Solubility of UF4, ThF4, and CeF3 in a LiF-NaF-KF melt

Article abstract of DOI:10.1134/S1066362211050079

The solubility of UF₄, ThF₄, and CeF₃ in fluoride melt of the composition 45 mol % LiF-12 mol % NaF-43 mol % KF in the temperature interval 773-973 K was determined. The solubility of the fluorides increases with an increase in the melt temperature. The CeF₃ solubility in the LiF-NaF-KF system is high: 19.9 mol % at 923 K and 23.3 mol % at 973 K. Experimental data on the solubility of CeF₃ (PuF ₃ imitator) were compared to the calculated data on the PuF ₃ solubility in the LiF-NaF-KF melt. The results showed that CeF3 can be considered as PuF3 imitator in FLINAK melt.

Full text of DOI:10.1134/S1066362211050079

ISSN 1066-3622, Radiochemistry, 2011, Vol. 53, No. 5, pp. 491–493. © Pleiades Publishing, Inc., 2011.
Original Russian Text © M.B. Seregin, A.P. Parshin, A.Yu. Kuznetsov, L.I. Ponomarev, S.A. Mel’nikov, A.A. Mikhalichenko, A.A. Rzheutskii, R.N. Manuilov,
2011, published in Radiokhimiya, 2011, Vol. 53, No. 5, pp. 416–418.
Solubility of UF4, ThF4, and CeF3 in a LiF–NaF–KF Melt
M. B. Sereginа, A. P. Parshina, A. Yu. Kuznetsov*a, L. I. Ponomarevb, S. A. Mel’nikova,
A. A. Mikhalichenkoa, A. A. Rzheutskiia, and R. N. Manuilova
a Leading Research Institute of Chemical Technology, Public Joint-Stock Company,
Kashirskoe shosse 33, Moscow, 115409 Russia; * e-mail: A_Quznetsov@vniiht.ru
b ANO KITs MYuKATEKS, Moscow, Russia
Received December 27, 2010
Abstract—The solubility of UF4, ThF4, and CeF3 in fluoride melt of the composition 45 mol % LiF–12 mol %
NaF–43 mol % KF in the temperature interval 773–973 K was determined. The solubility of the fluorides in-
creases with an increase in the melt temperature. The CeF3 solubility in the LiF–NaF–KF system is high:
19.9 mol % at 923 K and 23.3 mol % at 973 K. Experimental data on the solubility of CeF3 (PuF3 imitator)
were compared to the calculated data on the PuF3 solubility in the LiF–NaF–KF melt. The results showed that
CeF3 can be considered as PuF3 imitator in FLINAK melt.
Keywords: uranium tetrafluoride, thorium tetrafluoride, cerium trifluoride, fluoride melts, solubility
DOI: 10.1134/S1066362211050079
The performance of a molten salt reactor (MSRs),
in particular, of a transmutation reactor (MSTR) [1, 2],
depends not only on its design and nuclear-physical
characteristics, but also on the choice of a fuel salt
(FS). The commonly chosen fuel salts are low-melting
fluoride systems like LiF–BeF2, NaF–ZrF4, LiF–
NaF–BeF2, etc. The fuel salt and its components
should meet the following main requirements: low
neutron capture cross section; low melting point; low
cost of fuel salt; relatively low toxicity; high solubility
of fluorides of U, Pu, Am, Cm, and fission products
(FPs) in the melt; and corrosion resistance of structural
materials in the fuel salt under the conditions of MSR
operation.
With FLINAK system, it is possible to perform
electrochemical processes for the FS decontamination
from the accumulated FPs. In particular, electrolysis of
this system allows separation of UF4 and trifluorides of
Nd, Gd, and Eu [4].
7
7
Data on the solubility of actinide and fission prod-
uct fluorides in the LiF–NaF–KF system are practi-
cally lacking. The lack of such data complicates
evaluation of the applicability of the LiF–NaF–KF
melt as a fuel salt for MSTR and development of a
concept of the MSTR nuclear fuel cycle.
In this study we performed systematic measurements
of the solubility of UF4, ThF4, and CeF3 in the system
45 mol % LiF–12 mol % NaF–43 mol % KF (Тm =
725 K) in the temperature interval 773–973 K.
Search for a fuel salt for a molten salt reactor that
would fully meet the nuclear-physical and physico-
chemical requirements and could be efficiently regen-
erated is still continued. One of promising fluoride
compositions for use in an innovation MSTR is the
LiF–NaF–KF system.
Uranium, thorium, and cerium fluorides were pre-
pared by hydrofluorination of the corresponding metal
oxides of ultrapure grade. Lithium, sodium, and potas-
sium fluorides were prepared according to [5].
Today, among all the promising fuel compositions
considered for MSR, the LiF–NaF–KF system is the
least studied. The corrosion resistance of a number of
structural materials in the LiF–NaF–KF melt was stud-
ied. The most resistant in the FLINAK system are al-
loys with high Ni content and low Cr content, in par-
ticular, Ni-203 and Hastelloy-N [3].
The procedure for preparing a fuel salt melt con-
sisted in thorough mixing of the components in the
required proportion, followed by fusion under Ar at
973–1023 K in a graphite crucible. The fluorides being
studied were taken as preliminarily fused cylindrical
specimens. Experiments were performed in a hermeti-
cally sealed retort with controllable atmosphere. The
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SEREGIN et al.
773 K to 11.7 mol % at 973 K. In accordance with data
of Fig. 1, the temperature dependence of the UF4 solu-
bility can be subdivided into two temperature intervals.
In the interval 773–873 K, the temperature dependence
of the UF4 solubility can be described by the equation
С(UF4) = 0.015Т – 8.30, R2 = 0.935, and in the interval
873–973 K, by the equation С(UF4) = 0.070Т – 56.10,
R2 = 0.941.
Fig. 1. Temperature dependence of the UF4 solubility in
45 mol % LiF–12 mol % NaF–43 mol % KF melt.
As seen from Fig. 1 and the above equations, in the
temperature interval 873–973 K an increase in tem-
perature leads to a sharper increase in the UF4 solubil-
ity than in the interval 773–873 K.
The ThF4 solubility in 45 mol % LiF–12 mol %
NaF–43 mol % KF melt at 773 and 825 K is low: 0.03
and 0.05 mol %, respectively. Figure 2 shows the ex-
perimental data on the ThF4 solubility in the tempera-
ture interval 873–973 K in which the ThF4 solubility
considerably increases: from 4.52 mol % at 873 K to
7.63 mol % at 973 K. The temperature dependence of
the solubility can be described by the linear equation
С(ThF4) = 0.031Т – 22.38, R2 = 0.928.
Fig. 2. Temperature dependence of the ThF4 solubility in
45 mol % LiF–12 mol % NaF–43 mol % KF melt.
Barton et al. [10] studied the CeF3 solubility in
LiF–BeF2 melts with different component ratios and
compared it to the PuF3 solubility in the similar melts.
They found that the difference between the CeF3 and
PuF3 solubilities in a wide interval of BeF2 concentra-
tions is approximately constant, 14% on the average.
Barton et al. concluded that cerium trifluoride can be
taken as plutonium trifluoride imitator in LiF–BeF2
melts. It cannot be judged unambiguously whether this
conclusion can be applied to the LiF–NaF–KF melt,
but preliminary data show that CeF3 can be used for
estimating the PuF3 solubility.
Fig. 3. Temperature dependence of the CeF3 solubility in
45 mol % LiF–12 mol % NaF–43 mol % KF melt.
melt temperature was maintained with an accuracy of
±0.5 K.
The procedure for determining the solubility of the
fluorides consisted in fusion of a weighed portion of
the prepared homogeneous mixture LiF–NaF–KF,
heating of the melt to the required temperature, and
keeping of each cylindrical specimen, taken separately,
in this melt for 12 h. The procedure for determining
the solubility of fluorides is described in more detail
elsewhere [6].
We examined the influence of temperature on the
CeF3 solubility in the LiF–NaF–KF melt in the tem-
perature interval 773–973 K. The results are shown in
Fig. 3. The CeF3 solubility increases from 0.42 mol %
at 773 K to 23.3 mol % at 973 K. The temperature de-
pendence of the CeF3 solubility can be satisfactorily
described by the linear equation С(CeF3) = 0.126T –
99.47, R2 = 0.960.
The U content of the melt was determined by Sa-
kharov’s method [7]; the Th content, gravimetrically
[8]; the Ce content, by titration [8]; and the F content,
according to [9]. The relative uncertainty of the deter-
mination of U, Th, and Ce was ±0.5%.
As noted above, the experimental data on the solu-
bility of plutonium fluorides in LiF–NaF–KF melt are
practically absent. Beneš and Konings [11] made an
attempt of computational modeling of the PuF3 solubil-
ity in 43.9 mol % LiF–14.2 mol % NaF–41.9 mol %
KF melt. They calculated the equations for the tem-
perature dependence of the PuF3 solubility: logS = –
1.416 – 2.670 × 10–3T + 3.859 × 10–6T2 (interval 823–
The experimental data on the UF4 solubility in
45 mol % LiF–12 mol % NaF–43 mol % KF melt in
the temperature interval 773–973 K are plotted in
Fig. 1.
The UF4 solubility increases from 3.18 mol % at
RADIOCHEMISTRY Vol. 53 No. 5 2011

SOLUBILITY OF UF4, ThF4, AND CeF3 IN A LiF–NaF–KF MELT
493
tem. Measurement of the PuF3 solubility in reference
temperature points will furnish information on the
PuF3 solubility in the LiF–NaF–KF melt in a wide
temperature interval.
ACKNOWLEDGMENTS
The study was supported by the Rosatom State Cor-
poration, State Contract no. Н.4f.45.03.09.1037 of
March 5, 2009.
Fig. 4. Comparison of the (1) experimental temperature
dependence of the solubility of CeF3 (PuF3 imitator) and
(2) data for PuF3, calculated using the equations from [11].
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As can be seen, the slopes of the straight lines are
practically equal (differ by only 0.4%); therefore, it
can be expected that CeF3 will adequately imitate PuF3
in the LiF–NaF–KF melt also.
The results of studies [10, 12–14] in which the
solubility of PuF3 and CeF3 in 2LiF–BeF2 melt was
examined also count in favor of this assumption. The
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BeF2 melt and obtained the values of 0.56 mol % at
873 K and 0.90 mol % at 923 K. On the basis of the
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ity in the LiF–NaF–KF system will be considerably
higher than in the 2LiF–BeF2 system.
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solubility of CeF3 in the LiF–NaF–KF melt. We also
measured the solubility of UF4 and ThF4 in the interval
773–973 K and found that CeF3 can be used as PuF3
imitator in performing studies in the LiF–NaF–KF sys-
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