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MICROCOPY RESOLUTION TEST CHART
NATIONAL BUREAU OF STANDARDS - 1963
.
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.
ORNV P-1726
Conf-651041-2
HOV 1 8 1965
i
i
innud
PREPARATION, COATING, EVALUATION, AND IRRADIATION TESTING OF SOL-GEL
OXIDE MICROSPHERES
R. G. Wymer and J. H. Coobs
September 27, 1965
LEGAL NOTICE
· RELEASED FOR ARUNOURICIONIST
This report was proprred as an account of Government sponsored work. Neither the Unitod
States, nor the Commisolon, nor any person acting on behalf of the Commission:
A. Makes any warrunty or ropresentation, expressed or implied, with respect to the accu-
racy, complotoners, or usefulness of the informatio contained in this report, or that tho use
of any information, apparatu, method, or proceso disclosod in this report may not infringe
privitoly owned righta; or
B. Assumes any liabilities with rospect to the use of, or for damagor resulting from tho
use of any information, apparatus, method, or procevo disclosed in this roport.
As used in the above. "por son acting on beball of the Commission" Includes may on-
ployse or contractor of the Commission, or employee of such contractor, to the extent that
such amployee or contractor of the Commission, or employce of such contractor prepare
disnominatos, or provide. rccosa to, any information pursuant to his employwont or contract
with the Commission, or his employment with such contractor.

IN NUCLEAR SCIENCE ABSTRACTS
OAK RIDGE NATIONAL LABORATORY
Oak Ridge, Tennessee
Operated by
UNION CARBIDE NUCLEAR CORPORATION
for the
U. S. ATOMIC ENERGY COMMISSION
*Research sponsored by the U. S. Atomic Energy Commission under contract with the
Union Carbide Corporation.
A paper to be presented at the Nuclear and Engineering Ceramics Conference to be held
at Harwell, England, October 25-27, 1965.
...
.............
PREPARATION, COATING, EVALUATION, AND IRRADIATION TESTING OF SOL-GEL
OXIDE MICROSPHERES
R. G. Wymer and J. H. Coobs
ABSTRACT
Microspheres of Thoz, PuOz, ThO2-UOZ, ThO2-PuOz, rare earth oxides, and
americium and curium oxides were prepared at ORNL by sol-gel techniques. Thoria,
Tho,-U0g, and UO, microspheres were coated with pyrolytic carbon in fluidized beds
and evaluation has begun.
Sphere forming was accomplished by dispersing sols in 2-ethyl-1-hexanol con-
taining appropriate surfactants or adulterants. Engineering feasibility of microsphere
forming was demonstrated using a variety of sol dispersing techniques.
Sol-gel microspheres, because of their sphericity and uniform size, are well
suited to coating in fluidized beds and have been used extensively in coating studies
:
.
at ORNL. The thermal and chemical stabilities of pyrolytic-carbon-coated Thoz,
en m2
Tho,-U0g, and UO, microspheres were demonstrated by their ability to withstand
severe heat treatments (100 hr at 1900°C in vacuum, or 15 min at temperatures as high
as 2600°C) without failure, provided the coating thickness is over about 65 m.
Irradiation results indicate that carbon-coated oxide microspheres will be
satisfactory fuels for high-temperature, gas-cooled reactors.
PREPARATION, COATING, EVALUATION, AND IRRADIATION TESTING OF SOL-GEL
OXIDE MICROSPHERES
.
R. G. Wymer and J. H. Coobs
INTRODUCTION
A variety of sol-gel methods for the preparation of microspheres and other nuclear
fuel particle shapes have been reported.":4,04 These methods all have in common the
preparation of colloidal dispersions of hydrous oxides, followed by formation of gels and
subsequent sintering to a high density ceramic. The principal advantages of sol-gel
methods over other methods of fabricating ceramics are the relatively simple operations
made possible by the fact that many of the steps involve aqueous solutions and mild con-
ditions, and the attainment of extraordinarily high density in the final product by use of
relatively low sintering temperatures. These advantages lead to simplification in remote
handling methods and seem certain to result in a lowering of fuel element fabrication
costs.
At Oak Ridge National Laboratory by far the largest fraction of the work to date
has been carried out on thoria and thoria-<10% urania fuels. However, a considerable
amount of development has been carried out on urania sol-gel processes for microspheres,
and preliminary work on plutonia, plutonia-urania, and plutonia-thoria microspheres
looks very promising. A small development effort has been carried forward on prepara-
tion of rare earth oxide microspheres by sol-gel methods, and although the results are
highly preliminary, mention wili be made of them. The sol-gel work at ORNL has been
mostly carried out with nitrate salts and solutions as starting materials, and only these
are discussed in this paper.
The compatability of carbon coatings with oxide fuel kernels is a matter of
interest to reactor fuel producers, and some studies have been carried out with thoria
and urania. In addition, a limited number of irradiation studies on some of these
materials are far enough along to warrant their discussion at this time.
THORIA SOL-GEL PROCESS
By way of review the basic chemical flowsheet for the thoria sol-gel process is
presented in Fig. 1. This flowsheet differs from all other sol-gel process flowsheets
in that the basic colloid unit is prepared by steam denitration at temperatures rising
-
-
-
- -
to about 500°C. In the other processes, solutions or wet precipitates of nitrate,
chloride, or formale sc!ts are peptized by anior, removal and pH adjustment. Attempts
-
-
TTT
at ORNL to produce a dispersible U(IV) oxide by high temperature denitration have
been only partially successful. Plutonium and rare earth salts appear to be even more
sensitive to temperature than uranium, producing practically no dispersible oxide unless
temperatures are kept quite low. More will be said later on this point, especially as
:
regards plutonium."
Figure 2 shows the steps used io produce oxide and carbide microspheres of thoria
and thoria-urania. Only a limited amount of development has been carried out on the
formation of carbides,
and this will not be discussed here. The formation of amounts
of microspheres in excess of several tens of grams is carried out in a tapered glass column
re
about 5 ft tall. The system used is shown schematically in Fig. 3. Although several types
of sol dispersing heads have been evaluated, and development is continuing on several
*
*
*
of them, most of the microspheres produced at ORNL have been produced using the two-
ORNL-LR-DWG 74842.31

H20, HNO3
UO2(NO3)2
SOLUTION
(0.2M)
ca
THORIUM NITRATE
SOLUTION
(~2M)
STEAM
DENITRATION
1185-475°C
SOL PREPARATION
BLENDING, 30°C
AMMONIA
ADDITION
Tho2
VOETHO?
Hyo
.
TAL
STEAM
350-450°C
DENSE UO2-ThO2
TO SIZING AND
VIBRATORY
COMPACTION
GELATION
EVAPORATION
CALCINATION
80-35°C
AIR, 300°C/hr TO 1150°C
1150°C, Ihr
ARGON-4% HYDROGEN
UO3-ThO2
1150°C, 4 hr
GEL
ARGON - COOL TO 100°C (DENSITY: ~7kg/liter)
Fig. 1. Schematic Flowsheet for Sol-Gel Process.
p ann
.in
.
wa
y
... ..
.-.-
-.-
-
....
.
.
.
---
.
-
----
:
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-
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-
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.
-
.
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-
.
-
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..
ORNL-DWG 64-2091
CHANNEL BLACK

.
2
OXIDE SOL -
OXIDE-CARBON
SOL PREPARATION
BLENDING
MICROSPHERE FORMING
DRY ORGANIC
DISPERSION IN
ORGANIC SOLVENT -
PARTIAL DEHYDRATION
WET
SOLVENT
SOLVENT
OXIDE
GEL SPHERES
LOXIDE-CARBON
GEL
CONVERSION -
SINTERING
CALCINATION
ThO2-AIR, 1150°C, 4 hr
ThO2-U02-ARGON-4% H2
1150°C, 4 hr
VACUUM, 1750°C, 1 hr
DENSE
ThO2 OR ThO2-UO2
MICROSPHERES
THORIUM OR THORIUM-URANIUM
CARBIDE MICROSPHERES
Fig. 2. Steps in the Preparation o' Thoria and Thoria Dicorbide Microspheres.
.ORNL DWG 61-824E11

TWO-FLUID NOZZLE
AQUEOUS SOL FROM
SYRINGE PUMP
TO DISTILLATION
SYSTEM
6-in. DIAM -
12 in.
3-in. DIAM -
0-0.2 GPM
0-3 GPM
40 in.
CENTRIFUGAL
PUMP
EU CUNO
FILTER
FROM
DISTILLATION SYSTEM
UL
2-in. DIAM
3-in. DIAM
to
& in
in.
6 in.
71
ORGANIC SOLVENT ENTERS
COLUMN TANGENTIALLY

COLLECT GEL
SPHERES
Fig. 3. Schematic Flow Diagram of Equipment for Forming Gel Microspheres.
fluid nozzle shown in Fig. 4. Table 1 lists some of the organic drying liquids which
have been used and some of the surfactants found to be effective in preventing
particle clustering and sticking to the column wal's and in preventing formation of
misshapen particles. In general, the amount of surfactant used is in the range 0.1 to
0.5 vol % in the organic drying liquid.
Table 1. Drying Liquids and Surfactants Used in Forming Thoria Microspheros
Drying Liquids
2-ethyl-1-hexanol
2-methyl-1-pentanol
Surfactants
Ethomeen S/15 (a tertiary amine)
Span 80 (sorbitan monooleate)
Paraplex G-62 (plasticizer in polyvinylchloride plastic)
Typical product microspheres are shown in Fig. 5. From these figures an idea
of the shrinkage may be obtained.
Typical size yield data obtained for thoria microspheres are shown in Table 2.
Results obtained by pouring sol into a beaker of stirred 2-ethyl-1-hexanol are shown
for comparison.
ORNL OWG. 64.8759

-
AQUEOUS SOL
ORGANIC
0.010-0.030 in. ID
0.005-0.010 in. WALL THICKNESS
- 1/8-3/16
in. 10
10 in.
Fig. 4. Two-Flui: Nozzle for Forming Sol Droplets,


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Fig. 5. Gel and 1150*C Sintered 8% Uroniu-Thoria Microsphoras.

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Table 2. Size Yields for Thoria Microspheres
94 hr run; 20.2 kg Thon
Two-fiuid Nozzle: (wt % in size range)
<1504
210-250 u
250-297 y.
150-210 u.
17.3
9.9
50.8
22.0
Stirred System: (wt % in size ronge)
Relative Stirring Speed <150 M
150-210 u
250 g
210-250 -
20.7
330
31.8
24.0
23.5
350
34.2
27.4
22.8
15.6
370
42.5
35,0
21.2
1,3
URANIA SOL-GEL PROCESS
The sols used in the preparation of Vo, microspheres by the sol-gel technique
were prepared by precipitation of hydrous U(IV) oxide and its subsequent dispersion.
A flowsheet for the laboratory preparation of U(IV) sol is given in Fig. 6. Sol prepa-
ration begins with the preparation of a uranous nitrate solution by the catalytic re-
duction with hydrogen of a uranyl nitrate solution containing excess nitric acid. The
solution is filtered to remove the catalyst. Formic acid is added to the filtered solu-
tion. The hydrous oxide is formed by precipitating U(IV) with an NH, OH solution
containing hydroxylamine. The precipitated hydrous oxide is collected by filtration
1:
-.,
ORNL DWG. 65-9845
.
---.-

.
REDUCTION
.-
es ...
.....
2520 ml SOLUTION
0.5 M VOZ(NO3)2 - 1.3 M HNOZ
0.26 MUREA
30-60 mg PD CATALYST
...-
*
-
.-
_
PO CATALYSTS
FILTRATION
U(NO3)2 SOLUTION
CONC. FORMIC ACID

FORMIC ACID ADDITION
SOLUTION MADE 0.3 M IN FORMIC ACID
PRECIPITATION
3.0 M NH OH
0.5 M N244H20
U(IV) HYDROUS CXIDE
SUSPENSION
1- ? LITERS H, O

FILTRATION AND WASHING
24 cm dia. BUCHNER FUNNEL
WITH NO. 42 WHATMAN PAPER
-
hombo WASTE ELECTROLYTE
-
-
-
UO, FILTER CAKE
-
-
-

SOL FORMATION
HEAT AT 60-65°C AND STIR
UO, SOL
Fig. 6. Flowsheet for the 10.3 bratory Preparation of Urania Sol.
and washed to remove excess electrolyte. The washed filter cake is then heated at 60
to 65°C to produce a fluid, stable sol. Precautions are taken to protect the material
from air oxidation by the use of a blanketing gas (argon) during all stages of the process.
Because it was desirable to work with enriched uranium, the sol preparations were care :
ried out batchwise using 300 g of uranium in each preparation. Thus, several batches
could be worked with concurrently in the same area without fear of criticality incidents.
Sol concentrations of 1.3 to 1.7 MUO, are achieved. Formic acid is used in the prepa-
ration because higher concentrations of U(IV) (about 90% of total U) permit higher sol
concentrations, and because better filtration rates are obtained..
To form microspheres, urania sol is dispersed into dropluis at the top of the column
shown earlier, and the droplets fall down through the countercurrently flowing 2-ethyl-
-- -
---
1-hexanol. As the droplets fall, water is extracted until finally sufficient water has
-
--
-
-
been removed and the drops are set to gel microspheres. Two organic surfactants,
-
-
.. -
Amine "O" and Span "80", are added to the organic solvent to prevent spheres sticking
.
.
-.•-
---
--
to the column wall and to prevent "clustering" of spheres. 'Each of the surfactants is
-
...
-
present to the extent of about 0.5 vol %. The spheres settle down to a product col-
-
-
-
-
-
lection vessel at the bottom of the column, where they are collected batchwise.
-
Before firing to final densification at high temperature, the microspheres are
dried at a low temperature (below 200°C) to distill out most of the alcohol and water.
With uranium sols, this column routinely produces yields of 90% of the calcined
microspheres from a 300-g batch in the 125- to 177-p-diam size. In this 125 to 177 u
4.
-.
*
*
;
.
fraction about 3/4 of the spheres (by weight) are of 149- to 177–p size. With urania,
Luania,
ORNL microsphere forming experience has primarily been in the 50- to 250--diam
size range.
2
-
T
V
.
Figure 7 shows typical urania microspheres. The bright spots are light reflections
my
-
from the surfaces of the spheres. Polished sections of these same spheres are shown in
Fig. 8. Although it doesn't appear at this magnification, there is a uniformly distri-
buted microporosity in these spheres which tends to coalesce to form macropores at
about 1700°C.
One characteristic of the urania microspheres formed by this technique merits
special mention. That is that the carbon content of the UO, spheres is often high-
i.e., of the order of several thousand parts per million. Of course, if the next step is
coating the microspheres with pyrolytically deposited carbon, then the presence of
carbon in the sphere is of no concern. Table 3 shows some typical product analyses.
iHeating the spheres in co, removes carbon, but leads to a loss in average particle
strength, as shown in the table.
Fig. 7. Typical vo, Microspheres (Diam = 125-177 m)
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Table 3. Sintering Data for 125- to 177--Diameter UO, Microspheres
Sintered in Hg at indicated temperature for 4 hr
Sintering:
., Temp.
(°C)
Heat-up
Schedule
Wt Loss on
Sintering Gel
(W+ %)
Wt. Req'd
,to Crush
Carbon
Content
(opm)
Product
Density
(% of theor.)
1150
6.9
871
4600
97.8
1250
1050
4900
97.6
b
1150
10.2
354
460
~100
1250
313
200
~100
b
1250
9.4
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<100
~100
Q. Hy to sintering temperature at 300°C/hr
b. CO2 to 450°C at 50°C/hr; CO2 to 600°C at 25°C/hr; Co, to 950°C at 50°C/hr;
.Hj to sintering temperature at 300°C/hr
.
PLUTONIA SOL-GEL PROCESS
The development of a plutonia sol-gel process, while preliminary, is promising.
..
The sols used in the preparation of Puo, microspheres by the sol-gel technique were
made by a method which is a rather significant departure from the techniques used for
both thoria and urania. In early studies of the preparation of plutonia sols it became :
apparent that quite careful control of temperature and other process variables is re-
quired. The chemical flowsheet for plutonia sol preparation is shown in Fig. 9.

-
---
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wire 09-4843
:
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.
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:
.
.....
Pu(NO3)3 10-20 g/liter
1-2 M HNOZ
1-4 MOLES HNO,
PER MOLE Pu 3
PRECIPITATION
100% EXCESS OF
2 M NH, OH
WASHING
WATER WASHING TO
FILTRATE pH of 7-8
PEPTIZATION
DIGESTION AT 80°C
TO COLOR END POINT
(LIGHT GREEN TO VERY
DARK GREEN)
RESUSPENSION
WATER ADDITION
WITH AGITATION
NO, REMOVAL
HEAT TO DRY NESS;
BAKE AT 120-200°C
PLUTONIUM SOL
Pu CONC. 1-3 M
(NO, CONC. 0.1-0.4 MOLÉS NOZ/MOLE Pu)
Fig. 9. Flowsheet for the Laboratory Preparation of Plutonio Sol.
Dilute plutonium nitrate solution (10 to 20 g/liter), which contains 1 to 2 M
excess HNOz, is precipitated by dropwise addition of the plutonium solution into a
100% excess of 2 M NH OH with rapid agitation.
While other procipitation methods can be used, good results have been ob-
tained with this method; and it is important that the plutonium be neutralized
rapidly.
Essentially complete removal of contaminant nitrate and ammonium ions is
tate, the filter cake is suspended in water and refiltered. Three washings are usually
sufficient to reduce the pH of the filtrate to less than 8.0, which indicates satis-
factory contaminant ion removal. Filtration is readily accomplished by vacuum,
filtration with a medium frit glass filter. The filter cake cannot be allowed to dry
'since the dried hydroxide will not peptize.
Peptization
The washed plutonium hydroxide can be peptized by digestion at 70 to 80°C
with dilute HNO3. Complete peptization is indicated by a color change from light
green to a nearly transparent dark green.
It has been established by spectrophotometric anaiysis that the resultant sol
consists entirely of Pu(IV) polymer, and it is thought that the degree of polymeriza-
tion can be controlled by process variables such as concentration, time, temperature,
and nitrate concentration.
The minimum nitrate concentration necessary for complete peptization is one
mole of HNO, per mole of plutonium. At this concentration, a digestion time of
about 4 hr at 80°C is required. Recent work indicates that it may be desirable to
add more than the minimum nitrate concentration during peptization. Concentra-
tions as high as 4 moles of HNO, per mole of plutonium can be used. At nitrate/Pu
mole ratios of 2 or more, the digestion time is reduced to 10 to 15 min. When a
nitrate/Pu mole ratio of 1 is used, the maximum plutonium concentration obtainable
in the final sol is about 1 M; with a nitrate/Pu mole ratio of 2.5, sol concentrations
as high as 3.2 M have been obtained.
The plutonium polymer solution prepared in this step is a true sol; however,
this material is not suitable for fuel particle preparation. When such sols are taken
to dryness and calcined, friable, low-density oxide results; also, attempts to form
microspheres from such sols have not been successful. In addition, the high nitrate
concentration is nor conducive to successful mixing with other actinide sols to pro-
duce dense, strong oxides.
Removal of the excess nitrate ion is accomplished by baking the dried polymer
solution. This step is critical, since excessive baking results in material which can-
not be redispersed. In all preparations, attempts are made to bake until all but a
small fraction of the solids can be redispersed. This is taken as the minimum nitrate
obtainable for a given sol preparation and varies from a mole ratio of 0.1 to 0.4,
depending on precise conditions of the previous steps.
10
At present it is not possible to pre determine the exact temperature and baking
time required for a given preparation, and a series of successive baking and suspension
steps are necessary. In general, baking times of 8 to 12 hr are necessary at 120°C;
baking times of only 30 to 60 min are required at 200°C.
The final sol is prepared by resuspending the baked solid in excess water and
evaporating to the desired plutonium concentration. Plutonium concentrations in ex-
cess of 3 M can now be achieved. Good sol stability is obtained when the residual
nitrate concentration does not exceed 0.4 moles of nitrate per mole of plutonium.
While nitrate/plutonium mole ratios of slightly less than 0.1 have been obtained in
some preparations, ratios between 0.2 and 0.3 are typical.
The appearance of thoria and plutonia sois and their mixture is shown in Fig. 10.
It is evident from this figure that the sols are compatible with each other. Perhaps an
idea of the present scale of this work is also conveyed by the figure. Plutonia sols
are also compatible with urania sols. There is no evidence that mixed sols cannot be
made in all ratios of PuOg to Tho, and PuOy to Uog. The final products in both
cases are hard, dense materials. Figure 11 shows PuO, microspheres before and after
sintering at 1150°C in air. Figure 12 shows a 10% PuO.-90% Tho, mixture before
and after sintering in air at 1150°C; and Fig. 13 shows a 25% PuO, -75% UO, mixture
before and after sintering in hydrogen at 1150°C.
Although these materials have not been characterized, densities determined by
toluene immersion show them to be greater than 95% of theoretical density. The entire
composition range has been investigated for microsphere formation of thoria-plutonia,
and plutonia-urania mixtures up to about a 1/1 mole ratio have been explored in
prelimincry work.

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We have also prepared americium and curium-244 microspheres by sol-gel
techniques. Curium-244 poses a special problem in that radiolytic gas formation is
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tharu
Considerations of utilizing sol-gel microspheres as oxides rather converting them to
carbides led to a series of coating and compatibility experiments, first with Tho, and
finally with fueled microspheres.
.
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respect to behavior in a fluidized-bed coater. Early coating experiments established
that uniform coating could readily be deposited on the spherical particles under a
variety of coating conditions. The results of an extensive study of the properties of
coatings deposited on UC, from methane' were found to be directly applicable in
.

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12
to oxide microspheres, with but one limitation. Direct application of coatings on
bare ThO2, (Th, U)O, or UO, particles, or on oxide particles having a porous
coating layer deposited from acetylene must be carried out at temperatures of
1600°C or below to avoid conversion of a portion of the oxide to carbide. However,
coatings deposited from methane at these lower temperatures are generally favored
for inner coatings because they are low in density. And after deposition of such a
layer of pyrolytic carbon, which is quite impermeable although its density may be
as low as 1,4 g/cm", a high-density outer layer may be deposited at temperatures
as high as 2000°C without difficulty. This means that oxide fuel particles may be
readily coated with two- or three-layer pyrolytic carbon coatings that have char-
acteristics virtually identical to those which performed successfully on carbide
particles, 10
STABILITY OF PYROLYTIC-CARBON-COATED OXIDES AT HIGH TEMPERATURES
Successful deposition of pyrolytic-carbon coatings on oxide particles did not
remove the serious concern about the basic thermodynamic instability of carbon with
oxides at high temperatures. That this is a very real concern was emphasized by the
reported catastrophic failure at 1800°C of porous UO, particles coated with dense
pyrolytic carbon deposited from acetylene." Therefore, a program was initiated at
ORNL to determine the thermal and in adiation stability of coated oxide fuel particles
under conditions likely to be encountered by HTGR fuel elements during fabrication
and operation.
13
Thoria microspheres coated with various thicknesses of carbon deposited from
methane at 1400°C or from acetylene at 900 and 1100°C were used in the thermal
treatments. Individual batches of the 150 to 250--diam microspheres had coating
thicknesses ranging from about 15 u up to greater than 200 h. Samples from 21
different batches were heat treated in vacuum (10 torr) at 1100°C for 1000 hr,
at 1400°C for 200 hr, at 1800°C for 4 hr, at 2000°C for 50 hr, and at 2400 and
2700°C for 2 min. Other samples were heated in one atmosphere of argon at 2000°C
for up to 100 hr and at 2200°C for 2 hr. The results were evaluated by microradiog-
raphy, metallography, x-ray diffraction, and electron-microprobe analysis."
All samples survived the heat treatments at 1100 and 1400°C without evidence
of reaction between coatings and particles or of heavy metal migration. Those batches
having minimum coating thicknesses of 30 u or greater survived the 1800°C heat treat-
ment, and all batches having coatings 65 u or thicker survived even the most severe
heat treatments. At these temperatures rupture of the coatings occurred with some
particles in those batches having relatively thin low-density coatings and was followed
by complete conversion of the particles to carbide.
These results indicate that coating thickness is an important factor in determining
compatibility at very high temperatures. Apparently, coatings thicker than about 60 u
are required in order to withstand the equilibrium CO pressures developed at tempera-
above
tures about 1800°C by the reaction metal oxide + carbon - > metal carbide + carbon
monoxide. Approximately 100-w-thick coatings are now generally accepted as neces-
sary for successful irradiation performance of coated carbides. This indicates that the
14
.
:
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.
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•
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• • •
•
•
pressure developed within the coated particle by accumulated fission gases is much
greater than the equilibrium pressu, e of CO at 2000°C, and that at normal irradia-
•
•
• •
•
•
•
•
tion temperatures (<1500°C) the equilibrium pressure of CO would not contribute
significantly to the total internal pressure.
It is considered most significant that no migration of thorium was observed
by microradiography in any intact coatings, regardless of the time and temperature
employed. This is illustrated in Fig. 15, which shows microradiographs of a batch
having an average coating thickness of 120 u after heat treatment at 2000°C for
2 hr and 2700°C for 2 min. In both cases a gap was formed between particle and
coating by anisotropic growth of the low-density coating, and some high-density
material was present in this gap. However, no other effect of the heat treatment
may be detected.
The high-density material frequently observed in the gap was examined in
greater detail in another sample from this same batch that had been heated for 100
hr at 2000°C in vacuo. The as-polished microstructure shown in Fig. 16 exhibits
this material as translucent, angulor particles. The material was identified by
x-ray diffraction and electron microprobe analysis as single crystals of thoria.
1 111111 11141* 2411 111111
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Fig. 16.
Appearance of Pyrolytic-Carbon-Coated Thoria Particles After Heat Treatment for 100 hr at
2000°C in Vacuo. (a) A microradiograph of several particles. (b) An as-polished section of
a coated particle showing high-density material within the gap.
PERFORMANCE OF COATED-OXIDES UNDER IRRADIATION
After successful application on oxide microspheres of two- and three-layer
coatings having favorable characteristics, and demonstration of the high-temperature
stability of such coated particles, several irradiation tests involving coated oxides
were initiated. Sweep-capsule experiments are favored for this testing because a:
continuous account of the behavior of the test specimen is provided by analysis of
helium that is passed through the capsule. This gas is monitored continuously for
total activity and sampled periodically for fission-gas analysis." To date three
batches of carbon-coated sol-gel oxide microspheres have been tested in sweep or
static capsule experiments and one of these batches plus a batch of coated thoria
was also teured in a helium-cooled loop experiment. The characteristics of the
coatings on these four batches of particles are given in Table 4, and the test condi-
tions and results of the irradiation experiments are summarized in Table 5.
Sol-gel (Th, U)O, particles containing 8% UO, and coated with a two-layer
.
coating performed well at 1200°C to a low burnup in sweep-capsule experiment
B9-19. No failed particles were found during postirradiation examination, but some
cracking of the inner layer of the coating was revealed. In a few cases these cracks
had penetrated to the inner layer, as shown in Fig. 17, but none of the fractures ex-
tended into the outer layer. Similar behavior was observed with an earlier batch
(OR-182), which was also irradiated to low burnup at 1200°C in a static capsule,
as shown in Fig. 18a.
Table 4 Properties of Coatings Deposited on Sol- Gel Oxide Microspheres
Sample Designation
OR-182
(Th, U1O2?
OR-205 OR-206 HB-23
Thoz (Th,U)O, uo,
Type of fuel particle
Average fuel particle
diameter, u
217
243
206
148
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First Layer
Coating temperature, °C
CH, flow rate, cm min-'cm-2
Coating thickness, u .
Second Layer
Coating temperature, °C
CH, flow rate, cm min-'cm
Coating thickness, u
Third Layer
Coating temperature, °C
3 .-1 -2
CH, flow rate, cmºmincm
Coating thickness, i
1800
0.33
42
0.17
0.17
1.20
78
65
20
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-
1900
0.05
60
"Particles contain 8 wt % UO,, highly enriched.
Samoin sünismo--JEJ
Acetylene was used for inner coating on HB-23.
-
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.
Table 5. Irradiation Test Conditions and Fission-Ges Release Datu for Unsupported
Coated Sol-Ge! Oxide Microspheres
Sample
Experiment Designation
Type of Fuel
Particle
Coating
Structure
Test Burnup
Temp. (At. % heavy
(°C) metal
R/B for
88%
B9-19
OR-206
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1200
+370
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Fig. 17.
Duplex-Coated (Th,U)O, Particles from Batch OR-206. (a) Unirradiated. (b) Irradiated to
a burnup of 0.6 at % heavy metal at 1260°C in experiment B9-19. As polished.

R-26040
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Fig. 18. Duplex-Coated (Th,U)O, Particles after Irradiation. (a) Particles from batch OR-182 irradiated
to 0.25 at % heavy metal burnup in experiment 01-8. (b) Particles from batch OR-206 irradiated
to 2.7 at % heavy metal burnup in ORR Loop 1, experiment 14. As polished.
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Much higher burnup of the (Th, U10, particles was achieved during an 8-month
irradiation at 1370°C in the ORR loop No. 1 facility.' In this experiment a 50-g
sample of coated particles from the same batch tested in B9-19 were mixed with coated
Tho, particles and irradiated to 2.7 at. % burnup of heavy metal, which corresponds to
44% consumption of ºu. The loop was cooled with a mixture of helium and neon,
which was replaced at irregular intervals as the concentrations of contaminant gases
(CO and CO2) became excessive. Equilibrium concentrations of fission-product gases
in the coolant were used to calculate the fractional release, which increased by about
a factor of two during the experiment. Although some few particles showed evidence
of a pitting type of attack from reaction with contaminants, their behavior during the
experiment was quite satisfactory, and metallographic examination of a sample of the
particles showed very little damage. The particle shown in Fig. 18b is typical of the
sample, although some particles showed more damage to the inner portion of the first
coating.
coated particles
The most significant irradiation of oxide fuels is the recent successful sweep-
capsule experiment at 1600°C. The small (150-diam) UO, microspheres were
deliberately coated with a thick three-layer coating for use in a high-burnup experi-
ment at 1400°C and were tested in the first high-temperature sweep-capsule experi-
ment to a burnup of 25 at. % U as well. The behavior of these particles was excellent,
as shown in Fig. 19. No detectable damage to the coatings and only moderate swelling
of the vo, microspheres were observed. In this latter respect, coated oxide fuels be-
have quite favorabl... whereas, carbide fuel particles would have swelled and virtually
consumed the inner layer of the coating after similar exposure.
R-26041
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Fig. 19. Triplex-Coated Sol-Gel UO 2 Particles from Batch HB-23. (a) Unirradiated. (b) Irradiated
to 25 at % heavy metal burnup. at 1600°C in experiment C1-17. As polished.
17
.
The fractional release of rare-gas fission products during this sweep experiment
was most impressive. As seen from the data plotted in Fig. 20, the fractional release
was about an order of magnitude lower than from other experiments on coated oxides,
and more than two orders of magnitude lower than from the best pyrolytic-carbon-
coated carbide fuels. Since the observed release during irradiation can be attributed
to contamination of the coatings with fuel,'' these results reflect the greater thermal
stability of coated oxide particles. Typically, coated carbides have much higher
contamination levels in the coatings due to fuel migration during deposition; further-
more, the contamination level (and the associated release rate of fission-product
gases) of coated carbides would normally increase during irradiation at high tempera-
tures due to continued movement of fuel into the coating.
From the results of these experiments we conclude that carbon-coated oxide
fuel microspheres will perform very favorably in comparison with coated carbide
particles. The oxide fuel particles do not attack the coating during irradiation and
retain their general shape even after irradiation to high burnup. In addition, oxide
fuel particles present no special handling problems and may be coated with high-
density coatings at high temperatures without significant contamination of the
coating
ACKNOWLEDGEMENTS
The authors wish to acknowledge the contributions of their many co-workers in
the Chemical Technology and Metals and Ceramics Divisions of Oak Ridge National
Laboratory whose work is included here.
ORNI. DWG. 65.10308
tone...
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TTTTTTT
UNCOATED UC, (-4 a/0 BURNUP OF U)'
815°C
.
10-2
annov ...
.--.-
-..-
R/B, RATIO OF RELEASE RATE TO BIRTH RATE
TRIPLEX COATED UC)
(18.7 a/. BURNUP OF ū)
137000
:
ILLLII
..
.
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DUPLEX COATED UC2
(18.8 a/0 BURNUP OF U)
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DUPLEX COATED (Th,U)O,
al 10.6 0/0 BURNUP OF HEAVY METAL)
10-6.10.6.
TRIPLEX COATED UO
10.7
Lull
(25 a/BURNUP OF Ú
,
000°c
C
85 mkri...
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135
: 8887
10-8WLLLLLL
y
106
LLLLL
405
HALF LIF
104
100
.
.
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.
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•
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.
..
....
Fig. 20. R/8 vs Half Life for Release of Inert Gases from Pyrolytic-Carbon-
Coated Fuel Particles..
REFERENCES
1. R. G. Wymer and D. A. Douglas, Status and Progress Report for Thorium
Fuel Cycle Development for Period Ending Dec. 31, 1963, ORNL-361T
(July, 1965).
2. T. Amanuma and Kaoru Naruki, "Adaptation of the Sol-Gel Process for
the Preparation of Massive VO2 of 97 to 99% Theoretical Density," Report
on the International Conference on Beryllium Oxide and on Visits to Nuclear
Research Facilities of Australia and Japan, ORNL-64-1-63 (1963).
3. M. E. A. Hermans and H. S. G. Slooten, "Preparation of UO, and Tho,
Powders in the Subsieve Range," Third United Nations International Con-
ference on the Peaceful Uses of Atomic Energy, A/Conf. 28/P/634 (1964).
4. G. Cogliati, R. De Leone, G. R. Guidotti, R. Lanz, L. Lorenzini, E. Mezi,
and G. Scibona, "The Preparation of Dense Particles of Thorium and Uranium
Oxide," Third United Nations International Conference on the Peaceful Uses
of Atomic Energy, A/Conf. 287P7555 (1964).
*
5. J. P. McBride, Preparation of UO, Microspheres by a Sol-Gel Technique,
ORNL-3874 (in press).
6. James L. Kelly, A. Todd Kleinsteuber, Sam D. Clinton, and Orlen C. Dean,
"Sol-Gel Process for Preparing Spheroidal Porticles of the Dicarbides of
Thorium and Thorium-Uranium Mixtures," I&EC Process Design and Develop-
ment, 4, 212-6 (1965).
7. J. B. Sayers, K. S. 6. Rose, J. H. Coobs, G. Hauser, and C. Vivante, "The
Irradiation Behavior of Coated Particle Fuel," pp 919-959 in "Carbides in
Nuclear Energy," MacMillan and Co., London (1964).
8. E. L. Long et al., "Postirradiation Examination of Fueled Graphite Spheres and
Coated Particles, "pp 56-79 in GCRP Semiann. Prog. Rep., March 31, 1965
ORNL-3807.
9. R. L. Beatty, F. L. Carlsen, and J. L. Cook, "Pyrolytic Carbon Cootings on
Ceramic Fuel Particles, " to be published in Nuclear Applications.
..
10. R. L. Beatty, "Pyrolytic-Carbon Coating Studies," pp 3-8, Gas-Cooled Reactor
Program Semiannual Progress Report, Sept. 30, 1964, ORNL-3731.
...-. ri40p, noe ovd's.--elves
:
"
.
do en medicin
11. A. Auriol, C. David, G. Kurka, and E. LeBoulbin, "Carbon-Coated Particles & You
Dispersion Fuels," pp 462-480 in Ceramic-Matrix Fuels Containing Coated
Particles, TID-7654.
a
teres
n
:::..
i

.
.
12. R. L. Hamner, R. L. Beatty, and J. L. Cook, "Effects of Heat Treatment on
Pyrolytic-Carbon-Coated Oxice Particles," pp 12-20 in GCRP Semiann.
Prog. Rep., March 31, 1965, CPNL-3807.
13. P. E. Reagan, F. L. Carlsen, and R. M. Carroll, "Fission-Gas Release from
Pyrolytic-Carbon-Coated Fuel Particles During irradiation, " Nucl. Sci.
Engr. 18, No. 3, 301-308 (1961).
:
S
14. D. B. Trauger, Some Major Fuel-Irradiation Test Facilities at the Oak Ridge
National Laboratory, April 1964, ORNL-3574.
15. P. E. Reagan, R. M. Carroll, T. W. Fulton, and J. G. Morgan, "Instantaneous
Fission-Gas-Release Experiments with Coated particles," pp 84-90 in GCRP
Semiann. Prog. Rep., March 31, 1964, ORNL-3619.
16. R. W. Dayton, W. V. Goeddel, and W. O. Harms, "Ceramic Coated-Particle
Nuclear Fuels," Third United Nations International Conference on the Peace-
ful Uses of Atomic Energy, A/Conf. 28/P/235 (1964).
.
.
.
llet
.
.
12
END
DATE FILMED
12/ 23/ 65







.