ORNL-1030.txt

 
 
 
 

 

 
   
   
     

ORNL 1030
Reactors-Resea
3 4456 D28y831 5 M and Powe

[y

B (NN

 

 

cory B

g!?r“ -
AN INVESTIGATION OF @

ThF4<FUSED SALT SOLUTIONS FOR
HOMOGENEOUS ‘BREEDER ‘REACTORS

J. 0. Blomeke

OAK RIDGE NATIONAL LABORATORY

UL L R T R 1Y -
CIRCULATION SECTION
4500N ROOM 175

LIBRARY LOAN COPY
DO NOT TRANSFER TO ANOTHER S0
If you wish someone else to see this
report, send in name with report and
the library wil| arrange a loan.

 

»

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OPERATED BY
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A DIVIBION OF UNION CARBIDE AND CARBCN CORPOMATION

o

e 1
POBT OFFICE BOX B Lay :
OAK RIDOE. TENNESSER @
i

»

 

 
Report Number ORNL-1030
This document consists of
23 peges.

No. 4 of 125 Series A.

Contract No. W-T405, eng 26

CHEMICAL TECHNOLOGY DIVISION

AN INVESTIGATION OF ThF), - FUSED SALT SOLUTIONS FOR
HOMOGENEOUS BREEDER REACTORS

J. 0. Blomeke

Work by:

Jd. 0. Blomeke
C. P. Johnston

DATE ISSUED:

JUN 19 1951

CAK RIDGE NATIONAL LABORATORY
Operated by
CARBIDE AND CARBON CHEMICALS COMPANY
A Division of Unlon Carblde and Carbon Corporation
Pogt Office Box P
Oak Ridge, Tennessee

 

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ORNL-~1030
Reactors-Research
and Power
28. M. T. Kelley
29. W. H. Pennington
30. W. K. Eister
31. F. R. Bruce
32, F. L, Culler
33. J. 0. Davis

34. H. XK. Jackson
35. J. 0. Blomeke
36. W. R. Crimes
37. R. R. Hood
38. C. H. Secoy

emicals Company (Y-12 Area)

Technology (Kaufmann)
for Aeronautics

 

 
_ -3- ORNL-1030

Abstract

A consideration of the characteristics of fused salt-ThF) solutions
suitable for use in homogeneous reactors is presented, together with a

brief survey of the literature pertaining to such solutions and a summary

of the experimental work accomplished.

 

 
— b ORNL-1030

Sumary

Fused salt solutions containing thorium, primarily by virtue of their
generally low vepor pressures, will probably assume a role of increasing im-
portance as blanket and fuel solutions of high temperature power and breeder
reactors. The literature offers little information on Tth solfitions with
other salts and contains no reference to a specific solution which would be
satisfactory for homogensous reactor use. Experimental work has been initl-
ated with a view to finding an acceptable ThF) solution. Binary systems of
ThF), with LiF, MgFo, PbFe,.A1F3 and UF), have been investigated and some pre-
liminary measurements have been made on the ternmary, ThFy-LiF-MgF,, and the
quaternary, ThFy-LiF-MgF,-NaF. To date, the mixtures found which are of most
interest are a ThF}-LiF binary eutectic comtaining 26 mole % ThF), and melting
at 550°C, and a quaternary eutectic containing 20 mole % ThF), 62.3 mole %
LiF, 12.7% NaF and 5% MgFo, melting at 530°C. The concentration of Th in both
of these mixtures is greater than 1000 g/liter and the ratios of the neutron
capture cross section of Th-232 to the sum of the cross sections of the other
constituents of the mixture are 73 in the case of the binary and 16 for the
quaternary.

It is believed that a further reduction of the melting point of ThFh can
be obtained in fluoride solutions meeting other requirements for reactor use

and that the search for such solutions should be continued when the design and

construction of homogenecus U-233 power-breeders comes nearer to actuality.

 

 
’ -5- ORNL-1030

Fused salt solutions for service as either reactor fuels or breeding
blankets in high-temperature power or breeder reactors appear to combine many
of the more desirable features of both aqueous media and liquid metal and
alloy systems. They might be expected to possess the low vapor pressures
characteristic of liquid metals and alloys while, at the same time, they might
contain the high concentrations of fuel or breeding material expected of aque-
ous systems. Since the Chemical Technology Division is a group concerned pri-
merily with the chemical processing of reactor product solutions, it was felt
that some familiarity with fused salts and especially with problems likely to
arise 1n their processing should be acquired.

A considerable effort has been underwey for some time In the Materials
Chemistry Division to find a UF), -fused fluoride solution suitable for use as
a fuel for the aircraft reactor of the ANP Project and a survey of the liter-

(1)

ature relative to this problem has been made. Other work of this nature
has been carried out at Battelle.(g) Relatively little experimental work has
been done with ThFy in such solutions and so far as 1s known, no organized
program of research on this subject 1s at present underway. Consequently, it
was believed that a search for a fused fluoride solution containing Th which
would be suiteble for use in a U-233 breeder reactor would serve the double
purpose of initlating studies along a line of great potential interest to the
.pianming and design of future reactors and, at the same time, would serve as

& starting point in the study of the chemical processing of fused salt systems

in general. ‘

 
- = -6- ORNL-1030

Introduction (continued)

The experimental work covered by this report represents only the first
step of a search for a thorium solution satisfactory for use in some future
homogeneous reactor. Considerably more effort will probably be necessary
before such a solution is found.

The remainder of this report is divided into three parts. The first part
deals with the specifications for an acceptable thorium reactor solution; the
second deals with the results of a literature survey made at one stage of the
problem; and the last part is devoted to a description and discussion of the

experimental work accomplished.

Thorium Reactor Solutions

Thor imm-fused salt solutions could find use in either of two general
applications in breeder reactors. Ome application is that in which the solution
would be situated around and on the outside of the reactor core in the form of
& so-called blanket; the second application might be in a self-moderated type
of breeder in which the thorium would be present in the fuel solution together
with U-233 and a moderator, e.g. beryllium.

With the assistance of R. B. Briggs of thg Long Rahge‘Planning Group,
several preliminary specifications, summerized in Table I, were §¥0posed which
could serve as a guide in the search for these thorium solutions. The four

“ properties considered (cross section, Th composition, vapor pressure and melt-

ing point) do not represent either a complete or ah in#iclate list of the

-

 
Teble I

ORNL-1030

Ceneral Requirements for Thorium Reactor Solutions

 

 

 

 

Characteristic Blanket Self Moderated Fuel
OMh-232 10
10
Z 0other components 2 >>
Composition = 1000 g/L Th Th = 1 atom

Vapor Pressure

Melting Point

 

£ 760 mm at 500-
800°¢

£1300°C

 

U-233 = 0.02 atom
Moderator = 100 atoms

{760 m at 500-800°C

< 300°¢

 

 

 
 

‘ -8- ORNL-1030

Thorium Reactor Solutions {comtinued)

specifications required; they are intended to give merely a rough picture of
vhat, in the light of current thinking, would be desirable characteristics for
an eventual thorium reactor solution to possess.

In both applications, the nuclear considerations are of parsmount im-
portance. If the specifications of Table I are accepted, it becomes apparent
that one is limited to only & small number of fluorides having sufficlently
low capture cross sections and favorable vepor pressures to enable them to be
used in a reactor solution with thorium. These salts are listed with their
neutron capture cross sections and melting points in Table II.

It 'is realized that of the compounds listed in Table II, ZrF) might prove
unusable because of its tendency to sublime at temperatures of the order of
gseversl hundred degrees and BiF3 might eventually prove troublesome because of
difficulty in preventing the displacement of Bi**from the melt by manmy of the
structural metals of which the container walls would normally be made. Other
f;uorides of slightly higher cross section than those listed in Table II might
be considered for use but could, of course, be present only in relatively

smaller concentrations.

A Literature Survey

 

A survey of the project and open literature was made to supplement the
1)
search made by Grimes and Hill.( Special emphasis wes placed on phase studies

* of the particular salts listed in Table II. The results of this survey which

 
- -9~ ORNL-1030

Table II

Cross Sections and Melting Points of Several Imorganic Fluorides

 

 

Melting Polmt

Compound ¢ Barng
BeF, 800 0.03
11'F gl 0,03k
BiFs 727 0.045
MgF» 1263 0.08
PbF, 813% 0.21
ZxF), 872 0.22
AlFq 1040 0.23
ThF), 1080% T

 

 

 

%% The crogs section of F in this teble has been taken as
0.0l barns. '

* The figures so marked are experimental determimations made
as part of the present work.

 
aln. -10- ORNL-1030

A Literature Survey (continued)

 

were adjudged to be of the most interest to this problem are noted in Table III.
The ThF} -KF and ThF}-RbF reference was included in Table III despite the
unfavorable nuclear characteristics of K and Rb because it represented the only
specific reference to ThF), eutectic mixtures found in the literature.
Zachariasen(1l) has studied double salt formation in the systems NeF-ThF, and
KFwThFh by the X-ray diffraction method but he apparently did not investigate
the complete phase diagrams of the?e systems. ThF) has also been found to form

a very stable complex with Rb, having the formmula, Rb3ThF7.(3)

Experimental
Apperatus
The equipment used for this work was the same as that used by other work-

ers for thermal analysis studies of UF), salt mixtures.(le)

In essence, it con-
sisted of a 5-inch chromel-wound pot furnace capable of operation at tempera-
tures up to 1100°C. The tempersture of the furnace was controlled by means of
a variable transformer connected in series with the A. C. supply. The salt
mixtures were heated in graphite crucibles which fit into the furnace in such
a manner that an atmosphere of No could be malntained over the melt during
heating and cooling periods. Temperatures were measured by means of a chromel-
alumel thermocouple situated on the ingide of a graphite stirrer which extended

to the bottom of the crucible. The temperatures were measured and recorded by

" a Brown "Electronik" potentiometer and tests indicated that the temperatures

.

 

 
Table III

ORNL~1030

Some Published Phase Relationships of the Fluorides of Table IT

 

 

 

 

 

Eutectic Composition Eutectic Temp.

System Mole Percent oC Reference
ThF), -KF 17 ThF), 664 (3)
33 ThF) 750
57 ThF), 878
80 ThF), 954
ThF),RbF 15 ThFy 664 (3)
37 ThF), 762
80 ThFh_ 1000
LiF-A1Fy 1.5 AlFq 706 ()
37 AJI‘3 691
LiF-AlF4 36 AlFg 710 (5)
LiF-MgF, 33 MgF, Th2 (6)
LiF-MgF, 53 MgF, 718 (7)
LiF-BeFo 52 BeFp 360 (8)
LiF~MgF,~NaF, 10 MgF,, 43 NaF 630 (6)
29 MSFE 3 12 NaF 68!4-

MgF,=BeF, Complete Miscibility (9)
BiF4-FbFp Complete Miscibility (10)

 

 

 
 

t 1. ORNL-1030

so -recorded were accurate to + 5°C.
Materials
IhE),

The thorium fluoride used was obtained from the Iowa State College. The
thorium analyzed, gravimetrically, 75.1% and the fluoride, 25.5%. The the-
oretical Th content is 75.3%. A spsctrographic analysis indicated the sample
wvas essentially free of rare earths. The melting point of this ThFh was found
by experiment to be 1080 + 5°C. No reference could be found to a previous melt-
ing point determination for this compound.

ALF3

The A1F3 was prepared from a stock of Baker and Adamson.AIF3-XH20 by heat-
ing the hydrated material in an atmosphere of HF to about 600°C over a pericd
of 3 to 5 hours. The vendor reported impurities in the A1F3-xHéO amounting to
less than 0.014%. The dehyrated product amalyzed 32.5% Al and 67.9% F.(the-
oretical Al = 32.1%). The high volatility of AlF; in the neighborhood of
1000°C prevented an experimental determination of its melting point with the
equipment on hand.

MgFo

The MgFs used in this work was purchased from Eimer and Amend and was re-
ported by them to be 99% pure. A spectrographic analysis indicated the major
impurities to be Ca, Na, Cr, Fe and Ta.

PbF2
The FbF» used was Baker and Adamson "Purified” material and was not

.

 
2 -13- ORNL-1030

PbF, (continued)
analyzed chemically. The melting point was found experimentally to be 813 +

o o (13)
5°C, which may be compared with a literature value of 822°C,

Lir

The LiF was material purchased from the Maywcod Chemical Works. No
analysis of the LiF was carried out but an experimental determination of its
melting point (845°C) agreed exactly with the most relisble value obtained
from the literature.(lh) On the basis of this agrsement and its cleen appear-
ance, it 1s belleved that this was materlal of high purity.

U

The UF} used was obtained from K-25 through the ORNL SF Accountability
Office.

Prior to their use in this work, all of the chemicals were dried by heat-
ing in an oven at 110-115°C for 24 hours and were stored in dessicators upon
removal from the oven.

Results
bRy, -LiF

The phase dlagram for this system 1s given in Figure 1. A eutectic con-
taining about 26 mole % ThFj is formed which melte at 550°C. Some difficulty
wag experienced in obtalning liquidus points from cooling curves of mixtures
in the vicinity of the eutectic becausze of very pronounced super cooling. A
compound with an incongruent melting point at about 925° 13 formed at T5 mole
% ThF).

 

 
 

_— -2 ORI 2030
ThF), -MgFp

The ThF)-rich side of this system up to 60 mole % MgF, was investigated
and the results are shown in Figure 2. Two eutectics were found which melted
at 915° and 925°C corresponding to compositions of 25 mole % and 40 mole %
ThFy, , respectively. A compound with a congruent melting point of 937° was
indicated at 33 mole % MgF, and can be represented by the formula, MgThoFyg.
The investigation of this binary system was not carried further than 60% MgFo
because of the temperature limitatioms of the equipment but it seems probable
that no further eutectics of immediste interest to this problem would be found
at higher MgF2 concentrations.
Th¥), -PbFo

The proposed phase dlaegram for this system is given in Figure 3. Two
eutectics were obtained, one at 35 mole % ThF)y , melting at 9250 and a second
at 62 mole % ThFl which melted at 880°C. Some indications of a third eutectic
melting at about 760° and containing less than 2 mole % ThF), were obtained but
its presence was _not definitely established. Two compounds with congruent
melting points at about 950° and 942°C were indicated, corresponding to the
formlas Pb17Th3F46 and PbThFg, respectively. A third compound having en
incongruent melting point of sbout 1045°C was Indicated with a formuls, PbThgF3g.

Measurements of this binary proved unsatisfactory in a sense because of
a reduction of the Pbtt to elemental Pb by the graphite crucible and stirrer.
This reduction was not observed to occur appreciably at temperatufes less than

800° but became a greater problem with increasing temperature.

A

 
 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

-15- ORNL-1030
" Drawing # 11461
1100 ] I [ l I 6 J—
FIGURE | _—
PHASE DIAGRAM OF ThF, - LiF BINARY SYSTEM "
o
1000
900 s——
)
\O
N
800 AN
S q
" N\
S 70 O
]—
<
as \\
s /
=
= 600
- o
S00
400
0 10 20 30 40 50 60 70 80 90 100
MOLE PERCENT ThF,
3

CHEM-TECH-DIV---LAB-SEC-~JEF--8-1[-51 - |

 

 
 

 

 

°C

TEMPERATURE

-16-

ORNL-1030
Drawing i 11462

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

1200
10O 0,/
0
\ /
.0
1000 5 0/
\ /O’bo/. S _¢—0 -
® ®—0
900
800 ) el FIGURE 2 |
( ThF4~MgF, BINARY SYSTEM
MthzFlo
700
600
0o 10 20 30 40 50 60 70 80
MOLE PERCENT MgF,

CHEM-TECH-DIV--LAB-SEC--JEF~ 8-11-8I- 2

 

 
 

 

°C

TEMPERATURE

1200

1100

1000

900

800

700

600

-17 -

ORNL-1030
Drawing # 11443

 

 

 

 

 

 

 

 

 

PHASE DIAGRAM OF ThF,— PbF, BINARY SYSTEM

FIGURE 3

 

 

 

 

 

 

 

§=—o—0— \

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

/° T T
° l
OI |
\ 1
S |
l
|
1
I
\ ‘
FoThs ol
0 10 20 30 40 50 60 70 80 90 100

MOLE PERCENT Th F4

o

CHEM-TECH-DIV-LAB-SEC~JEF-- 5-11°5Il- 3

 

 

 
s ~18- ORNL-1030

ThF), -AJ.F3
Considerable effort was expended on this system with only moderate suc-

cess. The AJ.F3 sublimed at temperatures from 900 to 1100°¢ to such en extent
that reliable results could not be obtained with mixtures containing more than
20 mole % AIF3. A reproducible eutectic halt was cbtained at about 950°C in
the cooling curves of this binary system but the composition of the eutectic
mixture could not be determined. It is estimated that it lay somewhere in the
vicinity of 25 mole % AlF3.

THF), -UF)

The isomorphism of ThF) and UF;L(]'T) would lead one to expect them to be
miscible in both the solid and liquid states. Although supercooling prevented
the accurate determination by thermal analysis of a phase diagram for this
system, indications were that liquidus and solidus curves existed which ex-
hibited neither a maximum nor minimm point. X-ray anslyses of several of the
gsolidified melts showed that solid solutions were formed.

ThF}, -LiF-MgF o

This was the only ternary system investigated to any sppreciable extent
and it was studied in only an exploratory mammer. An indication of a ternary
eutectic of high ThFj content melting at 690° was obtained but since 'bh:fs tem-
perature was greatly in excess of the desired melting point, no formal attempt
was made to establish the composition.

ThF), -L1F-NaF-MgF,
A reference found in the literaature to the NaF-LiF-MgF, eutectics(3)

 

 
g ~19- ORNEL-1030
ThF) -LiF-NaF-MgFo (continued)

seemed worthy of further investigation, especially the secondary eutectic con-
taining only 124 NaF. Experiments were carried out which coasisted, at Pirst,
of adding ThFy to the 29% Mg, 12% NeF, 59% LiF eutectic mixture and finally,
cf changing the concentrations of the other components empirically. In this
way, & quaternary eutectic was obtained which melted at 530°C. The approximste

composition of this eutectic, expressed in mole percentages was:

20.0% TLFy,
62,3 LiF
12,7 Na¥F
2.0 MgFo

UF), -LiF-NaF-MeF

Proceeding along the same lines as described above for the case of the
ThFh quaternary mixture, a eutectic was found in this system which melted at

about 460°C. Tts approximate composition was:

25.0%  UF),

58,2 LiF

11.8 NaF

5.0 MgF,
Discussion

Of the systems studied, two eutectic mixtures stand out in the respect
that their melting points are both more than 100D under the next lowest melt-
ing mixture found. The eutectics referred to are the TXF)-LiF binary eutectic,
- containing 26 mole % ThF) and melting at 550° and the ThF)-LiF-NeF-MgF, quater-

nary, containing 20 mole % ThF) and melting at 53006. The concentration of Th

-~

 

 
 

. 4 -20~- CRNL-1030

Discussion (continued)

in these eutectics corresponds to about 2000 g/L in the binary and 1500 g/L in
the quaternary. The neutron capture cross section ratios (Table I) are 73 and
16, respectively end the vapor pressures at 800°C were,by cbservation, much
less than 760 mm. On the other hand, the melting points are still considerably
higher than the 300°C taken as the maximum desirable melting point.

The ThF)-FPbF,, ThF) -MgF, and ThFh-A1F3 binary systems show no eutectic of
sufficiently low melting point to be of immediate interest to this prcblem.
Thelr principle value lies in the contribution they would meke to & study of
ternary systems contalning these components. From the same viewpoint, binary
systems of ThF) with BeFp, BiF3 and ZrFy should be investigated.

The UF)y-LiF-NaF-MgF, quaternary eutectic is similar im both meliing point
and ursnium concentration to a UF)-LiF-NaF ternary eutectic reported previously
as being under consideration as a fuel for the ANP reactor.(l5) Although indi-
cations are that the unavailability of L17 isotope will preclude, for the time
being, the use of a LiF constituted fuel in this reactor, it should be pointed
out that, other factors being equal, the better neutroen economy of this guater-
nary would likely make it more acceptable than the termary eutectic.

The similarity between the diagram found for the ThF), -LiF system and that
obtained by Grimes, et. al.(12) for UFh-LiF is rather remerkable. A UF), ~-LiF
eutectic melting at 480°C was found at about 26 mole % UF), and likewise a com-
pound, LIU3F13, with an incongruent melting point was obtained. The same in-

veatigatérs(ls) have found & eutectic in the UF,-FbF, system at 62 mole % UF),

a_—

 
i -21- ORBL-1030

Discussion (comtinued)

which melts at about 730° and have obtained a diagram for the UF,-PbF, system
vhich conforms in most principle respects to the character of the diagram pre-
sented in Figure 3. No attempt was made to rigorously defime the extemt of
the similarity between ThF) end UF, systems but from indications obtained in
this investigation, a close coordination between the work done on these two
general systems should be maintained.

From the results obtained thus far in this work, the possibllity seems
rather remote that a mixture meoeting all the specifications set for this prob-
lem will be found. On the other hand, there appears to be a very real 1ikli-
hood that mixtures which come nearer meeting them than anything so far found
can be obtained and such mixtures might eventually prove as satisfactory for
reactor use as the "ideal" one considered here. The work done to date can be
considered as no more than a start on such a search and whenever the importence
of this problem is adjudged more immediate, a thorough and more nearly complete

solution to the problem should be allowed.

%“’A
/ ! J. 0. Blomeke
A -22~ CRNL-1030

(1)

(2)

(3)

(%)
(5)
(6)

(7)
(8)

(9)

(10)

(11)

(12)

(13)

(14)
(15)

Bibliography

Grimes, W. R. and D. G. Hill, "High Temperature Fuel Systems - A
Literature Survey,” Y-657, (July 20, 1950).

Chase, L. Hey Ro C. Crooks, J. N. Pattison, J. J. Ward and J. W. Clegg,
"Chemist:;'y of Liguid Fuels for Nuclear Reactors," BMI-T-53, (January
15, 1951

Dergunov, E. P. and A. G. Bergman, Doklady Akad. Nauk. S.S.S.R. 60,
391-4 (1948).

Puschin, N. A. and A. V. Baskoff, Z. anorg. Chem. 81, 347 (1913).

Fedotiev, P. P. and K. Timofeev, Z. anorg. allgem. Chem. 206, 263 (1932).

 

Bergman, A. G. and E. P. Dergtmov, Compt. rend. a.cad. gci. U.R.S.S. 31,
755 (1941).

Hynieki, V. P. and P. F. Antipine, Chinie et industrie 17, 601 (1927).

I({oy, 1))e1.'l.a M., Rustum Roy and E. E. Osborn, J. Am. Ceram. Soc. 33, 85
1950

Venturello, Giovanni, Atti. acad. sc¢i. Torino, Classe se¢i. fis., mat.
nat. 76 I., 556-63 (1941); Chem. Zentr. 1942 I., 111k,

Croatto, Ugo, Gazz. chim. ital. 7k, 20 (1944).

Zacheriasen, W. H., J. Am., Chem. Soc. 70, 2147 (1948). See also CC-3401,
(Jan. 10, 1946), and CC-3426, (Feb. 9, 1946).

 

"Chemistry of Liquid Fuel Systems,” The Aircraft Nuclear Propulsion
Project Quarterly Progress Report for Period Ending August 31, 1950,
ORNL-858, p. 104 £f (December 4, 1950).

Tables of Selected Values of Chemical Thermodynamic Properties, Series 2,
UcSo N&‘t"l.BUI- Stand&rds, 191"7, P- 27"10

2 Pc 91"'1.

"Chemistry of Liquid Fuels,” The Aircraft Nuclear Propulsion Project
Quarterly Progress Report for Period Ending December 10, 1950, ORNL-919
P. 234 £f (February 26, 1951).

 

 

 
— -23- ORNL-1030

(16) Grimes, W. R., et al., Unpublished results.

(17) zachariasen, W. H., "The Crystal Structure of Fluorides of Th, U,
Np and Pu," MDDC-1151, (Jenuary 11, 1947).