CA1083336A - Sintered ceramics having controlled density and porosity - Google Patents

Abstract

ABSTRACT OF THE DISCLOSURE

A process for controlling the density of a sintered body of a nuclear fuel material by forming discrete porosity in the body is presented and involves admixing a nuclear fuel material in powder form with ammonium oxalate followed by compaction of the mixture into a form that is heated and sintered. The practice of the foregoing process results in the production of a composition of matter in the form of a sintered, highly stable structure suitable for use in a nuclear reactor as nuclear fuel material.

Description

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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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 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 ~3--- . ...... .

~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 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 , ~ : , . ~

~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 .:: ,.: , ::, . : ~. ., :,, ~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.

;
. . : , ~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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~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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~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) -- 10 -- .

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~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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~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 .

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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~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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~(183336 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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Claims (12)

The embodiments of the invention in which an exclu-sive property or privilege is claimed are defined as follows:

1. A composition of matter in the form of a compacted structure suitable for sintering comprising a mixture of a nuclear fuel material having a particle size from about 0.02 to about 0.5 microns and from about 0.1 to about 3.0 weight percent ammonium oxalate with the ammonium oxalate being present in clumps of at least about 10 microns in size, and said compacted structure having a density ranging from about 30% to about 70%
of theoretical.

2. A composition according to claim 1 in which the nuclear fuel material is uranium oxide.

3. A composition according to claim 1 in which the nuclear fuel material is uranium dioxide.

4. A composition according to claim 1 in which the ammonium oxalate is present in clumps ranging from about 30 to about 60 microns in size.

5. A composition according to claim 1 in which the nuclear fuel material comprises a mixture of uranium dioxide and plutonium dioxide.

6. A composition according to claim 1 in which the nuclear fuel material comprises a mixture of uranium dioxide and gadolinium oxide.

7. A process for controlling the density of a sintered body of nuclear fuel material by forming discrete porosity therein comprising the steps of (a) providing a powder of the nuclear fuel material with a particle size from about 0.02 to about 0.5 microns, (b) admixing said powder of nuclear fuel material with an additive of a powder of ammonium oxalate in an amount of about 0.1 to about 3.0 percent by weight with the ammonium oxalate being present in clumps of at least about 10 microns in size, (c) forming the resulting mixture into a green body having a density ranging from about 30% to about 70% of theoretical, (d) heating said green body to decompose the ammonium oxalate into gases, and (e) heating the body to produce a sintered body having discrete porosity therein and an end-point density in the range of 90 to 96% of theoretical.

8. A process according to claim 7 in which the nuclear fuel material is comprised of uranium oxide.

9. A process according to claim 7 in which the nuclear fuel material is uranium dioxide.

10. A process according to claim 7 in which the ammonium oxalate is present in clumps ranging from about 30 to about 60 microns in size.

11. A process according to claim 7 in which the nuclear fuel material comprises a mixture of uranium dioxide and plutonium dioxide.

12. A process according to claim 7 in which the nuclear fuel material comprises a mixture of uranium dioxide and gadolinium oxide.