PRODUCTION DE COMBUSTIBLES NUCLEAIRES

PRODUCTION OF NUCLEAR FUEL PRODUCTS

Abrégé anglais

24-NF-04505
PRODUCTION OF NUCLEAR FUEL PRODUCTS
ABSTRACT OF THE DISCLOSURE
An improved method for compression molding of
typically brittle ceramic materials comprising uranium
dioxide which enhances their compacting and cohering
properties in the manufacture of nuclear fuel products.

Revendications

- 9 - 24-NF-04505
The embodiments of the invention in which an
exclusive property or privilege is claimed are defined
as follows:
1. A method for producing fissionable nuclear
fuel pellets containing uranium dioxide, consisting
essentially of the steps of:
(a) admixing particulate uranium dioxide nuclear
fuel with a fugitive binder comprising a combination of high
molecular weight acrylic acid polymer having an average
molecular weight of at least about 400,000 and ammonium
bicarbonate;
(b) forming the resultant admixture by pressing
into a coherent compact; and
(c) sintering the resultant admixture of said
particulate uranium dioxide with acrylic acid polymer and
ammonium bicarbonate fugitive binder formed into a compact
to expel binder material and integrate the particulate
uranium dioxide nuclear fuel into a uniform body.
2. The method of claim 1, wherein the fugitive
binder is admixed with particulate uranium dioxide nuclear
fuel in amount of about 0.5% to about 7% by weight of said
binder based upon the weight of the uranium dioxide fuel.
3. The method of claim 1, wherein the fugitive
binder comprises acrylic acid polymer and ammonium
bicarbonate in approximate proportions of about 1 part by
weight of acrylic acid polymer to about 4 to about 8 parts
by weight of ammonium bicarbonate.
4. The method of claim 1, wherein the acrylic acid
polymer comprises polymers having molecular weights within
the approximate range of from about 400,000 up to about
4,000,000.
5. The method of claim 1, wherein the acrylic
acid polymer and ammonium bicarbonate are in the approximate
proportions of about 1 part by weight of the acid polymer
to about 6 parts by weight of the ammonium bicarbonate.

- 10 - 24-NF-04505
6. A method of producing fissionable nuclear fuel
pellets containing uranium dioxide, consisting essentially
of the steps of:
(a) admixing particulate uranium dioxide
nuclear fuel with a fugitive binder comprising a
combination of about 1 part by weight of high molecular
weight acrylic acid polymer having an average molecular
weight of at least about 400,000 and about 4 to about 8
parts by weight of ammonium bicarbonate;
(b) forming the resultant admixture by pressing
into a coherent compact; and
(c) sintering the resultant admixture of said
particulate uranium dioxide with acrylic acid polymer and
ammonium bicarbonate fugitive binder formed into a compact
to expel binder material and integrate the particulate
uranium dioxide nuclear fuel into a uniform body.
7. The method of claim 6, wherein the fugitive
binder is admixed with particulate uranium dioxide nuclear
fuel in amount of about 0.5% to about 7% by weight of
said binder based upon the weight of the uranium dioxide
fuel.
8. The method of claim 6, wherein the acrylic
acid polymer and ammonium bicarbonate are in the approximate
proportions of about 1 part by weight of said polymer to
about 6 parts by weight of the ammonium bicarbonate.
9. The method of claim 6, wherein the acrylic
acid polymer comprises polymers having molecular weight
within the approximate range of from about 400,000 up
to about 4,000,000.
10. The method of claim 6, wherein the acrylic
acid polymer contains carboxyl group having the structure
<IMG>

- 11 - 24-NF-04505
11. A method for producing fissionable nuclear
fuel pellets containing uranium dioxide, consisting
essentially of the steps of:
(a) admixing particulate uranium dioxide nuclear
fuel with a fugitive binder comprising a combination of
about 1 part by weight of high molecular weight acrylic
acid polymer having an average molecular weight of at
least about 400,000 and about 6 to about 8 parts by weight
of ammonium bicarbonate, said fugitive binder being
admixed with the uranium dioxide nuclear fuel in amount
of about 0.5% to about 7% by weight of said binder based
upon the weight of the uranium dioxide fuel;
(b) forming the resultant admixture of uranium
dioxide fuel and binder by pressing into a coherent compact;
and
(c) sintering the compact of the admixture of
said particulate uranium dioxide with acrylic acid polymer
and ammonium bicarbonate fugitive binder to expel binder
material and integrate the particulate uranium dioxide
nuclear fuel into a uniform body.
12. The method of claim 11, wherein the acrylic
acid polymer comprises polymers having molecular weights
within the approximate range of at least about 400,000
up to about 4,000,000.
13. The method of claim 11, wherein the
acrylic acid polymer contains carboxyl groups having the
structure
<IMG>
14. A coherent compact of consolidated
particles consisting essentially of the product of
admixing particulate uranium dioxide nuclear fuel with a

- 12 - 24-NF-04505
fugitive binder comprising a combination of a high
molecular weight acrylic acid polymer having an average
molecular weight of at least about 400,000 and ammonium
bicarbonate.
15. The coherent compact of claim 14, wherein
the acrylic acid polymer comprises polymers having
molecular weights within the approximate range of at
least about 400,000 up to about 4,000,000.
16. The coherent compact of claim 14, wherein
the acrylic acid polymer contains carboxyl groups having
the structure
<IMG>
17. A coherent compact of consolidated particles
consisting essentially of the product of admixing
particulate uranium dioxide nuclear fuel with a fugitive
binder comprising a combination of acrylic acid polymer
comprising polymers having molecular weights within the
approximate range of at least about 400,000 up to about
4,000,000 and ammonium bicarbonate.
18. The coherent compact of claim 17, wherein
the acrylic acid polymer contains carboxyl groups having
the structure
<IMG>

Description

-1- 24-NF-04505
PRODUCTION OF NUCLFAR FUE~ PRODUCTS
-
FIELD OF THE'INVENTION
This :invention relates generally to the ceramic
art and the formation of sintered bodies from particulate
oxide materials. It is particularly concerned with a
process for producing consolidated units of particulate
ceramic materials including the compressing of such
particles into coherent and handleable compacts for
subsequent sintering to integrated bodies. The invention
is specifically directed to the manufacture of nuclear
fuel products from particulate materials containing
uranium dioxide.
BACKGROUND OF THE INVEN~I'ON
FissionabIe nuclear fuel comprises a variety of
compositions and forms of fissionable materials, including
ceramic compounds of uranium, plutonium and thorium.
FueI compounds for commercial energy generating reacto~s
typically comprise oxides of uranium, plutonium and
thorium, and mixtures thereof. The generally most suitable
and commonly used fuel for such commercial nuclear reactors
is uranium dioxide~ which can be combined with minor
amounts of other fuel materials and include neutron flux
controlling additives such as gadolinium.
Commercially produced uranium dioxide is a fine,
fairly porous powder, a form which is not suitable as such
for use as fuel in commercial reactors. A number of means
have been developed and used to convert powdered uranium
dioxide into a form suitable for use as a fuel in power
generating nuclear reactors. One commonly used technique
~A
~4~

~823~ 24-NF-04505
2--
has been to sinter appropriately sized bodies of the
powdered uranium dioxide material at high temperatures
to develop strong diffusion bonds between the individual
powder particles.
~lowever, the sinterlng technique requires a
preliminary compressing of the loose powder into a shaped,
and self-retaining compacted body of particles of sufficient
strength and integrity to survive handling and the sintering
procedure. The operation of compressing fine particles into
lQ a body or coherent compact with acceptable low reject
levels, and with the strength and uniformity for enduring
subsequent handling and firing has been a subject of
considerable concern and investigation in the nuclear fuel
industry.
Conventional organic or plastic binders commonly
used in powder fabrication have been considered to be
unsuitable in nuclear fuel processing operations.
Entrainment of any binder residues such as carbon within
the sintered nuclear fuel product is unacceptable in
reactor service. Moreover, the presence of any organic
binder among the particles inhibits the formation during
sintering of strong diffusion bonds between the particles,
and adverseIy affects the density of the sintered product.
The complete removal of binders, or their decomposition
products~ prior to sintering is especially difficult, and
usually requires a costly additional operation in the fuel
manufacture.
Accordingly, a common method has been to die
press uranium dioxide powder into appropriately sized
"green" (unfired) compacts without the assistance of any
binder. This approach however has resulted in very costly
high rates of rejects and scrap material recycling because
of the weakness of such green, binder-free compacts of
powder.
U.S. Patent No. 4,061,700, issued
December 6, 1977, to Gallivan, and assigned to the same

~ 23~ 2~-NF-04505
--3--
assignee as this applica-tion, discloses a distinctive
group of fugitive binders that improved the production
of sintered pellets of particulate nuclear fuel materials
for nuclear reactors. The fugitive binders of this patent
function without contaminating the resulting fuel product,
and they permit the formation of effective bonds between
sintered particles during firing without deleteriously
affecting the desired porosity of the fused pellet.
The disclosure of the said U.S. Patent No.
4,061,700, and of U.S. Patent Numbers 3,803,273,
issued April 9, 1974 to Hill et ali 3,923,933l issued
December 2, 1975 to Lay; and 3,927,154, issued
December 16l 1975 to Carter, assigned to the same assignee
as the instant application, are related to significant
aspects of producing nuclear fuel pellets from
particulate fissionable ceramic material for reactor
service.
The prior art techniques or means such as
disclosed in U.S. Patent No. 4,061,700, have been found
wanting in some conditions and circumstances. For
instance, it has been observed that the fugitive binders
of the aforesaid patent do not provide consistent results
as to pellet strength and integrity irrespective of the
blending conditions and particle characteristics of the
uranium dioxide powder. Specifically the severity o~
agitation in blending/ relative humidity and temperature,
and duration of storage, as well as the uranium oxide
powder properties such as size, surface area and moisture
content are all factors that apparently can detract from
3~ the uniformity of the physical attributes provided by
such fugitive binders.
This invention deals with the typical brittle
nature of ceramic materials and problems imposed thereby
when compressed molding such materials comprising uranium
dioxide powder and also occurring in the resulting molded
compact. As is well known, ceramic materials are generally

~2~823~ 24-NF-04505
--4--
of a relatively brittle consistency as opposed to a plastic
or conformable consistency. Thus, rather than gradually
deEorming over a period of progressively increasing applied
compressive stress approaching the breaking point as is
the case with a plastic material, ceramics tend to rigidly
resist substantially all deformation until the breaking
point is reached whereupon they abruptly fracture with the
resulting fissure or fissures instantly progressing through
the mass fragmenting it. An apt illustration of this
brittle and unyielding property and the fracture character-
istics of a ceramic is the crushing of a glass marble.
On the other hand, a plastic material will gradually yield
and deform with progressively increasing compressive stress
until reaching its breaking point and rupturing, and
eommonly the propagation of the resulting facture is of a
slower rate and does not eontinue to the extent of frag-
menting the mass. Thus a plastic type of material is more
amenable to eompression molding than the brittle type of
materials.
This inherent brittle characteristic in uranium
dioxide powder, or its lack of plasticity, constitutes a
significant shortcoming when subjected to compression
moldin~ operations and in the properties of the molded
products.
25 SUMM~RY OF THE INVENTION
This invention comprises a method of producing
coherent compacts from particulate ceramic material wherein
the ceramic material is rendered more conformable and
eohesive while undergoing eompression molding. The
invention ineludes the use of a unique fugitive binder for
partieulate ceramic material comprising a combination of
aerylie aeid polymers and ammonium bicarbonate.
OBJECTS OF THE INVENTION
It is a primary object of this invention to provide
an improved method for eompression molding particulate
ceramie materials.

~ 23~ 24-NF-04505
--5--
It is another object of this invention to provide
means for overcoming the brittle nature of ceramic
particles and to impart lubricity to such a material
whereby it can be compression molded under essentially
all compacting conditions and at high rates with a lower
level of surface imperfections and cracks.
A further object of this invention is to provide
a new and improved fugitive binder for improving the
compression molding of particulate ceramic materials
comprising uranium dioxide into coherent compacts, and
increasing the resistance of such compacts of ceramic
materials to fracture and end flaking during compression
molding and thereafter, including the sintered products
of the molded material.
A still further object of this invention is to
provide a novel fugitive binder for producing nuclear fuel
pellets comprising uranium dioxide from particulate
ceramic material wherein the particulate ceramic is
compression molded to a coherent compact at fast rates
with minimal rejects due to punch sticking or fractures
in the coherent compact.
DETAILED DESCRIPTION OF THE INVENTION
This invention comprises a method for producing a
fissionable nuclear fuel product in pellet-like form from
particulate ceramic material utilizing a fugitive binder
which is subsequently removed during the sintering
operation. The invention includes a new fugitive binder
for compression molding of particulate ceramic material
comprising uranium dioxide powder to form coherent compacts
of apt dimensions, and thereafter sintering the compacts
to produce integrated bodies of fissionable nuclear fuel
suitable for use in nuclear reactors.
The particulate fissionable nuclear fuel materials
for use in this invention comprise various materials used
as nuclear fuels for nuclear reactors, including ceramic
compounds such as oxides of uranium, plutonium and

~2~8~3~ 24 NF-04505
--6--
thorium. Preferred fuel compounds consist of uranium
oxide, plutonium oxide, thorium oxide~ and mixtures
thereof.
The particulate nuclear fuel material in the
practice of the invention can also include various additives
such as neutron absorbing materials comprising gadolinium
to moderate neutron flux densities.
Fugitive binders suitable for use in the practice
of this invention comprise a combination of acrylic acid
polymer or polymers and ammonium bicarbonate. The polymers
of acrylic acid for use in this invention are of relatively
high molecular weights, namely above about 400,000, and
preferably of a range of about 400,000 up to about
4,000,000 molecular weight. They include such relatively
high molecular weight acrylic acid polymers containing
carboxyl groups having the structure
~1 1 }
H C = O
I
OX
One commercial source of suitable acrylic acid
polymeric material for use in this invention comprises
B.F. Goodrich's "CARBOPOL~resins.
The ratio of the said acrylic acid polymer to
ammonium bicarbonate in the combination fugitive binder
of this invention is of the approximate proportions of about
l part by weight of acrylic acid polymer to about 4 to
about 8 parts by weight of ammonium bicarbonate. A
preferred embodiment of the invention comprises a ratio
of about l part by weight of the acrylic acid polymer
to about 6 parts by weight of ammonium bicarbonate.
The combination binder is preferably employed in
amounts of from about 0.5% to about 7% by weight based
upon the weight of the nuclear fuel material. Amounts
.

~ 23~ 24-NF-04505
--7--
in excesses beyond about 7% do not generally provide a
proportionally commensurate benefit ln bonding capacity,
and may introduce unwanted effects which compromise any
advantages or -the costs of including greater amounts of
the bonding agent.
In accordance with the method of this invention,
the fugitive binder combination is added without water
to the particulate ceramic fuel material containing
uranium oxide and the binder blended substantially
uniformly through the particulate material. Thereafter,
the blend of particulate ceramic material and added
combination binder is compressed into a coherent compact
of suitable dimensions in accordance with procedures and
means of the art. The method of this invention enables
the effective use of such blends in high speed, continuous
production rotary press devices and operation.
The "green" (unfired) coherent compacts thus
formed are then sintered in accordance with the practices
and procedures of the art to expel binder material and
integrate the ceramic particles into a uniform and
continuous body. The sintered product, typically in the
form of a pellet, is thereafter ground to specified
dimensions for its designated service.
Blending of the added binder can be effected
with any appropriate "dry" mixing apparatus including
low shear blenders such as fluidized bed, slab ana ribbon
blenders, and high shear or intensive blenders such as
vibratory mills, ball mills and centrifugal mills~
One such blending apparatus comprises vibratory
30 mills of the type described in pages 8-29 to 8-30 to
Perry and Chilton's 5th edition of Chemical Engineering
Handbook, McGraw-Hill Book Co.
The blending of the particulate material with
the binder without the addition of water, should include a
dwell time of at least about 5 minutes and preferably at

~Z~8~3~ 24-NF-04505
--8--
least about 10 minutes. This dwell perlod provides for
the combination binder to produce an effective bonding
mechanism and also the lubrication of the britt]e mixture
of ceramic particles to render them more plastic and
resistant to sticking to confining walls in compression
molding.
Upon attaining a substantially uniform blend of
the added combination binder with the ceramic material,
the blend thereof can be compression molded into coherent
compacts by substantially any effective means or device
in accordance with the technology of this field such as is
described in the prior art, including the above cited
patents. The coherent compacts are then sintered to expel
binder material therefrom and integrate the ceramic
particles into an essentially continuous body of substantially
uniform consistency, and relatively high strength and good
resistance to fracture.
An example of a preferred procedure for the
practice of the method of this invention is as follows.
A charge of enriched uranium dioxide powder,
granulated to a substantially uniform particle size, is
deposited in a vibratory mill (Sweco Inc. Vibro-Energy
mill) for blending with a fugitive binder. A dry mixture
of acrylic acid polymer (s.F. Goodrich's CARsoPOL resin,
25 Type 907, approx. mol. wt. ~50,000) and ammonium bicar-
bonate in a ratio of about 1 to about 6 parts by weight
is added to the charge of uranium dioxide powder in such
a quantity to provide about 109% by weight of said
combination binder based upon the weight of the powder.
The uranium dioxide powder and added binder
is blended for a period of about 10 minutes to achieve good
uniformity, and aged about 3 days. The blended and aged
mixture of binder and powder is then compressed molded
to a coherent compact at a pressure of about 25,000 pounds
per square inch. The coherent compacts thus produced
can then be sintered in a conventional manner.