JPS6341508B2 - - Google Patents

Description

[Detailed description of the invention]

FIELD OF THE INVENTION The present invention relates to ceramic technology and the production of sintered bodies from particulate oxide materials. More specifically, the present invention relates to a method for producing a solid particulate ceramic material, which method includes the step of forming such ceramic particles into an easily handled agglomerated compact during subsequent integration by sintering. Regarding. In particular, the present invention is directed to the production of nuclear fuel products from granular ceramic materials containing uranium dioxide. DESCRIPTION OF RELATED APPLICATIONS The present invention is filed by George L. Gaines Giunia, Basilicia A. Piacente, Walliam G. Ward, Peter C. Smith, Timothy G. Ghalivuan, and George L. Gaines. , Jr.,
Patricia A. Piacente, William J. Ward,
Peter C. Smith, Timothy J. Gallivan & Harry
U.S. Patent Application No. 331,492 filed December 17, 1981 in the name of George M.Laska and George L. Gaines, Jr. & William
No. 273,900 filed June 15, 1981 in the name of J. Ward. All of the above applications are assigned to the same assignee as the present application, and the contents of their specifications are incorporated herein by reference. BACKGROUND OF THE INVENTION Fissile nuclear fuel is comprised of fissile materials of various compositions and forms, including ceramic compounds of uranium, plutonium, and thorium. Nuclear fuel for commercial power reactors typically consists of uranium oxide, plutonium oxide, thorium oxide, and mixtures thereof. The generally most preferred and commonly used nuclear fuel material for such commercial power reactors is uranium dioxide. Uranium dioxide may be mixed with small amounts of other nuclear fuel materials and may also contain additives for neutron flux control (eg, gadolinium). Commercially produced uranium dioxide is a fairly porous, fine powder that, as is, is not suitable for use in commercial power reactors. Various means have been developed and used to convert powdered uranium dioxide into a form suitable for use as fuel in commercial power reactors. One example of a technique that has been commonly used is to sinter appropriately sized blocks of uranium dioxide powder material at high temperatures to form strong diffusion bonds between individual powder particles. In any case, such sintering techniques require that loose powder particles be previously formed into a compacted body of a specific shape, and that such compacted bodies have sufficient strength and bonding to withstand handling and sintering operations. It must be self-retaining and have a certain gender. The process of forming compacted agglomerates from fine particles with sufficient strength and homogeneity to withstand subsequent handling and sintering while keeping the defect rate sufficiently low is a research topic of great interest in the nuclear fuel industry. It was hot. Organic or plastic binders conventionally used during powder processing are considered unsuitable in nuclear fuel processing operations. The introduction of binder residues, such as carbon, into the sintered tube fuel product is unacceptable for nuclear reactor applications. Moreover, the presence of organic binders between the particles prevents the formation of strong diffusion bonds between the particles during sintering, and also adversely affects the density of the sintered product. Furthermore, it is particularly difficult to completely remove the binder or its decomposition products prior to sintering, so that costly additional operations are typically required during the production of nuclear fuel. Therefore, it has been conventional practice to form green compacts of suitable dimensions by die compaction of uranium dioxide powder without the aid of binders. However, the low strength of such green compacts without binder leads to high rejection rates and very high costs for recycling the scrap material. 1977, assigned to the same assignee as the present invention.
Gallivan, U.S. Pat. No. 4,061,700, issued Dec. 6, discloses a special type of fugitive binder that improves the production of sintered pellets of granular nuclear fuel material for nuclear reactors. The fugitive binder of this patent acts without contaminating the resulting nuclear fuel product, yet allows effective bonds to form between particles during sintering without adversely affecting the porosity of the sintered pellet. do. In addition to U.S. Pat. No. 4,061,700, cited above, U.S. Pat.
Nos. 3803273, 3923933 and 3927154 also relate to important problems in the field of producing nuclear fuel pellets from fissile granular ceramic materials for nuclear reactors. Conventional techniques and means, such as those disclosed in US Pat. No. 4,061,700, have proven unsatisfactory under certain conditions and circumstances. For example, when using the fugitive binder of the above patent,
It has been observed that irrespective of the formulation conditions and particle properties of the uranium dioxide powder, inconsistent results regarding pellet strength and cohesion are obtained. That is, the degree of agitation during compounding, the relative humidity and temperature, and the severity of the storage period, as well as uranium dioxide powder properties such as size, surface area, and moisture content, are all physical attributes imparted by the fugitive binder. This can be a factor that clearly reduces the uniformity of the image. The amine-type fugitive binders described in the aforementioned U.S. Patent Application Nos. 331,492 and 273,900 include
It has been found that there is a significant improvement in the processing properties of granular ceramic materials containing uranium dioxide and in the physical properties of compacts formed therefrom. However, the conventional means described above have not been able to alleviate the inherent brittleness of certain ceramic materials made of uranium dioxide powder, that is, to consistently and rapidly compression mold them into agglomerated compacts with high fracture resistance. It has been insufficient to provide such materials with the degree of plasticity that is possible. The present invention now relates to the inherent brittleness of ceramic materials and to the problems this presents when compression molding granular such materials and the problems that occur in the resulting molded articles. As is known, ceramic materials are generally brittle rather than plastic. That is, in the case of plastic materials, deformation occurs gradually as the applied compressive stress increases and approaches the breaking point, whereas ceramic materials tend to undergo almost no deformation. However, when the fracture point is reached, a sudden fracture occurs, and the resulting cracks immediately spread throughout the material, causing it to fracture. A good example of the non-yielding and fracture properties of ceramic materials is the crushing of glass spheres. On the other hand, plastic materials gradually bend and deform as compressive stress increases, and eventually break when they reach a breaking point. Even in such cases, the resulting cracks typically propagate slowly and do not progress to the extent that they result in total fracture. Therefore, plastic type materials are easier to compression mold than brittle type materials. Thus, the inherent brittleness (or lack of plasticity) of uranium dioxide powder causes serious problems when subjected to compression molding operations, and also constitutes a significant drawback in the properties of the molded articles. SUMMARY OF THE INVENTION The present invention provides a method for producing an agglomerated compact from a granular ceramic material, which method imparts plasticity to the granular ceramic material during compression molding by adjusting the water content in the material. (i.e., alleviating brittleness). Naturally, the method of the present invention includes a series of operations that affect the above-mentioned components, among which adjusting the water content to a specified value during the compression molding process is essential to the present invention. OBJECTS OF THE INVENTION The primary object of the invention is to provide a method for facilitating compression molding of granular ceramic materials. It also provides a means for mitigating the brittleness of granular ceramic materials and imparting plasticity to such materials, thereby allowing compression molding of granular ceramic materials at high speeds with low rejection rates under nearly all compression conditions. It is also one of the objects of the present invention to do so. Furthermore, the work of forming an agglomerated compact by compression molding of a granular ceramic material containing uranium dioxide powder is facilitated, and the compacted agglomerate (and its sintered product) is easily formed during compression molding and subsequent steps. It is also an object of the present invention to provide a method for increasing the fracture resistance and resistance to crack propagation. Furthermore, there is provided a method for producing nuclear fuel pellets in which agglomerated compacts are formed by compression molding of a granular ceramic material containing uranium dioxide powder, the method comprising crushing unsintered agglomerated compacts or sintered products thereof. Another object of the present invention is to provide a method that allows high-speed compression molding while minimizing the number of defective products. DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a method for producing pelletized fissionable nuclear fuel products from granular ceramic material using a fugitive binder that is removed during a subsequent sintering operation. Such a method involves forming an agglomerated compact of suitable dimensions by compression molding a blend of a granular ceramic material containing uranium dioxide powder and an amine-containing fugitive binder, and then sintering the agglomerated compact. The method includes steps for forming an integral structure of fissile nuclear fuel material suitable for use in a nuclear reactor. The granular ceramic nuclear fuel material used in the present invention is composed of various materials that can be used as nuclear fuel in a nuclear reactor, and includes ceramic compounds including oxides of uranium, plutonium, and thorium. It will be done. Suitable ceramic compounds for fuel include uranium oxide, plutonium oxide, thorium oxide, and mixtures thereof. The granular ceramic nuclear fuel material used in the present invention may also contain various additives, such as a neutron absorber comprising gadolinium for neutron flux density adjustment. Suitable fugitive binders for use in the present invention include the aforementioned U.S. Patent Application Nos. 331492 and 273900;
and the amine-containing compounds disclosed in that specification. These amine compounds are selected from the group consisting of amine carbonates and carbamates, and specific examples include ethylenediamine, monomethylamine, 3,3-diaminodipropylamine, 1,3-diaminopropane, 1,6
- Carbonates or carbamates such as diaminohexane, n-butylamine, diethylenetriamine and 1,7-diaminoheptane.
Preferably, such fugitive binders are used in a proportion of about 0.5% to about 7% (by weight) based on the weight of the particulate ceramic nuclear fuel material. Amounts greater than about 7% (by weight) generally not only do not provide a commensurate increase in bonding strength, but also have undesirable consequences that negate the desired benefits and increase the cost of large amounts of bonding agent. will also occur. In accordance with the method of the present invention, a fugitive binder as described above and water are added to a granular ceramic nuclear fuel material containing uranium dioxide, and the binder is substantially uniformly incorporated with the water into the nuclear fuel material. The content in the blend of granular ceramic nuclear fuel material and added binder is then reduced by any suitable means to achieve a specified water content level (e.g., by evaporating excess water above the specified level). The amount of water is adjusted to less than about 3500 parts by weight per million parts by weight of the formulation. Note that a suitable water content range is about 1000 to about 3000 ppm, and a particularly suitable water content range is about 1500±200 ppm. Thereafter, a blend of granular ceramic nuclear fuel material having a water content of less than about 3500 ppm as defined by the present invention and an added binder is compression molded according to conventional methods in the art to form the appropriate dimensions. An agglomerated compact is formed. The method of the invention makes it possible to effectively use the above-mentioned formulations in rotary presses for high-speed continuous production. By sintering the compacted agglomerates thus obtained according to conventional methods in the art, the binder is expelled and the ceramic particles are integrated into a homogeneous continuous body. The sintered product, which is usually in the form of pellets, is then ground to defined dimensions depending on the intended use. The water introduced into the granular ceramic nuclear fuel material, either independently or together with fugitive binders, is preferably as little as possible in order to reduce the amount of water that must be removed to meet the requirements of the invention. . The added water serves to more effectively and rapidly disperse the amine-containing binder into the granular ceramic nuclear fuel material and to perform its binding function as known in the art.
Typically, such water forms the solvent or suspending medium for the amine-containing binder, so it is most effective to add the amine-containing binder in the form of an aqueous solution or suspension, depending on the solubility of the amine compound used. It is. However, it is also possible to add such water independently. Furthermore, some water may already be present in the granular ceramic nuclear fuel material prior to the introduction of water and binder. In any event, the water content in the granular ceramic nuclear fuel material and binder formulation must be maintained at a specified level of approximately 3500 ppm or less throughout the compression molding process. Incorporation of the added binder can be carried out using any suitable "dry" mixing equipment. Examples of such equipment include low shear mixers such as fluidized bed mixers, slab blenders and ribbon blenders, and high shear or high intensity mixers such as vibratory mills, ball mills and centrifugal mills. Suitable mixing devices are described in Perry & Chilton, eds., Chemical Engineering Handbook.
Engineering Handbook, 5th Edition, published by McGraw-Hill Book Company.
This is a vibratory mill of the type described on pages 8-29-30 of Book Co.). In order to adjust the water content in the blend of granular ceramic nuclear fuel material and added binder to the required level of less than about 3500 ppm, any device that can control the amount of water retained therein may be used. Any suitable means can be used. For example, increasing the temperature of the formulation, adjusting the ambient humidity,
Water can be removed by evaporation by passing a relatively dry gas through the formulation, or by any combination of these methods. According to a preferred embodiment of the invention, the water content of the formulation is adjusted in a fluidized bed mixer. In this case, the necessary evaporation to adjust the water content of the formulation to the required level will be achieved by the use of fluidizing gas. When blending particulate ceramic nuclear fuel material with a binder containing an amine compound in the presence of water, a residence time of at least about 20 minutes, preferably, prior to adjusting the water content of the blend to the required level according to the present invention, is required. A residence time of at least about 40 minutes should be ensured. Such residence time serves to create an effective bonding mechanism for the amine compound and to modify the brittleness of the ceramic particles thereby imparting plasticity to them to facilitate compression molding. Agglomerated compacts are formed by compression molding the blend of granular ceramic nuclear fuel material and added binder after adjusting the water content to the required level of the present invention while maintaining the water content. Ru. For this purpose, virtually any available means or apparatus can be used according to the art, such as those described in the literature, including the above-mentioned US patents and patent applications. Next, by sintering the compacted agglomerated body of the compression-molded binder-containing granular ceramic nuclear fuel material,
The binder is driven out and the ceramic particles are integrated into a substantially continuous body having substantially uniform density, relatively high strength and good crush resistance. A suitable procedure for carrying out the method of the invention will be illustrated below. Enriched uranium dioxide powder, granulated to a substantially uniform particle size for blending with a fugitive binder, was placed in a vibratory mill [Vibro-Energy Milco, Sweco Inc.]. charged. A 48% (by weight) aqueous solution of ethylenediaminecarbamate is added to the uranium dioxide powder. In this case, the amount used may be such that 1.63% (by weight) of water and 1.5% (by weight) of solid ethylenediamine carbamate are supplied based on the weight of the uranium dioxide powder. Mixing the uranium dioxide powder and the ethylene diamine carbamate solution for about 20 minutes provides a substantially homogeneous blend of the components. After transferring such a formulation to a fluidized bed mixer, at 20°C
By supplying fluidizing nitrogen gas with a slightly higher temperature and/or lower dew point,
Water is removed from the formulation over a period of approximately 90 minutes.
In accordance with the present invention, such evaporation operations are continued until the water content of the formulation is reduced to about 3500 ppm or less, preferably about 1500 ppm. After the water content in the uranium dioxide powder and ethylenediamine carbamate mixture is adjusted to below about 3,500 ppm, it is compressed under a pressure of about 22,000 to 25,000 pounds per square inch using a rotary press for continuous production. Molding is performed. If the agglomerated compacted body thus obtained is sintered according to a conventional method,
Fuel pellet products can be manufactured. In order to demonstrate the effects of the present invention, a series of evaluation tests were conducted according to the procedures exemplified above. In addition,
All conditions were the same except that drying time and moisture content were varied as indicated in the table below. Tensile strength (TS), plasticity index (PI) and F ³ were evaluated by performing diametrical compression tests on the compacted bodies obtained from each test series. Naeo, PI is a relative measure of plastic deformation;
^F3 is a relative measure of resistance to crack slowing or crack propagation after fracture initiation.

【table】

[Table] Furthermore, another series of evaluation tests was conducted according to the above procedure, and the following data were obtained.

[Table] Corrected values (that is, values excluding bound water).

[Brief explanation of the drawing]

The drawing is a flow sheet illustrating the steps of the method of the invention.

Claims (1)

[Scope of Claims] 1. (a) a fugitive binder containing an amine compound and water are added and blended to a granular ceramic nuclear fuel material containing uranium dioxide, and (b) the granular ceramic nuclear fuel material and the fugitive binder are blended together. (c) adjust the water content in the blend with about 3,500 parts by weight or less per million parts by weight of said blend;
The steps of forming an agglomerated compact by compression molding said formulation having a water content of less than 3500 ppm imparts plasticity to said granular ceramic nuclear fuel material and facilitates its processing. A method for producing an agglomerated compact of granular ceramic nuclear fuel material. 2. The method of claim 1, wherein the fugitive binder comprises an aqueous solution of the amine compound. 3. The method of claim 1, wherein the fugitive binder comprises at least one amine compound selected from the group consisting of amine carbonates and carbamates. 4. The method of claim 1, wherein the fugitive binder comprises ethylenediaminecarbamate. 5 The fugitive binder is about 0.5% to about 7% (by weight) based on the weight of the granular ceramic nuclear fuel material.
2. The method of claim 1, comprising an amine compound of 6 (a) Adding and blending a fugitive binder containing at least one amine compound selected from the group consisting of amine carbonates and carbamates and water to a granular ceramic nuclear fuel material containing uranium dioxide; b) adjusting the water content in the blend of the granular ceramic nuclear fuel material and the fugitive binder to about 3,500 parts by weight or less per 1 million parts by weight of the blend;
and (c) forming an agglomerated compact by compression molding the resulting blend having a water content of less than about 3500 ppm, thereby imparting plasticity to the granular ceramic nuclear fuel material. A method for producing an agglomerated compressed body of granular ceramic nuclear fuel material, characterized in that the process thereof is facilitated. 7. The water content in the blend of the granular ceramic nuclear fuel material and the fugitive binder is
7. The method of claim 6, wherein the amount is adjusted to about 1,000 to about 3,000 parts by weight per million parts by weight. 8. The method of claim 6, wherein the fugitive binder comprises ethylenediaminecarbamate. 9 The fugitive binder is about 0.5% to about 7% (by weight) based on the weight of the granular ceramic nuclear fuel material.
7. The method of claim 6, comprising an amine compound of 10. The method of claim 6, wherein the water content in the blend of said particulate ceramic nuclear fuel material and said fugitive binder is adjusted to about 1500 parts by weight per million parts by weight of said blend. 11 (a) adding and blending a fugitive binder containing an amine compound and water to a granular ceramic nuclear fuel material containing uranium dioxide; (b) adding and blending a fugitive binder containing an amine compound and water; adjusting the moisture content to about 1500 parts by weight per million parts by weight of said formulation; (c) forming an agglomerated compact by compression molding said formulation having a moisture content of about 1500 ppm thus obtained; and (d) sintering the agglomerated compact to drive out the fugitive binder and integrate the granular ceramic nuclear fuel material as a homogeneous continuum, resulting in the granular ceramic nuclear fuel material being sintered. A method for producing an agglomerated compact of granular ceramic nuclear fuel material, characterized in that plasticity is imparted to the material to facilitate its processing. 12. The method of claim 11, wherein the fugitive binder comprises an aqueous solution of the amine compound. 13. Claim 1, wherein the fugitive binder comprises at least one amine compound selected from the group consisting of amine carbonates and carbamates.
The method described in Section 1. 14. The method of claim 11, wherein the fugitive binder contains ethylenediamine. 15. The method of claim 11, wherein the fugitive binder comprises ethylenediaminecarbamate. 16 The fugitive binder has a weight of about 0.5 to about 7 (by weight) based on the weight of the granular ceramic nuclear fuel material.
% of the amine compound. 17 (a) Adding and blending a fugitive binder comprising an aqueous solution of at least one amine compound selected from the group consisting of amine carbonates and carbamates to granular ceramic nuclear fuel material comprising uranium dioxide; b) adjusting the water content in the blend of the granular ceramic nuclear fuel material and the fugitive binder to about 3,500 parts by weight or less per 1 million parts by weight of the blend;
(c) forming an agglomerated compact by compression molding said blend having a water content of about 3500 ppm or less; and (d) forming said agglomerated compact by sintering said agglomerated compact. characterized in that as a result of the steps of driving out the fugitive binder and integrating the granular ceramic nuclear fuel material into a homogeneous continuum, the granular ceramic nuclear fuel material is imparted with plasticity to facilitate its processing. A method for producing an agglomerated compact of granular ceramic nuclear fuel material. 18 The fugitive binder has a weight of about 0.5 to about 7 (by weight) based on the weight of the granular ceramic nuclear fuel material.
Claim 1 containing % amine compound
The method described in Section 7. 19. The method of claim 17, wherein the water content in the blend of the granular ceramic nuclear fuel material and the fugitive binder is adjusted to between about 1000 and about 3000 parts by weight per million parts by weight of the blend. . 20. The method of claim 17, wherein the fugitive binder contains ethylenediaminecarbamate. 21 (a) Applying a fugitive binder comprising an aqueous solution of at least one amine compound selected from the group consisting of carbonates and carbamates of amines to a granular ceramic nuclear fuel material containing uranium dioxide by the weight of said granular ceramic nuclear fuel material. (b) the water content in the blend of the granular ceramic nuclear fuel material and the fugitive binder is determined to be approximately 0.5% to about 7% (by weight) based on the weight of the blend; (c) forming an agglomerated compact by compression molding said blend having a water content of about 1500 ppm, and (d) said agglomerated compact. steps of driving out the fugitive binder and integrating the granular ceramic nuclear fuel material as a homogeneous continuum by sintering the granular ceramic nuclear fuel material, imparting plasticity to the granular ceramic nuclear fuel material and improving its properties. A method for producing an agglomerated compact of granular ceramic nuclear fuel material, which is characterized by ease of processing. 22. The method of claim 21, wherein the fugitive binder comprises ethylenediamine carbonate. 23. The method of claim 21, wherein the fugitive binder contains ethylenediaminecarbamate.