United States Patent (19) 11) 4,061,700 Gallivan 45) Dec

United States Patent (19) 11) 4,061,700 Gallivan 45) Dec. 6, 1977

54 FUGITIVE BINDER FOR NUCLEAR FUEL Attorney, Agent, or Firm-Ivor J. James, Jr.; Samuel E. MATERIALS Turner; Sam E. Laub 75 Inventor: Timothy Joseph Gallivan, 57 ABSTRACT Wilmington, N.C. A process for fabricating a body of a nuclear fuel mate rial has the steps of admixing the nuclear fuel material in 73) Assignee: General Electric Company, San Jose, powder form with a binder of a compound or its hydra Calif. tion products containing ammonium cations and anions selected from the group consisting of carbonate anions, 21 Appl. No.: 612,084 bicarbonate anions, carbamate anions and mixtures of such anions, forming the resulting mixture into a green 22 Filed: Sept. 10, 1975 body such as by die pressing, heating the green body to 51) Int. C.? ...... G21C 21/00 decompose substantially all of the binder into gases, 52 U.S. Cl...... 264/05; 252/301.1 R further heating the body to produce a sintered body, 58) Field of Search ...... 264/.5; 252/301.1 R; and cooling the sintered body in a controlled atmo 176/89, 90 sphere. Preferred binders used in the practice of this invention include ammonium bicarbonate, ammonium (56) References Cited carbonate, ammonium bicarbonate carbamate, ammo nium sesquicarbonate, ammonium carbamate and mix U.S. PATENT DOCUMENTS tures thereof. This invention includes a composition of 3,312,631 4/1967 Smith et al...... 264/.5 X matter in the form of a compacted structure suitable for 3,330,889 7/1967 Samos et al...... 264/.5 sintering comprising a mixture of a nuclear fuel material 3,345,437 10/1967 Flack et al...... 264/.5 and a binder of a compound or its hydration products 3,728,421 4/1973 Noothout ...... 264/.5 3,812,050 5/1974 Steele ...... 264/.5 containing ammonium cations and anions selected from 3,953,286 4/1976 Watson et al...... 264/.5X the group consisting of carbonate anions, bicarbonate 3,963,828 6/1976 Becker ...... 252/301.1 RX anions, carbamate anions and mixtures of such anions. Primary Examiner-Peter A. Nelson 9 Claims, 4 Drawing Figures

350

250

Armmoniurn Bicorbonote -- Uranium Dioxide

OO Uranium OO Dioxide

O O 2O 30 4O 50 SO PRESSING PRESSURE x IOoo psi U.S. Patent Dec. 6, 1977 Sheet 1 of 3 4,061,700

3OO

25 O

Ammonium Bicarbonate 2OO w -- Uronium Dioxide

OO / Uronium OO Dioxide

O O 2O 3O 4O 5O SO PRESSING PRESSURE X OOO, psi

Afg. / U.S. Patent Dec. 6, 1977 Sheet 2 of 3 4,061,700

U.S. Patent Dec. 6, 1977 Sheet 3 of 3 4,061,700

Ammonium Corbonate - Uranium Dioxide 3OO

25O

2 O O

OO%. Uronium OO Dioxide0.

O O 2O 3O 4O 5O PRESSING PRESSURE x IOOO psi

Afg. 4 4,061,700 1. 2 order of 90% to 95% of theoretical density are specified FUGTIVE BINDER FOR NUCLEAR FUEL and occasionally a density as low as 85% of theoretical MATERIALS is specified. Most pressed uranium dioxide powder, BACKGROUND OF THE INVENTION however, will sinter to final densities of about 96% to 5 98% of theoretical. Therefore, to obtain sintered bodies The present invention relates generally to the art of with lower densities the time and temperature must be forming and sintering ceramic powders and is particu carefully controlled to allow the shrinkage of the body larly concerned with a method for sintering a uranium to proceed only to the desired value. This is inherently dioxide nuclear fuel body having a fugitive binder. more difficult than the use of a process which is allowed Various materials are used as nuclear fuels for nuclear O to go to completion. Specifically, small variations dur reactors including ceramic compounds of uranium, ing sintering can result in large variations or no signifi plutonium and thorium with particularly preferred cant variations in the sintered body of compacted pow compounds being uranium oxide, plutonium oxide, tho der depending on a number of factors such as the pow rium oxide and mixtures thereof. An especially pre der chemistry, particle size and agglomeration. Gener ferred nuclear fuel for use in nuclear reactors is uranium 15 ally, however, a change in sintering time such as, for dioxide. example an hour or two, does not significantly change Uranium dioxide is produced commercially as a fine, the density of the final sintered product. Also, when fairly porous powder which cannot be used directly as sintered bodies having the desired low density have nuclear fuel. It is not a free-flowing powder but clumps been attained by carefully controlling sintering time and and agglomerates, making it difficult to pack in reactor 20 temperature, it has been found that these sintered bod tubes to the desired density. ies, when placed in the reactor, frequently undergo The specific composition of a given commercial ura additional sintering within the reactor thereby interfer nium dioxide powder may also prevent it from being ing with proper reactor operation. used directly as a nuclear fuel. Uranium dioxide is an A number of techniques have been used in the past to exception to the law of definite proportions since 25 reduce the density of the sintered body other than vary "UO' actually denotes a single, stable phase that may ing time and temperature. For example, one technique vary in composition from UO1.7 to UO2.25. Because ther has been to press the uranium dioxide powder, break it mal conductivity decreases with increasing O/U ratios, up and repress it. The problem with this technique is uranium dioxide having as low an O/U ratio as possible that the resulting sintered body has large interconnect is preferred. However, since uranium dioxide powder 30 ing pores throughout the body which extend out to the oxidizes easily in air and absorbs moisture readily, the surface resulting in a large exposed surface area which O/U ratio of this powder is significantly in excess of can absorb into the body significant amounts of gases, that acceptable for fuel. and in particular water in the form of water vapor. A number of methods have been used to make ura During reactor operation these gases are liberated pro nium dioxide powder suitable as a nuclear fuel. Pres 35 viding a possible source of corrosion for the fuel clad ently, the most common method is to die press the pow ding. Another method involves adding a plastic of se der into cylindrically-shaped green bodies of specific lected particle size to the uranium dioxide powder. The size without the assistance of fugitive binders since the admixed powder is then pressed and sintered, however complete removal of these binders and their decomposi the decomposition of the plastic during sintering usually tion products is difficult to achieve prior to sintering. results in carbon residues which contaminate the nu The entrainment of binder residues is unacceptable in clear fuel. sintered nuclear fuels. Sintering atmospheres may range In U.S. patent application Ser. No. 437,837 filed Jan. from about 1000 C to about 2400 C with the particular 30, 1974 (and now abandoned) in the name of Kenneth sintering temperature depending largely on the sinter W. Lay and assigned to the same assignee as the present ing atmosphere. For example, when wet hydrogen gas 45 invention, there is disclosed a process for controlling is used as the sintering atmosphere, its water vapor the end-point density of a sintered uranium dioxide accelerates the sintering rate thereby allowing the use nuclear fuel body and the resulting product. Uranium of correspondingly lower sintering temperatures such dioxide powder having a size ranging up to 10 microns as a temperature of about 1700' C. The sintering opera is admixed with a precursor to uranium dioxide, such as tion is designed to densify the bodies and bring them 50 ammonium diuranate, having an average agglomerated down to the desired O/U ratio or close to the desired particle size ranging from about 20 microns to 1 milli O/U ratio. meter and the mixture is formed into a pressed compact Although uranium dioxide suitable as a nuclear fuel or green body. The body of the precursor and the ura can have an O/U ratio ranging from 1.7 to 2,015, as a nium dioxide has a density lower than that of the ura practical matter, a ratio of 2.00 and suitably as high as 55 nium dioxide powder and the precursor is used in an 2.015 has been used since it can be consistently pro amount which results in discrete low density regions in duced in commercial sintering operations. In some in the green body which range from about 5% to 25% by stances, it may be desirable to maintain the O/U ratio of volume of the green body. The green body is sintered to the uranium dioxide at a level higher than 2.00 at sinter decompose the precursor and produce a sintered body ing temperature. For example, it may be more suitable 60 having discrete low density porous regions which re under the particular manufacturing process to produce duce the end-point density of the sintered body by at a nuclear fuel having an O/U ratio as high as 2.195, and least 2%. The sintered body has an end-point density then later treat the sintered product in a reducing atmo ranging from 85% to 95% of theoretical. sphere to obtain the desired O/U ratio. In copending U.S. patent application Ser. No. One of the principal specifications for uranium diox 65 598,839, filed July 24, 1975 (and now abandoned) and ide sintered bodies to be used for a nuclear reactor is assigned to the same assignee as the present invention their density. The actual value may vary but in general there is disclosed a process for controlling the final or uranium dioxide sintered bodies having densities of the end-point density of a sintered uranium dioxide nuclear 4,061,700 3. 4. fuel body by adding ammonium oxalate to a nuclear fuel particulate form with the binder, forming the resulting material such as uranium dioxide before pressing into a mixture into a green body having a density ranging green body. This addition results in discrete low density from about 30% to about 70% of theoretical density of porous regions in the sintered body which correspond the nuclear fuel material, heating said green body to to the ammonium oxalate particles. The end-point den decompose substantially all the binder into gases, fur sity of the sintered body is, therefore, a function of the ther heating the body to produce a sintered body and amount of ammonium oxalate added. cooling the sintered body in a controlled atmosphere. As previously mentioned, conventional organic or This invention also provides a composition of matter plastic binders are unsuitable for use in powder fabrica that is suitable for sintering in the form of a compacted tion since they tend to contaminate the interior of the O structure comprising a mixture of a nuclear fuel material sintered body with impurities such as hydrides. These and a binder of a compound or its hydration products binders are normally converted to gases during the containing ammonium cations and anions selected from sintering step and these gases must be removed, requir the group consisting of carbonate anions, bicarbonate ing special apparatus or procedures. In addition, upon anions, carbamate anions and mixtures of such anions decomposition, these prior art binder materials often 15 and preferably a binder selected from the group consist leave deposits of organic materials in the equipment ing of ammonium bicarbonate, ammonium carbonate utilized to sinter the article, thereby complicating the and mixtures thereof. maintenance procedures for the equipment. In the sintering process, it is desirable to develop OBJECTS OF THE INVENTION strong diffusion bonds between the individual particles 20 It is an object of this invention to provide an additive without significantly reducing the interconnecting po of a binder of a compound or its hydration products rosity of the body. The use of organic binders along containing ammonium cations and anions selected from with normal compacting pressures and sintering tem the group consisting of carbonate anions, bicarbonate peratures inhibits the formation of these strong bonds. anions, carbamate anions and mixtures of such anions The higher compacting pressures and sintering temper 25 for a nuclear fuel material that serves to bind the parti atures required to develop such bonds sharply reduce cles of nuclear fuel into green shapes suitable for sinter the desired porosity. IIlg. There is a particular need, therefore, in the art of Another preferred object of this invention is to pro preparing sintered bodies for nuclear reactors by pow vide a binder selected from the group consisting of der ceramic techniques for a binder which will impart 30 ammonium bicarbonate, ammonium carbonate, ammo an adequate degree of green strength without contami nium bicarbonate carbamate, ammonium sesquicarbon nating the interior of such bodies and which will permit, ate, ammonium carbamate or mixtures thereof as an through sintering, the formation of strong bonds be addition to nuclear fuel materials and which, upon heat tween particles without deleteriously affecting the po ing at moderate temperatures before a sintering process, rosity. 35 decompose into gases and leave substantially no impuri ties in the sintered structures of the nuclear fuel mate SUMMARY OF THE INVENTION rial. This invention presents the improvement of utilizing Still another object of this invention is to provide a a binder of a compound or its hydration products con process for sintering green shapes of a nuclear fuel ma taining ammonium cations and anions selected from the terial using a binder of a compound or its hydration group consisting of carbonate anions, bicarbonate an product containing ammonium cations and anions se ions, carbamate anions and mixtures of such anions, lected from the group consisting of carbonate ions, preferably a binder selected from the group consisting bicarbonate anions, carbamate anions and mixtures of of ammonium bicarbonate, ammonium carbonate, am such anions. monium bicarbonate carbamate, ammonium sesquicar 45 Other objects and advantages of this invention will bonate, ammonium carbamate and mixtures thereof, in a become apparent from the following specification and powder ceramic process for imparting green strength to the appended claims. articles cold pressed from nuclear fuel powders of vary ing particle size and a particular shape or configuration DESCRIPTION OF THE DRAWINGS for which it is desired to maintain a certain degree of 50 FIG. 1 presents a graph of tensile strength versus die porosity, uniformity of pore size, a lack of interconnec pressing pressure for one group of green pellets without tions between the pores and the shape or configuration a binder and one group of green pellets with a binder of the base material particles in the final article after disclosed in this invention. sintering. The binders disclosed in this invention are FIGS. 2 and 3 present photomicrographs (at a magni efficient binders for use in nuclear fuels, and further the 55 fication of 25 and 100 times respectively) of uranium binders enable the realization of defect free, pressed dioxide pellets produced according to the teachings of bodies of nuclear fuel materials and tensile strength in Example 2. the bodies comparable to strengths achieved with long FIG. 4 presents a graph of tensile strength versus die chain hydrocarbon binders. Further the binders in this pressing pressure for one group of unsintered pellets invention leave substantially no impurities in the nu without a binder and one group of unsintered pellets clear fuel material since these binders decompose upon with a binder disclosed in this invention. heating into ammonia (NH3), carbon dioxide (CO2) and water (H2O) (or water vapor) at temperatures as low as DETAILED DESCRIPTION OF THE 3O C. INVENTION The binder addition to nuclear fuel material as pres 65 It has now been discovered that a process for sinter ented in this invention enables the practice of a process ing a green body of a nuclear fuel material having high for forming and sintering a body of a nuclear fuel hav reliability can be achieved by admixing a fugitive binder ing the steps of admixing the nuclear fuel material in of a compound or its hydration products containing 4,061,700 5 6 ammonium cations and anions selected from the group In the present invention the binder should have cer consisting of carbonate anions, bicarbonate anions, car tain characteristics. It must be substantially comprised bamate anions and mixtures of such anions with a nu of a compound or its hydration products containing clear fuel material in powder form. In greater detail the annonium cations and anions selected from the group process can be conducted by practicing the steps of consisting of carbonate anions, bicarbonate anions, car providing a powder of the nuclear fuel material, admix bamate anions and mixtures of such anions and free of ing the nuclear fuel material with a binder of a com impurities so that it can be mixed with uranium dioxide pound or its hydration products containing ammonium powder and pressed and sintered without leaving any cations and anions selected from the group consisting of undesired impurities after heating with particularly carbonate anions, bicarbonate anions, carbamate anions 10 preferred binders being ammonium bicarbonate and and mixtures of such anions, forming the resulting mix ammonium carbonate and mixtures thereof. It has been ture into a green body having a density ranging from found that commercially available ammonium bicar about 30% to about 70% of theoretical density, heating bonate contains virtually no impurities and commer the green body sufficiently to decompose the binder cially available ammonium carbonate also contains vir into gases and thereafter heating the body to produce a 15 tually no impurities except for other ammonium com sintered body having a controlled porosity and a con pounds as listed in the foregoing paragraph. Thermo trolled density. gravimetric analysis confirms that there is a complete The practice of the foregoing process results in the volatilization of ammonium bicarbonate and ammonium production of a composition of matter in the form of a carbonate at heating rates typically used for reductive compacted structure suitable for sintering and is com 20 atmospheric UO sintering. Ammonium bicarbonate prised of a mixture of a nuclear fuel material and a and ammonium carbonate when heated to the tempera binder of a compound or its hydration products contain ture range of decomposition, decompose to form ammo ing ammonium cations and anions selected from the nia, carbon dioxide and water at significant rates leaving group consisting of carbonate anions, bicarbonate an substantially no contaminants (impurities) in the fuel ions, carbamate anions and mixtures of such anions. 25 and no undesirable residues in the sintering furnace. As used herein, nuclear fuel material is intended to Additionally the ammonium bicarbonate and the ammo cover the various materials used as nuclear fuels for nium carbonate are used in small particle sizes of 400 nuclear reactors including ceramic compounds such as mesh or less in order to achieve maximum plastic flow oxides of uranium, plutonium and thorium with particu of the binder into the interstices of the nuclear fuel larly preferred compounds being uranium oxide, pluto 30 material. Ammonium carbonate is used as the binder nium oxide, thorium oxide and mixtures hereof. An when the combination of binding and density reducing especially preferred nuclear fuel for use in this invention pores is desired in the nuclear fuel. Ammonium bicar is uranium oxide, particularly uranium dioxide. Further bonate is used as the binder when it is desired to avoid the term nuclear fuel is intended to cover a mixture of the formation of density reducing pores in the nuclear the oxides of plutonium and uranium and the addition of 35 fuel material. The plasticity of ammonium bicarbonate one or more additives to the nuclear fuel material such and ammonium carbonate may be demonstrated by the as gadolinium oxide (Gd2O3). fact that these compounds can be die pressed to green In carrying out the present process which will be densities as high as 90% of theoretical density at moder discussed for the preferred use of uranium dioxide, the ate pressing pressures. uranium dioxide powder (or particles) used generally The amount of binder added to the nuclear fuel mate has an oxygen to uranium atomic ratio greater than 2.00 rial generally ranges from about 0.5 to about 7.0 weight and can range up to 2.25. The size of the uranium diox percent depending on the formability of the nuclear fuel ide powder or particles ranges up to about 10 microns material. For example formable uranium dioxide pow and there is no limit on lower particle size. Such particle ders require less of an addition of the binder while less sizes allow the sintering to be carried out within a rea 45 readily formable powders require larger amounts of sonable length of time and at temperatures practical for binder. When the selected binder is ammonium carbon commercial applications. For most applications, to ob ate, the amount of the addition of this binder is depen tain rapid sintering, the uranium dioxide powder has a dent upon the desired sintered density for the nuclear size ranging up to about 1 micron. Commercial uranium fuel material. dioxide powders are preferred and these are of small 50 Homogenous blending of the binder with the nuclear particle size, usually sub-micron generally ranging from fuel material is practiced to develop fully the binding about 0.02 micron to 0.5 micron. action of the binder on the nuclear fuel material. Where Compositions suitable for use as a binder in the prac porosity or a lower density is not desired, the homoge tice of this invention either alone or in mixtures, include nous blending of the binder with the nuclear fuel mate ammonium bicarbonate, ammonium carbonate, ammo 55 rial avoids the formation of agglomerates of the binder nium bicarbonate carbamate, ammonium sesquicarbon since such agglomerates can volatize during sintering ate, ammonium carbamate and mixtures thereof. When leaving pores in the sintered nuclear fuel material which mixed with nuclear fuel materials, these binders and the pores reduce the density of the nuclear fuel material in nuclear fuel material are believed to undergo the phe sintered bodies. When it is felt that agglomerates of the nomenon of adhesion forming an ammonium derivative binder exist in the nuclear fuel material after mixing, a of the carbonate series such as milling process such as jet milling or hammer milling is practiced so that the agglomerates are destroyed. The (NH), UO(CO), (NH)6CUO) (CO) blended and milled powder may then be predensified by (H2O).H2O, (NH), UOCO) (HO), (NH) low pressure die pressing followed by granulation (UO), (CO)3(OH)(HO), NH. 65 through a sizing screen to promote flowability of the UO(CO)(OH)(HO) mixture. In order to control the density of sintered bodies of and UOCO3.H2O, or mixtures of these. nuclear fuel material, pore formers such as ammonium 4,061,700 7 8 oxalate or a uranium precursor may be added to the The sintered uranium dioxide bodies are preferably nuclear fuel material along with the binders of this in cooled in the same atmosphere in which they were vention. The pore former can be mixed either at the sintered. same time as the binders disclosed in this invention or This invention provides several advantages in the during a subsequent mixing step. In the event that the sintering of nuclear fuel materials and in the resulting nuclear fuel material, binder and pore former are mixed sintered pellets. The addition of the binders of this in and then milled to promote homogeneity, the process vention, particularly ammonium bicarbonate, ammo ing is conducted to yield an acceptable particle size nium carbonate, or mixtures thereof, does not leave any after milling to assure the formation of pores during undesirable residue in the sintered pellets. Thermo sintering. O gravimetric analysis has shown that ammonium bicar The resulting mixture of nuclear fuel material with bonate and ammonium carbonate decompose com the binders of this invention, with or without pore for mer, can be formed into a green body, generally a cylin pletely into ammonia (NH3), carbon dioxide (CO2) and drical pellet by a number of techniques such as pressing water vapor (H2O). The early decomposition of ammo (particularly die pressing). Specifically, the mixture is 5 nium bicarbonate and ammonium carbonate prevents compressed into a form in which it has the required the entrapment of undesirable gases in the microstruc mechanical strength for handling and which, after sin ture of the nuclear fuel material during the sintering tering, is of the size which satisfies reactor specification. process. Pellets incorporating ammonium bicarbonate The presence of the binders of this invention in the or ammonium carbonate according to the teachings of nuclear fuel material significantly enhances both the 20 this invention can be sintered using conventional wet strength and integrity of the resulting green body. The hydrogen as a sintering gas or controlled atmosphere green body can have a density ranging from about 30% sintering under an atmosphere comprising a mixture of to 70% of theoretical, but usually it has a density rang hydrogen and carbon dioxide. The process is conducted ing from about 40% to 60% of theoretical, and prefer so that the gases from the decomposition of ammonium ably about 50% of theoretical. 25 bicarbonate or ammonium carbonate are excluded from The green body is sintered in an atmosphere which the sintering atmosphere such as by using the counter depends on the particular manufacturing process. Spe current flow of the sintering atmosphere in a sintering cifically, it is an atmosphere which can be used to sinter furnace. uranium dioxide alone in the production of uranium The invention is further illustrated by the following dioxide nuclear fuel and also it must be an atmosphere examples. which is compatible with the gases resulting from the decomposition of ammonium bicarbonate. For example, EXAMPLE 1. a number of atmospheres can be used such as an inert Ammonium bicarbonate was hammer milled to an atmosphere, a reducing atmosphere (e.g. dry hydrogen) average particle size of about 20 microns. or a controlled atmosphere comprised of a mixture of 35 Uranium dioxide having an oxygen to uranium ratio gases (e.g. a mixture of hydrogen and carbon dioxide as of about 2.05 and an average particle size of 0.7 microns set forth in U.S. Pat. No. 3,872,022) which in equilib was blended with the ammonium bicarbonate in a ratio rium produces a partial pressure of oxygen sufficient to of 1.3 grams of ammonium bicarbonate to 98.7 grams of maintain the uranium dioxide at a desired oxygen to uranium dioxide. Three thousand grams of blended uranium ratio. 40 powder were prepared in this manner. The rate of heating to sintering temperature is limited The blended powders were hammer milled to destroy largely by how fast the by-product gases are removed any uranium dioxide aggregates in order to assure a prior to achieving a sintering temperature and generally homogenous distribution of ammonium bicarbonate in this depends on the gas flow rate through the furnace the uranium dioxide powder. and its uniformity therein as well as the amount of mate 45 The hammer milled powder was die pressed at 6700 rial in the furnace. Specifically, the rate of flow of gas psi to increase its bulk density and the resulting struc through the furnace, which ordinarily is substantially tures were crushed through a 12 mesh screen to pro the same gas flow used in the sintering atmosphere, mote both flowability and control of the agglomerate should be sufficient to remove the gases resulting from S. decomposition of ammonium bicarbonate before sinter 50 ing temperature is reached. Generally, best results are The resulting powder was die pressed into cylindrical obtained when the rate of heating to decompose the fuel pellets using pressures ranging from 26,000 to binder ranges from about 50° C per hour to about 300 54,000 psi. As a reference, 2 groups of fuel pellets were C per hour. After decomposition of the binder is com also die pressed at the same pressures from the original pleted and byproduct gases substantially removed from 55 batch of uranium dioxide powder. One reference group the furnace, the rate of heating can then be increased, if (hereinafter Group 1) contained no binder and received desired, to a range of about 300° C to 500 C per hour no other processing prior to pressing. The other refer and as high as 800 C per hour but not be so rapid as to ence group (hereinafter Group 2) also contained no crack the bodies. binder but was hammer milled, die pressed, and crushed Upon completion of sintering, the sintered body is through a sizing screen prior to die pressing. usually cooled to room temperature. The rate of cool Tensile strengths as measured by diametrical con ing of the sintered body is not critical in the present pression tests were conducted on the binder pellets and process, but it should not be so rapid as to crack the the 2 reference groups. The binderless reference pellets sintered body. Specifically, the rate of cooling can be of Group 2 were too weak for tensile strength measure the same as the cooling rates normally or usually used in 65 ments. The tensile strength vs. die pressing pressure commercial sintering furnaces. These cooling rates may curves for the remaining pellets are shown in FIG. 1. range from 100 C to about 800° C per hour, and gener The pellets containing binder clearly possess superior ally, from about 400 C per hour to 600 C per hour. tensile strength for all pressing pressures. 4,061,700 9 10 dioxide powder was processed through 6 hours of ball EXAMPLE 2 milling without the addition of a binder and die pressed Three hundred and sixty kilograms of uranium diox into pellets. This batch of powder also possessed poor ide powder having an O/U ratio of 2.04 and an average flowability. particle size of 0.6 microns were mixed with an ammo Tensile strengths as measured by diametral compres nium bicarbonate binder following the procedure in sion tests were made on the ammonium carbonate con Example 1. The powders were die pressed into cylindri taining pellets and the reference pellets. The results cal fuel pellets at a green density ranging from 4.9 to 5.1 demonstrated that the use of ammonium carbonate as a grams/cm3using 0.536 inch diameter tooling. The green binder significantly increases tensile strengths at all fuel pellets were randomly loaded 3 inches deep in 10 pressing pressures. molybdenum sintering boats. The balance of both groups of pellets were sintered in The sintering boats were stoked into a continuous the furnace according to the procedure described in furnace having an atmosphere of dissociated ammonia Example 2. The sintered pellets fabricated with the using a 45 inches/hour push rate and a temperature rise ammonium carbonate binder yielded sintered theoreti of 8 C/minute. 5 cal density curves approximately 2.6% lower than the The furnace was of sufficient length to assure a resi reference pellets without the ammonium carbonate dence time of 4 hours at the peak sintering temperature binder. Therefore ammonium carbonate can be used to of 1720 C for the pellets. The atmosphere was com give a combination of binding action and pore forming prised of dissociated ammonia having a dew point of 67 action for the sintered pellets. Centigrade. One hundred and ninety ft/hour of dissoci 20 The carbon analysis, total outgas, and O/U measure ated ammonia was introduced at a point of the furnace ments on the pellets fabricated with the assistance of length from the pellet entry end for impurity removal ammonium carbonate were respectively 5 ppm, 3 mi and another 225 ft3/hour was introduced at the pellet croliters/gram, and 2.003. The reference group of pel removal end of the furnace to provide a clean sintering lets without the ammonium carbonate binder had a atmosphere. The gases were removed at the pellet entry 25 end of the furnace so that the gas flow was countercur carbon content of 6 ppm, total outgas content of 3 mi rent to the passage of the boats through the furnace. croliters/gram, and an O/U ration of 2004. All analyses For the sintered pellets, the average oxygen to ura were conducted the same for both series of pellets. nium ratio was 2.003, the average carbon was 7.50 ppm, As will be apparent to those skilled in the art, various the average hydrogen was 0.131 ppm, the average nitro 30 modifications and changes may be made in the inven gen was 12.89 ppm, and the average total outgas was tion described herein. It is accordingly the intention 3.41 microliters/gram. that the invention be construed in the broadest manner Typical photomicrographs of a sectioned pellet, at 25 within the spirit and scope as set forth in the accompa times and 100 times magnification, are shown in FIGS. nying claims. 2 and 3 respectively. The structure shows a uniform 35 What is claimed is: distribution of fine pores in a uranium dioxide matrix. 1. A process for sintering a body of nuclear fuel mate The pore sizes are similar to those observed in uranium rial comprising the steps of dioxide pellets fabricated without the assistance of a a. admixing the nuclear fuel material in a particulate binder. No additional pore forming was observed from form with a binder having a particle size less than the amount of the ammonium bicarbonate binder used 400 mesh so as to achieve a uniform dispersal of in this Example. said binder in the nuclear fuel material so that said The pellets fabricated from the binder containing binder and said nuclear fuel material undergo adhe uranium dioxide were center ground to a desired diame sion, said binder being comprised of ammonium ter with a 96.6% yield of good quality pellets. In con bicarbonate, ammonium bicarbonate carbamate, trast, another batch of uranium dioxide pellets fabri 45 ammonium sesquicarbonate, ammonium carbamate cated by the same procedure, but without binder, had and mixtures thereof, only a 77% yield of good quality pellets. b. forming the resulting mixture by pressing into a green body having a density ranging from about EXAMPLE 3 30% to about 70% of theoretical density, Five thousand grams of uranium dioxide powder 50 c. heating said green body at a temperature sufficient having an oxygen to uranium ratio of 2.04 were placed to decompose substantially all of the binder into in a 2 gallon rubber lined ball mill, one-half filled with gases that enter an atmosphere maintained over inch stainless steel balls. The powder was dry milled said green body, for 6 hours. d. heating the body at a temperature sufficient to Reagent grade ammonium carbonate binder was 55 produce a sintered body and further decompose hammer milled to about 20 microns in particle size. The any binder residues that enter the atmosphere binder was added to the uranium dioxide in the ball mill maintained over said body, and and milled for 15 minutes. The ball mill was emptied e, cooling the sintered body in the atmosphere main and the balls screened from the binder containing pow tained over said body. der. 2. A process according to claim 1 in which the admix Cylindrical fuel pellets were pressed from the pow ing step is conducted to give from about 0.5 to about 7.0 der at pressures ranging from 15,000 to 29,000 psi. Since weight percent binder in the mixture with the nuclear the powder was not die pressed to increase its bulk fuel material. density and the resulting structures were crushed 3. A process according to claim 1 in which the nu through a sizing screen, poor powder flowability re 65 clear fuel material is comprised of uranium dioxide and sulted, making die pressing difficult. However, good plutonium dioxide. fuel pellets were obtained during the die pressing. As a 4. A process according to claim 1 in which the nu reference, another portion of the same batch of uranium clear fuel material is uranium dioxide. 4,061,700 1. 12 7. A process according to claim 1 in which the nu 5. A process according to claim 1 in which the binder clear fuel material is uranium oxide. 8. A process according to claim 1 in which the binder is ammonium bicarbonate. is ammonium sesquicarbonate. 9. A process according to claim 1 in which the binder 6. A process according to claim 1 in which the binder 5 is ammonium carbamate. is ammonium bicarbonate carbamate. t