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H
T.
HA '
Hanford Laboratories Operation General Electric Co., Richland Wash.
Uranium
T
Oxide-Liquid Metal Slurries
N
OWER REACTOR
operation use of a
molten metal as supporting medium for
a fissionable material eliminates expen-
sive fuel cladding, reduces decontam-
ination requirements, permits continual
withdrawal for chemical processing, and
provides freedom from radiolytic de-
composition.
The liquid metal must have a rela-
tively low melting point, fairly wide
liquid range, and low neutron cross
section. Because stabili ty of the slurry
varies inversely with differences in den-
sity benveen its constituents, the metal
should have a density similar to that of
the suspended material. Bismuth and
lead, \vhich have densities of about 10
grams per cc., are promising.
The molten metal may support the
fissionable material in solution or as a
slurry containing a suspended inter-
metall ic or oxide. Solubilities of the
fissionable metals are limited at prac-
tical temperatures, yet high enough to
modify intermetallic particle size by
solution and recrystallization. Oxides
offer a wider temperature range of
particle size stability. However,
fis-
sionable oxides may not be wetted by the
liquid metal at reasonable temperatures.
Whether wetting occurs is determined
by the balance of forces at the inter-
face formed by the solid and liquid.
This relationship is described mathe-
matically by
;SA
= y s L + yL cos
( 1 )
in which the surface tensions are those of
the solid, the solid-liquid interface, and
the liquid, respectively; contact angle
0 is measured through the liquid.
When 0 is 90' C. or less, the surface is
said to be wetted.
Wetting is favored
by a high solid surface tension, low liquid
and interfacial tensions, or combinations.
To
attain maximum solid surface ten-
sion, the surface must be free of adsorbed
atoms.
The contact angle of bismuth on
uranium dioxide decreases from 138.5
at 380' C. to 92' at 1242' C. in argon
( 4 ) .
The wetting temperature is there-
fore greater than 1250' C. Similar
angles were found for lead on uranium
dioxide.
Initial attempts to wet uranium oxide
with bismuth failed. However, in 1956
experimenters at the Knolls Atomic
Present address, Phillips Petroleum Co.
Idaho Falls, Idaho.
P o ~ e rLaboratory dispersed uranium
oxide by adding various metals as oxygen
getters 7) , presumably raising the oxide
surface energy. Although no uranium
oxide was visible at the surface of the
product in successful experiments,
patches were noted in the interior.
Magnesium. sodium, titanium, and ura-
nium (as hydride) \\ere satisfactory;
lithium and tin
v
ere not.
Experimental
Methods and Materials.
Two
methods of slurry preparation were used
successfully, magnesium gettering and
in xitu
preparation from uranium and
bismuth sesquioxide. I n the former
sufficient magnesium was added to the
uranium oxide charge to reduce the
oxygen-uranium ratio to 1.6, assuming
that all the magnesium was oxidized.
Preparation temperature was usually
700' C .; higher temperatures were used
for comparison to the in situ method.
The in situ method is based upon the
reaction
U
+
2/3 Bi203 UOz
+ 3 Bi ;
AF =
-150
kcal 2 )
Above
840
C. the bismuth oxide is
molten. Th e solubility of uranium in
bismuth at this temperature is about
10
weight 7 0 ,
so
that a completely liquid
reaction path is possible.
Materials were prepared as shown in
Table
I.
~~
Table I. Preparation
of
Materials
Material Preparation
U Degreased cleanedin8S 03,
washed in chilled water and
acetone dried in air
UOS HZ
eduction
of
UOa; sieved to
< 140 microns with mean par-
ticle size of 3 microns
Bi203
Bi Mg Reagent grade
Oxidation
of
Bi in air stream
Slurries were produced in capsules
machined from 304-L stainless steel?
2
X
1 to 5 X 2I/s inch in inner
diameter. Capsule contents were added
in air, no attempt being made to blanket
the opera tion with inert gas except when
the lids were welded into place.
Capsules were heated either in a muffle
furnace with rocker agitation or in a
9600-cycle-per-second Tocco induction
unit with manual or rocker agitation.
A specially designed rocker was used
with the Tocco unit 3 ) .
T o avoid sectioning of each capsule
and tedious analyses, a gamma absorp-
tiometer was constructed for nonde-
structive examination of the slurries.
I t consisted of a thulium-170 source, an
automatic traversing platform, a 1/16 X
3 /8
inch collimating slit in a lead block,
and a sodium iodide scintillation crystal
with associated electronic equipment.
The instrument was capable of deter-
mining the uraniumcon tentof a uranium-
bismuth capsule within 1% absolute.
Measurements on uranium oxide systems
were invalidai.ed by gas pockets in the
slurry.
The experiments at the Knolls Lab-
oratory were confirmed in these labora-
tories at higher uranium concentra-
tions. A 50-gram batch of slurry con-
taining 13.2 iveight 7 0 uranium as di-
oxide, plus sufficient magnesium to re-
duce the oxygen to uranium ratio to 1.6,
was capsulated and heated 3 hours at
700' C . with rocking. Th e capsule was
air-cooled and cut open. Th e oxide had
dispersed within the melt. Th e casting
was trisected and found to contain 13.3,
12.1, and 14.0 weight 7 0 uranium in
descending order. (Uranium concentra-
tions are henceforth expressed paren-
thetically as weight per cent in descend-
ing order.)
In an identical experiment: save for
replacement of bismuth with the bis-
muth-45.5% lead eutectic composition,
analysis confirmed (15.2, 12.7, 11.7)
that the oxide could be dispersed by
this technique and that the suspension
was reasonably stable.
Gallium Experiments. To facilitate
observation and measuremen:? a dis-
persion which is unreactive to\vard glass
is desirable, but free uranium is un-
desirable in the gettered product.
Gallium will not attack glass, but \vi11
remove oxygen from bismuth and reduce
higher oxides of uranium to the dioxide.
It is low-melting and mobile.
However, a charge of uranium dioxide
in bismuth plus gallium was unwet when
heated 3 hours at 1100'
C.: 16
hours
at 900'
C . ?
and 8 hours at
1100
to
1175' C . with periodic oscillation. In
a second attempt a sealed \-).cor tube,
previously evacuated to
3
microns at
500' C., was heated to 850' C. for
6
days with periodic shaking by rods at-
tached to the tube ends. Th e upper
portion of the casting contained high
concentrations of uranium-up to 26
weight yO-and a spotty distribution of
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segregated oxide partially coated with
gallium. It was concluded that urani um
dioxide itself is not wet by bismuth at
these temperatures.
Th e desire to pre-
pare a dispersion free of additives with
tailored oxygen to uranium ratio
sug-
gested an in
situ
preparation.
A
charge
Qf uranium, bismuth oxide, and bismuth
was capsulated and heated (Table
11,
No.
1). When the capsule was opened,
the bismuth oxide had disappeared and
no uranium was found in the original
form. Finely dispersed ura niu m oxide
was visible as red particles during micro-
n Situ Method.
Figure 1
A
magnesium-gettered ura-
nium oxide-bismuth slurry showed no
oxide powder
O/U = 1.6
scopic examination under polarized light.
In other experiments in which the
oxygen-uranium ratio was close to 2
(Table 11,No. 2 ) , a considerable amount
of unwetted oxide was found at the top
and throughout the upper portions of
the casting.
The amount of oxygen present in the
free space of the capsule increased the
oxygen-uranium ratio to less than 1.97.
Because lack of wetting was not due to
oxygen gain, either some uranium was
removed by combination with
the
cap-
sule, or oxides with oxygen-uranium
ratios close to 2 cannot be wet at these
temperatures. In experiments in which
the original ratio was
1.95
an essentially
unwet powder was found on the surface
of
the product; in no experiment in
which the ratio was 1.67 or less has the
unwet powder occurred. As no mag-
nesium is present in the
in situ
prepara-
tions, and uranium dioxide is not wet
at these temperatures, it is concluded
that in this case wetting is produced
by reduction of the oxide-uranium ratio.
Magnesium Gettering.
As
gettering
of excess oxygen is not solely responsible
for the wetting obtained with mag-
nesium, alternatives were sought. From
thermodynamic data
2)
uranium mon-
oxide is more stable than the dioxide in
the range
300
to
800
C. Because of
its oxygen deficiency the surface energy
of the monoxide is probably higher than
the dioxide, a condition which would
promote wetting. However, uranium
monoxide has not been identified in
bulk. Surface films have yielded x-ray
diffraction data, but these lines were not
found in diffraction patterns of the dis-
persion.
Although no evidence exists of free
reduced forms of uranium, magnesia
may exist as a coating or bridge between
the oxide particles and bismuth-e.g.,
Bi..Mg-O..U-0. This mechanism re-
ceived support from later experiments
3) ,
in which wetting did not occur with a
higher oxide U308 ) , but did occur
with addition of magnesium to reduce
the oxygen-uranium ratio to
2.0.
To estimate the
relative stabilities of slurries prepared by
both methods, capsules containing
8
Stability of
Slurries.
Figure
2.
A micrograph of Figure
1
casting shows dispersion
in
segregated
region
50X)
weight urani um (Table
11 3
and
4)
were subjected to essentially identical
treatment. Th e .regular heating period
was followed by
30
minutes at
600
C.
with intermitten t shaking. After dis-
persion, the capsule was allowed to stand
10
minutes without movement. The
furnace was then carefully removed and
the capsule quenched by a directed
stream of water.
The magnesium - gettered casting
(Figure 1) showed no oxide powder a t the
surface or in the interior. This particu-
lar capsule showed a clean separation o f
slurry and bismuth inch from the
base. Chemical analysis (10.5,
11.5,
11.0, 10.1, 5.2,
0.0;
weighted average
7.9y0) confirmed that the slurry segre-
gated upward-i.e., the effective density
of the oxide was less than that of the
bismuth. In Figure 2 the large bal-
loons are gas pockets. Microscopic
inspection revealed many small uranium
dioxide particles clustered about the
periphery. Th e region below the in-
terface was completely free of uranium
dioxide.
The casting from the in
situ
prepara-
tion (Table
11 No.
4 ) revealed no region
Table
II.
Slurry Preparations We re Selected
to
Illustrate Points
O/U
Charge, Grams
c,
~~l~ Heating Condition.
K O u UOZ Biz03 Mg Bi Wt. Ratio Hours c.
1
30.6 ... 20.0
...
182.5 14 1 8 840
2 9.40 ... 12.0 . .. 213.5 4 1.96 1.5 950 1170
2.0 1170 1220
0.5 600 700
1 1000 1090
1 1090. 1140
3 ... 21.6 ... 0.78 213.0 8 1.60 1 875 930
4 18.8 ... 20.5
. . .
195.7 8 1.67
As in
No.
3
5 47.0 ... 51.1 ...
136.9 20 1.67
20 900
6 ... 53.3 ... 1.916 179.8 20 1.60 2 850
22 900
72 600
1 1000 200
1
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NUCLEAR TECHNOLOGY
of uranium freedom after 10 minutes
at 600
C.
(15.4, 10.3, 5.2, 5.3,
4 .9 ,
5.3; weighted average 7 .4%), and no un-
wetted powder. It was clear that the
effective density of the oxide was less
than the bismuth.
The stability may be expected to in-
crease with uran ium concentration, be-
cause of higher viscosity-e.g., a n
l
1,570
in situ
preparation with an oxygen-
uraniu m ratio of 1.67 was heated a t
700 to 900' C. for 32 hours with rocker
agitation and
1
hour at
1200 C.
with
manual shaking. Th e capsule was al-
lowed to cool withou t movement. It
is estimated that the contents were fluid
for 15 minutes. Upo n sectioning, no
unwet oxide \vas found and analysis
showed relatively little segregation (12.2,
12.5, 11.3, 11.2, 10.6; weighted average
11.6%).
Figure 3 is a micrograph of upper and
lower segments of this casting. Th e oxide
particles are 3 to 4 microns. Th e kid-
ney-shaped inclusions have a diamond
hardness of 303 under 50-gram load.
The corresponding value
for
uranium is
200 and for uranium ferride (UeFe)
is 319.
It
is probable that the particles
are uranium ferride, a product of cor-
rosion. Th e lowest segment shows uni-
formity of dispersion.
To identify the appearance of ura-
nium bismuthide, a casting was prepared
containing 11.77, uranium in bismuth
(Figure 4). Th e large crystals have a
hardness of 58 and do not resemble any
formations in the oxide systems.
The largest uranium concentration
achieved in a slurry fluid at 600
C .
has been
20
Lveight
7
(Table
11
5
and
6). After a quiescent period of 92
hours at 600 C. the slurries showed sur-
prisingly little segregation-e.g., 22.2,
25.5, 22.0, 21.1, 19.3, 10.9; weighted
average 19 . 9% . Though still mobile,
these slurries appare ntly are slushlike.
Sodium Addition
The upward seg-
regation of oxide apparently results
from incomplete wetting-i.e., 0