the
cathode, the oxygen is electrochemically ionized and the actinide
metal remains. The ionized O2− is transported through the salt and
discharges at the anode to form O2 gas. The distinctive feature of
this process is that the actinides never dissolve in salt.
It has been reported that a LiCl salt bath at 923 K is suitable for
reducing UO2 and UO2–PuO2 mixed oxide to their metals.4-8 In our
previous experiment on reducing UO2 particles charged in a cathode
basket,13 the reduction progressed from the outside to the center of
the basket, and an oxygen-concentrated zone was found at the metal/
oxide interface, which might have been due to the precipitation of
Li2O. It was indicated that LiCl at the metal/oxide interface was
saturated with Li2O and that the rate of transportation of O2−
through the molten salt to the bulk salt by diffusion determined the
oxide reduction rate. Therefore, the solubility and diffusion coefficient of O2− in the molten salt are essential for reducing the oxide in
a short time. Molten LiCl at 923 K has a high O2− solubility of about
12 mol %,17,18 which is one of the reasons why LiCl was selected
for the salt bath.
When spent nuclear fuels are processed in an electrolytic reduction cell, some salt-soluble fission products such as Cs, Sr, and Ba
accumulate in the LiCl salt bath. Thus, it is necessary to investigate
their effect on the reduction behavior of actinide oxides. Although
the O2− solubility in a molten salt strongly affects the oxide reduction rate, the solubilities in mixed chloride systems have not been
reported yet. In the present study, the Li2O solubility in molten LiCl
containing dissolved alkali 共Na, K, or Cs兲 or alkaline-earth 共Ca, Sr,
or Ba兲 metal chloride was measured as functions of the salt content
and temperature.
z
E-mail: [email protected]
Experimental
Anhydrous LiCl, NaCl, KCl, CsCl, CaCl2, SrCl2, and BaCl2 with
a purity of 99.99% were obtained from Aldrich-APL. Anhydrous
Li2O was supplied by Furuuchi Chemical Co. A schematic diagram
of the experimental apparatus used for the O2− solubility measurements is shown in Fig. 1. LiCl and Li2O, contained in a stainless
steel or titanium crucible 共48 or 50 mm inside diameter兲, were
heated to 923 K in a stainless steel thermowell attached to the floor
of an argon-atmosphere glove box 共O2 ⬍ 2 ppm, H2O ⬍ 1 ppm兲,
which was heated externally using an electric furnace. The amount
of Li2O exceeded its solubility in LiCl at 923 K. Then, a type-K
thermocouple sheathed in a stainless steel tube and a stirrer made of
a tantalum or molybdenum plate were immersed into the melt. After
the temperature was stabilized at 923 ⫾ 1 K, salt samples 共0.02–0.2
g兲 were taken by dipping a stainless steel tube into the surficial melt,
upon which the solidified salt became attached to the closed end of
the tube. Then, an alkali or alkaline-earth chloride 共NaCl, KCl,
CsCl, CaCl2, SrCl2, or BaCl2兲 was incrementally added. The temperature was varied in the range of 723–923 K, considering the
liquidus temperature for each salt content. Two salt samples were
taken at each salt content and temperature. It was verified that the
concentration of Li2O in the melt was equilibrated within 2–3 h after
changing the salt content or temperature. The salt samples were
dissolved in water, which was followed by neutralization analysis
using 0.01 N–HCl and a phenol red indicator to determine the oxide
ion concentration. In this measurement, the oxide ion concentration
is high enough to be precisely determined by the neutralization
analysis. The concentrations of alkali and alkaline-earth metals were
then determined using a Shimazu AA-680 atomic absorption flame
emission spectrophotometer 共AAS兲 or a Jarrell Ash IRIS Advantage
inductively coupled plasma atomic emission spectrometer 共ICPAES兲.
Results and Discussion
Li2O solubility in LiCl–NaCl, LiCl–KCl, LiCl–CsCl, LiCl–SrCl2,
and LiCl–BaCl2 systems.— NaCl, KCl, CsCl, SrCl2, or BaCl2 was
added to LiCl saturated with Li2O, and then the O2− concentration in
the melt was measured at 873 and 923 K. The melting point of LiCl
共879 K兲 was depressed to less than 873 K by dissolving Li2O. For
the LiCl–KCl system, the temperature was varied in the range 723–
923 K. The results for LiCl–NaCl, LiCl–KCl, LiCl–CsCl,
LiCl–SrCl2, and LiCl–BaCl2 systems are presented in Tables I-V,
respectively. The data are listed in the order of measurement. The
metal chloride content of each melt, XMClx 共mol %兲, in the tables
was defined by the following equation
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Journal of The Electrochemical Society, 157 共9兲 E135-E139 共2010兲
E136
Metal chloride
addition
Table II. Results of Li2O solubility measurement in LiCl–KCl
system.
Thermocouple
XKCl a
共mol %兲
0.0
5.0
Stirrer
10.0
15.0
20.0
30.0
LiCl-Li2O
melt
Li2O
precipitate
40.9
SUS or Ti crucible
a
Figure 1. Schematic diagram of the apparatus used for O2− solubility measurement.
XMClx = 100 ⫻ 关MClx兴/共关LiCl兴 + 关MClx兴兲
关3兴
Here, 关MClx兴 and 关LiCl兴 are molar amounts of metal chloride and
LiCl in the melt, respectively, which were calculated from the
weight of metal chloride added and the weight of the salt sample
removed for analysis. The molar ratio of Mx+ to Li+, which was
analyzed using the AAS and ICP-AES, clearly indicated that the
precipitate was Li2O. Thus, Na+, K+, Cs+, Sr2+, and Ba2+ did not
precipitate under any measurement conditions. The LiCl–5.0 mol %
NaCl melt, for instance, dissolved 10.1 mol % of Li2O at 923.6 K,
as shown in Table I. The molar ratio of Na+ to Li+ is then calculated
to be 0.0426, which agrees well with the analytical value of 0.042
by AAS, indicating that Na+ never precipitated. Therefore, the measured O2− concentration corresponds to the Li2O solubility. In this
Table I. Results of Li2O solubility measurement in LiCl–NaCl
system.
XNaCl a
共mol %兲
0.0
5.0
10.0
15.0
20.0
30.0
a
b
c
T
共K兲
SLi2O
共mol %兲
Na+ /Li+ b
共mol ratio兲
923.5
872.9
873.1
923.6
923.6
873.4
873.5
923.9
923.8
873.9
873.9
923.1
11.5
10.6
8.7
10.1
8.6
7.8
6.2
7.3
6.2
5.4
3.5
4.2
NAc
NA
NA
0.042
0.091
NA
NA
0.153
0.21
NA
NA
0.38
Calculated from weights of LiCl and NaCl added.
Analytical results of salt samples by AAS.
Not analyzed.
b
T
共K兲
SLi2O
共mol %兲
K+ /Li+ b
共mol ratio兲
923.4
872.7
873.1
923.9
922.4
872.5
873.0
822.8
924.1
923.9
872.7
822.5
722.9
774.2
823.4
873.1
924.2
724.0
772.2
823.8
873.8
922.8
11.3
9.9
8.2
9.7
7.9
6.6
5.2
4.4
6.4
5.1
4.1
3.4
1.10
1.41
1.88
2.4
3.1
0.47
0.65
0.86
1.14
1.51
NA
NA
NA
0.042
0.089
NA
0.151
NA
NA
0.21
NA
NA
NA
NA
NA
0.39
NA
NA
NA
NA
NA
0.66
Calculated from weights of LiCl and KCl added.
Analytical results of salt samples by AAS.
paper, the Li2O solubility, SLi2O 共mol %兲, was defined as the mol %
of O2− anions and is described by the following equation
SLi2O = 100 ⫻ 关O2−兴/共关O2−兴 + 关Cl−兴兲
关4兴
where 关O 兴 and 关Cl 兴 are the molar amounts of O and Cl in the
melt, respectively. Two salt samples were taken under each condition and were then subjected to neutralization analysis. Both resultant Li2O concentrations agree well, indicating that the accuracy of
the analysis was mostly within 1% and that the salt samples taken
from the melt contained no Li2O precipitate. The values of SLi2O in
the five tables are the averages of the two analytical results. In this
measurement, the influence of peroxide ions, O2−
2 , or superoxide
ions, O−2 ,19 need not be taken into account because the partial pressure of oxygen was very low in the argon-atmosphere glove box.
The Li2O solubility at 923 and 873 K is plotted against the salt
content in Fig. 2a and b, respectively. Obviously, the addition of
each alkali metal chloride to molten LiCl significantly decreases the
Li2O solubility. The Li2O solubility in LiCl–20 mol % KCl, for
instance, is less than half that in pure LiCl. The decrease in solubil2−
−
2−
−
Table III. Results of Li2O solubility measurement in LiCl–CsCl
system.
XCsCl a
共mol %兲
0.0
4.7
9.4
14.1
a
b
T
共K兲
SLi2O
共mol %兲
Cs+ /Li+ b
共mol ratio兲
924.0
873.8
872.7
923.8
922.8
872.6
873.1
923.1
11.8
10.7
8.1
9.5
7.5
6.6
4.9
5.8
NA
NA
NA
0.039
0.083
NA
NA
0.136
Calculated from weights of LiCl and CsCl added.
Analytical results of salt samples by AAS.
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Journal of The Electrochemical Society, 157 共9兲 E135-E139 共2010兲
16
0.0
2.5
5.0
10.0
15.0
11.1
SLi2O
共mol %兲
Sr2+ /Li+ b
共mol ratio兲
923.1
923.8
923.9
924.1
924.0
923.9
873.1
924.2
11.6
12.4
12.8
14.2
15.1
14.9
12.8
14.0
NA
0.0183
0.037
0.075
NA
0.115
NA
0.084
Calculated from weights of LiCl and SrCl2 added.
Analytical results of salt samples by AAS and ICP-AES.
ity increased with increasing atomic number among the alkali metals. For the alkaline-earth metals, the Li2O solubility slightly decreased when BaCl2 was added, whereas the addition of SrCl2
increased the Li2O solubility. As expected, the Li2O solubility decreased with a decrease in temperature.
In addition to the measurements for Tables I-VI, SLi2O in LiCl at
923 K was separately measured twice to obtain the values of 11.8
and 11.6 mol %. The value of SLi2O in LiCl at 923 K 共i.e., the
average of the eight experimental data兲 determined in this study was
11.64 ⫾ 0.04 mol %, which agrees well with the value of 12 mol
% in the literature.17,18 The values of SLi2O at 923 K in the binary
chloride systems can thus be represented as functions of the salt
content 共XMClx兲 by the empirical formulas
(a) 923 K
SrCl2
12
BaCl2
8
CsCl
NaCl
4
0
KCl
0
10
20
30
XMClx (mol%)
40
16
Li2O solubility, SLi2O (mol%)
a
b
T
共K兲
Li2O solubility, SLi2O (mol%)
Table IV. Results of Li2O solubility measurement in LiCl–SrCl2
system.
XSrCl2 a
共mol %兲
E137
(b) 873 K
SrCl2
12
BaCl2
8
4
CsCl
NaCl
KCl
2
LiCl–NaCl:SLi2O = 11.64 − 0.328 XNaCl + 0.0027 XNaCl
共XNaCl ⬍ 30 mol %兲
关5兴
LiCl–KCl:SLi2O = 11.64 − 0.407 XKCl + 0.0039
2
XKCl
共XKCl ⬍ 41 mol %兲
关6兴
0
0
10
20
30
XMClx (mol%)
40
Figure 2. Li2O solubility in LiCl–MClx systems 共M = Na, K, Cs, Sr, and
Ba兲 as a function of salt content at 共a兲 923 and 共b兲 873 K.
2
LiCl–CsCl:SLi2O = 11.64 − 0.487 XCsCl + 0.0053 XCsCl
共XCsCl ⬍ 14 mol %兲
关7兴
2
LiCl–SrCl2:SLi2O = 11.64 + 0.259 XSrCl2 − 0.0024 XSrCl2
共XSrCl2 ⬍ 15 mol %兲
关8兴
log SLi2O = 1.994 − 1.686 ⫻ 103 /T
2
LiCl–BaCl2:SLi2O = 11.64 − 0.034 XBaCl2 − 0.0046 XBaCl2
共XBaCl2 ⬍ 10 mol %兲
关9兴
Figure 3 shows the temperature dependence of Li2O solubility in
LiCl–KCl systems. As KCl is added to LiCl, the liquidus temperature is depressed,20 and the solubility measurement can be per-
Table V. Results of Li2O solubility measurement in LiCl–BaCl2
system.
XBaCl2
共mol %兲a
0.0
2.5
5.0
10.0
a
b
T
共K兲
SLi2O
共mol %兲
Ba2+ /Li+
共mol ratio兲b
923.9
924.1
923.5
872.6
873.0
923.4
11.7
11.7
11.4
9.6
9.1
10.8
NA
0.0174
0.039
NA
NA
0.082
Calculated from weights of LiCl and BaCl2 added.
Analytical results of salt samples by AAS and ICP-AES.
formed at a lower temperature. The logarithm of Li2O solubility
plotted against the reciprocal temperature gives an approximately
linear relationship. The value of SLi2O in the LiCl–KCl eutectic melt
共41 mol % KCl兲 is represented by the equation
共723 K ⬍ T ⬍ 923 K兲
关10兴
Because of its low melting point 共625 K兲, the LiCl–KCl eutectic
melt is commonly used as a molten salt bath. The Li2O solubility is
low in LiCl–KCl eutectic bath, e.g., 0.65 mol % at 773 K.
CaCl2 addition to LiCl saturated with Li2O.— In the experiment, weighed CaCl2 was added 5 times to the crucible charged
with LiCl 共1.179 mol兲 and Li2O 共0.230 mol兲 at 923 K. The results
are presented in Table VI and Fig. 4, which show the evaluated
amount of ions in the salt phase. The first two additions of CaCl2
共0.030 and 0.032 mol兲, which increased the total CaCl2 content to 5
mol %, did not appear to change the O2− concentration in the melt.
The analysis of the salt samples indicated that the Ca2+ concentration in the melt was very small 共⬃1000 ppm兲. Therefore, the added
CaCl2 reacted with O2− in the melt to give a CaO precipitate, causing the Li2O precipitate to dissolve in the melt to maintain the O2−
concentration. The measured O2− concentration was identical to the
Li2O solubility in LiCl. The third addition of CaCl2 共0.068 mol兲
completely removed the Li2O precipitate, resulting in the system
consisting of liquid LiCl–Li2O 共unsaturated兲 and solid CaO phases.
After the fourth addition of CaCl2 共0.076 mol兲, the system consisted
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Journal of The Electrochemical Society, 157 共9兲 E135-E139 共2010兲
E138
Table VI. Results of O2− solubility measurement in LiCl–CaCl2 system.
Amount of reagent added
a
Analytical results for salt sample
LiCl
共mol兲
Li2O
共mol兲
CaCl2
共mol兲
T
共K兲
1.179
—
—
—
—
—
0.230
—
—
—
—
—
—
0.030
0.032
0.068
0.076
0.086
923.6
923.6
923.1
923.8
923.3
923.5
O
2−
concentration
共mol %兲
11.8
11.7
10.9
6.2
6.8
6.8
Resultant components
Ca /Li
共mol ratio兲
Liquid phase
Solid phase
0.000
0.00100
0.00095
0.00098
0.064
0.123
LiCl–Li2O
LiCl–Li2O a
LiCl–Li2O a
LiCl–Li2O a
LiCl–Li2O–CaO
LiCl–CaCl2–CaO
Li2O
Li2O, CaO
Li2O, CaO
CaO
CaO
CaO
2+
+
Liquid phase contained ⬃1000 ppm Ca2+.
of liquid LiCl–Li2O–CaO and solid CaO phases. Finally, the fifth
addition of CaCl2 共0.086 mol兲 increased the CaCl2 content of the
melt to 3.8 mol %. The CaO solubility in LiCl containing less than
3.8 mol % CaCl2 was estimated to be ⬃6.8 mol %.
Thermodynamic considerations.— Table VII shows the standard
Gibbs free energies of formation 共⌬Gf0兲 for oxides of alkali and
alkaline-earth metals at 923 K.21 Clearly, the solubility of Li2O decreases when chlorides of alkali and alkaline-earth metals, whose
value of ⌬Gf0 for the oxide is more positive than that for Li2O, are
Li2O solubility, SLi2O (mol%)
20
XKCl = 0
XKCl = 5
XKCl = 10
XKCl = 15
10
5
Li2O + 共2/x兲MClx = 2LiCl + M共2/x兲O
共M = Na,K,Cs,Mg,Ca,Sr, and Ba兲
XKCl = 20
2
1
关11兴
XKCl = 30
0.5
XKCl = 41
0.2
1.0
1.1
1.2
1000 / T(K)
1.3
1.4
Figure 3. Temperature dependence of Li2O solubility in LiCl–KCl systems
for various KCl contents, XKCl 共mol %兲.
Amount of ions in salt phase (mol)
added to the LiCl melt, as shown in Fig. 2. It seems reasonable that
the addition of such metal chlorides to the LiCl melt can reduce the
stability of O2− ions because some of the Li+ ions surrounding O2−
ions may be displaced by the metal ions added to the melt. As a
result, the activity coefficient of O2− increases and the solubility of
Li2O decreases. Among the alkali metals, ⌬Gf0 for the oxide increases with increasing atomic number, which clearly affects the
extent to which the solubility of Li2O is depressed. ⌬Gf0 for BaO is
slightly more positive than that for Li2O, corresponding to the mild
effect of BaCl2 addition on Li2O solubility. ⌬Gf0 for SrO is slightly
more negative than that for Li2O; thus, adding SrCl2 to LiCl can
enhance the solubility of Li2O in the melt.
The changes in the standard Gibbs free energy 共⌬G0兲 in the
following reactions were calculated using a thermodynamic
database21 and are presented in Table VII
2.0
The chlorides and oxides are in their standard states at 923 K. The
values of ⌬G0 for alkali metal chlorides and BaCl2 are positive,
which is consistent with the experimental finding that the only precipitate was Li2O. In the LiCl–SrCl2 system, Li2O precipitated when
the SrCl2 content was less than 15.0 mol %. Because ⌬G0 for SrCl2
is slightly negative, it is expected that SrO precipitates instead of
Li2O when the SrCl2 content in the melt increases. ⌬G0 for CaCl2 is
more negative, and CaO precipitates instead of Li2O, independent of
the CaCl2 content. CaO can dissolve in the LiCl melt, as indicated in
Fig. 4. MgO crucibles exhibit excellent corrosion resistance in
LiCl–Li2O systems,7 indicating that MgO does not dissolve in the
LiCl melt. ⌬G0 for MgCl2 is much more negative than that for
CaCl2. The consideration of ⌬Gf0 for the oxide and ⌬G0 for Reac-
Li+
1.5
Cl-
1.0
Table VII. Standard Gibbs free energies of formation „⌬Gf0… for
M„2Õx…O and changes in standard Gibbs free energy „⌬G0… for the
reaction Li2O + „2Õx…MClx = 2 LiCl + M„2Õx…O at 923 K, where
x = 1 for alkali metals and x = 2 for alkaline-earth metals.
0.5
M
⌬Gf0 of M共2/x兲O
共kJ/mol兲
⌬G0
共kJ/mol兲
Cs
K
Na
Li
Ba
Sr
Ca
Mg
⫺227
⫺233
⫺286
⫺476
⫺460
⫺498
⫺541
⫺502
287
273
173
0
60
⫺5
⫺80
⫺201
O20.0
0
Ca2+
0.1
0.2
0.3
Cumulative CaCl2 added (mol)
0.4
Figure 4. Change in the content of salt phase as CaCl2 was added to LiCl
saturated with Li2O at 923 K.
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Journal of The Electrochemical Society, 157 共9兲 E135-E139 共2010兲
E139
tion 11 is helpful in comprehending the O2− solubility in LiCl–MClx
systems.
Nuclear Fuel,” entrusted to CRIEPI by the Ministry of Education,
Culture, Sports, Science and Technology of Japan 共MEXT兲.
Conclusion
Central Research Institute of Electric Power Industry assisted in meeting
the publication costs of this article.
The solubility of oxide ions in LiCl-rich LiCl–MClx 共M = Na, K,
Cs, Ca, Sr, or Ba兲 melts was measured as a function of the salt
content.
In LiCl–NaCl, LiCl–KCl, LiCl–CsCl, LiCl–SrCl2, and
LiCl–BaCl2 systems, the precipitate was Li2O under an oversaturated condition. The solubility of Li2O in molten LiCl at 923 K was
determined to be 11.64 ⫾ 0.04 mol %. The solubility significantly
decreased when alkali metal chlorides 共NaCl, KCl, and CsCl兲 were
added to LiCl. The decrease in solubility increased with increasing
atomic number among the alkali metals. The logarithm of Li2O solubility in the LiCl–KCl systems plotted against the reciprocal temperature gave an approximately linear relationship over the temperature range 723–923 K. For the alkaline-earth metals, the Li2O
solubility slightly decreased when BaCl2 was added, whereas the
addition of SrCl2 increased the Li2O solubility. Empirical formulas
for the relationship between Li2O solubility and salt content were
presented.
The addition of CaCl2 to LiCl saturated with Li2O gave a CaO
precipitate. The solubility of CaO in LiCl was estimated to be
⬃6.8 mol % at 923 K. MgO crucibles show excellent corrosion
resistance in LiCl–Li2O systems, indicating that MgO does not dissolve in LiCl.
An electrolytic reduction process for actinide oxides in a LiCl
electrolyte at 923 K has been developed, and the solubility of oxide
ions in the electrolyte strongly affects the rate of oxide reduction.
When spent nuclear fuels are processed in an electrolytic reduction
cell, some salt-soluble fission products such as Cs, Sr, and Ba accumulate in the LiCl electrolyte. Cs is the most problematic element in
terms of hindering the reduction of actinide oxides.
Acknowledgment
The present work is the result of “Development of Electrochemical Reduction Process for Oxide Nuclear Fuel” and “Application of
Electrochemical Reduction to Pyrochemical Reprocessing for Oxide
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