US3632095A

US3632095A - Carrier for uranium oxide pellets

Info

Publication number: US3632095A
Application number: US765072A
Authority: US
Inventors: James A Rode; Paul W Hubert
Original assignee: United Nuclear Corp
Current assignee: General Atomics Corp; United Nuclear Corp
Priority date: 1968-10-04
Filing date: 1968-10-04
Publication date: 1972-01-04
Anticipated expiration: 1989-01-04
Also published as: DE1949989A1; GB1222937A; DE1949989B2; FR2019957A1; SE343161B; DE1949989C3

Abstract

A carrier of uranium oxide pellets suitable for use in dewaxing and sintering of said pellets has a molybdenum or stainless steel base on which there are multiple layers of corrugated molybdenum sheets for holding the pellets within the depressions of the parallel corrugations. The molybdenum sheets are placed above each other with the corrugations running in about the same general direction. The separation of these sheets is by the uranium oxide pellets during the sintering operation and by molybdenum cylinders of a larger diameter than the pellets during the dewaxing operation.

Description

States atem [72] lnventors JamesA.Rode

[56] References Cited UNITED STATES PATENTS St. Louis;

Paul W. Hubert, Pevely, both 0t Mo.

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ABSTRACT: A carrier of uranium oxide pellets suitable for use in dewaxing and sinterin denum or stainless steel base on which there are multi layers of corrugated molybdenum sheets for holdin lets within the depressions of the parallel corru molybdenum sheets are placed above each other with the corrugations running in about the same general direction. The separation of these sheets is by the uranium oxide ing the sintering operation and by molybdenum c larger diameter than the pellets during the dewaxing operation.

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SHEET 2 BF 2 INVENTORS JAMES A. RODE PAUL W. HUBERT BY QM luau,

CARRIER FOR URANIUM OXIDE PELLETS FIELD OF THE INVENTION This invention relates to a carrier for uranium oxide pellets in the dewaxing and sintering operations of the process for preparing uranium oxide pellets for the nuclear reactor.

SUMMARY OF THE INVENTION The carrier of this invention has a metallic support base resistant to the corrosive emironment during the dewaxing and sintering of the uranium oxide pellets. On the support base is a first molybdenum sheet which is removable therefrom. The sheet has parallel corrugations thereon with depressions to hold the pellets. At least one additional corrugated molybdenum sheet used for holding additional pellets is placed above the first sheet which is supported by either the pellets during the sintering operation or by a plurality of molybdenum cylinders of a diameter greater than the pellets during the dewaxing operation. For the sintering operation one or more sheets of molybdenum may be advantageously placed above the last layer of pellets to serve as a heat shield for more even distribution of the heat among the pellets to provide a more uniform sintered product.

BRIEF DESCRIPTION OF THE DRAWINGS The accompanying drawings illustrate the preferred embodiments of this invention wherein FIG. 1 is a perspective view of a carrier suitable for holding uranium oxide pellets for dewaxing and FIG. 2 is a perspective of a carrier suitable for holding the pellets for sintering.

DESCRIPTION OF THE PREFERRED EMBODIMENTS In the production of uranium oxide pellets, finely ground or preferably micronized uranium dioxide powder is initially mixed with an organic binder such as polyvinyl alcohol and Carbowax. The admixture is then agglomerated and pressed under a pressure in the range between 30 to 80 t.s.i. to produce cylindrical pellets of a sufficiently high green strength to withstand subsequent treatments and to provide strong and sound finished pellets. The organic binder used for pelletizing must be first removed to eliminate high contamination of the final products by primarily carbon and nitrogen. Generally, the binder is removed by a treatment commonly called dewaxing," using a mixture of steam and a small amount of hydrogen or crack ammonia, the latter being used to prevent oxidation of U and at a temperature of about 800 C. After the binder is removed, the dewaxed pellets are sintered at a temperature of about 1,650 C. in a reducing atmosphere to about 95 percent or more of the theoretical density.

For commercial production of uranium oxide pellets a large amount of pellets is being handled in a short period of time either automatically or semiautomatically. The design of a carrier for handling the pellets for dewaxing and sintering becomes an important aspect of a successful commercial uranium oxide pelletizing plant. We have discovered that uranium oxide pellets can be handled efficiently and with minimum manual manipulation using the carrier of this invention for the dewaxing and sintering operations and more particularly the carrier of this invention produces a more uniformly sintered product. Further to illustrate this invention two specific embodiments of this invention are described hereinbelow with reference to the accompanying drawings.

Referring initially to MG. 1 the carrier of this invention which is specially adapted for dewaxing operations has a supporting base II which has a rectilinear flat main body 12 and two

railings

13 and 14 on the opposite sides of the base II. The supporting base 11 should be constructed from a metal which is substantially resistant to corrosion at the ambient environment during dewaxing operations. We found stainless steel or lnconel eminently suitable for this purpose. The supporting base 11 may be molded or extruded into the form shown in FIG. 1.

On top of the supporting base II are two rectangular molybdenum sheets 15 and I6 placed side by side with their opposite outer edges parallel to and preferably abutting the railings I3 and 14. The molybdenum sheets are corrugated with substantially parallel depressions 17 extending the whole length of the sheet. The depressions [7 of the corrugations are designed to hold multiple rows of uranium pellets 24 with each row separated from its adjacent rows by a fixed distance of about one to two times the diameter of the pellets.

Above the first layer of molybdenum sheets 15 and 16 are two similarly corrugated molybdenum sheets 18 and I9 each separated from its corresponding adjacent sheet at a vertical. distance greater than the diameter of the pellets by two rows of molybdenum cylinders 20 placed at the outermost rows of depressions of the corrugated sheets. The use of molybdenum cylinders provided a space above the pellets thus allowing the decomposed vaporous material during dewaxing to escape therefrom. Two or more layers of similar corrugated molybdenum sheets 21 may be provided for holding additional pellets for dewaxing with each separated from its adjacently above or below sheets with molybdenum cylinders 20.

The corrugated molybdenum sheets may be made by any method. They can be molded, stamped or pressed, for example, from a flat sheet to the desired form. The thickness of the corrugated sheet may be varied depending on the size of the pellets and on the load exerted thereon. Because of the corrugation which has the advantage of strengthening the sheet, a thickness about one-tenth inch has been found adequate for handling pellets of common sizes.

For sintering, the carrier described above may be used. However, since most volatile materials have escaped from the pellets in the dewaxing operation, the space provided between the corrugated sheets by the molybdenum cylinders may be eliminated. Also, if a stainless steel or an lnconel supporting base were used for the dewaxing operation, it may be advantageous to change the base to a more temperature resistant material such as molybdenum because sintering of uranium dioxide pellets is conducted at a much higher temperature, i.e., about 1,650 C. We found the modification of the carrier can be accomplished simply by pushing each of the two stacks of uranium oxide pellets on the corrugated molybdenum sheets onto a molybdenum supporting base and by removing the molybdenum cylinders from the edge depressions thereof. The resultant carrier now is in the form substantially as shown in FIG. 2. In this drawing, the supporting base 22 made of molybdenum has generally the same configuration as the supporting base for the dewaxing carrier 10, except it is about half the width. The corrugated sheets 23 now are separated solely by the dewaxed pellets 24. Dewaxing does not increase the diameter of the pellets and therefore it will be understood that the pellets 24 in FIG. 2 have about the same diameter as the pellets shown in FIG. I.

To form a heat shield which would allow a more even heat distribution within the stack of pellets and hence a more uniform sintered products, one or more layers or corrugated molybdenum sheets 25 may be placed above the topmost layer of pellets 26. The molybdenum sheets used as heat shields may be separated from each other with molybdenum spacers 27 placed about four comers and at the center between the sheets 25.

The carrier of this invention can be utilized in modern automatic or semiautomatic pelletizing plants for the manufacture of ceramic grade U0 fuel elements. In the modern pelletizing plants, U0, pellets are compacted continuously. These pellets are then placed on the corrugated sheet either individually or one row at a time. Once the required number of layers of pellets are filled, the carrier is sent to the dewaxing furnace for the removal of the binder therein. Subsequent to dewaxing, the stacks of pellets are directly transferred to a new supporting base; thereon the molybdenum spacers are removed and the heat shield is placed thereon. After the modification, the carrier is sent into the sintering furnace for the required treatmentv Because the pellets are orderly arranged in rows, handling the sintered product subsequently is equally simplified.

We claim:

l. A carrier for uranium oxide pellets suitable for use in the dewaxing and sintering operations of said pellets which comprises:

a. a metallic support resistant to corrosive actions under the temperatures and ambient conditions of the dewaxing and sintering operations;

b. a first corrugated molybdenum sheet positioned on said metallic support and being removable therefrom, said molybdenum sheet having substantially parallel corrugations with depressions for holding a plurality of cylindrical uranium pellets prepared for subjection to at least one of said dewaxing or sintering operations and c. at least one additional corrugated molybdenum sheet having substantially parallel corrugations of the same dimensions as said first sheet for holding additional uranium pellets, each of said additional molybdenum sheets being positioned above and removable from a lower molybdenum sheet separated either by the pellets placed therebetween or by a plurality of molybdenum spacers placed between the sheets and the thickness of such spacers being adapted to be greater than the diameter of the pellets.

2. A carrier for uranium oxide pellets suitable for use in the dewaxing of said pellets which comprises:

a. a metallic support resistant to corrosive actions under the temperature and ambient conditions of the dewaxing operation;

b. a first corrugated molybdenum sheet removably positioned on said metallic support, said sheet having substantially parallel corrugations with depressions for holding a plurality of cylindrical uranium pellets prepared for subjection to said dewaxing operation; and

c. at least one additional corrugated molybdenum sheet having substantially parallel corrugations of the same dimensions as those of said first sheet for holding additional uranium pellets, each of said additional sheets being positioned above and removable from a lower molybdenum sheet separated by a plurality of molybdenum spacers placed between the sheets, the thickness of such spacers being adapted to be greater than the diameter of said pellets so as to leave spaces above the pellets for the escape of vaporous material during the dewaxing operation.

3. A carrier according to claim 2 wherein the metallic support is made of stainless steel or molybdenum.

4. A carrier according to claim 2 wherein the molybdenum spacers for the corrugated sheets are molybdenum cylinders having a diameter adapted to be greater than the diameter of the pellets and at least some of said molybdenum cylinders are positioned on the corrugations adjacent to the edges of the sheets.

5. A carrier according to claim 2 wherein the carrier has at least three sheets of corrugated molybdenum for carrying the pellets for the dewaxing operation.

6. A carrier for uranium oxide pellets according to claim 2 in which the distance between the corrugations in the molybdenum sheets is adapted to be from about 1 to about 2 times the diameter of the pellets.

7. A carrier for uranium oxide pellets suitable for use in the sintering of said pellets which comprises:

a. a metallic support resistant to corrosive action under the temperature and ambient conditions of the sintering operation;

b. a first corrugated molybdenum sheet positioned on said support and removable therefrom, said sheet having substantially parallel corrugations with depressions for holding a plurality of cylindrical uranium pellets prepared for subjection to the sintering operation; and

c. at least one additional corrugated molybdenum sheet having substantially parallel corrugations of the same dimensions as those of said first sheet for holding additional uranium pellets, each of said additional sheets being positioned above and removable from a lower molybdenum sheet separated therefrom by the pellets placed thercbetween. 8. A carrier according to claim 7 wherein the metallic support is made of stainless steel or molybdenum.

9. A carrier according to claim 7 wherein the carrier has at least five sheets of corrugated molybdenum for carrying the pellets for the sintering operation, and at least one corrugated molybdenum sheet which does not carry pellets and which is spaced above the top sheet that carries pellets to form a heat shield and produce an even heat distribution within the mass of pellets.

10. A carrier for uranium oxide pellets according to claim 7 in which the distance between the corrugations in the molybdenum sheets is adapted to be from about i to about 2 times the diameter of the pellets.

Claims

2. A carrier for uranium oxide pellets suitable for use in the dewaxing of said pellets which comprises: a. a metallic support resistant to corrosive actions under the temperature and ambient conditions of the dewaxing operation; b. a first corrugated molybdenum sheet removably positioned on said metallic support, said sheet having substantially parallel corrugations with depressions for holding a plurality of cylindrical uranium pellets prepared for subjection to said dewaxing operation; and c. at least one additional corrugated molybdenum sheet having substantially parallel corrugations of the same dimensions as those of said first sheet for holding additional uranium pellets, each of said additional sheets being positioned above and removable from a lower molybdenum sheet separated by a plurality of molybdenum spacers placed between the sheets, the thickness of such spacers being adapted to be greater than the diameter of said pellets so as to leave spaces above the pellets for the escape of vaporous material during the dewaxing operation.
3. A carrier according to claim 2 wherein the metallic support is made of stainless steel or molybdenum.
4. A carrier according to claim 2 wherein the molybdenum spacers for the corrugated sheets are molybdenum cylinders having a diameter adapted to be greater than the diameter of the pellets and at least some of said molybdenum cylinders are positioned on the corrugations adjacent to the edges of the sheets.
5. A carrier according to claim 2 whereIn the carrier has at least three sheets of corrugated molybdenum for carrying the pellets for the dewaxing operation.
6. A carrier for uranium oxide pellets according to claim 2 in which the distance between the corrugations in the molybdenum sheets is adapted to be from about 1 to about 2 times the diameter of the pellets.
7. A carrier for uranium oxide pellets suitable for use in the sintering of said pellets which comprises: a. a metallic support resistant to corrosive action under the temperature and ambient conditions of the sintering operation; b. a first corrugated molybdenum sheet positioned on said support and removable therefrom, said sheet having substantially parallel corrugations with depressions for holding a plurality of cylindrical uranium pellets prepared for subjection to the sintering operation; and c. at least one additional corrugated molybdenum sheet having substantially parallel corrugations of the same dimensions as those of said first sheet for holding additional uranium pellets, each of said additional sheets being positioned above and removable from a lower molybdenum sheet separated therefrom by the pellets placed therebetween.
8. A carrier according to claim 7 wherein the metallic support is made of stainless steel or molybdenum.
9. A carrier according to claim 7 wherein the carrier has at least five sheets of corrugated molybdenum for carrying the pellets for the sintering operation, and at least one corrugated molybdenum sheet which does not carry pellets and which is spaced above the top sheet that carries pellets to form a heat shield and produce an even heat distribution within the mass of pellets.
10. A carrier for uranium oxide pellets according to claim 7 in which the distance between the corrugations in the molybdenum sheets is adapted to be from about 1 to about 2 times the diameter of the pellets.