Uranium Oxide Liquid Metal Slurries

7/24/2019 Uranium Oxide Liquid Metal Slurries

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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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NO. 2 FEBRUARY

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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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INDUSTRIAL AND ENGINEERING CHEMISTRY


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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

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Uranium Oxide Liquid Metal Slurries