Method of improving the green strength of nuclear fuel pellets

A process for imparting increased strength and physical durability in green bodies or pellets formed of particulate oxides of uranium, plutonium and the like in the production of pelletized fissionable nuclear fuel. The green or unfired pellets comprise a fugitive binder dispersed through the particulate oxide fuel material.

This invention relates to manufacturing techniques and procedures 
comprising compressing particulate ceramic materials into compacted, 
coherent and handleable bodies for subsequent sintering to produce 
integrated units or products, and it particularly relates to a method of 
forming green or presintered pellets of particulate fissionable nuclear 
fuel material having increased physical strength and integrity for 
enduring subsequent handling or processing, such as sintering and grinding 
to dimensions, and their final utilization. 
Various materials are used as fissionable 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 
fissionable 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 desired density. 
The specific composition of a given commercial uranium dioxide powder may 
also prevent it from being used directly as a nuclear fuel. Uranium 
dioxide is an exception to the law of definite proportions since "UO.sub.2 
" actually denotes a single, stable phase that may vary in composition 
from UO.sub.1.7 to UO.sub.2.25. Because thermal conductivity decreases 
with increasing O/U ratios, uranium dioxide having as low an O/U ratio as 
possible is preferred. However, since uranium dioxide powder oxidizes 
easily in air and absorbs moisture readily, the O/U ratio of this powder 
is significantly in excess of that acceptable for fuel. 
Although uranium dioxide suitable as a fissionable nuclear fuel can have an 
O/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 since it can be 
consistently produced in commercial sintering operations. In some 
instances, it may be desirable to maintain the O/U ratio of the uranium 
dioxide 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 having an O/U ratio as high as 2.195, and then 
later treat the sintered product in a reducing atmosphere to obtain the 
desired O/U ratio. 
A number of methods have been used to make uranium dioxide powder suitable 
as a fissionable nuclear fuel. Formerly, the most common method was to die 
press the powder into cylindrically-shaped green bodies of specific size 
without the assistance of binders since the complete removal of binders 
and their decomposition products was difficult to achieve prior to 
sintering. The entrainment of binder residues is considered unacceptable 
in sintered nuclear fuels. 
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 inhibits 
the formation of strong bonds unless a presintering treatment is applied 
to remove the binder. The higher compacting pressures and sintering 
temperatures required to develop such bonds sharply reduce the desired 
porosity. 
Sintering atmospheres may range from about 1000.degree. C. to about 
2400.degree. C. with the particular sintering temperature depending 
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.degree. C. The sintering 
operation is designed to densify the bodies and bring them down to the 
desired O/U ratio or close to the desired O/U ratio. 
Conventional organic or plastic binders are unsuitable for use in powder 
fabrication of nuclear fuel since they tend to contaminate the interior of 
the sintered body with impurities such as carbon, and their removal 
requires a separate binder removal treatment or operation. In addition, 
upon decomposition, these binder materials often leave deposits of organic 
materials in the equipment utlized to sinter the article, thereby 
complicating the maintenance procedures for the equipment. 
U.S. Pat. No. 4,061,700, issued Dec. 6, 1977 to Gallivan, and assigned to 
the same assignee as this application, discloses a group of new fugitive 
binders that produce improved sintered bodies of nuclear fuel materials 
for nuclear reactors by powder ceramic techniques without contaminating 
the resultant fuel or manufacturing systems, and which permit, through 
sintering, the formation of strong bonds between the sintered particles 
without deleteriously affecting porosity. 
The improved fugitive binders of said U.S. Pat. No. 4,061,700 comprise a 
compound or its hydration products containing ammonium cations and anions 
selected from the group consisting 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 carbamate, ammonium sesquicarbonate, 
ammonium carbamate and mixtures thereof. 
The binders disclosed in this patent are efficient binders for use in 
nuclear fuels, and further the binders enable the realization of defect 
free, pressed bodies of nuclear fuel materials and tensile strength in the 
bodies comparable to strengths achieved with long chain hydrocarbon 
binders. Further, the binders in this patent leave substantially no 
impurities in the nuclear fuel material since these binders decompose upon 
heating into ammonia (NH.sub.3), carbon dioxide (CO.sub.2) and water 
(H.sub.2 O) (or water vapor) at temperatures as low as 30.degree. C. 
The disclosure of the aforesaid U.S. Pat. No. 4,061,700, and U.S. Pat. Nos. 
3,803,273; 3,923,933; and 3,927,154, also assigned to the same assignee as 
the subject application, each relating to significant aspects in the 
subject field of producing nuclear fuel pellets or bodies from particulate 
fissionable ceramic material, are all incorporated herein by reference. 
Notwithstanding the significant contributions of the inventions of above 
patents to this field and their specific advances in that technology, 
there remains a need to further increase the green or unfired strength and 
durability of consolidated bodies or pellets of such particulate ceramic 
nuclear fuel materials prior to their sintering and thereafter, to thereby 
reduce the high number of rejects and production costs incurred during 
manufacture resulting from imperfections or flaws attributable to marginal 
green or unfired strength or physical integrity. 
SUMMARY OF THE INVENTION 
This invention comprises a method for producing green or unfired compressed 
bodies or pellets of particulate fissionable ceramic fuel materials with 
fugitive binders of the type and materials set forth in the above cited 
patents, having significantly greater strength and physical integrity 
prior to firing or in the green stage, and thereafter, and the improved 
products derived therefrom. In addition to the specific components or 
ingredients given, this invention comprises a combination of sequenced 
manufacturing steps or operations including an essential aging period 
effected or carried out intermediate to certain of such sequenced steps or 
operations of the overall procedure. 
The method of this invention enables the practice of a process with an 
exceptionally low level of rejects or physical imperfections for the 
formation and subsequent sintering of bodies of fissionable nuclear fuel, 
comprising the steps of admixing the nuclear fuel material in particulate 
form with the binder, forming the resulting mixture into a green body 
having a density ranging from about 30% to about 70% of theoretical 
density of the nuclear fuel material, heating said green body to decompose 
substantially all the binder into gases, further heating the body to 
produce a sintered body and cooling the sintered body in a controlled 
atmosphere. 
This invention also provides a composition of matter that is suitable for 
sintering in the form of a compacted, coherent handleable structure 
comprising a mixture of a nuclear fuel material and a binder of a compound 
or its hydration products containing ammonium cations and anions selected 
from the group consisting of carbonate anions, bicarbonate anions, 
carbamate anions and mixtures of such anions and preferably a binder 
selected from the group consisting of ammonium bicarbonate, ammonium 
carbonate and mixtures thereof. 
OBJECTS OF THE INVENTION 
It is an object of this invention to provide an improved method of 
manufacturing compacted, coherent and handleable bodies or pellets of 
nuclear fuel from particulate fissionable ceramic material, and the 
compressed products of such method. 
It is also an object of this invention to provide a method of improving the 
strength and physical integrity of green or unfired compressed and 
integrated bodies or pellets of nuclear fuel comprising particulate 
fissionable material and a fugitive binder, and the physically enhanced 
products thereof. 
It is a further object of this invention to provide a method for forming 
and sintering a body of nuclear fuel comprising the steps of admixing the 
nuclear fuel material in fine particulate form with a binder, forming the 
resulting mixture into a coherent and handleable green body, heating said 
green body to dispel any binder ingredients and to produce a durable 
sintered body wherein the number of rejects due to inadequate strength or 
physical durability of the green or as yet unsintered bodies, and in turn 
production costs, are significantly reduced. 
It is another object of this invention to provide a method for preparing 
particulate admixtures for producing green or unfired compressed bodies or 
pellets of particulate fissionable nuclear fuel materials admixed with a 
fugitive binder that are resistant to pressing flaws or deformities 
whereby the compressed bodies or pellets formed therefrom are 
substantially uniform throughout in configuration and physical integrity, 
and substantially free of physical imperfections or irregularities such as 
end flakes, radial cracks, fractures, chips and the like debilitating 
defects that impair the physical integrity of the units and cause their 
inability to meet specifications for nuclear fuel or simply their ultimate 
failure in physical structure. 
A still further object of this invention is to provide a method for 
preparing particulate admixtures of fissionable nuclear fuel material with 
a binder for producing compressed and sintered bodies or pellets wherein 
the density of the sintered product is controlled by the inclusion of a 
pore forming substance such as ammonium oxalate.

DETAILED DESCRIPTION OF THE INVENTION 
This invention comprises the discovery of an improved manufacturing 
procedure for the formation of coherent pellets or bodies of compacted 
particulate ceramic materials having markedly enhanced physical strength 
and resistance to pressing deformities, a minimum of physical impediments 
or flaws, and which are durable and handleable when subjected to factory 
production operations. This new procedure for the formation of such firmly 
integrated or coherent units from particulate ceramic materials admixed 
with fugitive binders comprises a combination of a specific sequence of 
manufacturing steps or operations including an essential period for the 
maturing or reacting of the fugitive binder of blended admixtures of the 
ingredients prior to significant subsequent handling or processing 
thereof, such as the subsequent consolidation of the particulate 
components to a compacted shape and the sintering thereof, or following 
work thereon including grinding or cutting to shape or given dimensions. 
This invention entails the admixing of a nuclear fuel or similar ceramic 
material in powder or relatively fine particulate form with a binder of a 
compound or its hydration products containing ammonium cations and anions 
selected from the group consisting of carbonate anions, bicarbonate 
anions, carbonate anions and mixtures of such anions. 
The particulate nuclear fuel materials comprise 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 
thereof. 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 as 
gadolinium oxide (Gd.sub.2 O.sub.3). 
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 2l00 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 
particle size. Such particle sizes allow the sintering to be carried out 
within a reasonable length of time and at temperatures practical for 
commercial applications. For most applications, to obtain rapid sintering, 
the uranium dioxide powder has a size ranging up to about 1 micron. 
Commercial uranium dioxide powders are preferred and these are of small 
particle size, usually sub-micron generally ranging from about 0.02 micron 
to 0.5 micron. 
Compositions suitable for use as a binder in the practice of this invention 
either alone or in mixtures, include ammonium bicarbonate, ammonium 
carbonate, ammonium bicarbonate carbamate, ammonium sesquicarbonate, 
ammonium carbamate and mixtures thereof. When mixed with nuclear fuel 
materials, these binders and the nuclear fuel material are believed to 
undergo the phenomenon of adhesion forming an ammonium derivative of the 
carbonate series such as (NH.sub.4).sub.4 [UO.sub.2 (CO.sub.3).sub.3 ]; 
(NH.sub.4).sub.6 [(UO.sub.2).sub.2 (CO.sub.3).sub.5 (H.sub.2 O).sub.2 
]H.sub.2 O; (NH.sub.4).sub.2 (CO.sub.3).sub.2 (H.sub.2 O).sub.2 ]; 
(NH.sub.4).sub.3 [(UO.sub.2).sub.2 (CO.sub.3).sub.3)(OH)(H.sub.2 O).sub.5 
]; NH.sub.4 [UO.sub.2 (CO.sub.3)(OH) (H.sub.2 O).sub.3 ]and UO.sub.2 
CO.sub.3 H.sub.2 O, or mixtures of these. 
In the present invention the binder preferably has certain characteristics. 
It should be substantially comprised of a compound or its hydration 
products containing ammonium cations and anions selected from the group 
consisting of carbonate anions, bicarbonate anions, carbamate anions and 
mixtures of such anions and free of impurities so that it can be mixed 
with uranium dioxide powder and pressed and sintered without leaving any 
undesired impurities after heating with particularly preferred binders 
being ammonium bicarbonate and ammonium carbonate and mixtures thereof. It 
has been found that commercially available ammonium bicarbonate contains 
virtually no impurities and commercially available ammonium carbonate also 
contains virtually no impurities except for other ammonium compounds as 
listed in the foregoing paragraph. Thermogravimetric analysis confirms 
that there is a complete volatilization of ammonium bicarbonate and 
ammonium carbonate at heating rates typically used for reductive 
atmospheric UO.sub.2 sintering. Ammonium bicarbonate and ammonium 
carbonate when heated to the temperature range of decomposition, decompose 
to form ammonia, carbon dioxide and water at significant rates leaving 
substantially no contaminates (impurities) in the fuel and no undesirable 
residues in the sintering furnace. Additionally the ammonium bicarbonate 
and the ammonium carbonate are used in small particle sizes of 400 mesh or 
less in order to achieve maximum surface coverage of the binder on the 
surface of the nuclear fuel material. 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 demonstrated by the fact that these compounds 
can be die pressed to green densities as high as 90% of theoretical 
density at moderate pressing pressures. 
The amount of binder 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 less readily formable 
powders require larger amounts of binder. When the selected binder is 
ammonium carbonate, the amount of the addition of this binder is dependent 
upon the desired sintered density of the nuclear fuel material. 
Homogeneous blending of the binder with the nuclear fuel material is 
practiced to develop fully the binding action of the binder on the nuclear 
fuel material. Where porosity or a lower density is not desired, the 
homogenous blending of the binder with the nuclear fuel material avoids 
the formation of agglomerates of the binder since such agglomerates can 
volatize during sintering leaving pores in the sintered nuclear fuel 
material which pores reduce the density of the nuclear fuel material in 
sintered bodies. When it is 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 agglomerates are 
destroyed. The blended and milled powder may then be predensified by low 
pressure die pressing followed by granulation through a sizing screen to 
promote flowability of the mixture. 
In carrying out this invention, it is preferred in order to achieve an 
optimum degree of uniformity of blending and freedom from non-homogeneous 
agglomerates therein that the binder be added to the particulate ceramic 
material by pneumatically injecting the binder into a mass of the 
particulate ceramic while suspended or fluidized in a fluid bed system and 
therein continuing the fluidized blending thereof until a substantially 
uniform dispersion of the binder about the particles of ceramic fuel 
material is achieved. A preferred fluid bed system for the addition and 
mixing of such ingredients is disclosed in U.S. Pat. Nos. 4,172,667, 
issued Oct. 30, 1979, and 4,168,914, issued Sept. 25, 1979. 
Blending of the combined particulate binder and ceramic material preferably 
should be continued for a period of at least about 10 minutes to insure a 
high degree of homogeneity and to induce the formation of more handleable, 
small agglomerates of the blended ingredients. 
In accordance with this invention the blend of such particulate ceramic 
material with the binder component described above is held for a 
relatively brief period of greater than 48 hours and preferably at least 
about 72 hours in a substantially quiescent state to age or mature the 
binder, prior to proceeding with the usual manufacturing operations or 
steps including compressing the particulate admixture into a consolidated 
or compacted coherent mass or body such as a pellet, and the subsequent 
sintering of such integrated bodies. 
It appears that during this period, binders of the type or composition 
specified, undergo a degree of a decomposition reaction and conversion to 
products that provide an enhanced binding effect upon the ceramic 
particles which is markedly superior to that afforded by its precursor. 
Following completion of said aging period or intermission, the matured 
mixture of particulate nuclear fuel material with the binder can be formed 
into a green body, generally a cylindrical pellet by a number of 
techniques such as pressing (particularly die pressing). Specifically, the 
mixture is compressed into a form in which it has the required mechanical 
strength for handling and which, after sintering, is of the size which 
satisfies reactor specification. The aging of the binders of this 
invention in the nuclear fuel material significantly enhances both the 
strength and integrity of the resulting green body. The green body can 
have a density 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 particualr 
manufacturing process. Specifically, it is an atmosphere which can be used 
to sinter uranium dioxide alone in the production of uranium dioxide 
nuclear fuel and also it must be an atmosphere which is compatible with 
the gases resulting from any decomposition of binder ingredients. For 
example, a number of atmospheres 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 as set forth in U.S. Pat. No. 3,872,022) which in equilibrium 
produces a partial pressure of oxygen sufficient 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 achieving a sintering 
temperature and generally this depends on the gas flow rate through the 
furnace and its uniformity therein as well as the amount of material in 
the furnace. Specifically, the rate of flow of gas through the furnace, 
which ordinarily is substantially the same gas flow used in the sintering 
atmosphere, should be sufficient to remove the gases resulting from 
decomposition of binder material before sintering temperature is reached. 
Generally, best results are obtained when the rate of heating to decompose 
and volatilize all binder materials ranges from about 50.degree. C. per 
hour to about 300.degree. C. per hour. After decomposition of the binder 
material is completed and byproduct gases substantially removed from the 
furnace, the rate of heating can then be increased, if desired, to a range 
of about 300.degree. C. to 500.degree. C. per hour and as high as 
800.degree. C. per hour but not be so rapid 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 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 usually used in commercial sintering furnaces. 
These cooling rates may range from 100.degree. C. to about 800.degree. C. 
per hour, and generally, from about 400.degree. C. per hour to 600.degree. 
C. per hour. The sintered uranium dioxide bodies are preferably cooled in 
the same atmosphere in which they were sintered. 
To govern the densities of the sintered bodies of ceramic fuel material of 
this invention, pore formers such as ammonium oxalate or a uranium 
precursor can be added to the fuel material along with the binders in the 
practice of this invention. The pore formers, when used, are preferably 
reduced to a uniformly fine granular form and premixed with the 
particulate ceramic material. 
The green or unfired nuclear fuel pelleted product formed by the new 
process of this invention and exhibiting improved strength and physical 
integrity is illustrated in FIG. 1 of the drawing. 
The following comprises an example of a preferred embodiment for the 
practice of this invention and an illustration of the pronounced 
improvement in tensile strength of the products produced thereby. 
Uniformly fine powdered ammonium bicarbonate was introduced into uranium 
dioxide particles in a ratio of about 2.7 weight percent based on the 
UO.sub.2 in the fluid bed system and apparatus of U.S. Pat. No. 4,172,667. 
The particulate admixture of NH.sub.4 HCO.sub.3 and UO.sub.2 was fluidized 
and agitated within the system for about 10 minutes, whereupon the 
resultant homogeneous blend of the particles was aged under quiescent or 
static condition for 72 hours prior to subsequent processing including 
pressing and compacting the particles into coherent integrated bodies or 
pellets. The process was thereafter completed by die pressing into 
cylindrical fuel pellets in accordance with the procedures set forth in 
U.S. Pat. No. 4,061,700. 
The pellets produced from the thus aged product exhibited significantly 
greater strength and integrity in the green or unfired state with a 
tensile strength increased by a factor of about twofold over unaged 
admixtures prepared in a like manner except for aging intermission. After 
the routine firing of the compressed pellets, the aged pellet samples 
survived the grinding operation to achieve precise dimensions thereof to 
the extent of about 95% recovery whereas like produced but unaged pellets 
had a grinding recovery of less than about 50%. 
Further identically prepared samples of admixtures of ammonium bicarbonate 
binder with uranium dioxide were aged for several different periods of 
time, namely 24 hours, 48 hours and 72 hours, and compared with a sample 
likewise processed but without any binder. The results of this evaluation, 
measured in tensile strength, psi, are shown in the graph of FIG. 2 of the 
drawing.