FFR_chap12.txt

CHAPTER 12

CHEMICAL ASPECTS OF MOLTEN-FLUORIDE-SALT
REACTOR FUELS*

The search for a liquid for use at high temperatures and low pressures
in a fluid-fueled reactor led to the choice of either fluorides or chlorides
because of the requirements of radiation stability and solubility of appre-
clable quantities of uranium and thorium. The chlorides (based on the CI37
isotope) are most suitable for fast reactor use, but the low thermal-neutron
absorption cross section of fluorine makes the fluorides a uniquely desirable
cholce for a high-temperature fluid-fueled reactor in the thermal or epi-
thermal neutron region.

Since for most molten-salt reactors considered to date the required con-
centrations of UFy and ThEFy have been moderately low, the molten-salt
mixtures can be considered, to a first approximation, as base or solvent
salt mixtures, to which the fissionable or fertile fluorides are added. For
the fuel, the relatively small amounts of Uly required make the correspond-
ing binary or ternary mixtures of the diluents nearly controlling with regard
to physical properties such as the melting point.

12-1. CHoicE oF BASE OR SOLVENT SALTS

The temperature dependence of the corrosion of nickel-base alloys by
Huoride salts is deseribed in Chapter 13. From the data given there, 1300°F
(704°C 18 taken as an upper limit for the molten-salt-to-metal interface
temperature. To provide some leeway for radiation heating of the metal
wills and to provide a safety margin, the maximum bulk temperature of
the molten-salt fuel at the design condition will probably not exceed 1225°F.,
Iv « eireulating-fuel reactor, in which heat is extracted from the fuel in an
external heat exchanger, the temperature difference between the inlet and
outlet of the reactor will be at least {00°I7. The provision of a margin of
wifety of 100°1° between minimum operating temperature and melting
poitt makes salts with melting points above 1025°F of little interest at
present. and therefore this discussion is limited largely to salt mixtures
huving melting points no higher than 1022°F (550°C). One of the basic
features desired 1In the molten-salt reactor is a low pressure in the fuel
svstent, =0 only fluorides with o low vapor pressure at the peak operating
temperature ( ~700°C) are considered.

*By W. R. Grimes, D. R. Cuneo, F. F. Blankenship, G. W. Keilholtz, H. F.
Poppendiek, and M. T. Robinson.

569
570 CHEMICAL ASPECTS: MOLTEN-FLUORIDE-SALT FUELS  [cHap. 12

 

 

Frg. 12-1. The system LiF-NaF-KF [A. G. Bergman and E. . Dergunov,
Compt. rend. acad. sci. U.R.S.S., 31, 754 (1941)].

Of the pure fluorides of molten-salt reactor interest, only Bel’s meets
the melting-point requirement, and it is too viscous for use in the pure
state. Thus only mixtures of two or more fluoride salts provide useful
melting points and physical properties.

The alkali-metal fluorides and the fluorides of beryllium and zirconium
have been given the most serious attention for reactor use. Lead and bis-
muth fluorides, which might otherwise be useful because of their low neutron
absorption, have been eliminated because they are readily reduced to the
metallic state by structural metals such as iron and chromium.

Binary mixtures of alkali fluorides that have sufficiently low melting
points are an equimolar mixture of KF and Lil", which has a melting point
of 490°C, and a mixture of 60 mole 9 RbI with 40 mole 9} Lil’, which has
a melting point of 470°C. Up to 10 mole 9, UKy can be added to these
alkall fluoride systems without increasing the melting point above the
550°C Iimit. A melting-point diagram for the ternary system Lil'-Nal'-KI,
Fig. 12-1, indicates a eutectic with a lower melting point than the melting
points of the simple binary Lil'—KF system. This eutectic has interesting
properties as a heat-transfer fluid for molten-salt reactor systems, and
data on its physical properties are given in Tables 12-1 and 12-2. The
KF-LiF and RbF-LiI binaries and their ternary systems with Nal® are
the only available systems of the alkali-metal fluorides alone which have
TapLe 12-1

MprTivGg Points, HeaT CapAaciTIES, AND EQUATIONS FOR DENSITY
AND Viscosity oF Typrcar MoLTEN FLUoORIDES

 

Liquid density,

Viscosity, centipoise

 

 

 

 

 

 

 

 

 

 

 

 

 

¢ it Melting g/ce Heat capacity ‘
"”‘”)l““(;}”“’ point, p=A— BT(°C) at 700°C, n= AeB/TCK) |
mole e °C — cal/gram — At 600°C
A B A b
X 1077

LiF-BeF s

(69-31) 05 2.16 40 0.65 0.118 3624 7.5
LiF-Bel's

(50-50) 350 2.46 40 0.67 0.0189 6174 22 .2
NaF-Bel’;

(H7-43) 3060 2.27 37 052 (.0346 5164 12.8
Nak-7rF,

(50-30) 510 3.79 93 (.28 (0.0709 4168 8.4
Lik-NaF-KF

(46.5-11.5-42) 454 2.53 73 0.45 0.0400 4170 4.75
LiF-NaF-Bekq

(35-27-38) 338 2.22 41 0.59 0.0338 4738 7.8

 

(121

A0 TOTOTID

Asvd

SLIIVS LNHATOR HO

[LS
572 CHEMICAL ASPECTS. MOLTEN-FLUORIDE-SALT FUELS [CHAP. 12

1000

 

900

800

  
 

Temperatuyre, °C
~
o
S

o
o
o

500 —
~r 3 ~ ~
s A '
| SIS 2 ]S 3
400 el e [ e e
o o o O o
zll |12 |Z z z
© vy | ~ o
300
NaF 10 20 30 40 50 60 70 80 %0  zFy

ZrF4 . mole %

F1a. 12-2. The system NaF-ZrF,.

low melting points at low uranium concentrations. They would have
utility as special purpose reactor fuel solvents if no mixtures with better

properties were available.

 

 

 

 

TaBLE 12-2
TuermaL ConpucTiviTY oF TYPicaL FLUORIDE MIXTURES
Thermal conduetivity,
Composition, Btu/ (hr) (ft) (°F)
mole 9,
Solid Liquid
LiF-NaF-KF (46.5-11.5-42) 2.7 2.6
NaF-BeF, (57-43) 2.4

 

 

 

 

 

Mixtures with melting points in the range of interest may be obtained
over relatively wide limits of concentration if ZrF4 or BeF2 is a component
of the system. Phase relationships in the NaF-ZrFy system are shown in
Iig. 12-2. There is a broad region of low-melting-point compositions that

have between 40 and 55 mole 9 ZrF,.
 

   
 

 

 

   

 

   
   

 

 

 

 

 

 

 

 

  

 

 

 

 

 

 

  

 

 

 

 
 

 

 

    

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

12-1] CHOICE OF BASE OR SOLVENT SALTS 573
900 I | 1 T T
800 . 1 R : -
| i
| |
700 | 1= T ——
| |
nU .
o 600 Lif + Liquid o ]
2 i
5 !
g
&
£ 500 e
-
‘ BeFp + Liguid
400 = . LipBeF4 e .
+ Liguid }
LiF + LigBeFy 1 T ! f
Pt . LioBeF4 + BeF
300 e b 22 TR L —
2 !
L1 LigBeF4 o . : !
4 E L|iB&F3 + B§F2 |
200 l 1 | LiBeF3 = I [ 1 |
LiF 10 20 30 40 50 60 70 80 90 BeF)
BeF7 mole % LiBeF3 + Befp
Fi1c. 12-3. The system LiF-BeF.
i | |
800 — — t L —
: |
|
700 - — . oe=OmlData ——— |+
|
|
¢ a — NayBeF 4 +LIQUID :
g 600 - T - : —
2 | |
5 | | S
@ i i |
L ‘ | | ]
- \ : _ | /
a— NogBeFy 4 NoF | | 3'~ NaBeFy Bef, + LIQUID
‘ , | A+ 1QuID ‘
400 f— | A=A A - BeFyp+ 3'—NaBeF3 =
| \/ : “/ ’
a— N(_;IZBEFA + 55— NoBeF3 — W \ -
— N 5 — NaBeF + LIQUID
300+ - a’— Na,BeF , + NaF 8 — NaBeFq —g————oer f . B:EFQ Tis Nug;F3 .
4 — NaBeF, + — NaBef "— Na,BeF ; _
c;es ¥ . quA\‘\i‘ NasbelF 4 /BeF2+3 NdeF3
200 v — NUQBeF4 + NaF i — T I i
NaF 10 20 30 40 50 &0 70 80 90 BeFg
BeF,, mole %

Fi1g. 12-4. The system NaF-BeF.
574 CHEMICAL ASPECTS: MOLTEN-FLUORIDE-SALT FUELS  [cHAP. 12

BeFs
Dotted Lines Represent 549
Incompletely Defined
Phase Boundaries and
Alkemade Lines

      
 
  

The Symbol TC Represents All Temperatures Are in °C

a Compound Whose Exact
Composition Hos Not
Been Determined

370

NaF-Befg
80

356

iF-Be et -—-‘4\
EE =7l
T =X e N

700 - 20
750 (NaF - LiF-BeFo) . 8250
’ - // / 70

LiF 800 750 700 64§ 700 750 800 850 %00 950 NaF
844 980

F16. 12-5. The system LiF-NaF-BeFo.

The lowest melting binary systems are those containing Bel's and LiF
or NaI". Since Bel's offers the best cross section of all the useful diluents,
fuels based on these binary systems are likely to be of highest interest in
thermal reactor designs.

The binary system LiF-BeF2 has melting points below 500°C over the
concentration range from 33 to 80 mole 9, BeF2. The presently accepted
LiF-Bel's system diagram presented in Fig. 12-3 differs substantially from
previously published diagrams [1-3]. It is characterized by a single eutectic
between Bel'e and 2Lil" - BeF 2 that freezes at 356°C and contains 52 mole
T Bel's. The compound 2LiF - BeFz melts incongruently to Lil' and
liquid at 460°C; LiF - Bel'z is formed by the reaction of solid BeFs and
solid 2LiI" - BeF2 below 274°C.

The diagram of the Nal'-BeFy system (Fig. 12-4) is similar to that of
the LiI'-Bel'; system. The ternary system combining both NaF and LilF
with Bel's, shown in Fig. 12-5, offers a wide variety of low-melting compo-
sitions. Some of these are potentially useful as low-melting heat-transfer
liquids, as well as for reactor fuels.
TaBLE 12-3

MEeLTIiNG Points, HEaT CapaciTies, AND EqQuATIiONs ForR DENSITY
AND ViscosiTy oF FUueL BEARING SaLTs

 

Liquid density,

Viscosity, centipoise

 

 

 

 

Composition Melting g/cc Heat capacity
: mg’le o point, p=A — BTCC) at 700°C, n= AcB/TCK
/0 °C - cal/gram At 600°C
A B A B
X 10—5

LiF-Bel»-UF4

(67-30.5-2.5) 464 2.38 40 0.57 8.4
NaF-BeF 2*~UF4

(55.5-42-2.5) 400 2.50 43 0.46 10.5
NaF-ZrF,-UF,

(50-46—4) 520 3.93 93 0.26 0.0981 3895 8.5

 

 

 

 

 

 

 

 

 

[1-g1

SIIVS LNHATOS HO HSVH A0 HDIOHD

juby ]
576

CHEMICAL ASPECTS: MOLTEN-FLUORIDE-SALT FUELS  [cHAp. 12

TaBLE 124

TaerMmal ConbuctiviTy oF Typicarn FrLuoripE FurLs

 

 

 

 

 

Thermal conductivity,
Composition, Btu/ (hr)(ft) °F)
mole %,

Solid Liquid
LiF-NaF-KF-UF, (44.5-10.9-43.5-1.1) 2.0 2.3
NaF-ZrF,-UF4 (50-46-4) 0.5 1.3
NaF-ZrF~UF4 (53.5-40-6.5) 1.2
NaF-KF-UF, (46.5-26-27.5) 0.5

 

 

 

UF4
1035

All Temperatures Are in °C

950

NaF-2 UF4

7 NaF-6 u
5 NaF+3 UF4

2 NaF'UFy A

  
  
  

3 NaF-UFy4
750

765

850
ANZrFy
\ 918
7 NaF-6 ZrFy NaF-ZrFy 200
5NaF-2ZrF4 2NaF-ZiFg 3 NaF-2 ZrFy

%0 3 NaF-ZrFy

Fia. 12-6. The system NaF-ZrF;-UF4.
12-2] FUEL AND BLANKET SOLUTIONS 577

UF4

All Temperatures Are in °C
E-- Eutectic
P:: Peritectic LiF- 4UFy /...

UF |- Primary Phase Field

P P00

7LF6UF 4 \ VF

 

| 0
ALiF- UFy O\
5 - >
B |7LiF6UF, 05
A2
© N1 300 650 ;
CPOO _'}O“gd‘ \ 450 - 600~—— =T
NN o : =V
LiF 2LiF-BeF, P 400°g_ 400 BeF

Fig. 12-7. The system LiF-BeFo-UF,.

12-2. FueL AND BLANKET SOLUTIONS

12-2.1 Choice of uranium fluoride. Uranium hexafluoride is a highly
volatile compound, and it is obviously unsuitable as a component of a
liquid for use at high temperatures. The compound UO2Fs, which is rela-
tively nonvolatile, is a strong oxidant that would be very difficult to con-
tain. Fluorides of pentavalent uranium (Ul's,Usl', ete.) are not thermally
stable [4] and would be prohibitively strong oxidants even if they could be
stabilized in solution. Uranium trifluoride, when pure and under an inert
atmosphere, 1s stable even at temperatures above 1000°C [4,5]; however,
it is not so stable in molten fluoride solutions [6]. It disproportionates
appreciably in such media by the reaction

4 UFs == 3 UF;:+ U,

at temperatures below 800°C. Small amounts of UF3 are permissible in the
presence of relatively large concentrations of UF4 and may be beneficial
insofar as corrosion 18 concerned. It is necessary, however, to use UF4
as the major uraniferous compound in the fuel.
578 CHEMICAL ASPECTS: MOLTEN-FLUORIDE-SALT FUELS [cHAP. 12

UFy4

   
   
  
     
 

All Temperatures Are in °C

E -=Eutectic
P- Peritectic

:Primury Phase Field

7NaF- éUF4

25

5NafF- 3UF4 800

7NaF- 6UF4

  
 
 

5NaF-3UF4

  
     

 

. ) . 4 . )
E E E BeFg
2NqF'8eF2/ \NaF ‘BeFy

F1G. 12-8. The system NaF-Bel2-UF,.

12-2.2 Combination of UF, with base salts. The fuel for the Aircraft
Reactor Experiment (Chapter 16) was a mixture of UF4 with the NaF-ZrI'4
base salt. The ternary diagram for this system is shown in Fig. 12-6. The
compounds ZrF4 and UF4 have very similar unit cell parameters [4] and
are isomorphous. They form a continuous series of solid solutions with a
minimum melting point of 765°C for the solution containing 23 mole 9
UF4. This minimum is responsible for a broad shallow trough which pene-
trates the ternary diagram to about the 45 mole 9, Nal' composition. A
continuous series of solid solutions without a maximum or a minimum
exists between a—3Nal - UFy and 3Nal’ - ZrF4; in this solution series the
temperature drops sharply with decreasing ZrFs concentration. A con-
tinuous solid-solution series without a maximum or a minimum also exists
between the Isomorphous congruent compounds 7NaF -6UFs and
7Nal - 6ZrF4; the liquidus decreases with increasing Zrl'y content. These
two solid solutions share a boundary curve over a considerable composition
range. The predominance of the primary phase fields of the three solid
solutions presumably accounts for the complete absence of a ternary
eutectic in this complex system. The liquidus surface over the area below
8 mole 9, UI'4 and between 60 and 40 mole 9, NaF is relatively flat. All
fuel compositions within this region have acceptable melting points. Minor
12-2] FUEL AND BLANKET SOLUTIONS 579

 

ThF,
1080°C
\060
1000 7
950°
900°
850°

800

) Oo

550°C %%, 750
I,
I G-
a 0,0
2N\ A\
N 2
TN
b, 502N
LiF AN\ = BeF
845°C ligBeF,  LiBeFg (?) 543°C
475°C 360°C

F1g. 12-9. The system LiF-BeF:-ThF,.

advantages in physical and thermal properties acerue from choosing mix-
tures with minimum ZrI's content in this composition range. Typical
physical and thermal properties are given in Tables 12-3 and 12-4.

The nuclear studies in Chapter 14 indicate that the combination of
BeFs with NaF or with LiF (provided the separated Li? isotope can be
used) are more suitable as reactor fuels. The diagram of Fig. 12-7 reveals
that melting temperatures below 500°C can be obtained over wide com-
position ranges in the three-component system Lil'-Bel's-UF4. The lack
of a low-melting eutectic in the NaI'-UI'4 binary system is responsible for
melting points below 500°C being available over a considerably smaller
concentration interval in the NaF-Bel's-UF4 system (Fig. 12-8) than in
its LiF—Bels—-UF4 counterpart.

The four-component system LiF-NalF-BeF.-UF4; has not been com-
pletely diagrammed. It is obvious, however, from examination of Fig. 12-5
that the ternary solvent LiF-Nal'-Bel's offers a wide variety of low-melting
compositions; it has been established that considerable quantities (up to
at least 10 mole 9;) of UF4 can be added to this ternary system without
elevation of the melting point to above 500°C.
580 CHEMICAL ASPECTS: MOLTEN-FLUORIDE-SALT FUELS [cHAP. 12

   
 
 
 
  

7 LiF-6 ThFy

LiF-2 ThF4
T E=Eutectic
P—Peritectic
Liquidus
Temperatures
Are in °C

   

 

 
 

LiF

 

 
  

0 25
2 LiF-BeF;
BeFy fmole %)

Fic. 12-10. The system LiF-BeF.-ThFy in the concentration range 50 to 100
mole 9, LiF.

12-2.3 Systems containing thorium fluoride. All the normal compounds
of thorium are quadrivalent; accordingly, any use of thorium in molten
fluoride melts must be as ThF4. A diagram of the LiF-BeFe—ThTF ternary
system, which i1s based solely on thermal data, is shown as Fig. 12-9.
Recent studies in the 50 to 100 mole 9 LiF' concentration range have
demonstrated (Fig. 12-10) that the thermal data are qualitatively correct.
Breeder reactor blanket or breeder reactor fuel solvent compositions in
which the maximum ThF4 concentration is restricted to that available in
salts having less than a 550°C liquidus may be chosen from an area of the
phase diagram (I'ig. 12-10) in which the upper limits of ThI'4 concentra-
tion are obtained in the composition

75 mole 9, LiF-16 mole 9, ThF4-9 mole 9, BeFs,,
69.5 mole 9, LiF-21 mole 9, ThF4+-9.5 mole 9, BeFs,
68 mole 9, LiF-22 mole 9, ThI'+-10 mole 9, Bel's.

12-2.4 Systems containing Thy and UF;. The LiF-Bel'>-UF4 and the
LiF-BeFo—ThF4 ternary systems are very similar; the two eutectics in the
LiF-BelF3—ThI'y system are at temperatures and compositions virtually
identical with those shown by the UF4-bearing system. The very great
12-3] PROPERTIES OF FLUORIDE MIXTURES 581

similarity of these two ternary systems and preliminary examination of
the Lil'-Bel"'s>-ThF4-UF4 quaternary system suggests that fractional re-
placement of UF4 by ThIy will have lLittle effect on the freezing tem-
perature over the composition range of interest as reactor fuel.

12-2.5 Systems containing PuF;. The behavior of plutonium fluorides
i molten fluoride mixtures has received considerably less study. Plu-
tonium tetrafluoride will probably prove very soluble, as have Uy and
Thl'y, in suitable fluoride-salt diluents, but is likely to prove too strong an
oxidant to be compatible with presently available structural alloys. The
trifluoride of plutonium dissolves to the extent of 0.25 to 0.45 mole 9, in
LiI'-Bel’s mixtures containing 25 to 50 mole ¢, Bel's. As indicated in
Chapter 14, it is believed that such concentrations are in excess of those
required to fuel a high-temperature plutonium burner.

12-3. Puysicar anp THeErMAL ProPERTIES OF FLUORIDE MIXTURES

The melting points, heat capacities, and equations for density and vis-
cosity of a range of molten mixtures of possible interest as reactor fuels are
presented above in Tables 12-1 and 12-3, and thermal-conductivity values
are given in Tables 12-2 and 12-4; the methods by which the data were ob-
tained are described here. The temperatures above which the materials
are completely in the liquid state were determined in phase equilibrium
studies. The methods used included (1) thermal analysis, (2) differential-
thermal analysis, (3) quenching from high-temperature equilibrium states,
(4) visual observation of the melting process, and (5) phase separation by
filtration at high temperatures. Measurements of density were made by
weighing, with an analytical balance, a plummet suspended in the molten
mixture. Iiuthalpies, heats of fusion, and heat capacities were determined
from measurements of heat liberated when samples in capsules of Ni or
Inconel were dropped from various temperatures into calorimeters; both
ice calorimeters and large copper-block calorimeters were used. Measure-
ments of the viscosities of the molten salts were made with the use of a
capillary efflux apparatus and a modified Brookfield rotating-cylinder
device; agreement between the measurements made by the two methods
indicated that the numbers obtamed were within 4 1095.

Thermal conductivities of the molten mixtures were measured in an
apparatus similar to that deseribed by Lucks and Deem [7], in which the
heating plate is movable so that the thickness of the liquid specimen can
be varied. The uncertainty in these values is probably less than 4 25%.
The variation of the thermal conductivity of a molten fluoride salt with
temperature is relatively small. The conductivities of solid fluoride mix-
tures were measured by use of a steady-state technique in which heat was
passed through a solid slab.
H82 CHEMICAL ASPECTS: MOLTEN-FLUORIDE-SALT FUELS  [cHAP. 12

The vapor pressures of PuF; [8], UF4 [9], and ThF4 are negligibly small
at temperatures that are likely to be practical for reactor operations. Of
the fluoride mixtures likely to be of interest as diluents for high-temperature
reactor fuels, only AlF3, BeF» [9], and ZrF4 [10-12] have appreciable
vapor pressures below 700°C.

Measurements of total pressure in equilibrium with NaF-ZrF,~UF,
melts between 800 and 1000°C with the use of an apparatus similar to that
described by Rodebush and Dixon [13] yielded the data shown in Table
12-5. Sense et al. [14], who used a transport method to evaluate partial

TaBLE 12-5

Varor Pressures oF FrLuoripE MixTUres CoNTAINING ZRF4

 

 

 

 

COIIEEI(; s1(1;7;0n, Vapor pressure constants* Vapor pressure
at 900°C,
mm Hg
NaF ZrFy UF, A B
X103
100 7.792 9.171 0.9
100 12 542 11.360 617
57 43 7.340 7.289 14
50 50 7.635 7.213 32
50 46 4 7.888 7.551 28
53 43 4 7.37 7.105 21

 

 

 

 

 

 

 

 

*For the equation log P (mm Hg) = A — (B/T), where T is in °K,

pressures in the NaF-ZrF4 system, obtained slightly different values for
the vapor pressures and showed that the vapor phase above these liquids
is quite complex. The vapor-pressure values obtained from both investi-
gations are less than 2 mm Hg for the equimolar NaF-ZrF4 mixture at
700°C. However, since the vapor is nearly pure ZrF4, and since ZrF4 does
not melt under low pressures of its vapor, even this modest vapor pressure
leads to engineering difficulties; all lines, equipment, and connections ex-
posed to the vapor must be protected from sublimed ZrF4 “snow.”
Measurements made with the Rodebush apparatus have shown that the
vapor pressure above liquids of analogous composition decreases with in-
creasing size of the alkali cation. All these systems show large negative
deviations from Raoult’s law, which are a consequence of the large, posi-
tive, excess, partial-molal entropies of solution of ZrF,. This phenomenon
has been interpreted qualitatively as an effect of substituting nonbridging
TaBLE 12-6

Varor Pressures oF NaF-BeFs MixTures*

 

Composition,

Vapor pressure constantsf

 

 

 

 

 

mole % Temperature Vapor pressure
interval, NaF Bel', NaF - BeFs at 800°C,
°C mm Hg
Nak BekF, A B A B A B
x 104 X 10% x 101
26 74 785-977 10.43 1.096 9.77 1.206 1.69
41 59 802988 1006 1.085 9.79 1.187 0.94
50 50 796-996 9.52 1.071 9 82 1.187 0 41
60 40 855-1025 9 392 1.1667 9.080 1.1063 (.09
75 25 857-1035 9.237 1.2175 8.2 1.12 0.02

 

 

 

 

 

 

 

 

 

 

*Compiled from data obtained by Sensc et al. [15].
tFor the equation log /> (mm Hg) = A — (B/T), where 7' is in °K.

 

SHUAIXIN JAIH0NTd A0 STILIHdONd [e-2T

€8¢
H84 CHEMICAL ASPECTS: MOLTEN-FLUORIDE-SALT FUELS  [cHaP. 12

fluoride ions for fluoride bridges between zirconium ions as the alkali
fluoride concentration is increased in the melt [12].

Vapor pressure data obtained by the transport method for NaF-BeF,
mixtures [15] are shown in Table 12-6, which indicates that the vapor
phases are not pure BeFs;. While pressures above LiF-BeFz must be ex-
pected to be higher than those shown for NaF-BeF: mixtures, the values
of Table 12-6 suggest that the “snow” problem with BeFo mixtures is
much less severe than with ZrFs melts.

Physical property values indicate that the molten fluoride salts are, in
general, adequate heat-transfer media. It is apparent, however, from
vapor pressure measurements and from spectrophotometric examination of
analogous chloride systems that such melts have complex structures and
are far from ideal solutions,

12-4. PropucTtoN AND PURIFICATION OF FLUORIDE MIXTURES

Since commercial fluorides that have a low concentration of the usual
nuclear poisons are available, the production of fluoride mixtures is largely
a purification process designed to minimize corrosion and to ensure the
removal of oxides, oxyfluorides, and sulfur, rather than to improve the
neutron economy. The fluorides are purified by high-temperature treat-
ment with anhydrous HF and Ho gases, and are subsequently stored in
sealed nickel containers under an atmosphere of helium.

12-4.1 Purification equipment. A schematic diagram of the purification
and storage vessels used for preparation of fuel for the Aircraft Reactor
Experiment (Chapter 16) is shown in Fig. 12-11. The reaction vessel in
which the chemical processing is accomplished and the receiver vessel into
which the purified mixture is ultimately transferred are vertical cylindrical
containers of high-purity low-carbon nickel. The top of the reactor vessel
1s pierced by a charging port which is capped well above the heated zone
by a Teflon-gasketed flange. The tops of both the receiver and the reaction
vessels are pilerced by short risers which terminate in Swagelok fittings,
through which gas lines, thermowells, etc., can be introduced. A transfer
line terminates near the bottom of the reactor vessel and near the top of
the receiver; entry of this tube is effected through copper-gasketed flanges
on l-in.-diameter tubes which pierce the tops of both vessels. This transfer
line contains a filter of micrometallic sintered nickel and a sampler which
collects a specimen of liquid during transfer. Through one of the risers in
the receiver a tube extends to the receiver bottom; this tube, which is
sealed outside the vessel, serves as a means for transfer of the purified
mixture to other equipment.

This assembly is connected to a manifold through which He, Hs, HF,
or vacuum can be supplied to either vessel. By a combination of large tube
124] PURIFICATION OF FLUORIDE MIXTURES 085

   

N
e~ | 3/8-in. Nickel
|
»

Transfer Line

U .
Filter
(S

 

 

 

 

 

 

Reaction Vessel

Receiver Vessel/

 

Fia. 12-11. Diagram of purification and storage system.

furnaces, resistance heaters, and lagging, sections of the apparatus can be
brought independently to controlled temperatures in excess of 800°C.

12—4.2 Purification processing. The raw materials, in batches of proper
composition, are blended and charged into the reaction vessel. The material
is melted and heated to 700°C under an atmosphere of anhydrous HF to
remove Ho(O with a minimum of hydrolysis. The HF is replaced with Ho
for a period of 1 hr, during which the temperature is raised to 800°C, to
reduce U5t and U®* to U4 (in the case of simulated fuel mixtures), and
sulfur compounds to S~ and extraneous oxidants (Fet**, for example) to
5806 CHEMICAL ASPECTS: MOLTEN-FLUORIDE-SALT FUELS  [cHar, 12

lower valence states. The hydrogen, as well as all subsequent reagent gases,
is fed at a rate of about 3 liters/min to the reaction vessel through the re-
ceiver and transfer line and, accordingly, it bubbles up through the molten
charge. The hydrogen is then replaced by anhydrous HF, which serves,
during a 2- to 3-hr period at 800°C, to volatilize HeS and HCI and to con-
vert oxides and oxyfluorides of uranium and zirconium to tetrafluorides at
the expense of dissolution of considerable NiF; into the melt through re-
action of HF with the container. A final 24- to 30-hr treatment at 800°C
with Hy suffices to reduce this NiFe and the contained FeF. to soluble
metals.

At the conclusion of the purification treatment a pressure of helium
above the salt in the reactor vessel is used to force the melt through the
transfer line with its filter and sampler into the receiver. The metallic
iron and nickel are left in the reactor vessel or on the sintered nickel filter.
The purified melt is permitted to freeze under an atmosphere of helium
in the receiver vessel.

12-5. RADIATION STABILITY OF FLUORIDE MIXTURES

When fission of an active constituent occurs in a molten fluoride solu-
tion, both electromagnetic radiations and particles of very high energy
and intensity originate within the fluid. Local overheating as a consequence
of rapid slowing down of fission fragments by the fluid is probably of little
consequence in a reactor where the liquid is forced to flow turbulently and
where rapid and intimate mixing occurs. Moreover, the bonding in such
liquids 1s essentially completely ionic. Such a solution, which has neither
covalent bonds to sever nor a lattice to disrupt, should be quite resistant to
damage by particulate or electromagnetic radiation.

More than 100 exposures to reactor radiation of various fluoride mix-
tures containing UF4 in capsules of Inconel have been conducted; in these
tests the fluid was not dehberately agitated. The power level of cach test
was fixed by selecting the U235 content of the test mixture. Thermal neu-
tron fluxes have ranged from 10V to 10™ neutrons/(cm?)(sec) and power
levels have varied from 80 to 8000 w/cm?. The capsules have, in general,
been exposed at 1500°F for 300 hr, although several tests have been con-
ducted for 600 to 800 hr. A list of the materials that have been studied is
presented in Table 12-7. Methods of examination of the fuels after irra-
diation have included (1) freezing-point determinations, (2) chemical
analysis, (3) examination with a shielded petrographic microscope, (4) as-
say by mass spectrography, and (5) examination by a gamma-ray spectro-
scope. The condition of the container was checked with a shielded metal-
lograph.

No changes in the fuel, except for the expected burnup of U?32 have
been observed as a consequence of irradiation. Corrosion of the Inconel
 

 

 

 

 

 

 

 

 

 

 

12-5] RADIATION STABILITY OF FLUORIDE MIXTURES H&87
TaBLE 12-7
MovreN Savts WaicH Have BEEN Stupiep
iN IN-PiLe CaprsurLk TESTS
. Composition,
System mole
NaF-KF-UF, 46.5-26-27.5
Nal-BeFo-UFy 25-60-15
NaF-BeFo-UF4 47-51-2
NaF-BeF-UF, 50-46-4
NalF-ZrF4UF, 63-25-12
NaF-ZrF4«-UF, 53.5-40-6.5
NalF-7ZrF~UF, 50-48-2
Nal-ZrF—UF; 50-48-2
TaBLE 12-8
DescripTions oF INncoNEL ForceED-CircuraTioN Looprs
OperaTep IN THE LITR anp tae MTR
Loop designation
LITR LITR MTR
Horizontal Vertical Horizontal
NaF-ZrF,~UF4 composition,
mole §5 62.5-12.5-25 | 63-25-12 | 53.5-40-6.5
Maximum fission power, w/cm? 400 500 800
Total power, kw 2.8 10 20
Dilution factor™® 180 7.3 b5
Maximum fuel temperature, °F 1500 1600 1500
Fuel temperature differential, °F 30 250 155
Fuel Reynolds number 6000 3000 5000
Operating time, hr 645 332 467
Time at full power, hr 475 235 271

 

 

 

 

 

*Ratio of volume of fuel in system to volume of fuel in reactor core.

 
588 CHEMICAL ASPECTS: MOLTEN-FLUORIDE-SALT FUELS  [CHAP. 12

capsules to a depth of less than 4 mils in 300 hr was found; such corrosion
is comparable to that found in unirradiated control specimens [16]. In
capsules which suffered accidental excursions in temperatures to above
2000°F, grain growth of the Inconel occurred and corrosion to a depth of
12 mils was found. Such increases in corrosion were almost certainly the
result of the serious overheating rather than a consequence of the radiation
field.

Tests have also been made in which the fissioning fuel is pumped through
a system in which a thermal gradient is maintained in the fluid. These tests
included the Aireraft Reactor Iixperiment (deseribed in Chapter 16) and
three types of forced-circulation loop tests. A large loop, in which the
pump was outside the reactor shield, was operated in a horizontal beam
hole of the LITR.* A smaller loop was operated in a vertical position in
the LITR lattice with the pump just outside the lattice. A third loop was
operated completely within a beam-hole of the MTR.7 The operating con-
ditions for these three loops are given in Table 12-8.

The corrosion that occurred in these loop tests, which were of short
duration and which provided relatively small temperature gradients, was
to a depth of less than 4 mils and, as in the capsule tests, was comparable
to that found in similar tests outside the radiation field [16]. Therefore
it is concluded that within the obvious limitations of the experience up to
the present time there is no effect of radiation on the fuel and no accelera-
tion of corrosion by the radiation field.

12-6. BEgavior orF FisstoN Propucts

When fission of an active metal occurs in a molten solution of its fluo-
ride, the fission fragments must originate in energy states and ionization
levels very far from those normally encountered. These fragments, how-
ever, quickly lose energy through collisions in the melt and come to equi-
librium as common chemiecal entities. The valence states which they ulti-
mately assume are determined by the necessity for cation-anion equivalence
in the melt and the requirement that redox equilibrium be established
among components of the melt and constituents of the metallic container.

Structural metals such as Inconel in contact with a molten fluoride so-
lution are not stable to Fo, Ul's, or Ul's. It 1s clear, therefore, that when
fission of uranium as UFy4 takes place, the ultimate equilibrium must be
such that four cation equivalents are furnished to satisfy the fluoride ions
released. Thermochemical data, from which the stability of fission-product
fluorides in complex dilute solution could be predicted, are lacking in

*Low Intensity Test Reactor, a tank type research reactor located at Oak
Ridge, Tennessee.
fMaterials Testing Reactor, a tank type research reactor located at Arco, Idaho.
12-6] BEHAVIOR OF FISSION PRODUCTS H89

many cases. No precise definition of the valence state of all fission-product
fluorides can be given; it is, accordingly, not certain whether the fission
process results in oxidation of the container metal as a consequence of de-
positing the more noble fission products i the metallic state.

12-6.1 Fission products of well-defined valence. The noble gases. The
fission products krypton and xenon can exist only as elements. The
solubilitiecs of the noble gases in NaF-ZrFs (53-47 mole 9) [17],
NaF-ZrF4~UF4 (50-46—-1 mole %) [17], and LiF-Nal-KF (46.5-11.5-42
mole %) obey Henry's law, increase with increasing temperature, decrease
with increasing atomic weight of the solute, and vary appreciably with
composition of the solute. The Henry’s law constants and the heats of
solution for the noble gases in the NaF-ZrF, and LiF-NaF-KF mixtures
are given in Table 12-9. The solubility of krypton in the NakF-ZrF4 mix-
ture appears to be about 3 X107% moles/(cm?)(atm).

TasLE 12-9

SoLUBILITIES AT (G00°C aAxp HEATS OF SOLUTION FOR NOBLE
Gases 1y MovLreN FrLuoripes MIXTURES

 

5 In NaF-7rF4 In LiF-NaF-KF

 

 

 

(53-47 mole &, (46.5-11.5-42 mole %)
Gas
Heat of Heat of
K* solution, K* solution,
keal/mole keal /mole
X108 X108
Helium 21.6 +£1 6.2 11.34£0.7 8.0
Neon 11.3 £0.3 7.8 4.440.2 8.9
Argon 5.1 £0.15 8.2
Xenon 1.94+0.2 11.1

 

 

 

 

 

 

 

*Henry’s law constant in moles of gas per cubic centimeter of solvent per
atmosphere,

The positive heat of solution ensures that blanketing or sparging of the
fuel with helium or argon in a low-temperature region of the reactor cannot
lead to difficulty due to decreased solubility and bubble formation in higher
temperature regions of the system. Small-scale in-pile tests have revealed
that, as these solubility data suggest, xenon at low concentration is re-
tained in a stagnant melt but is readily removed by sparging with helium.
Only a very small fraction of the anticipated xenon poisoning was observed
590 CHEMICAL ASPECTS. MOLTEN-FLUORIDE-SALT FUELS  [cHAP. 12

during operation of the Aireraft Reactor Experiment, even though the sys-
tem contained no special apparatus for xenon removal [18]. It seems
certain that krypton and xenon isotopes of reasonable half-life can be
readily removed from all practical molten-salt reactors.

Elements of Groups [-A, II-A, I11-B, and IV-B. The fission products
Rb, Cs, Sr, Ba, Zr, Y, and the lanthanides form very stable fluorides; they
should, accordingly, exist in the molten fluoride fuel in their ordinary
valence states. High concentrations of ZrFy and the alkali and alkaline
earth fluorides can be dissolved in LiF-NaF-KF, LiI'2-BeFs, or Nal'-ZrF,
mixtures at 600°C. The solubilitiés at 600°C of Y5 and of selected rare-
earth fluorides in NaF-ZrF, (53-47 mole 9 ) and LiF-BeF 2 (65-35 mole %)
are shown in Table 12-10. For these materials the solubility increases

TasLe 12-10

SoLUBILITY oF YF3 axp or SoME Rare-FEarta FLUroripEs
IN Nal-ZrFy axp 1x LaF-BulFs ar 600°C

 

 

 

 

 

 

i
| Solubility, mole ¢, MFE;
Fluoride . | . -
In Nalb 7rk, i In LifF-Bel,
i (57—43 mole ©) g (62-38 mole Yp)
| _
YT 3.6 f
LaFg ‘ 2.1 “
Cng ‘ 2.3 .48
smkEs | 2.5 |

 

 

 

about 0.5% /°C' and increases slightly with increasing atomic number in
the lanthanide series; the saturating phase is the simple trifluoride. For
solutions containing more than one rare earth the primary phase is a solid
solution of the rare-carth trifluorides; the ratio of rare-earth cations in the
molten solution is virtually identical with the ratio in the precipitated solid
solution. Quite high burnups would be required before a molten fluoride
reactor could saturate its fuel with any of these fission products.

12-6.2 Fission products of uncertain valence. The valence states as-
sumed by the nonmetallic elements Se, Te, Br, and I must depend strongly
on the oxidation potential defined by the container and the fluoride melt,
and the states are not at present well defined. The sparse thermochemical
data suggest that if they were in the pure state the fluorides of Ge, As,
Nb, Mo, Ru, Rh, Pd, Ag, Cd, Sn, and Sh would be reduced to the cor-
responding metal by the chromium in Inconel. While fluorides of some of
12-7] FUEL REPROCESSING 591

these elements may be stabilized in dilute molten solution in the melt, it is
possible that none of this group exists as a compound in the equilibrium
mixture. An appreciable, and probably large, fraction of the niobium and
ruthenium produced in the Aireraft Reactor Experiment was deposited
in or on the Inconel walls of the fluid circuit; a detectable, hut probably
small, fraction of the ruthenium was volatilized, presumably as RulF'5, from
the melt.

12-6.3 Oxidizing nature of the fission process. The fission of a mole of
UF4 would yield more equivalents of cation than of anion if the noble gas
isotopes of half-life greater than 10 min were lost and if all other elements
formed fluorides of their Towest reported valence state. If this were the
case .the system would, presumably, retain cation-anion equivalence by
reduction of fluorides of the most noble fission products to metal and
perhaps by reduction of some U*™ to U3*T. If, however, all the elements
of uncertain valence state listed in Article 12-6.2 deposit as metals, the
balance would be in the opposite direction. Only about 3.2 equivalents of
combined cations result, and since the number of active anion equivalents
i= & minimum of 4 (from the four fluorines of UF4), the deficiency must
he alleviated by oxidation of the container. The evidence from the Aireraft
Reactor Experiment, the in-pile loops, and the in-pile capsules has not
shown the fission process to cause serious oxidation of the container; it is
possible that these experiments burned too little uraniurm to yield significant
results. If fission of UF4 1s shown to be oxidizing, the detrimental effect
could be overcome by deliberate and occasional addition of a reducing agent
to create a small and stable concentration of soluble UF3 in the fuel
mixture.

12-7. FUuEL REPROCESSING

Numerous conventional processes such as solvent extraction, selective
precipitation, and preferential ion exchange could be readily applied to
molten fluoride fuels after solution in water. However, these liquids are
readily amenable to remote handling and serve as media in which chemical
reactions can be conducted. Most development efforts have, accordingly,
been concerned with direct and nonaqueous reprocessing methods.

Recovery of uranium from solid fuel elements by dissolution of the
element in a fluoride bath followed by application of anhydrous HF and
subsequent volatilization of the uranium as UFg has been described
[19,20]. The volatilization step accomplishes a good separation from Cs,
Sr, aud the rare earths, fair separation from Zr, and-poor separation from
Nb and Ru. The fission products I, Te, and Mo volatilize completely from
the melt. The nonvolatile fission products are discarded in the fluoride
solvent. Further decontamination of the UFy is effected by selective ab-
592 CHEMICAL ASPECTS: MOLTEN-FLUORIDE-SALT FUELS  [cHAP. 12

sorption and desorption on beds of NaF. At 100°C, UFg is absorbed on the
bed by the reversible reaction

UFs (g) + 3NaF—=3NaF - UFg,

which was first reported by Martin, Albers, and Dust [21]. Niobium ac-
tivity, along with activity attributable to particulate matter, is also
absorbed; ruthenium activity, however, largely passes through the bed.
Subsequent desorption of the UFg4 at temperatures up to 400°C is accom-
plished without desorption of the niobium. The desorbed UFy is passed
through a second NaF bed held at 400°C as a final step and is subsequently
recovered in refrigerated traps. The decontaminations obtained are
greater than 10° for gross beta and gamma emitters, greater than 107 for
Cs, Sr, and lanthanides, greater than 10° for Nbh, and about 10% for Ru.
Uranium was recovered from the molten-salt fuel of the Aircraft Reactor
Experiment by this method, and its utility for molten-fluoride fuel systems
or breeder blankets was demonstrated. Recovery of plutonium or thorium,
however, is not possible with this process.

There are numerous possible methods for reprocessing molten-salt fuels.
The behavior of the rare-earth fluorides indicates that some decontamina-
tion of molten-fluoride fuels may be obtained by substitution of Cel’; or
Lals, in a sidestream circuit, for rare earths of higher cross section. It
seems likely that Pul’y can be recovered with the rarc-earth fluorides and
subsequently separated from them after oxidation to Pul’y. [Further, it
appears that both selective precipitation of various fission-product ele-
ments and active constituents as oxides, and selective chemisorption of
these materials on solid oxide beds are capable of development into valu-
able separation procedures. Only preliminary studies of these and other
possible processes have heen made.
593

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2, A. V. Novosrrova et al,, Thermal and X-ray Analysis of the Lithium-
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11. W. FiscHER, Institut fiir Anorganische Chemie, Technicshe Hochschule,
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17. W. R. Grimes ct al,, Solubility of Noble Gases in Molten Fluorides.
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mole 93), J. Phys. Chem. (in press).
594 CHEMICAL ASPECTS: MOLTEN-FLUORIDE-SALT FUELS  [cHAP. 12

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