Note: Descriptions are shown in the official language in which they were submitted.
1083336 24 NF 04079
The present invention relates generally to the
art of sintering ceramic powders and more particularly
is concerned with a method for controlling the end-point
density of a sintered uranium dioxide nuclear fuel~ -
body.
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 esp~cially 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. The specific composition of certain
commercial uranium dioxide powders also prevents uranium
dioxide from being used directly a~ a nuclear fuel.
Uranium dioxide is one of the exceptions to the law of
definite proportions since the term "UO2" is generally
used to denote a single, stable phase that actually
varies in composition from U02.00 to U02 25. Because
thermal conductivity decrease~ with increasing 0/U
ratios, uranium dioxide having as low an 0/U ratio as
possible is preferred. However, since uranium dioxide powder
oxidizes easily in air and absorbs moisture readily, the
O/U ratio of the fine powder i~ si~nificantly in excess
of that acceptable for fuel.
A number of methods have been used to make
uranium dioxide powder suitable as a nuclear fuel.
Presently, the most common method i8 to press the powder
into cylindrically-shaped green bodies of specific size
which are sintered in a suitable sintering atmosphere at
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1083336 24 NF 04075
a temperature which can range from about 1000c to 2400c
with the particular sintering temperature depending largely
on the fineness of the powder and the composition of the
sintering atmosphere. For example, when wet hydrogen
gas is used as the sintering atmosphere, a sintering
temperature in the range of 1600C to 1800C is preferred.
When a controlled oxidiæing atmosphere is used for sintering
(as described in U. S. Patent No. 3,872,022 - Hollander
et al - March 18, 1975) a temperature in the range of
900-1500C is desirable. The sintering operation is `
designed to densify the bodies and bring them within
the proper O/U ratio.
Uranium dioxide suitable as a nuclear fuel
can have an O/U ratio ranging from about 2.0 to 2.015, and,
as a practical matter, uranium dioxide can be consistently
produced in this range in commercial sintering operations.
In some instances, it may be desirable to maintain the
O/U ratio of the uranium dioxide at a level appreciably
higher than 2.00 at sintering temperature depending largely
upon the particular manufacturing process. For example,
it may be more suitable under the particular manufacturing
process to produce a nuclear fuel having an o/U ratio a~
high as 2.195, and then later treat the sintered product -
in a reducing atmosphere to obtain the desired O/U ratio.
One of the principal specifications for uranium
dioxide sintered bodies to be used as fuel for a nuclear
reactor is density. The actual density may vary but
in general uranium dioxide sintered bodies having
densities of the order of 90% to 95% of theoretical
30 ~ density and occasionally a ~o~nlty a~ low as 85% of
theoretical. Most pres~ed uranium dioxide powders,
however, will sinter to final densities of about 96
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24 NF 040~9
1083336
to 98% of theoretical. Therefore, to ob~ain si~tered
bodies with lower densities, preferably in the range of
94 to 97% of theoretical density, the ~intering time
and temperature mu3t be carefully controlled to allow
the shrinkage of the body to proceed only to the desired
density. This is inherently more dif~icult than the use
of a process which goes to completion, and ~pecifically,
small variations during sintering can result in large
variations in the sintered body of compacted powder.
Some other variations in powder properties, such
as particle size and state of agglomeration also affect
the density of the sintered body. It has been found
that when sintered bodies having the desired density have
been obtained by carefully controlling sintering time
~eoc;7
and temperature, and thesa are placed in a crc~ctor,
these bodies frequently undergo additional densification
within the reactor thereby interfering with proper reactor
opsration.
A number of techniques have been used in the
past to reduce the density of the sintered body other
than varying process conditions. For example, one
technique has been to press the uranium dioxide powder to
higher than final pelletizing pressure, repress and to
sinter it. The problem with this technique is that the
resulting sintered body has large interconnecting
pores throughout the body which go out to the surface
resulting in a large surface area which can absorb into
the body significant amounts of gases and in paxticular
water during fuel fabrication. These gases and water
provide a source of corrosion for the fuel cladding during
reactor operation. Another technique i~ to add organic
materials which burn out in the sintering proce~s leaving
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24 NF 04079
~083336
stable porosity, However, these materials decompose to
leave carbon and thereby contam~nate the nuclear fuel.
Sill another approach is to control the final
~r end-point density of a sintered uranium aioxide nuclear
fuel body by adding a precursor to uranium dioxide such
as ammonium diuranate. Such an addition i~ made to the
uranium dioxide powder before pressing into a green body
as set forth in U. S. Patent No. 3,883,623, issued May
13, 1975 in the name of K. W. Lay. This addition results
in discrete low density regions in the sintered body
which correspond to the ammonium diuranat~ regions in the
green body. The end-point density of the sintered body i~
controlled by the amount of ammonium diuranate added.
The present invention presents a process for
controlling the final or end-point density of a sintered
body of nuclear fuel material by admixing ammonium
oxalate with the nuclear fuel material before pressing
into a green body. Upon heating the green body, the
ammonium oxalate decomposes and leaves discrete porosity ~ -
in the sintered body which correspond to the ammonium
oxalate regions in the green body. The end-point
density of the sintered body is therefore a function -
of the amount of ammonium oxalate added to the nuclear
fuel material. The present invention also present~ a
composition of matter in the form of a sintered highly
stable structure suitable for use in a nuclear reactor
as a nuclear fuel material.
It is a primary object of this invention to
provide an additive of ammonium oxalate to nuclear fuel
materials that serves to control the final sintered
density of bodies of the nuclear fuel material with the
final sintered density being preferably in the range of
24 NF 04079
33336
about 90 to about 97% of theoretical.
Another object of this invention is ~o provide
an additive of ammonium oxalate to nuclear fuel materials
that after sintering laaves substantially no impurities
in the sintered structures of the nuclear fuel material.
Another object of this invention is to control
the pore size and its dis~ribution in the final sintered
structure of nuclear fuel materials through the admixing
of ammonium oxalate to the nuclear fuel material prior
to sintering.
Still another object of this invention is to
provide a proce~s for controlling the final density of a
sintered body of nuclear fuel material involving the
admixing of ammonium oxalate to the nuclear fuel material
prior to sintering.
Other object~ and advantages of this invention
will become apparent from the following specification
and the appended claims.
Figures 1 and 2 present photomicrograph~ (at a
magnification of 50 times) of uranium dioxide pellets
produced according to the teachings of Examples 1 and
2 respectively.
It has now been discovered that a process for
controlling the density of a sintered body of a nuclear
fuel material by forming discrete porosity therein can
be conducted by practicing the steps of providing a
powder of the nuclear fuel material, admixing the
nuclear fuel material with an additive of a powder of am-
monium oxalate, forming the re~ulting mixture into a
green body having a desity ranging from about 30% to
about 70% of theoretical density, heating the green
body sufficiently to decompose the ammonium oxalate into
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24 NF 04079
~083336
gases and thereafter continuing to heat ~he body to
produce a sintered body having a discrete porosity and
a controlled end-point density.
The practice of the foregoing proce~s results
in the production of a composition of matter in the form
of a sintered highly stable s~ructure suitable for use in
a nuclear reactor.
As used herein, nuclear fuel material is
intended to cover the variuos materials used as nuclear
fuels for nuclear reactors including ceramic compounds
such as oxides and carbides 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, par~icularly 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 additivas to the nuclear
fuel material such a~ gadolinium oxide (Gd2O3).
As used herein, the term discrete porosity indi-
cates regions that are non-interconnecting and which are
primarily contained completely within the body, i.e., each
such region being surxounded by the nuclear fuel material.
In addition, the term end-point density of the sintered
body is the density of the sintered body as a whole, i.e.,
it is the final density of the whole sintered body.
The article of the present invention is a sintered
body of nuclear fuel material, preferably uranium dioxide,
containing a number of discrete porous regions which
correspond to those regions occupied by the pore former
in the green body. These porous regions lower the end-
point density of the sintered body by an amount ranging
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24 NF 04079
~983336
from about 2% to about 13%. The particular reduction in
end point density attained in the ~intered body depends
on the amount of pore former used. The present ~intered
body has an end-point density ranging from about 85% to
97% of theoretical, and preferably gO% to 97% of theoretical,
and an oxygen to metal atomic ratio ranging from about
2.00 to 2.034, and preferably, 2.00 to 2.010.
In carrying out the present process which
will be discussed for the preferred use of uranium dioxide,
the uranium dioxide powder or particles used generally
has an oxygen to uranium atomic ratio greater than 2.00
and can range up to 2.25. The crystallite size of the
uranium dioxide powder making up the larger particles
ranges up to about 10 microns and there is no limit on
the smaller size. Such particle sizes allow the
sintering to be carried out within a reasonable length
of time and at temperatures practical for commerci~l
applications. For most applications, to obtain rapid sintering, -
the uranium dioxide powder has a crystallite size rang-
ing up to 1 micron. Commercial uranium dioxide powders
are preferred and these are of small particle size,
u~ually sub-micron generally ranging from about 0O02
micron to about 0.5 micron.
In the present invention the ammonium oxalate
should have certain characteristics. It must be substan-
tially pure and free of impurities so that it can be
mixed with uranium dioxide powder and pressed, without
leaving any undesired impurities. This pore former of
ammonium oxalate, when hsated to its decomposition
temperature, decomposes to form ammonia, carbon dioxide
and water at 250C or greater leaving ~ubstantially no
contaminants (impurities) in the nuclear fuel material.
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24 NF 04079
~0133336
Such decomposition is very useful since it occurs well
below the temperature where sintering is believed to be
initiated. This decomposition is accompanied by the
complete volatilization of the decomposition products
which escape from the nuclear fuel material while the
fuel is still porous. Thus ammonium oxalate acts as a
pore former and leav~s the porosity in the ~uel at the
original locations o~ the ammonium oxalate. The size
of individual pores and the size distribution can be
controlled by varying the particle size of the particles
of ammonium oxalate added.
Generally, the particles of ammonium oxalate
added to uranium dioxide tend to clump together and
agglomerate, and therefore, it is the size of the
aggolomerate, i. e~, agglomerated particles, tha~
is given here. In the present invention, the ammonium
oxalate added to uranium dioxide should have an average
agglomerate or aggregat¢ (clump) size significantly
larger than that of the uranium dioxide powder. Specifi-
cally, the ammol~ium oxalate should have an average clump
size of at least about 10 microns and preferably in a range
of about 30 microns to about 60 microns~ This clump
size range is used so that when the mixture of ammonium
oxalate and uranium dioxide powder is pressed into a
green body, the green body ha~ a structure composed -
of a substantially uniform matrix of uranium dioxide
powder with discrets regions of ammonium oxalate.
Agglomerates or aggregates of the ammo~ium oxalate
in the uranium dioxide having a size L~P~ than 1
millimeter may result in low density regions which are
too large making the sintered body insufficiently uniform
to meet reactor specifications. On the other hand,
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24 NF 04079
~8~336
agglomerates or aggregat~s of the ammonium oxalate in the
uranium dioxias having a size significantly less than 10
microns would form a mixture which, when compacted and
sintered, would densify and not show a density significantly
di~ferent from that of the sintered uranium dioxide matrix
without the ammonium oxalate addition, i.e., such small
ammonium oxalate agglomerates or aggregates would not
result in discrete porosity which would significantly
lower the end-point dansity of th~ sintered body. The
term "clump" is used herein to co~er a collection of
particles of ammonium oxalate cohering with sufficient
strength as introduced into the nuclear fuel material
and capable of remaining essentially intact during
subsequent mixing and handling prior to heating.
The amount of ammonium oxalate used can vary
and depends largely on the degree of uniformity required
for the sintered body and the particular end-point
density required for the sintered body. To produce a
reduction of about 2% to about 13% in the end-point
density of the sintered body, ammonium oxalate should
be admixed with the uranium dioxide powder in an amount
ranging from about 0.1 to about 3.0% by weight, preferably
about 0.4 to about 2.5% by weight.
The final pore size distribution of sintered
pellets is also a function of the size distribution of
the ammonium oxalate powder. A preferred particle size
of ammonium oxalata powder i~ -270 to +400 mesh.
In carrying out the present process, the
uranium dioxide powder and ammonium oxalate are admixed
by any technique, such as stirring, which produces a
mixture wherein the agglomerates of the ammonium oxalate
with uranium dioxide are dispersed substantially uniformly
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24 NF 040 9
~83336
throughout the uranium dioxide powder. Such a mixture,
when pressed into a green body, allows the resulting
sintered body to have substantially uniform density
across the entire sintered body. Should the agglomerate
of ammonium oxalate with uranium dioxide be clumpsd
together in the uranium dioxide powd~r matrix, the
resulting sintered body is likely to have one big
hole therein making it substantially non-uniform and ~ !
thereby causing problems in mechanical strength and
other properties. Accordingly this process is conducted
to avoid clumping of the agglomerates of ammonium oxalate.
The re~ulting mixture of uranium dioxide powder
and ammonium oxalate can be formed into a green body,
generally a pellet or cylinder, by a number of techniques
such as pressing or extrusion. Specifically, the mixture
is compressed into a form in which it has the re~uired
mechanical strength for handling and which, after sintering,
is of the size which satisfies nuclear reactor specifications.
The green body can have a dansity ranging from about 30%
to 70% of theoretical, but usually it has a density
ranging from about 40 to 60% of theoretical, and preferably
about 50% of theoretical.
The green body is sintered in an atmosphere
which depends on the particular manufacturing process.
Specifically, it is an atmosphere which can be used to
sinter nuclear fuel materials alone, such as uranium
dioxide alone in the production of uranium dioxide
nuclear fuel. For example, a number of atmo~phere can
be used such as an inert atmosphere, a reduing atmos-
phere (e.g. dry hydrogen) or a controlled atmosphere
comprised of a mixture of gases (e.g. a mixture of hydrogen
and carbon dioxide as set forth in U. S. Patent 3,872,022)
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24 NF 04079
~L~8333~;
which in equilibrium produces a partial pressure of
oxygen suffiaient to maintain the uranium dioxide at a
desired oxygen to uranium ratio.
The rate of heating to sintering temperature
is limited largely by how fast the by-product gases are
removed prior to sintering and generally this depends on
the gas flow rate through the furance and its uniformity
therein as well as the amount of nuclear fuel material
in the furnace. Spe~ifically, the gas flow rate through
the furnace, which ordinarily is substantially the same
gas flow used as the sintering a~mosphere, should be
sufficient to remove the gases resulting from decomposition
of amonium oxalate before sintering temperature is reached.
Generally, best results are obtained when the rate of
heating to decompose the pore former ranges from about
50C per hour to about 300C pex hour. After decomposi- -
tion of the ammonium oxalate is completed and by-product
gases substantially removed from the furnace, the rate of
heating can then be increased, if desired, to a range of
~out 300C to 500C per hour and as high as 800C per
hour but should not be so rapid so as to crack the bodies.
Upon completion of sintering, the sintered
body is usually cooled to room temperature. The rate of
cooling of the sintered body is not cr~cal 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 usually
used in commercial sintering furnaces. These cooling rates
may range from 100C to about 800C per hour, and
generally, from about 400C per hour to 600C per hour.
The sintered uranium dioxide bodie~ are preferably cooled
in the same atmosphere in which they were sintered.
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24 NF 04079
~L083336
This invention provides several advantages in
the sintering of nuclear fuel materials and in the requlting
ntered pellets. The pellets produced by this method are
resistant to in-reactor densification as documented by
out-of-pile thermal ~imulation of densification test
The addition of ammonium oxalate does not leave any
undesirable residue in the sintered pellets. Thermo-
gravimetric analysis has shown that ammonium oxalate
decomposes completely into amonia (NH3), carbon dioxide
(C02) and water vapor (H20). The early decomposition of
ammonium oxalate prevents the entrapment of undesirable
gases in the microstructure of the nuclear fuel material
during the sintering process. Pellets incorporating
ammonium oxalate according to the teachings of this
invention can be sintered u3ing conventional wet hydrogen
as a sintering gas or controlled atmosphere sintering
under an atmosphere comprising a mixture of hydrogen
and carbon dioxide.
The invention is further illustrated by the
following examples: `
EXAMPLE I
Uranium dioxide powder having an oxygen to
uranium ratio of about 2.06 to 2.08 was used. The
uranium dioxide powder ranged in size from about .5 to
1 micron ~or its smallest particles to an average agglo-
merate ~ize of 840 microns.
Ammonium oxalate having an average agglomerate
particle size of about 37 to 74 microns was used.
number of green bodies were made as follows: 500 grams
of uranium dioxide powder waC isopressed at 10,000 psi
into a compacted slug. This slug was broken and granulated
through a 16 mesh screen to obtain a free flowing U02
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24 NF 04079
1~ 8 33 36
powder. This U02 powder was then bl~nded wi~h 5 grams
ammonium oxalate powder in a Nye blender for 10 minutes.
After blenaing the reQul~ing mixture was pressed into
green pellets having a density of abou~ 5.3 grams/cc.
Additional green bodies of only uranium dioxide
were made. Five hundred grams of the uranium dioxide
powder alone were pressed in the same manner as abvve
to produce green bodies in pellet form having a density
o~ 5.3 grams/cc.
The two different groups of green bodies were
placed in amolybdenum boat stacked thrae layers deep
and this boat was then placed in an alumina tube furnace
which was about 144 inches in total length with a heated
zone of about 70 inches in length. The furnace was
electrically heated by molybdenum wound resistancc wires~
The sintering atmosphere was wet with a gas flow rate
of about 40 cu.ft./Hr. The temperature of the sintering
zone was maintained at 1700C + 25C. The molybdenum
boat carrying the green pellets was pushed through the -
sintering furnace at a rate suffici~nt to obtain 4 hours
in preheating zone, 4 hours in sintering zone and about
4 hours in the cooling zone. The preheating zone has a
temperature ranging from 600C at the cool end to 900C
adjacent the sintering zone and the cooling zone has a
temperature ranging from 1100C at the end adjacent the
sintering zone to 50C at the other end.
The end point density of each of the sintered
bodies, given as percent of theoretical, was determined
by a standard technique, i.e. by a differential weight
technique by weighing in water and in air and calculating
the volume from the difference in weight and the known
density of water. This technique for measuring end-point
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24 NF 04079
~83336
density could be used since the sintered body produced in
accordance with the present invention has a subst~ntially
continuous outer surface so that any amount of the
water which may have entered the pores was insignificant
or within experimental error.
The average end-point den~ity of the sintered
bodies initially formed from uranium dioxide along was
99.12% of theoretical-density whereas the average end-
point density of the sintered bodie3 formed in accordance
with the present invention, i.e. initially formed from
uranium dioxide and ammonium oxalate, was 95.4% of
theoretical-density. The carbon content of the sintered
bodies ranged from 1 to 7 parts per million.
The sintered body formsd in accordance with
the present invention was sectioned, polished and examined
by standard metallographic techniques. A micrograph of
a sectioned pellet, magnified 50 times, is shown in
Figure 1. The micrograph shows a matrix or the continuous
phase o sintered uranium dioxide microstructure with
the porosity in the uranium dioxide distinct areas
(darker areas) within the matrix indicating where agglo-
merates of ammonium oxalate were present initially~ The
pore size produced by the ammonium oxalate addition
ranged from 10 to 70 microns ~ m) with an average pore
size of approximately 50~L m. When these pellets were
subjected to thermal simulated densification test at
1700C for 24 hours (equivalent to 5000 MWD/T in raactor),
the average increase in density was measured to be only
0.59%. This small increase in density indicates that
the fuel is substantially resistant to in-reactor densifi-
cation.
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24 NF 04079
~(183336
EXAMPLE 2
The procedure o~ Example 1 was repeated for
two groups of pellets. One group of pellets had only UO2
having a green density of 5.3 grams/cc. The other group
of pellets had .85% by weight ammonium oxalate (-270 ~
400 mesh) blended with uranium dioxide with an average
gresn density of 5.3 grams/cc. The pellets initially
having only uranium dioxide sintered to an average density
98~5% of theoretical density while the pellets with
ammonium oxalate addition had an average density of 95.6%
of theoretical density.
The carbon content o~ these pellets ranged from
1 to 4 ppm, the hydrogen content of these pellets ranged
from 0.19 to 0.32 ppm and the nitrogen content ranged
from 14 to 17 ppm.
A micrograph of a sectioned pellet, magnified
S0 times, is shown in Figure 2~ and the axplanation of
the micrograph of Figure 1 in Example 1 is applicable-h-
here. The pore size produced b~ ammonium oxalate addi-
tion range from 10~ m to 40JLm with an average pore
size of approximately 30~ m.
As will be apparent to those skilled in the
art, various modifications and changes may be made in the
method and composition described therein. It is
accordingly the intention that the invention be construed
in the broadest manner within the spirit and scope as
set forth in the accompanying claims.
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