US 3393256 A
Description (OCR text may contain errors)
July 16, 1968 I BE'R 3,393,256 PROCESS FOR MAINTAINING STRAIGHTNESS IN EXTRUDED CERAMIC SECTIONS Filed Aug. 10, 1967 INVENTOR.
BY MARIO J. ZAMBERNARD United States Patent 3,393,256 PROCESS FOR MAINTAINING STRAIGHTNESS IN EXTRUDED CERAMIC SECTIONS Mario 1. Zambernard, Homer City, Pa., assignor to the United States of America as represented by the United States Atomic Energy Commission Filed Aug. 10, 1967, Ser. No. 660,882 3 Claims. (Cl. 26 .5)
ABSTRACT OF THE DISCLOSURE A method for drying extruded rod-like ceramic sections prior to sintering to reduce the degree of warping and out of roundness in the final product. During the drying step of preparing U0 extruded sections, the latter are mounted in a rotatable tube by way of over-sized circular holes in the supporting end plates. As the sections rotate with the tube, they also roll around in their supporting holes. Drying gas delivered to the sections is then applied uniformly over all surface areas of the sections.
Background of the invention This invention was made in the course of, or under a contract with the United States Atomic Energy Commission.
In the preparation of certain types of extruded sections, such as extended cylindrical sections of U0 for use in nuclear fuel elements, it is necessary to maintain a high degree of straightness in the extruded sections during the drying and sintering cycle because of the strict dimensional requirements in their ultimate use as fuel elements. Drying prior to sintering is ordinarily accomplished by placing the extruded sections in an oven and maintaining a heated atmosphere until binder and moisture are evaporated.
For relatively short lengths of extruded sections of up to about four inches, bending or warping from the straight section in the range heretofore found to be present in the final product under existing methods of preparing the extruded sections were acceptable. If lengths beyond four inches are to be prepared, however, and this appears to be desirable in some situations, warping of the order of magnitude produced by previous methods produce results much less than satisfactory.
Summary of the invention The present invention overcomes the aforementioned difiiculties by permitting the preparation of extruded sections of the type herein described with far less warping that heretofore was thought to be possible in the final sintered sections by conventional methods of preparation. In accordance with this invention rod-like sections of ceramic U0 containing binder and plasticizer are dried in a revolving tube in order to expose all surface areas uniformly to the drying atmosphere. As a result of this method of drying, in which the sections are kept in continual motion, there is produced a sharply reduced bowing or warping and out of roundness in the final product of substantially greater length.
It is thus a principal object of this invention to provide a way of preparing extruded rod-like sections of reduced tendency to warp.
Other objects and advantages of this invention will be readily apparent from the following description of preferred embodiments of this invention.
Description of the drawing FIG. 1 is an isometric view of preferred apparatus for carrying out the principles of this invention.
FIG. 2 shows a partial section along one of the extruded sections of FIG. 1.
Description of the preferred embodiments In the preparation of fired extruded sections of U0 intended for loading into fuel rods for use in nuclear reactors, UO in powdered form is blended with a binder and plasticizer, mixed together to the proper consistency, and loaded into an extrusion chamber. The mixture is then evacuated of air and extruded into the desired length and shape. The extruded sections are then dried in accordance with this invention as will be described below to remove the binder and any moisture present, and finally fired in a furnace to sinter the final product. This process of preparing extruded sections of U0 except for the unique way hereinafter described for drying the sections prior to sintering, is well known in the art.
Referring to the figures, revolving dryer 10 consists of an elongated tube 12 having a pair of end identical plates 14 and 16. Plate 14 is provided with a plurality of openings 18 to support the ends of extruded sections 22 of circular cross-section. The opposite ends of the latter are supported in similar fashion by similar openings (not shown) in end plate 16. In addition, extending from each of plates 14 and 16 are shaft extensions 24 and 26, respectively, mounted on supports 28 and 32, respectively, for permitting the rotation of tube 12. Shaft 26 extends for coupling to a motor unit 34 which provides the power for rotating tube 12.
Shown somewhat schematically is a blower 36 whose purpose is to direct a stream of the drying atmosphere through and around tube 12 where openings (i.e. 18 in plate 14) are oversized to permit circulation through the interior of tube 12 as well as to permit sections 22 to roll around during rotation. Dryer 10 is located within an environment such as a room or a large chamber wherein the atmosphere may be controlled at the desirable consistency, temperature and pressure. In the examples described below, the drying atmosphere consisted of air at a temperature of about 22 C., at substantially atmospheric pressure with blower 36 consisting of a M: RR fan located at a distance of about two feet from the upstream end of dryer 10.
In the utilization of the dryer shown, fuel rods 22, following extrusion, are placed within tube 12 as illustrated, and the latter is rotated while fan 36 is operated to produce a steady flow of drying atmosphere through and around tube 12. Straightness in rods 22 is accomplished, it is believed, by introducing uniform stresses in the outer layers of the green extrusions during the drying cycle through the simultaneous rolling and rotating sequence of extrusions 22 arising from the rotation of tube 12.
Several extrusions of U0 were made in accordance with the principles of this invention, a few of which are listed in the following table:
TABLE Sample Weight of U02, grams 4, 000 4, 000 4, 000 Weight of binder (ammonium ginate), grams 30 30 30 Weight of plasticizer (glyeerine) grams 240 240 240 Water content (initial percent) WH20/WUO2 l5 19 19 Extrusion load, lbs 125, 000 125, 000 125, 000 Moisture content of Extr on, w/0 8. 28 9. 04 15. 6 Green density, percent T.D 39. 2 41. 6 43. 9 Fired density, percent I.D 85. 2 89. 6 87. 2
Rotational speeds for tube 12 were within the range of 15 rpm. to 3 rpm. It was found that better results were produced by beginning the drying process at the high end of the rotational speed range and gradually reducing the speed to the low end of the range as the drying process was completed. This effect is believed to be the result of the characteristics of the green sections which are more plastic and likely to take a set whereas a reduction in speed is desirable later as the extrusions have acquired some green strength and greater advantage can be taken of a smoother flow and distribution of the drying atmosphere.
In each of the above samples, for extruded rods of six inch lengths, bending from the straight section was within about 0.015 inch. This compares to bending of 0.040 to .050 inch for lengths of about four inches for the same extrusion process except that the rods are dried by placing them in a drying oven and not rolled and rotated in accordance with this invention. Thus, it is seen that the present invention reduces bowing by about /3 for lengths of about 50% longer than previously obtained. This represents a significant improvement which makes it possible to utilize fuel rods of much greater length than heretofore was thought to be possible.
A feature of the invention is that the rolling action permits positive control of the drying sequence as the rate of surface exposure changes and the evaporation rate can be controlled by varying the speed of revolution, heat input and drying atmosphere. The temperature, pressure and moisture content of the atmosphere can be made to be other than ambient air if desired, fan 36 can be operated to give any desired rate of gas flow, and motor 34 can be used to alter the rolling rate for optimum or desired conditions. Another important advantage of this invention is that the rotational speed of tube 12 can be varied to suit the particular circumstances and the characteristics of the green extrusions.
Fuller details and information on this type of process for producing extended and sintered powdered U0 ceramic sections is described in Symposium on Power Packed U0 Fuel Elements, CEND153, Nov. 30-Dec. 1, 1961.
It is thus seen that there has been provided an improved way of producing extruded sections of improved straightmess with a minimum of out of roundness. While only preferred embodiments of the invention have been described, it is understood that many variations thereof are possible without departing from the principles of this invention. For example, tube 12 may be provided with additional openings both in the end plates and in the outer wall, if desired. Furthermore, tube 12 can be provided with the heating coils to aid the drying process and also to burn out the binder thus combining, if desired, drying and presintering of the sections. While only circular sections have been described, other than circular can be dried if some rounding tendency can be tolerated. Hence the invention is not to be limited in scope by the preceding description but only by that of the following claims.
1. In a process of preparing solid extruded sections of circular cross-section from ingredients consisting of a mixture of a powdered principal material, binder, and plasticizer having the steps of blending the ingredients, mixing said ingredients to the proper consistency, extruding the mixture into at least one extruded section, exposing the section to a drying atmosphere, and sintering the dried section, the improvement comprising, during the step of drying, supporting said section at the ends thereof for unrestrained rolling, and rotating and rolling said section simultaneously to introduce uniform outer surface area exposure to the drying atmosphere for reducing the extent of warping present in the section after sintering, the rate of rotation being decreased as said section is dried.
2. The process of claim 1 having a plurality of Said sections, said sections being mounted for drying parallel to each other in a circumferential array about an axis which is the center of rotation of said array, each of said sections being free to roll while being rotated about the center of rotation.
3. The process of claim 2 wherein the starting materials include powdered U0 as the principal material.
References Cited UNITED STATES PATENTS 3,124,625 3/1964 Sheinberg et al. 264--.5 3,318,695 5/1967 Goslee et al. 264.5 3,319,351 5/1967 Sprissler et al. 34109 L. DEWAYNE RUTLEDGE, Primary Examiner.