Note: Descriptions are shown in the official language in which they were submitted.
~ 108~'~43 24-NF-04109
The present invention relates generally to the art of
fonming and sintering ceramic powders and is particularly
concerned with a method for sintering a uranium dioxide
nuclear fuel body having a fugitive binder.
Various materials are used as nuclear fuels for nuclear
reactors including ceramic compounds of uranium, plutonium
and thorium with particularly preferred compounds being
uranium oxide, plutonium oxide, thorium oxide and mixtures
thereof. An especially preferred nuclear fuel for use in
nuclear reactors is uranium dioxide.
Uranium dioxide is produced commercially as a fine, ~ -
fairly porous powder which cannot be used directly as
nuclear fuel. It is not a free-flowing powder but clumps and
agglomerates, making it difficult to pack in reactor tubes
to the aesired density.
The spocific composition of a given commerci~l uranium
dioxide powder may also prevent it from being used directly
a~ a nuclear fuel. Uranium dioxide is an exception to the
law of difinite proportions since "U02" actually denotes a
single, ~table phase that may vary in composition from
U01 7 to U02 25 Because thermal conductivity docre~e~ with
incrieasing 0/U ratios, uranium dioxide having as low an 0/U
ratio as possible i~ preferred. However, since uranium
dioxide powder oxidizes ea~ily in air and absorbs moi~ture
readily, the 0/U ratio of this powder is significantly in
exce~s of that acceptable for fue~.
v A ~ er of method6 have been used to make uranium di-
oxide powder ~uitable a~ a nuclear fuel. Pre~ently, the
most common ~ethod is to die press the powder into cylindri-
eally -sh3ped green bodies of specific size without the a~-
si~tance of fugitive binders since the complete removal of
these binders and their decomposition products i3 difficult
. ~
24_NF-04109
108;~ 3
to achieve prior to sintering. The entrainment of binder
residues is unacceptable in sintered nuclear fuels. Sin-
tering atmospheres may range from about 1000 C to about
2400 C with the particular sintering temperature depeinding
largely on the sintering atmosphere. For example, when wet
hydrogen gas is used as the sintering atmosphere, its water
vapor accelerates the sintering rate thereby allowing the
use of correspondingly lower sintering temperatures such as
a temperature of about 1700 C. The sintering operation is
designed to densify the bodies and bring them down to the
desired 0/U ratio or close to the desired 0/U ratio.
Although uranium dioxide suitable as a nuclear fuel can
have an 0/U ratio ranging from 1.7 to 2.015, as a practical
matter, a ratio of 2,00 and suitably as high as 2 015 has
been used ~ince it can be consistently produced in com-
mercial sintering operations. In some instances, it may
be desirable to maintain the 0/U ratio of the uranium di-
oxide at a level higher than 2 00 at sintering temperature.
For example, it may be more suitable under the particular
manufacturing process to produce a nuclear fuel havinq an
0/U ratio as high as 2 195, and then later treat the sin-
tered product in a reducing atmosphere to obtain the desired
0/U ratio.
One of the principal specifications for uranium dioxide
sintered bodies to be used for a nuclear reactor is their
density. The actual value may vary but in general uranium
dioxide sintered bodies having dengities of the order of
90% to 95~ of theoretical density are specified and occas-
ionally a density as low as 85% of theoretical i8 ~pecified.
Most pressed uranium dioxide powder, however, will ~intorf-to
final densities of about 96% to 98% of theoretical There-
fore, to obtain gintered bodies with lower densities the time
-~ 24_NF_04109
108;~43
and temperature must ~e carefully controlled to allow the
shrinkage of the body to proceed only to the desired value.
Thi~ is inherently more difficult than the use of a process
which i~ allowed to go to completion. Specifically, small
variations during sintering can result in large variations
or no significant variation~ in the sintered body of com-
pacted powder depending on a number of factors such as the
powder chemistry, particle size and agglomeration. Generally,
however, a change in sintering time such as, for example an
hour or two, does not significantly change the density of
the final ~intered product. Also, when sintered bodie~
having the desired low density have been attained by care-
fully controlling sintering time and temperature, it has
~een found that these sintered bodies, when placed in the
reactor, ~requently undergo additional sintering within the
reactor thereby interfering with proper reactor operation.
A number of technique~ have been usod in the past to
reduce the don~ity o~ the ~intered body other than varying
time and t~mperature, For example, one technique has been
to press the uranium dioxide powder, break it up and repress
it. The problem with this technique is that the resulting
sintered body ha~ large interconnecting pores throughout the
body which extend out to the ~urface resulting in a large ~ -
exposed surface area which can absorb into the body signi- -~
ficant amountR of gaæes, and in particular water in the form
of water vapor. ~uring reactor operation these gase~ are
liborated providing a possible source of corrosion for the
fuel cladding. Another method involves adding a plastic of
se~ected particle si2e to the uranium dioxide powder. The
admixed powder is then pres~ed and sintered, however~ the
decomposition of the plastic during sintering usually re~ults
in carbon residues which contaminate the nuclear fuel.
~08~443 24-NF-04109
In Canadian patent No. 1,012,243 dated June 21, 1977
in the name of Kenneth W. Lay and assigned to the same
assignee as the present invention, there is disclosed a
process for controlling the end-point density of a sintered
uranium dioxide nuclear fuel body and the resulting product.
Uranium dioxide powder having a size ranging up to 10 microns
is admixed with a precursor to uranium dioxide, such as
ammonium diuranate, having an average agglomerated particle
size ranging from about 20 microns to 1 millimeter and the
mixture is formed into a pressed compact or green body.
The body of the precursor and the uranium dioxide has a
density lower than that of the uranium dioxide powder and
the precursor is used in an amount which results in
discrete low density regions in the green body which range
from about 5% to 25% by volume of the green body. The
green body is sintered to decompose the precursor and
produce a sintered body having discrete low density porous
regions which reduce the end-point density of the
sintered body by at least 2~. The sintered body has an
end-point density ranging from 85% to 95~ of theoretical.
In Canadian patent application Serial No. 257,625
dated July 23, 1976 and assigned to the same assignee as the
present invention there is disclosed a process for controlling
the final or end-point density of a sintered uranium
dioxide nuclear fueI body by adding ammonium oxalate to a
nuclear fueI material such as uranium dioxide before pressing
into a green body. This addition results in discrete
low density porous regions in the sintered body which
correspond to the ammonlum oxalate particles. ~he end-point
density of the sintered body is, therefore, a function of the
amount of ammonium oxalate added.
-- 4 --
~08'~443 24-NF-04109
As previously mentioned, conventional organic or plastic
binder~ are unsuitable for use in powder fabrication since
they tend to contaminate the interior of the sintered body
with impurities such as hydrides. These binders are normally
B c~ r~
oon~crcd to gases during the sintering step and these gases
must be removed, requiring special apparatus or procedures.
}n addition, upond decomposition, these prior art binder
materials often leave deposits of organic materials in the
equipment utilized to sintered the article, thereby com-
plicating the maintenance procedures for the equipment.
In the sintering process, it is desirable to develop
strong diffusion bonds between the individual particles
without significantly reducing the interconnecting porosity
of the body. The use of organic binders along with normal
compacting pressures and sintering temperatures inhibits the
formation of these strong bonds. The higher compacting
pr-s~ures and sintering temperatures required to develop
such bonds sharply reduce the desired porosity.
There i8 a particular need, therefore, in the art of
preparing sintered bodie~ for nuclear reactors by powder
ceramic technigues for a binder which will impart an ad-
equate degree of green strength without contaminating the
interior of such bodies and which will permit, through
sintering~ the formation of strong bonds between particles
without deleteriously affecting the porosity.
This invention presents the improvement of utilizing a
binder of a compound or its hydration products containing
ammonium cations and anions selected from the group con-
sisting of carbonate anions, bicarbonate anions, carbamate
anions and mixtures of such anions, preferably a binder
selected from the group consisting of ammonium bicarbonate,
ammonium carbonate, ammonium bicarbonate ammonium sesqui_
_ 5 _
~o ~'~ 4~3 24-NF-04109
carbonate, ammonium carbamate and mixtures thereof, in a
powder ceramic process for imparting green strength to
articles cold pressed from nuclear fuel powders of varying
particles size and a particular shape or configuration for
which it i8 de~ired to maintain a certain degree of porosity,
uniformity of pore size, a lack of interconnections between
the pores and the shape or configuration of the base material
particles in the final article after sintering. The binders
disclosed in this invention are efficient ~inders for use in
nuclear fuel~, and further the binders enable the realization
of defect free, pressed bodies of nuclear fuel material~ and
ten~ile strength in the bodies comparable to strengths
achieved with long chain hydrocarbon binders. Further the
binders in this invention leave substantially no impurities
in the nuclear fuel material since these binders decompose
upon heating into ammonia (NH3), carbon dioxide (C02) and
water (H20) (or wat~r vapor) at temperatures as low a~ ~C
The binder addition to nuclear fuel material as presented
in thi~ invention enables the practice of a process for
forming and sintering a body of a nuclear fuel having the
step~ of admixing the nuclear fuel material in particulate
form with the binder, forming the re~ulting mixture into a
green body having a density ranging from about 30X to about
70X of theoretical density of the nuclear fuel material,
heating saia green body to decompose substantially a}l the
hinder into gases, further heating the body to produce a
~ntered body and cooling the sintered body in a controlled
atmosphere.
Thi8 invention also provide a composition of matter that
i~ ~uitable for sintering in the form of a compacted structure
comprising a mixture of a nuclear fuel material and a binder
of a compound or it~ hydration products containing ammonium
108~443 24_NF_04109
cations and ~nions selected from the group consisting of
carbonate anions, bicarbonate anions, carbamate anions and
mixtures of such aniona and preferably a binder selected
from the group consisting of ammonium bicarbonate, ammonium
carbonate and mixtures thereof.
It is an object of this invention to provide an additive
of a binder of a compound or its hydration products con-
tain~ng ammonium cations and anions selected from the group
consisting of carbonate anions, bicarbonate anions, car- -
bamate anions and mixtures to bind the particles of nuclear
fuel into green shapes suitable for sintering.
Another preferred object of this invention i8 to provide
a binder selected from the group consisting of am~onium bicar-
bonate, ammonium carbonate, ammonium bicarbonate carbamate,
ammonium sesquicarbonate, ammonium carbamate or mixtures
thereof as an addition to nuclear fuel materials and which,
upon heating at moderate temperaturos before a sintering
proceJ~, decompo~e into gases and leave sub~tant~ally no
impurities in the ~intered structure of the nuclear fuel
material.
Still another object of this invontion is to provide a
proce~s for ~intering green shapes of a nuclear fuel material
u~ing a binder of a compound or its hydration product con-
taining ammonium cations and anions solected from the group
consisting of carbonate ions, bicarbonate anion~, carbamate
anions and mixtures of such an~ons.
Other o~ject~ and advantages of this invention will
~come apparent ~rom the following ~pecification and the
appended claims.
Figure 1 presents a graph of tensile strengtb versu~
die pressing pressure for one group of green pellets without
a binder and one group of green pellets w~th a binder dis-
iO82~3 24-NF-04109
closed in this invention.
Figures 2 and 3 present photomicrographs (at a mignifi-
cation of 25 and 100 times respectively) of uranium dioxide
pellets produced according to the teachings of Example 2
Figure 4 presents a graph of tensile strength ver~us
die pressing pressure for one group of unsintered pellets
without a binder and one group of unsintered pellets with a
binder di~closed in this invention.
It has now been discovered that a process for sintering
a green body of a nuclear fuel material having high relia-
bility can be achieved by admixing a fugitive binder of a --
compound or its hydration products containing ammonium cations
and anions selected from the group consisting of carbonate
anion~, b$carbonate anions, carbamate anions and mixtures
of ~uch anions with a nuclear fuel material in powder form.
In greater detail the process can be conducted by practicing
the step~ of providing a powder of the nuclear fuel material,
ad~ixing the nuclear fuel material w~th a binder of a com-
pound or lt~ hydration product~ containing ammonium cation~
and anions solected from the group consisting of carbonate
anions, bicarbonate anions, carbamate anions and mixture~
of such anions, forming the resulting mixture into a green
body having a density ranging from about 30X to about 7~X
of theoretical density, he~ting the green body sufficiently
to decompose the ~inder into gases and thereafter heating
the body to produce a sintered body having a controlled
porosity and a controlled density.
The practice of the foregoing process results in the
production of a composition of matter in the form of a
compacted structure suitable for sintering and i8 comprised
of a mixture of a nuclear fuel material and a binder of a
compound or its hydration products containing ammonium cations
10 8 ~ ~ 4 3 24-NF-04109
and anions selected from the group consisting of carbonate - -
anions, bicarbonate anions, carbamate anions and mixtures of
such anions.
As used herein, nuclear fuel material is intended to
cover the various materials used as nuclear fuels for nuclear - -
reactors including ceramic compounds such as oxides of
uranium, plutonium and thorium with particularly preferred
compounds being uranium oxide, plutonium oxide, thorium
oxide and mixtures hereof An especially preferred nuclear
fuel for use in this invention is uranium oxide, particularly
uranium dioxide Further the term nuclear fuel is intended
to cover a mixture of the oxides of plutonium and uranium
and the addition of one or more additives to the nuclear
fuel material such a# gadolinium oxide ~Gd203).
In carrying out the pre~ent processes whieh will be
discussed for the preferred use of uranium dioxide, the
uranium dioxide powder (or partiele~) used generally has an
oxygen to uranium atomie ratio greater than 2.00 and can
range up to 2.25 The size of the uranium dioxide powder
or particles ranges up to about 10 microns and there is no
limit on lower partiele ~ize Such particle sizes allow
the sintering to be carried out within a reasonable length
of time and at temperatures practical for com~ercial
applications. For most applications, to obtain rapid sin-
tering, the uranium dioxide powder has a size ranging up
to about 1 micron. Commercial uranium dioxide powder~ are
preferred and these are of ~mall partiele size, u~ually
sub-micron generally ranging from about 0.02 micron to 0.
mieron.
Compositions suitable for use as a binder in the practiee
of this invention either alone or in mixtures, include am-
monium ~iearbon~te> a~on~um ear~onate, ammonium bicarbonate
~ 82~43 24_NF-04109
carbamate, ammonium sesquicar~onate, ammonium carbamate an~
mixtures thereof. When mixed with nuclear fuel materials,
these binders and the nuclear fuel material are believed to
undergo the phenomenon of adhesion forming ammonium derlva-
tive of the carbonate series such as
4 2 3 3'] (~H4)6 CU2)2 (C03)5 (H20) ~ H O
4 2 ~Uo2(co3)2 (H20) ~,
~H ) ~U2)2 (C3)3 (OH)(H2 5~ ,
~H4 [U02 (C03) ~OH)(~20)3~ and U02C03.~20,or mixtures of these.
In the present invention the binder should have certain
eharacteristies. It must be substantially comprised of a
eompound or its hydration products eontaining ammonium cations
and anions seleeted from the group eonsisting of carbonate
anions, biearbonate an~ons, carbamate anions and mixtures of
sueh anioas and free of impurities so that it ean be mixed
wlth uranium dioxide powder and pressed and sintered without
leaving any unde~ired impurities after heating with particu-
larly preferred binders being ammonium biearbonate and
ammonium earbonate and mixtures thereof. It has been found
that eommereially available ammonium bicarbonate contains
virtually no impurities and eommercially available ammonium
earbonate al~o eontains virtually no impurities exeept for
other ammonium eompounds a~ listed in the foregoing para-
graph. Thermogravimetric analysi~ confirms that there is
a eomplete volatilization of ammonium biearbonate and am-
monium ear~onate at heating rates typieally u~ed for re-
duetive atmospherie U02 sintering. Ammonium biearbonate
~nd ammonium carbonate ~hen heated to the temperature range
of decomposition, decompose to form ammonia, earbon dioxide
and water at signifieant rates leaving substantially no
eontaminates ~impurities) in the fuel and no undesirable
-- 10 --
108~4~3 24_NF_04}09
residues in the sintering furnace. Additionally the ammoni-
um bicarbonate and the ammonium carbonate are used in small
particle sizes of 400 mesh or less in order to achieve
maximum pla~tic flow of the binder into the interstices of
the nuclear fuel material. Ammonium carbonate is used as
the binder when the combination of binding and density
reducing pores is desired in the nuclear fuel. Ammonium
bicarbonate is used as the binder when it is desired to
avoid the formation of density reducing pores in the nuclear
fuel material, The plasticity of ammonium bicarbonate and
ammonium carbonate may be demon~trated by the fact that
these compounds can be die pressed to green densities as -
high as 90X of theoretical density at moderate pressing
pre~sures,
The amount of ~inder added to the nuclear fuel material
generally ranges from about 0.5 to about 7.0 weight percent
depending on the formability of the nuclear fuel material.
For example formable uranium dioxide powders require less of
an addition of the binder while le~s readily formable pow-
ders require larger amount~ of binder. When the selectedbinder i~ ammonium carbonate, the amount of the addition of
this binder is dependerlt upon the de~ired sintered density
for the nuclear fuel material.
Homogenous blending of the binder with the nuclear fuel
material is practiced to develop fully the binding action of
the b~nder on the nuclear fuel material. Where porosity or
a lower density i~ not desired, the homogeneous ~lending
of the binder with the nuclear fuel material avoids the
formation of agglomerates of the binder since ~uch agglo-
merates can volatize durin~ 6intering leaving pores in thesintered nuclear fuel material which pores reduce the density
of the nuclear fuel material in sintered ~odies. When it is
-- 11 --
-~- lO~X~3 24_NF-04109
felt that agglomerates of the binder exist in the nuclear
fuel material after mixing, a milling process such as jet
milling or hammer milling is practiced so that the agglo-
merates are destroyed. The blended and milled powder may
then be predensified by low pressure die pressing followed
by granulation through a sizing screen to flowability of
the mixture.
In order to control the density of sintered bodies of
nuclear fuel material, pore formers such as ammonium oxlate
or a uranium precursor may be added to the nuclear fuel
material along with the binders of this invention. The pore
former can be mixed either at the same time as the binders
di~closed in this invention or during a subsequent mixing
step In the event that the nuclear fuel material, binder
and pore former are mixed and then milled to promote hom-
ogeneity, the processing is conducted to yield an acceptable
particle ~izé~ after milling to as~ure the formation of
poros during sintering.
The ~esulting mixture of nuclear fuel material with the
binders of this invention, wit~30r without pore former, can
be formed into a green body, generally a cylindrical pellet
by a num~er of techniques such a~ pressing (particularly die
pressing) Specifically, the ~ixture i~ compressed into a
form in which it has the required mechanical strength for
handling and which, after sintering, i~ of the ~ize which
satisfies reactor specification. Ths pre~ence of the bindera
of this invention in the nuclear fuel material ~ignificantly
enhances both the strength and integrity of the re~ulting
green body. The green body can have a density ranging from
3~ about 30X to 70~ of theoretical, but usually it has a density
ranging from about ~0% to 60X of theoretical, and preferably
about 50% of theoretical.
- 12 _
1~8~ 3 24_NF-04109
The green body is sintered in an atmosphere which depends
on the particular manufacturing process. Specifically, it
$8 an atmosphere which can be used to sintered uranium
dioxide alone in the production of uranium dioxide nuclear
fuel and a 180 it must be an atmosphere which is compatible
with the gases resulting from the decomposition of ammonium
bicarbonate. For example, a number of atmosphere can be
used such as an inert atmosphere, a reducing atmosphere
(e g dry hydrogen) or a controlled atmosphere comprised of
a mixture of gases (e g. a mixture of hydrogen and carbon
dioxide a~ set forth in U.S. Patent No. 3,872,022 dated
March 18, 1975) which in equilibrium produces a partial
pre~sure of oxygen sufficient to maintain the uranium
dioxide at a desired oxygen to uranium ratio.
The rate of heating to sintering temperature i8 limited
largely by how fa~t the by-produce gase~ are romoved prior
to ach$eving a sintering temperature and generally this
depends on the ga~ flow rate through the furnace and its
uniformity therein as well as the amount of material in the
furnance. Specifically, the rate of flow of gas through
the furnace, which ordinarily i8 ub~ntially the same
gas flow used in the sintering atmosphere, should be suf-
ficient to remove the g~es re~ulting from decomposition of
ammonium bicarbonate before sintering temperature i~ reached.
Generally, best result~ are obtained when the rate of
heating to decompose the binder range~ from about 50C
per hour to about 300 C per hour After decompo~ition of
the binder i8 completed and by-product gases ~ubstantially
D fc~r~ce
IL~ removed from the ~urnancc, the rate of heating can then be
incroasea, if desired, to a rango of about 300 C to 500 C
per hour and as high as 800 C per hour but not be 80 rapid
as to crack the bodies
_ 13 -
24-NF-04109
~08~43
Upon completion of sintering, the sintered body is
usually cooled to room temperature. The rate of cooling
of the sintered body i8 not critical in the present
process, but it should not be so rapid as to crack the
sintered body Specifically, the rate of cooling can be
the same as the cooling rates normally or u~ually used in
commercial sintering furnaces. TheQe cooling rates may
range from 100 C to about 800 C per hour, and generally,
from about 400 C per hour to 600C per hour The sintered
uranium dioxide bodies are preferably cooled in the same
atmosphere in which they were sintered.
This invention provides several advantages in the
sintering of nuclear fuel materials and in the resulting
sintered pellets. The addition of the binders of this
invention, particularly ammonium bicarbonate, ammonium
carbonate, or mixture~ thereof, does not leave any un-
de-irable residuo in the ~intered pellet~. Thermogravi-
metric analy~ ha~ ~hown that ammonium bicarbonate and
ammonium carbonate decompose completely into ammonia (NH3),
carbon dioxide (C02) and water vapor (H20) The early
decomposition of ammonium bicarbonate and ammonium carbonate
prevents the entrapmont of undesirable ga~es in the micro-
structure of the nuclear fuel material during the sintering
process Pellets incorporating ammonium bicarbonate or
ammonium carbonate according to the teachings of thi~ in-
vention can be sintered using conventional wet hydrogen as
a sintering ga~ or controlled atmosphere sintering under
an atmosphere comprising a ~ixture of hydrogen and carbon
dioxide. The proces~ i9 conducted so that the gases from
the decompo~ition of ammonium bicarbonate or amm~nium car_
bonate are excluded from the sintering atmosphere such a~
~y using the countercurrent f~ow of the sintering at-
~ 8~3 24_NF-04109
mosphere in a sintering furnace.
The invention is further illustrated by the following
examples.
Ammonium bicarbonate Wa8 hammer milled to an average
particle size of about 20 microns,
Uranium dioxide having an oxygen to uranium ratio of
about 2.05 and an average particle size of 0,7 microns was
blended with the ammonium bicaxbonate in a ratio of 1.3
grams of ammonium bicarbonate to 98,7 grams of uranium
dioxide, Three thousand gram~ of blended powder were
prepared in this manner,
The blended powders were hammer milled to destroy any
uranium dioxide aggrates in order to as~ure a homogenous
distribution of ammonium bicarbonate in the uranium dioxide
powder,
The hammer milled powder wa~ die pressed at 6700 psi --
to increa~e ~ts bul~ density and the resulting structures
w~re cru~hed through a 12 mesh screen to promote both
flowability and control of the agglomerate size.
The resulting powder was die pre~sed into cylindrical
fuel pellets u~ing pressure~ ranging from 26,000 to 54,000
pBi, As a reference, 2 group~ of fuel pellets were also
die pressed at the ~ame pressures from the original batch
of uranium dioxide powder, One reference group (herein-
after ~roup 11 conta~ned no biDder and received no other
processing prior to pressing. The other reference group
(hereinafter Group 2) also contained no binder but WQS
hammer milled, die pressed, and crushed through a sizing
screen prior to die pressing.
Tensile ~trengths as mea~ured by diametrical com-
pre~sion tests were csnducted on the binder pellets and the
2 reference groups. The binder}es~ reference pellets of
_ 15 -
~08'~443 24-NF-04109
Group 2 were too weak for tensile strength measurements.
The tensile strength vs. die pre~sing pre~sure curves for
the reamining pellets are ~hown in Figure 1. m e pellets
containing binder clearly posses superior tensile strength
for all presQing pressures.
Three hundred and sixty kilograms of uranium dioxide
powder having an O/U ratio of 2.04 and an average part$cle
size of 0.6 microns were mixed with an ammonium bicarbonate
binder following the procedure in Example 1. The powders
were die pressed into cylindrical fuel pellets at a green
d-n-ity ranging from 4.9 to 5.1 grams/cm3 using 0.536
diameter tooling. The green fuel pellets were randomly
loaded 3 1/2 deep in molybdenum sintering boats.
The sintering boats were stoked into a continuous
furnace having an atmosphere of dis~ociated ammonia using a
45 inches/hour push rate and a temperature ri~e of 8C/
minute.
The furnace Wa8 of ~ufficient length to assure a
r-~id-nco timo of 4 hours at the peak sintering temperature
of 1720C for the pellets. The atmosphere was comprised of
dissociated ammonia having a dew point of 67 Centrigrade
One hundred and ninety ft 3/hour of di~ociated ammonla was
introduced at a point 1/3 of the furnace length from-tho
p~llet entry end for impurity removal and another 225 ft3/
hour was introduced at the petlet remova} end of furnace
to provide a clean sintering atmosphere. m e gases were
removed at the pellet entry end of the furnace 80 that
th0 gas flow w~s countorcurrent to the pa~sage of the boats
through the furnace. -
For the s~ntered pellets, the average oxygen to uranium
ratio was 2,003, the average carbon was 7.50 ppm, the
averago hydrogen was .131 ppm, the average nitrogen wa~ 12.89
_ 16 -
24_NF_04109
108'~443
ppm, and the average total outga~ was 3.41 microliters/gram.
Typical photomicrographs of a sectioned pellet, at 25
times and 100 times magnification, are shown in Figure~ 2
and 3 respectively The structure shows a uniform dis-
tribution of fine pores in a uranium dioxide matrix. The
pore sizes are similar to thoss observed in uranium dioxide
pellets fabricated without the assistance of a binder. No
additional pore forming was observed from the amount of
the ammonium ~icarbonate binder used in this Example
~he pellets fabsicated from the binder containing
uranium dioxide were center ground to a desired diameter
with a 96.6X yield of good quality pelletg. In contrast,
another batch of uranium dioxide pellets fabricated by the
same procedure, but without binder, had only a 77X yield
of good quality pellets.
Five thousand grams of uranium dioxide powder having
an oxygen to uranium ratio of 2 04 were placed in a 2 1/2
gallon ru~ber lined ball mill, one-half filled with 3/8"
stainle~s ~teel balls. The powder was dry milled for 6
hours
Reagent grade ammonium carbonate binder was hammer
milled to about 20 microns in particle size The binder
was added to the uranium dioxide in the ball mill and
milled for lS minutes, The ball mill was emptied and the
ballg screened from the binder containing powder
Cylindrical fuel pellets were pressed from the powder
at pressures rang~ng from 15,000 to 29,000 psi. Since the
powder was not die pregsed to increase its ~ul~ density and
the resulting strcutures were cru~hed through a sizing
screen, poor powder flowa~ility regulted, making die pres~-
ing_ difficult. However, good fuel pellets were obtained
during the die pressing. As a reference, another portion
of the same ~atch o~ uranium dioxide powder w~s proces~ed
0 ~ 3 24-NF-04109
through 6 hours of ball milling without the addition of a
binder and die pressed into pellets. This batch of powder
also possessed poor flowability.
Tensile 6trengths as measured by diametral compression
tests were made on the ammonium carbonate containing pellets
and the reference pellet~. The results demonstrated that
the use of ammonium car~onate as a binder significantly in-
creases tensile strengths at all pressing pressures.
The balance of both groups of pellets were sintered in
the furnace according to the procedure described in Example
2 The ~intered pellets fabricated with the ammonium car-
bonate binder yielded sintered theoretical density curves
approximately 2 6% lower than the reference pellets without
the ammonium carbonate binder. Therefore ammonium carbonate
can be used to give a combination of binding action and pore
forming action for tho sintered pellets
The carbon analy~is, total outgas, and 0/U measurement~
on the pellets fabricated with the assistance of ammonium
carbonate were respectively 5 ppm, 3 microliter~/gram, and
2.003 The reference group of pellets without the ammonium
earbonate binder had a carbon eontent of 6 ppm, total out-
gas content of 3 microliters/gram, and an 0/U ration of
2 004 All analyses were conducted the same for both s~ries ~ -
of pellets
As will be apparent to tho~e skilled in the art, various
modif~cations and ~hanges m~y be made in the invention des-
cribed herein. It i8 accordingly the intention that the
invention be con~trued in the broade~t manner within the
6pirit and scope as set forth in the accompany~ nq claims.
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