CA1082443A - Fugitive binder for nuclear fuel materials - Google Patents
Fugitive binder for nuclear fuel materialsInfo
- Publication number
- CA1082443A CA1082443A CA262,310A CA262310A CA1082443A CA 1082443 A CA1082443 A CA 1082443A CA 262310 A CA262310 A CA 262310A CA 1082443 A CA1082443 A CA 1082443A
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- Prior art keywords
- binder
- nuclear fuel
- ammonium
- fuel material
- anions
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- Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
- Y02—TECHNOLOGIES OR APPLICATIONS FOR MITIGATION OR ADAPTATION AGAINST CLIMATE CHANGE
- Y02E—REDUCTION OF GREENHOUSE GAS [GHG] EMISSIONS, RELATED TO ENERGY GENERATION, TRANSMISSION OR DISTRIBUTION
- Y02E30/00—Energy generation of nuclear origin
- Y02E30/30—Nuclear fission reactors
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Abstract
ABSTRACT OF THE DISCLOSURE
A process for fabricating a body of a nuclear fuel material has the steps of admixing the nuclear fuel material in power 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, carbamate anions and mixtures of such anions, forming the resulting mixture into a green body such as by die pressing, heating the green body to decompose sub-stantially all of the binder into gases, further heating the body to produce a sintered body, and cooling the sintered body in a controlled atmosphere. Preferred binders used in the practice of this invention include ammonium bicarbonate, ammonium carbonate, ammonium bicarbonate carbamate, ammonium sesquicarbonate, ammonium carbamate and mixtures thereof.
This invention includes a composition of matter in the form of a compacted structure suitable for sintering 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 car-bonate anions, bicarbonate anions, carbamate anions and mixtures of such anions.
A process for fabricating a body of a nuclear fuel material has the steps of admixing the nuclear fuel material in power 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, carbamate anions and mixtures of such anions, forming the resulting mixture into a green body such as by die pressing, heating the green body to decompose sub-stantially all of the binder into gases, further heating the body to produce a sintered body, and cooling the sintered body in a controlled atmosphere. Preferred binders used in the practice of this invention include ammonium bicarbonate, ammonium carbonate, ammonium bicarbonate carbamate, ammonium sesquicarbonate, ammonium carbamate and mixtures thereof.
This invention includes a composition of matter in the form of a compacted structure suitable for sintering 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 car-bonate anions, bicarbonate anions, carbamate anions and mixtures of such anions.
Description
~ 108~'~43 24-NF-04109 The present invention relates generally to the art of fonming and sintering ceramic powders and is particularly concerned with a method for sintering a uranium dioxide nuclear fuel body having a fugitive binder.
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 especially 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. It is not a free-flowing powder but clumps and agglomerates, making it difficult to pack in reactor tubes to the aesired density.
The spocific composition of a given commerci~l uranium dioxide powder may also prevent it from being used directly a~ a nuclear fuel. Uranium dioxide is an exception to the law of difinite proportions since "U02" actually denotes a single, ~table phase that may vary in composition from U01 7 to U02 25 Because thermal conductivity docre~e~ with incrieasing 0/U ratios, uranium dioxide having as low an 0/U
ratio as possible i~ preferred. However, since uranium dioxide powder oxidizes ea~ily in air and absorbs moi~ture readily, the 0/U ratio of this powder is significantly in exce~s of that acceptable for fue~.
v A ~ er of method6 have been used to make uranium di-oxide powder ~uitable a~ a nuclear fuel. Pre~ently, the most common ~ethod is to die press the powder into cylindri-eally -sh3ped green bodies of specific size without the a~-si~tance of fugitive binders since the complete removal of these binders and their decomposition products i3 difficult . ~
24_NF-04109 108;~ 3 to achieve prior to sintering. The entrainment of binder residues is unacceptable in sintered nuclear fuels. Sin-tering atmospheres may range from about 1000 C to about 2400 C with the particular sintering temperature depeinding 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 C. The sintering operation is designed to densify the bodies and bring them down to the desired 0/U ratio or close to the desired 0/U ratio.
Although uranium dioxide suitable as a nuclear fuel can have an 0/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 ~ince it can be consistently produced in com-mercial sintering operations. In some instances, it may be desirable to maintain the 0/U ratio of the uranium di-oxide 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 havinq an 0/U ratio as high as 2 195, and then later treat the sin-tered product in a reducing atmosphere to obtain the desired 0/U ratio.
One of the principal specifications for uranium dioxide sintered bodies to be used for a nuclear reactor is their density. The actual value may vary but in general uranium dioxide sintered bodies having dengities of the order of 90% to 95~ of theoretical density are specified and occas-ionally a density as low as 85% of theoretical i8 ~pecified.
Most pressed uranium dioxide powder, however, will ~intorf-to final densities of about 96% to 98% of theoretical There-fore, to obtain gintered bodies with lower densities the time -~ 24_NF_04109 108;~43 and temperature must ~e carefully controlled to allow the shrinkage of the body to proceed only to the desired value.
Thi~ is inherently more difficult than the use of a process which i~ allowed to go to completion. Specifically, small variations during sintering can result in large variations or no significant variation~ in the sintered body of com-pacted powder depending on a number of factors such as the powder chemistry, particle size and agglomeration. Generally, however, a change in sintering time such as, for example an hour or two, does not significantly change the density of the final ~intered product. Also, when sintered bodie~
having the desired low density have been attained by care-fully controlling sintering time and temperature, it has ~een found that these sintered bodies, when placed in the reactor, ~requently undergo additional sintering within the reactor thereby interfering with proper reactor operation.
A number of technique~ have been usod in the past to reduce the don~ity o~ the ~intered body other than varying time and t~mperature, For example, one technique has been to press the uranium dioxide powder, break it up and repress it. The problem with this technique is that the resulting sintered body ha~ large interconnecting pores throughout the body which extend out to the ~urface resulting in a large ~ -exposed surface area which can absorb into the body signi- -~
ficant amountR of gaæes, and in particular water in the form of water vapor. ~uring reactor operation these gase~ are liborated providing a possible source of corrosion for the fuel cladding. Another method involves adding a plastic of se~ected particle si2e to the uranium dioxide powder. The admixed powder is then pres~ed and sintered, however~ the decomposition of the plastic during sintering usually re~ults in carbon residues which contaminate the nuclear fuel.
~08~443 24-NF-04109 In Canadian patent No. 1,012,243 dated June 21, 1977 in the name of Kenneth W. Lay and assigned to the same assignee as the present invention, there is disclosed a process for controlling the end-point density of a sintered uranium dioxide nuclear fuel body and the resulting product.
Uranium dioxide powder having a size ranging up to 10 microns is admixed with a precursor to uranium dioxide, such as ammonium diuranate, having an average agglomerated particle size ranging from about 20 microns to 1 millimeter and the mixture is formed into a pressed compact or green body.
The body of the precursor and the uranium dioxide has a density lower than that of the uranium dioxide powder and the precursor is used in an amount which results in discrete low density regions in the green body which range from about 5% to 25% by volume of the green body. The green body is sintered to decompose the precursor and produce a sintered body having discrete low density porous regions which reduce the end-point density of the sintered body by at least 2~. The sintered body has an end-point density ranging from 85% to 95~ of theoretical.
In Canadian patent application Serial No. 257,625 dated July 23, 1976 and assigned to the same assignee as the present invention there is disclosed a process for controlling the final or end-point density of a sintered uranium dioxide nuclear fueI body by adding ammonium oxalate to a nuclear fueI material such as uranium dioxide before pressing into a green body. This addition results in discrete low density porous regions in the sintered body which correspond to the ammonlum oxalate particles. ~he end-point density of the sintered body is, therefore, a function of the amount of ammonium oxalate added.
~08'~443 24-NF-04109 As previously mentioned, conventional organic or plastic binder~ are unsuitable for use in powder fabrication since they tend to contaminate the interior of the sintered body with impurities such as hydrides. These binders are normally B c~ r~
oon~crcd to gases during the sintering step and these gases must be removed, requiring special apparatus or procedures.
}n addition, upond decomposition, these prior art binder materials often leave deposits of organic materials in the equipment utilized to sintered the article, thereby com-plicating the maintenance procedures for the equipment.
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 along with normal compacting pressures and sintering temperatures inhibits the formation of these strong bonds. The higher compacting pr-s~ures and sintering temperatures required to develop such bonds sharply reduce the desired porosity.
There i8 a particular need, therefore, in the art of preparing sintered bodie~ for nuclear reactors by powder ceramic technigues for a binder which will impart an ad-equate degree of green strength without contaminating the interior of such bodies and which will permit, through sintering~ the formation of strong bonds between particles without deleteriously affecting the porosity.
This invention presents the improvement of utilizing a binder of a compound or its hydration products containing ammonium cations and anions selected from the group con-sisting 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 ammonium sesqui_ _ 5 _ ~o ~'~ 4~3 24-NF-04109 carbonate, ammonium carbamate and mixtures thereof, in a powder ceramic process for imparting green strength to articles cold pressed from nuclear fuel powders of varying particles size and a particular shape or configuration for which it i8 de~ired to maintain a certain degree of porosity, uniformity of pore size, a lack of interconnections between the pores and the shape or configuration of the base material particles in the final article after sintering. The binders disclosed in this invention are efficient ~inders for use in nuclear fuel~, and further the binders enable the realization of defect free, pressed bodies of nuclear fuel material~ and ten~ile strength in the bodies comparable to strengths achieved with long chain hydrocarbon binders. Further the binders in this invention leave substantially no impurities in the nuclear fuel material since these binders decompose upon heating into ammonia (NH3), carbon dioxide (C02) and water (H20) (or wat~r vapor) at temperatures as low a~ ~C
The binder addition to nuclear fuel material as presented in thi~ invention enables the practice of a process for forming and sintering a body of a nuclear fuel having the step~ of admixing the nuclear fuel material in particulate form with the binder, forming the re~ulting mixture into a green body having a density ranging from about 30X to about 70X of theoretical density of the nuclear fuel material, heating saia green body to decompose substantially a}l the hinder into gases, further heating the body to produce a ~ntered body and cooling the sintered body in a controlled atmosphere.
Thi8 invention also provide a composition of matter that i~ ~uitable for sintering in the form of a compacted structure comprising a mixture of a nuclear fuel material and a binder of a compound or it~ hydration products containing ammonium 108~443 24_NF_04109 cations and ~nions selected from the group consisting of carbonate anions, bicarbonate anions, carbamate anions and mixtures of such aniona and preferably a binder selected from the group consisting of ammonium bicarbonate, ammonium carbonate and mixtures thereof.
It is an object of this invention to provide an additive of a binder of a compound or its hydration products con-tain~ng ammonium cations and anions selected from the group consisting of carbonate anions, bicarbonate anions, car- -bamate anions and mixtures to bind the particles of nuclear fuel into green shapes suitable for sintering.
Another preferred object of this invention i8 to provide a binder selected from the group consisting of am~onium bicar-bonate, ammonium carbonate, ammonium bicarbonate carbamate, ammonium sesquicarbonate, ammonium carbamate or mixtures thereof as an addition to nuclear fuel materials and which, upon heating at moderate temperaturos before a sintering proceJ~, decompo~e into gases and leave sub~tant~ally no impurities in the ~intered structure of the nuclear fuel material.
Still another object of this invontion is to provide a proce~s for ~intering green shapes of a nuclear fuel material u~ing a binder of a compound or its hydration product con-taining ammonium cations and anions solected from the group consisting of carbonate ions, bicarbonate anion~, carbamate anions and mixtures of such an~ons.
Other o~ject~ and advantages of this invention will ~come apparent ~rom the following ~pecification and the appended claims.
Figure 1 presents a graph of tensile strengtb versu~
die pressing pressure for one group of green pellets without a binder and one group of green pellets w~th a binder dis-iO82~3 24-NF-04109 closed in this invention.
Figures 2 and 3 present photomicrographs (at a mignifi-cation of 25 and 100 times respectively) of uranium dioxide pellets produced according to the teachings of Example 2 Figure 4 presents a graph of tensile strength ver~us die pressing pressure for one group of unsintered pellets without a binder and one group of unsintered pellets with a binder di~closed in this invention.
It has now been discovered that a process for sintering a green body of a nuclear fuel material having high relia-bility can be achieved by admixing a fugitive binder of a --compound or its hydration products containing ammonium cations and anions selected from the group consisting of carbonate anion~, b$carbonate anions, carbamate anions and mixtures of ~uch anions with a nuclear fuel material in powder form.
In greater detail the process can be conducted by practicing the step~ of providing a powder of the nuclear fuel material, ad~ixing the nuclear fuel material w~th a binder of a com-pound or lt~ hydration product~ containing ammonium cation~
and anions solected from the group consisting of carbonate anions, bicarbonate anions, carbamate anions and mixture~
of such anions, forming the resulting mixture into a green body having a density ranging from about 30X to about 7~X
of theoretical density, he~ting the green body sufficiently to decompose the ~inder into gases and thereafter heating the body to produce a sintered body having a controlled porosity and a controlled density.
The practice of the foregoing process results in the production of a composition of matter in the form of a compacted structure suitable for sintering and i8 comprised of a mixture of a nuclear fuel material and a binder of a compound or its hydration products containing ammonium cations 10 8 ~ ~ 4 3 24-NF-04109 and anions selected from the group consisting of carbonate - -anions, bicarbonate anions, carbamate anions and mixtures of such anions.
As used herein, nuclear fuel material is intended to cover 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 hereof 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 a# gadolinium oxide ~Gd203).
In carrying out the pre~ent processes whieh will be discussed for the preferred use of uranium dioxide, the uranium dioxide powder (or partiele~) used generally has an oxygen to uranium atomie ratio greater than 2.00 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 partiele ~ize Such particle sizes allow the sintering to be carried out within a reasonable length of time and at temperatures practical for com~ercial applications. For most applications, to obtain rapid sin-tering, the uranium dioxide powder has a size ranging up to about 1 micron. Commercial uranium dioxide powder~ are preferred and these are of ~mall partiele size, u~ually sub-micron generally ranging from about 0.02 micron to 0.
mieron.
Compositions suitable for use as a binder in the practiee of this invention either alone or in mixtures, include am-monium ~iearbon~te> a~on~um ear~onate, ammonium bicarbonate ~ 82~43 24_NF-04109 carbamate, ammonium sesquicar~onate, ammonium carbamate an~
mixtures thereof. When mixed with nuclear fuel materials, these binders and the nuclear fuel material are believed to undergo the phenomenon of adhesion forming ammonium derlva-tive of the carbonate series such as 4 2 3 3'] (~H4)6 CU2)2 (C03)5 (H20) ~ H O
4 2 ~Uo2(co3)2 (H20) ~, ~H ) ~U2)2 (C3)3 (OH)(H2 5~ , ~H4 [U02 (C03) ~OH)(~20)3~ and U02C03.~20,or mixtures of these.
In the present invention the binder should have certain eharacteristies. It must be substantially comprised of a eompound or its hydration products eontaining ammonium cations and anions seleeted from the group eonsisting of carbonate anions, biearbonate an~ons, carbamate anions and mixtures of sueh anioas and free of impurities so that it ean be mixed wlth uranium dioxide powder and pressed and sintered without leaving any unde~ired impurities after heating with particu-larly preferred binders being ammonium biearbonate and ammonium earbonate and mixtures thereof. It has been found that eommereially available ammonium bicarbonate contains virtually no impurities and eommercially available ammonium earbonate al~o eontains virtually no impurities exeept for other ammonium eompounds a~ listed in the foregoing para-graph. Thermogravimetric analysi~ confirms that there is a eomplete volatilization of ammonium biearbonate and am-monium ear~onate at heating rates typieally u~ed for re-duetive atmospherie U02 sintering. Ammonium biearbonate ~nd ammonium carbonate ~hen heated to the temperature range of decomposition, decompose to form ammonia, earbon dioxide and water at signifieant rates leaving substantially no eontaminates ~impurities) in the fuel and no undesirable 108~4~3 24_NF_04}09 residues in the sintering furnace. Additionally the ammoni-um bicarbonate and the ammonium carbonate are used in small particle sizes of 400 mesh or less in order to achieve maximum pla~tic flow of the binder into the interstices of the nuclear fuel material. Ammonium carbonate is used as the binder when the combination of binding and density reducing pores is desired in the nuclear fuel. 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 demon~trated by the fact that these compounds can be die pressed to green densities as -high as 90X of theoretical density at moderate pressing pre~sures, The amount of ~inder 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 le~s readily formable pow-ders require larger amount~ of binder. When the selectedbinder i~ ammonium carbonate, the amount of the addition of this binder is dependerlt upon the de~ired sintered density for the nuclear fuel material.
Homogenous blending of the binder with the nuclear fuel material is practiced to develop fully the binding action of the b~nder on the nuclear fuel material. Where porosity or a lower density i~ not desired, the homogeneous ~lending of the binder with the nuclear fuel material avoids the formation of agglomerates of the binder since ~uch agglo-merates can volatize durin~ 6intering leaving pores in thesintered nuclear fuel material which pores reduce the density of the nuclear fuel material in sintered ~odies. When it is -~- lO~X~3 24_NF-04109 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 agglo-merates are destroyed. The blended and milled powder may then be predensified by low pressure die pressing followed by granulation through a sizing screen to flowability of the mixture.
In order to control the density of sintered bodies of nuclear fuel material, pore formers such as ammonium oxlate or a uranium precursor may be added to the nuclear fuel material along with the binders of this invention. The pore former can be mixed either at the same time as the binders di~closed in this invention or during a subsequent mixing step In the event that the nuclear fuel material, binder and pore former are mixed and then milled to promote hom-ogeneity, the processing is conducted to yield an acceptable particle ~izé~ after milling to as~ure the formation of poros during sintering.
The ~esulting mixture of nuclear fuel material with the binders of this invention, wit~30r without pore former, can be formed into a green body, generally a cylindrical pellet by a num~er of techniques such a~ pressing (particularly die pressing) Specifically, the ~ixture i~ compressed into a form in which it has the required mechanical strength for handling and which, after sintering, i~ of the ~ize which satisfies reactor specification. Ths pre~ence of the bindera of this invention in the nuclear fuel material ~ignificantly enhances both the strength and integrity of the re~ulting green body. The green body can have a density ranging from 3~ about 30X to 70~ of theoretical, but usually it has a density ranging from about ~0% to 60X of theoretical, and preferably about 50% of theoretical.
- 12 _ 1~8~ 3 24_NF-04109 The green body is sintered in an atmosphere which depends on the particular manufacturing process. Specifically, it $8 an atmosphere which can be used to sintered uranium dioxide alone in the production of uranium dioxide nuclear fuel and a 180 it must be an atmosphere which is compatible with the gases resulting from the decomposition of ammonium bicarbonate. For example, a number of atmosphere can be used such as an inert atmosphere, a reducing atmosphere (e g dry hydrogen) or a controlled atmosphere comprised of a mixture of gases (e g. a mixture of hydrogen and carbon dioxide a~ set forth in U.S. Patent No. 3,872,022 dated March 18, 1975) which in equilibrium produces a partial pre~sure of oxygen sufficient to maintain the uranium dioxide at a desired oxygen to uranium ratio.
The rate of heating to sintering temperature i8 limited largely by how fa~t the by-produce gase~ are romoved prior to ach$eving a sintering temperature and generally this depends on the ga~ flow rate through the furnace and its uniformity therein as well as the amount of material in the furnance. Specifically, the rate of flow of gas through the furnace, which ordinarily i8 ub~ntially the same gas flow used in the sintering atmosphere, should be suf-ficient to remove the g~es re~ulting from decomposition of ammonium bicarbonate before sintering temperature i~ reached.
Generally, best result~ are obtained when the rate of heating to decompose the binder range~ from about 50C
per hour to about 300 C per hour After decompo~ition of the binder i8 completed and by-product gases ~ubstantially D fc~r~ce IL~ removed from the ~urnancc, the rate of heating can then be incroasea, if desired, to a rango of about 300 C to 500 C
per hour and as high as 800 C per hour but not be 80 rapid as to crack the bodies _ 13 -~08~43 Upon completion of sintering, the sintered body is usually cooled to room temperature. The rate of cooling of the sintered body i8 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 u~ually used in commercial sintering furnaces. TheQe cooling rates may range from 100 C to about 800 C per hour, and generally, from about 400 C per hour to 600C per hour The sintered uranium dioxide bodies are preferably cooled in the same atmosphere in which they were sintered.
This invention provides several advantages in the sintering of nuclear fuel materials and in the resulting sintered pellets. The addition of the binders of this invention, particularly ammonium bicarbonate, ammonium carbonate, or mixture~ thereof, does not leave any un-de-irable residuo in the ~intered pellet~. Thermogravi-metric analy~ ha~ ~hown that ammonium bicarbonate and ammonium carbonate decompose completely into ammonia (NH3), carbon dioxide (C02) and water vapor (H20) The early decomposition of ammonium bicarbonate and ammonium carbonate prevents the entrapmont of undesirable ga~es in the micro-structure of the nuclear fuel material during the sintering process Pellets incorporating ammonium bicarbonate or ammonium carbonate according to the teachings of thi~ in-vention can be sintered using conventional wet hydrogen as a sintering ga~ or controlled atmosphere sintering under an atmosphere comprising a ~ixture of hydrogen and carbon dioxide. The proces~ i9 conducted so that the gases from the decompo~ition of ammonium bicarbonate or amm~nium car_ bonate are excluded from the sintering atmosphere such a~
~y using the countercurrent f~ow of the sintering at-~ 8~3 24_NF-04109 mosphere in a sintering furnace.
The invention is further illustrated by the following examples.
Ammonium bicarbonate Wa8 hammer milled to an average particle size of about 20 microns, Uranium dioxide having an oxygen to uranium ratio of about 2.05 and an average particle size of 0,7 microns was blended with the ammonium bicaxbonate in a ratio of 1.3 grams of ammonium bicarbonate to 98,7 grams of uranium dioxide, Three thousand gram~ of blended powder were prepared in this manner, The blended powders were hammer milled to destroy any uranium dioxide aggrates in order to as~ure a homogenous distribution of ammonium bicarbonate in the uranium dioxide powder, The hammer milled powder wa~ die pressed at 6700 psi --to increa~e ~ts bul~ density and the resulting structures w~re cru~hed through a 12 mesh screen to promote both flowability and control of the agglomerate size.
The resulting powder was die pre~sed into cylindrical fuel pellets u~ing pressure~ ranging from 26,000 to 54,000 pBi, As a reference, 2 group~ of fuel pellets were also die pressed at the ~ame pressures from the original batch of uranium dioxide powder, One reference group (herein-after ~roup 11 conta~ned no biDder and received no other processing prior to pressing. The other reference group (hereinafter Group 2) also contained no binder but WQS
hammer milled, die pressed, and crushed through a sizing screen prior to die pressing.
Tensile ~trengths as mea~ured by diametrical com-pre~sion tests were csnducted on the binder pellets and the
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 especially 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. It is not a free-flowing powder but clumps and agglomerates, making it difficult to pack in reactor tubes to the aesired density.
The spocific composition of a given commerci~l uranium dioxide powder may also prevent it from being used directly a~ a nuclear fuel. Uranium dioxide is an exception to the law of difinite proportions since "U02" actually denotes a single, ~table phase that may vary in composition from U01 7 to U02 25 Because thermal conductivity docre~e~ with incrieasing 0/U ratios, uranium dioxide having as low an 0/U
ratio as possible i~ preferred. However, since uranium dioxide powder oxidizes ea~ily in air and absorbs moi~ture readily, the 0/U ratio of this powder is significantly in exce~s of that acceptable for fue~.
v A ~ er of method6 have been used to make uranium di-oxide powder ~uitable a~ a nuclear fuel. Pre~ently, the most common ~ethod is to die press the powder into cylindri-eally -sh3ped green bodies of specific size without the a~-si~tance of fugitive binders since the complete removal of these binders and their decomposition products i3 difficult . ~
24_NF-04109 108;~ 3 to achieve prior to sintering. The entrainment of binder residues is unacceptable in sintered nuclear fuels. Sin-tering atmospheres may range from about 1000 C to about 2400 C with the particular sintering temperature depeinding 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 C. The sintering operation is designed to densify the bodies and bring them down to the desired 0/U ratio or close to the desired 0/U ratio.
Although uranium dioxide suitable as a nuclear fuel can have an 0/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 ~ince it can be consistently produced in com-mercial sintering operations. In some instances, it may be desirable to maintain the 0/U ratio of the uranium di-oxide 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 havinq an 0/U ratio as high as 2 195, and then later treat the sin-tered product in a reducing atmosphere to obtain the desired 0/U ratio.
One of the principal specifications for uranium dioxide sintered bodies to be used for a nuclear reactor is their density. The actual value may vary but in general uranium dioxide sintered bodies having dengities of the order of 90% to 95~ of theoretical density are specified and occas-ionally a density as low as 85% of theoretical i8 ~pecified.
Most pressed uranium dioxide powder, however, will ~intorf-to final densities of about 96% to 98% of theoretical There-fore, to obtain gintered bodies with lower densities the time -~ 24_NF_04109 108;~43 and temperature must ~e carefully controlled to allow the shrinkage of the body to proceed only to the desired value.
Thi~ is inherently more difficult than the use of a process which i~ allowed to go to completion. Specifically, small variations during sintering can result in large variations or no significant variation~ in the sintered body of com-pacted powder depending on a number of factors such as the powder chemistry, particle size and agglomeration. Generally, however, a change in sintering time such as, for example an hour or two, does not significantly change the density of the final ~intered product. Also, when sintered bodie~
having the desired low density have been attained by care-fully controlling sintering time and temperature, it has ~een found that these sintered bodies, when placed in the reactor, ~requently undergo additional sintering within the reactor thereby interfering with proper reactor operation.
A number of technique~ have been usod in the past to reduce the don~ity o~ the ~intered body other than varying time and t~mperature, For example, one technique has been to press the uranium dioxide powder, break it up and repress it. The problem with this technique is that the resulting sintered body ha~ large interconnecting pores throughout the body which extend out to the ~urface resulting in a large ~ -exposed surface area which can absorb into the body signi- -~
ficant amountR of gaæes, and in particular water in the form of water vapor. ~uring reactor operation these gase~ are liborated providing a possible source of corrosion for the fuel cladding. Another method involves adding a plastic of se~ected particle si2e to the uranium dioxide powder. The admixed powder is then pres~ed and sintered, however~ the decomposition of the plastic during sintering usually re~ults in carbon residues which contaminate the nuclear fuel.
~08~443 24-NF-04109 In Canadian patent No. 1,012,243 dated June 21, 1977 in the name of Kenneth W. Lay and assigned to the same assignee as the present invention, there is disclosed a process for controlling the end-point density of a sintered uranium dioxide nuclear fuel body and the resulting product.
Uranium dioxide powder having a size ranging up to 10 microns is admixed with a precursor to uranium dioxide, such as ammonium diuranate, having an average agglomerated particle size ranging from about 20 microns to 1 millimeter and the mixture is formed into a pressed compact or green body.
The body of the precursor and the uranium dioxide has a density lower than that of the uranium dioxide powder and the precursor is used in an amount which results in discrete low density regions in the green body which range from about 5% to 25% by volume of the green body. The green body is sintered to decompose the precursor and produce a sintered body having discrete low density porous regions which reduce the end-point density of the sintered body by at least 2~. The sintered body has an end-point density ranging from 85% to 95~ of theoretical.
In Canadian patent application Serial No. 257,625 dated July 23, 1976 and assigned to the same assignee as the present invention there is disclosed a process for controlling the final or end-point density of a sintered uranium dioxide nuclear fueI body by adding ammonium oxalate to a nuclear fueI material such as uranium dioxide before pressing into a green body. This addition results in discrete low density porous regions in the sintered body which correspond to the ammonlum oxalate particles. ~he end-point density of the sintered body is, therefore, a function of the amount of ammonium oxalate added.
~08'~443 24-NF-04109 As previously mentioned, conventional organic or plastic binder~ are unsuitable for use in powder fabrication since they tend to contaminate the interior of the sintered body with impurities such as hydrides. These binders are normally B c~ r~
oon~crcd to gases during the sintering step and these gases must be removed, requiring special apparatus or procedures.
}n addition, upond decomposition, these prior art binder materials often leave deposits of organic materials in the equipment utilized to sintered the article, thereby com-plicating the maintenance procedures for the equipment.
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 along with normal compacting pressures and sintering temperatures inhibits the formation of these strong bonds. The higher compacting pr-s~ures and sintering temperatures required to develop such bonds sharply reduce the desired porosity.
There i8 a particular need, therefore, in the art of preparing sintered bodie~ for nuclear reactors by powder ceramic technigues for a binder which will impart an ad-equate degree of green strength without contaminating the interior of such bodies and which will permit, through sintering~ the formation of strong bonds between particles without deleteriously affecting the porosity.
This invention presents the improvement of utilizing a binder of a compound or its hydration products containing ammonium cations and anions selected from the group con-sisting 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 ammonium sesqui_ _ 5 _ ~o ~'~ 4~3 24-NF-04109 carbonate, ammonium carbamate and mixtures thereof, in a powder ceramic process for imparting green strength to articles cold pressed from nuclear fuel powders of varying particles size and a particular shape or configuration for which it i8 de~ired to maintain a certain degree of porosity, uniformity of pore size, a lack of interconnections between the pores and the shape or configuration of the base material particles in the final article after sintering. The binders disclosed in this invention are efficient ~inders for use in nuclear fuel~, and further the binders enable the realization of defect free, pressed bodies of nuclear fuel material~ and ten~ile strength in the bodies comparable to strengths achieved with long chain hydrocarbon binders. Further the binders in this invention leave substantially no impurities in the nuclear fuel material since these binders decompose upon heating into ammonia (NH3), carbon dioxide (C02) and water (H20) (or wat~r vapor) at temperatures as low a~ ~C
The binder addition to nuclear fuel material as presented in thi~ invention enables the practice of a process for forming and sintering a body of a nuclear fuel having the step~ of admixing the nuclear fuel material in particulate form with the binder, forming the re~ulting mixture into a green body having a density ranging from about 30X to about 70X of theoretical density of the nuclear fuel material, heating saia green body to decompose substantially a}l the hinder into gases, further heating the body to produce a ~ntered body and cooling the sintered body in a controlled atmosphere.
Thi8 invention also provide a composition of matter that i~ ~uitable for sintering in the form of a compacted structure comprising a mixture of a nuclear fuel material and a binder of a compound or it~ hydration products containing ammonium 108~443 24_NF_04109 cations and ~nions selected from the group consisting of carbonate anions, bicarbonate anions, carbamate anions and mixtures of such aniona and preferably a binder selected from the group consisting of ammonium bicarbonate, ammonium carbonate and mixtures thereof.
It is an object of this invention to provide an additive of a binder of a compound or its hydration products con-tain~ng ammonium cations and anions selected from the group consisting of carbonate anions, bicarbonate anions, car- -bamate anions and mixtures to bind the particles of nuclear fuel into green shapes suitable for sintering.
Another preferred object of this invention i8 to provide a binder selected from the group consisting of am~onium bicar-bonate, ammonium carbonate, ammonium bicarbonate carbamate, ammonium sesquicarbonate, ammonium carbamate or mixtures thereof as an addition to nuclear fuel materials and which, upon heating at moderate temperaturos before a sintering proceJ~, decompo~e into gases and leave sub~tant~ally no impurities in the ~intered structure of the nuclear fuel material.
Still another object of this invontion is to provide a proce~s for ~intering green shapes of a nuclear fuel material u~ing a binder of a compound or its hydration product con-taining ammonium cations and anions solected from the group consisting of carbonate ions, bicarbonate anion~, carbamate anions and mixtures of such an~ons.
Other o~ject~ and advantages of this invention will ~come apparent ~rom the following ~pecification and the appended claims.
Figure 1 presents a graph of tensile strengtb versu~
die pressing pressure for one group of green pellets without a binder and one group of green pellets w~th a binder dis-iO82~3 24-NF-04109 closed in this invention.
Figures 2 and 3 present photomicrographs (at a mignifi-cation of 25 and 100 times respectively) of uranium dioxide pellets produced according to the teachings of Example 2 Figure 4 presents a graph of tensile strength ver~us die pressing pressure for one group of unsintered pellets without a binder and one group of unsintered pellets with a binder di~closed in this invention.
It has now been discovered that a process for sintering a green body of a nuclear fuel material having high relia-bility can be achieved by admixing a fugitive binder of a --compound or its hydration products containing ammonium cations and anions selected from the group consisting of carbonate anion~, b$carbonate anions, carbamate anions and mixtures of ~uch anions with a nuclear fuel material in powder form.
In greater detail the process can be conducted by practicing the step~ of providing a powder of the nuclear fuel material, ad~ixing the nuclear fuel material w~th a binder of a com-pound or lt~ hydration product~ containing ammonium cation~
and anions solected from the group consisting of carbonate anions, bicarbonate anions, carbamate anions and mixture~
of such anions, forming the resulting mixture into a green body having a density ranging from about 30X to about 7~X
of theoretical density, he~ting the green body sufficiently to decompose the ~inder into gases and thereafter heating the body to produce a sintered body having a controlled porosity and a controlled density.
The practice of the foregoing process results in the production of a composition of matter in the form of a compacted structure suitable for sintering and i8 comprised of a mixture of a nuclear fuel material and a binder of a compound or its hydration products containing ammonium cations 10 8 ~ ~ 4 3 24-NF-04109 and anions selected from the group consisting of carbonate - -anions, bicarbonate anions, carbamate anions and mixtures of such anions.
As used herein, nuclear fuel material is intended to cover 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 hereof 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 a# gadolinium oxide ~Gd203).
In carrying out the pre~ent processes whieh will be discussed for the preferred use of uranium dioxide, the uranium dioxide powder (or partiele~) used generally has an oxygen to uranium atomie ratio greater than 2.00 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 partiele ~ize Such particle sizes allow the sintering to be carried out within a reasonable length of time and at temperatures practical for com~ercial applications. For most applications, to obtain rapid sin-tering, the uranium dioxide powder has a size ranging up to about 1 micron. Commercial uranium dioxide powder~ are preferred and these are of ~mall partiele size, u~ually sub-micron generally ranging from about 0.02 micron to 0.
mieron.
Compositions suitable for use as a binder in the practiee of this invention either alone or in mixtures, include am-monium ~iearbon~te> a~on~um ear~onate, ammonium bicarbonate ~ 82~43 24_NF-04109 carbamate, ammonium sesquicar~onate, ammonium carbamate an~
mixtures thereof. When mixed with nuclear fuel materials, these binders and the nuclear fuel material are believed to undergo the phenomenon of adhesion forming ammonium derlva-tive of the carbonate series such as 4 2 3 3'] (~H4)6 CU2)2 (C03)5 (H20) ~ H O
4 2 ~Uo2(co3)2 (H20) ~, ~H ) ~U2)2 (C3)3 (OH)(H2 5~ , ~H4 [U02 (C03) ~OH)(~20)3~ and U02C03.~20,or mixtures of these.
In the present invention the binder should have certain eharacteristies. It must be substantially comprised of a eompound or its hydration products eontaining ammonium cations and anions seleeted from the group eonsisting of carbonate anions, biearbonate an~ons, carbamate anions and mixtures of sueh anioas and free of impurities so that it ean be mixed wlth uranium dioxide powder and pressed and sintered without leaving any unde~ired impurities after heating with particu-larly preferred binders being ammonium biearbonate and ammonium earbonate and mixtures thereof. It has been found that eommereially available ammonium bicarbonate contains virtually no impurities and eommercially available ammonium earbonate al~o eontains virtually no impurities exeept for other ammonium eompounds a~ listed in the foregoing para-graph. Thermogravimetric analysi~ confirms that there is a eomplete volatilization of ammonium biearbonate and am-monium ear~onate at heating rates typieally u~ed for re-duetive atmospherie U02 sintering. Ammonium biearbonate ~nd ammonium carbonate ~hen heated to the temperature range of decomposition, decompose to form ammonia, earbon dioxide and water at signifieant rates leaving substantially no eontaminates ~impurities) in the fuel and no undesirable 108~4~3 24_NF_04}09 residues in the sintering furnace. Additionally the ammoni-um bicarbonate and the ammonium carbonate are used in small particle sizes of 400 mesh or less in order to achieve maximum pla~tic flow of the binder into the interstices of the nuclear fuel material. Ammonium carbonate is used as the binder when the combination of binding and density reducing pores is desired in the nuclear fuel. 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 demon~trated by the fact that these compounds can be die pressed to green densities as -high as 90X of theoretical density at moderate pressing pre~sures, The amount of ~inder 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 le~s readily formable pow-ders require larger amount~ of binder. When the selectedbinder i~ ammonium carbonate, the amount of the addition of this binder is dependerlt upon the de~ired sintered density for the nuclear fuel material.
Homogenous blending of the binder with the nuclear fuel material is practiced to develop fully the binding action of the b~nder on the nuclear fuel material. Where porosity or a lower density i~ not desired, the homogeneous ~lending of the binder with the nuclear fuel material avoids the formation of agglomerates of the binder since ~uch agglo-merates can volatize durin~ 6intering leaving pores in thesintered nuclear fuel material which pores reduce the density of the nuclear fuel material in sintered ~odies. When it is -~- lO~X~3 24_NF-04109 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 agglo-merates are destroyed. The blended and milled powder may then be predensified by low pressure die pressing followed by granulation through a sizing screen to flowability of the mixture.
In order to control the density of sintered bodies of nuclear fuel material, pore formers such as ammonium oxlate or a uranium precursor may be added to the nuclear fuel material along with the binders of this invention. The pore former can be mixed either at the same time as the binders di~closed in this invention or during a subsequent mixing step In the event that the nuclear fuel material, binder and pore former are mixed and then milled to promote hom-ogeneity, the processing is conducted to yield an acceptable particle ~izé~ after milling to as~ure the formation of poros during sintering.
The ~esulting mixture of nuclear fuel material with the binders of this invention, wit~30r without pore former, can be formed into a green body, generally a cylindrical pellet by a num~er of techniques such a~ pressing (particularly die pressing) Specifically, the ~ixture i~ compressed into a form in which it has the required mechanical strength for handling and which, after sintering, i~ of the ~ize which satisfies reactor specification. Ths pre~ence of the bindera of this invention in the nuclear fuel material ~ignificantly enhances both the strength and integrity of the re~ulting green body. The green body can have a density ranging from 3~ about 30X to 70~ of theoretical, but usually it has a density ranging from about ~0% to 60X of theoretical, and preferably about 50% of theoretical.
- 12 _ 1~8~ 3 24_NF-04109 The green body is sintered in an atmosphere which depends on the particular manufacturing process. Specifically, it $8 an atmosphere which can be used to sintered uranium dioxide alone in the production of uranium dioxide nuclear fuel and a 180 it must be an atmosphere which is compatible with the gases resulting from the decomposition of ammonium bicarbonate. For example, a number of atmosphere can be used such as an inert atmosphere, a reducing atmosphere (e g dry hydrogen) or a controlled atmosphere comprised of a mixture of gases (e g. a mixture of hydrogen and carbon dioxide a~ set forth in U.S. Patent No. 3,872,022 dated March 18, 1975) which in equilibrium produces a partial pre~sure of oxygen sufficient to maintain the uranium dioxide at a desired oxygen to uranium ratio.
The rate of heating to sintering temperature i8 limited largely by how fa~t the by-produce gase~ are romoved prior to ach$eving a sintering temperature and generally this depends on the ga~ flow rate through the furnace and its uniformity therein as well as the amount of material in the furnance. Specifically, the rate of flow of gas through the furnace, which ordinarily i8 ub~ntially the same gas flow used in the sintering atmosphere, should be suf-ficient to remove the g~es re~ulting from decomposition of ammonium bicarbonate before sintering temperature i~ reached.
Generally, best result~ are obtained when the rate of heating to decompose the binder range~ from about 50C
per hour to about 300 C per hour After decompo~ition of the binder i8 completed and by-product gases ~ubstantially D fc~r~ce IL~ removed from the ~urnancc, the rate of heating can then be incroasea, if desired, to a rango of about 300 C to 500 C
per hour and as high as 800 C per hour but not be 80 rapid as to crack the bodies _ 13 -~08~43 Upon completion of sintering, the sintered body is usually cooled to room temperature. The rate of cooling of the sintered body i8 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 u~ually used in commercial sintering furnaces. TheQe cooling rates may range from 100 C to about 800 C per hour, and generally, from about 400 C per hour to 600C per hour The sintered uranium dioxide bodies are preferably cooled in the same atmosphere in which they were sintered.
This invention provides several advantages in the sintering of nuclear fuel materials and in the resulting sintered pellets. The addition of the binders of this invention, particularly ammonium bicarbonate, ammonium carbonate, or mixture~ thereof, does not leave any un-de-irable residuo in the ~intered pellet~. Thermogravi-metric analy~ ha~ ~hown that ammonium bicarbonate and ammonium carbonate decompose completely into ammonia (NH3), carbon dioxide (C02) and water vapor (H20) The early decomposition of ammonium bicarbonate and ammonium carbonate prevents the entrapmont of undesirable ga~es in the micro-structure of the nuclear fuel material during the sintering process Pellets incorporating ammonium bicarbonate or ammonium carbonate according to the teachings of thi~ in-vention can be sintered using conventional wet hydrogen as a sintering ga~ or controlled atmosphere sintering under an atmosphere comprising a ~ixture of hydrogen and carbon dioxide. The proces~ i9 conducted so that the gases from the decompo~ition of ammonium bicarbonate or amm~nium car_ bonate are excluded from the sintering atmosphere such a~
~y using the countercurrent f~ow of the sintering at-~ 8~3 24_NF-04109 mosphere in a sintering furnace.
The invention is further illustrated by the following examples.
Ammonium bicarbonate Wa8 hammer milled to an average particle size of about 20 microns, Uranium dioxide having an oxygen to uranium ratio of about 2.05 and an average particle size of 0,7 microns was blended with the ammonium bicaxbonate in a ratio of 1.3 grams of ammonium bicarbonate to 98,7 grams of uranium dioxide, Three thousand gram~ of blended powder were prepared in this manner, The blended powders were hammer milled to destroy any uranium dioxide aggrates in order to as~ure a homogenous distribution of ammonium bicarbonate in the uranium dioxide powder, The hammer milled powder wa~ die pressed at 6700 psi --to increa~e ~ts bul~ density and the resulting structures w~re cru~hed through a 12 mesh screen to promote both flowability and control of the agglomerate size.
The resulting powder was die pre~sed into cylindrical fuel pellets u~ing pressure~ ranging from 26,000 to 54,000 pBi, As a reference, 2 group~ of fuel pellets were also die pressed at the ~ame pressures from the original batch of uranium dioxide powder, One reference group (herein-after ~roup 11 conta~ned no biDder and received no other processing prior to pressing. The other reference group (hereinafter Group 2) also contained no binder but WQS
hammer milled, die pressed, and crushed through a sizing screen prior to die pressing.
Tensile ~trengths as mea~ured by diametrical com-pre~sion tests were csnducted on the binder pellets and the
2 reference groups. The binder}es~ reference pellets of _ 15 -~08'~443 24-NF-04109 Group 2 were too weak for tensile strength measurements.
The tensile strength vs. die pre~sing pre~sure curves for the reamining pellets are ~hown in Figure 1. m e pellets containing binder clearly posses superior tensile strength for all presQing pressures.
Three hundred and sixty kilograms of uranium dioxide powder having an O/U ratio of 2.04 and an average part$cle size of 0.6 microns were mixed with an ammonium bicarbonate binder following the procedure in Example 1. The powders were die pressed into cylindrical fuel pellets at a green d-n-ity ranging from 4.9 to 5.1 grams/cm3 using 0.536 diameter tooling. The green fuel pellets were randomly loaded 3 1/2 deep in molybdenum sintering boats.
The sintering boats were stoked into a continuous furnace having an atmosphere of dis~ociated ammonia using a 45 inches/hour push rate and a temperature ri~e of 8C/
minute.
The furnace Wa8 of ~ufficient length to assure a r-~id-nco timo of 4 hours at the peak sintering temperature of 1720C for the pellets. The atmosphere was comprised of dissociated ammonia having a dew point of 67 Centrigrade One hundred and ninety ft 3/hour of di~ociated ammonla was introduced at a point 1/3 of the furnace length from-tho p~llet entry end for impurity removal and another 225 ft3/
hour was introduced at the petlet remova} end of furnace to provide a clean sintering atmosphere. m e gases were removed at the pellet entry end of the furnace 80 that th0 gas flow w~s countorcurrent to the pa~sage of the boats through the furnace. -For the s~ntered pellets, the average oxygen to uranium ratio was 2,003, the average carbon was 7.50 ppm, the averago hydrogen was .131 ppm, the average nitrogen wa~ 12.89 _ 16 -24_NF_04109 108'~443 ppm, and the average total outga~ was 3.41 microliters/gram.
Typical photomicrographs of a sectioned pellet, at 25 times and 100 times magnification, are shown in Figure~ 2 and 3 respectively The structure shows a uniform dis-tribution of fine pores in a uranium dioxide matrix. The pore sizes are similar to thoss observed in uranium dioxide pellets fabricated without the assistance of a binder. No additional pore forming was observed from the amount of the ammonium ~icarbonate binder used in this Example ~he pellets fabsicated from the binder containing uranium dioxide were center ground to a desired diameter with a 96.6X yield of good quality pelletg. In contrast, another batch of uranium dioxide pellets fabricated by the same procedure, but without binder, had only a 77X yield of good quality pellets.
Five thousand grams of uranium dioxide powder having an oxygen to uranium ratio of 2 04 were placed in a 2 1/2 gallon ru~ber lined ball mill, one-half filled with 3/8"
stainle~s ~teel balls. The powder was dry milled for 6 hours Reagent grade ammonium carbonate binder was hammer milled to about 20 microns in particle size The binder was added to the uranium dioxide in the ball mill and milled for lS minutes, The ball mill was emptied and the ballg screened from the binder containing powder Cylindrical fuel pellets were pressed from the powder at pressures rang~ng from 15,000 to 29,000 psi. Since the powder was not die pregsed to increase its ~ul~ density and the resulting strcutures were cru~hed through a sizing screen, poor powder flowa~ility regulted, making die pres~-ing_ difficult. However, good fuel pellets were obtained during the die pressing. As a reference, another portion of the same ~atch o~ uranium dioxide powder w~s proces~ed 0 ~ 3 24-NF-04109 through 6 hours of ball milling without the addition of a binder and die pressed into pellets. This batch of powder also possessed poor flowability.
Tensile 6trengths as measured by diametral compression tests were made on the ammonium carbonate containing pellets and the reference pellet~. The results demonstrated that the use of ammonium car~onate as a binder significantly in-creases tensile strengths at all pressing pressures.
The balance of both groups of pellets were sintered in the furnace according to the procedure described in Example 2 The ~intered pellets fabricated with the ammonium car-bonate binder yielded sintered theoretical density curves approximately 2 6% lower than the reference pellets without the ammonium carbonate binder. Therefore ammonium carbonate can be used to give a combination of binding action and pore forming action for tho sintered pellets The carbon analy~is, total outgas, and 0/U measurement~
on the pellets fabricated with the assistance of ammonium carbonate were respectively 5 ppm, 3 microliter~/gram, and 2.003 The reference group of pellets without the ammonium earbonate binder had a carbon eontent of 6 ppm, total out-gas content of 3 microliters/gram, and an 0/U ration of 2 004 All analyses were conducted the same for both s~ries ~ -of pellets As will be apparent to tho~e skilled in the art, various modif~cations and ~hanges m~y be made in the invention des-cribed herein. It i8 accordingly the intention that the invention be con~trued in the broade~t manner within the 6pirit and scope as set forth in the accompany~ nq claims.
_ 18 -
The tensile strength vs. die pre~sing pre~sure curves for the reamining pellets are ~hown in Figure 1. m e pellets containing binder clearly posses superior tensile strength for all presQing pressures.
Three hundred and sixty kilograms of uranium dioxide powder having an O/U ratio of 2.04 and an average part$cle size of 0.6 microns were mixed with an ammonium bicarbonate binder following the procedure in Example 1. The powders were die pressed into cylindrical fuel pellets at a green d-n-ity ranging from 4.9 to 5.1 grams/cm3 using 0.536 diameter tooling. The green fuel pellets were randomly loaded 3 1/2 deep in molybdenum sintering boats.
The sintering boats were stoked into a continuous furnace having an atmosphere of dis~ociated ammonia using a 45 inches/hour push rate and a temperature ri~e of 8C/
minute.
The furnace Wa8 of ~ufficient length to assure a r-~id-nco timo of 4 hours at the peak sintering temperature of 1720C for the pellets. The atmosphere was comprised of dissociated ammonia having a dew point of 67 Centrigrade One hundred and ninety ft 3/hour of di~ociated ammonla was introduced at a point 1/3 of the furnace length from-tho p~llet entry end for impurity removal and another 225 ft3/
hour was introduced at the petlet remova} end of furnace to provide a clean sintering atmosphere. m e gases were removed at the pellet entry end of the furnace 80 that th0 gas flow w~s countorcurrent to the pa~sage of the boats through the furnace. -For the s~ntered pellets, the average oxygen to uranium ratio was 2,003, the average carbon was 7.50 ppm, the averago hydrogen was .131 ppm, the average nitrogen wa~ 12.89 _ 16 -24_NF_04109 108'~443 ppm, and the average total outga~ was 3.41 microliters/gram.
Typical photomicrographs of a sectioned pellet, at 25 times and 100 times magnification, are shown in Figure~ 2 and 3 respectively The structure shows a uniform dis-tribution of fine pores in a uranium dioxide matrix. The pore sizes are similar to thoss observed in uranium dioxide pellets fabricated without the assistance of a binder. No additional pore forming was observed from the amount of the ammonium ~icarbonate binder used in this Example ~he pellets fabsicated from the binder containing uranium dioxide were center ground to a desired diameter with a 96.6X yield of good quality pelletg. In contrast, another batch of uranium dioxide pellets fabricated by the same procedure, but without binder, had only a 77X yield of good quality pellets.
Five thousand grams of uranium dioxide powder having an oxygen to uranium ratio of 2 04 were placed in a 2 1/2 gallon ru~ber lined ball mill, one-half filled with 3/8"
stainle~s ~teel balls. The powder was dry milled for 6 hours Reagent grade ammonium carbonate binder was hammer milled to about 20 microns in particle size The binder was added to the uranium dioxide in the ball mill and milled for lS minutes, The ball mill was emptied and the ballg screened from the binder containing powder Cylindrical fuel pellets were pressed from the powder at pressures rang~ng from 15,000 to 29,000 psi. Since the powder was not die pregsed to increase its ~ul~ density and the resulting strcutures were cru~hed through a sizing screen, poor powder flowa~ility regulted, making die pres~-ing_ difficult. However, good fuel pellets were obtained during the die pressing. As a reference, another portion of the same ~atch o~ uranium dioxide powder w~s proces~ed 0 ~ 3 24-NF-04109 through 6 hours of ball milling without the addition of a binder and die pressed into pellets. This batch of powder also possessed poor flowability.
Tensile 6trengths as measured by diametral compression tests were made on the ammonium carbonate containing pellets and the reference pellet~. The results demonstrated that the use of ammonium car~onate as a binder significantly in-creases tensile strengths at all pressing pressures.
The balance of both groups of pellets were sintered in the furnace according to the procedure described in Example 2 The ~intered pellets fabricated with the ammonium car-bonate binder yielded sintered theoretical density curves approximately 2 6% lower than the reference pellets without the ammonium carbonate binder. Therefore ammonium carbonate can be used to give a combination of binding action and pore forming action for tho sintered pellets The carbon analy~is, total outgas, and 0/U measurement~
on the pellets fabricated with the assistance of ammonium carbonate were respectively 5 ppm, 3 microliter~/gram, and 2.003 The reference group of pellets without the ammonium earbonate binder had a carbon eontent of 6 ppm, total out-gas content of 3 microliters/gram, and an 0/U ration of 2 004 All analyses were conducted the same for both s~ries ~ -of pellets As will be apparent to tho~e skilled in the art, various modif~cations and ~hanges m~y be made in the invention des-cribed herein. It i8 accordingly the intention that the invention be con~trued in the broade~t manner within the 6pirit and scope as set forth in the accompany~ nq claims.
_ 18 -
Claims (19)
1. A composition of matter in the form of a compacted structure suitable for sintering comprising a mixture of a nuclear fuel material, and from about 0.5 to about 7.0 percent by weight of a binder, said binder being substantially uniformly dispersed in the nuclear fuel material so that said binder and said nuclear fuel material adhere, and said binder being selected from the group consisting of ammonium bicarbonate, ammonium bicarbonate carbamate, ammonium sesquicarbonate, ammonium carbamate and mixtures thereof.
2. A composition according to claim 1 in which the nuclear fuel material comprises uranium oxide.
3. A composition according to claim 1 in which the binder is ammonium bicarbonate.
4. A composition according to claim 1 in which the nuclear fuel material comprises uranium dioxide.
5. A composition according to claim 1 in which the binder is ammonium bicarbonate carbamate.
6. A composition according to claim 1 in which the nuclear fuel material comprises a mixture of uranium dioxide and plutonium dioxide.
7. A composition according to claim 1 in which the nuclear fuel material comprises a mixture of uranium dioxide and gadolinium oxide.
8. A composition according to claim 1 in which the nuclear fuel material comprises uranium dioxide having a particle size ranging from about 0.02 to about 0.5 micron.
9. A composition according to claim 1 in which the binder is ammonium sesquicarbonate.
10. A composition according to claim 1 in which the binder is ammonium carbamate.
11. A process for sintering a body of nuclear fuel material comprising the steps of:
a) admixing the nuclear fuel material in a particulate form with a binder having a particle size less than 400 mesh so as to achieve a uniform dispersal of said binder in the nuclear fuel material so that said binder and said nuclear fuel material undergo adhesion, said binder being comprised of ammonium bicarbonate, ammonium bicarbonate carbamate, ammonium sesqui-carbonate, ammonium carbamate and mixtures thereof, b) forming the resulting mixture by pressing into a green body having a density ranging from about 30% to about 70%
of theoretical density, c) heating said green body at a temperature sufficient to decompose substantially all of the binder into gases that enter an atmosphere maintained over said green body, d) heating the body at a temperature sufficient to produce a sintered body and further decompose any binder residues that enter the atmosphere maintained over said body, and e) cooling the sintered body in the atmosphere maintained over said body.
a) admixing the nuclear fuel material in a particulate form with a binder having a particle size less than 400 mesh so as to achieve a uniform dispersal of said binder in the nuclear fuel material so that said binder and said nuclear fuel material undergo adhesion, said binder being comprised of ammonium bicarbonate, ammonium bicarbonate carbamate, ammonium sesqui-carbonate, ammonium carbamate and mixtures thereof, b) forming the resulting mixture by pressing into a green body having a density ranging from about 30% to about 70%
of theoretical density, c) heating said green body at a temperature sufficient to decompose substantially all of the binder into gases that enter an atmosphere maintained over said green body, d) heating the body at a temperature sufficient to produce a sintered body and further decompose any binder residues that enter the atmosphere maintained over said body, and e) cooling the sintered body in the atmosphere maintained over said body.
12. A process according to claim 11 in which the admixing step is conducted to give from about 0.5 to about 7.0 weight percent binder in the mixture with the nuclear fuel material.
13. A process according to claim 11 in which the nuclear fuel material is comprised of uranium dioxide and plutonium dioxide.
14. A process according to claim 11 in which the nuclear fuel material is uranium dioxide.
15. A process according to claim 11 in which the binder is ammonium bicarbonate.
16. A process according to claim 11 in which the binder is ammonium bicarbonate carbamate.
17. A process according to claim 11 in which the nuclear fuel material is uranium oxide.
18. A process according to claim 11 in which the binder is ammonium sesquicarbonate.
19. A process according to claim 11 in which the binder is ammonium carbamate.
Priority Applications (1)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| CA262,310A CA1082443A (en) | 1976-09-29 | 1976-09-29 | Fugitive binder for nuclear fuel materials |
Applications Claiming Priority (1)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| CA262,310A CA1082443A (en) | 1976-09-29 | 1976-09-29 | Fugitive binder for nuclear fuel materials |
Publications (1)
| Publication Number | Publication Date |
|---|---|
| CA1082443A true CA1082443A (en) | 1980-07-29 |
Family
ID=4106964
Family Applications (1)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| CA262,310A Expired CA1082443A (en) | 1976-09-29 | 1976-09-29 | Fugitive binder for nuclear fuel materials |
Country Status (1)
| Country | Link |
|---|---|
| CA (1) | CA1082443A (en) |
-
1976
- 1976-09-29 CA CA262,310A patent/CA1082443A/en not_active Expired
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