Production of nuclear fuel products

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. The uranium dioxide is mixed with a fugitive binder comprising a high molecular weight acrylic acid polymer and ammonium bicarbonate. The mixture is then pressed and sintered to expel the binder and to give a fissionable nuclear fuel pellet.

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 INVENTION 
Fissionable nuclear fuel comprises a variety of compositions and forms of 
fissionable materials, including ceramic compounds of uranium, plutonium 
and thorium. Fuel compounds for commercial energy generating reactors 
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 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. 
However, the sintering 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 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 adversely 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. Pat. No. 4,061,700, issued Dec. 6, 1977, to Gallivan, and assigned to 
the same assignee as this application, 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. Pat. No. 4,061,700, and of U.S. Pat. Nos. 
3,803,273; 3,923,933; and 3,927,154, assigned to the same assignee as the 
instant application, and relating to significant aspects in the subject 
field of producing nuclear fuel pellets from particulate fissionable 
ceramic material for reactor service, are all incorporated herein by 
reference. 
The prior art techniques or means such as disclosed in U.S. Pat. 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 
of 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 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 compression molding such materials 
comprising uranium dioxide powder and also occurring in the resulting 
molded compact. As is well known, ceramic materials are generally of a 
relatively brittle consistency as opposed to a plastic or conformable 
consistency. Thus, rather than gradually deforming 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 
characteristics 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 commonly the propagation of the resulting 
fracture is of a slower rate and does not continue to the extent of 
fragmenting the mass. Thus a plastic type of material is more amenable to 
compression 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 molding operations and in the properties of the molded 
products. 
SUMMARY 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 cohesive while undergoing compression molding. The 
invention includes the use of a unique fugitive binder for particulate 
ceramic material comprising a combination of acrylic:acid polymers of 
relatively high molecular weight and ammonium bicarbonate. 
OBJECTS OF THE INVENTION 
It is a primary object of this invention to provide an improved method for 
compression molding particulate ceramic materials. 
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 ceramic compounds consisting of oxides of uranium used 
as nuclear fuel for nuclear reactors. 
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 
##STR1## 
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 1 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 1 part by weight of the 
acrylic acid polymer to about 6 parts by weight of ammonium bicarbonate. 
The combination binder is preferable employed in amounts of from about 0.5% 
to about 7% by weight based upon the weight of the nuclear fuel material. 
Amounts in excesses beyond about 7% do not generally provide a 
proportionally commensurate benefit in 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 
consisting of 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 
and ribbon blenders, and high shear or intensive blenders such as 
vibratory mills, ball mills and centrifugal mills. 
One such blending apparatus comprises vibratory 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 least about 10 minutes. This dwell period provides for 
the combination binder to produce an effective bonding mechanism and also 
the lubrication of the brittle 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 and pending 
applications. 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 (B. F. Goodrich's CARBOPOL resin, Type 907, approx. 
mol. wt. 450,000) and ammonium bicarbonate 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 1.9% 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.