US3177578A - Method of making a fibrous fissionable member - Google Patents

Description

United States Patent ()fiice 3,177,578 Patented Apr. 13, 1965 3,177,578 METHOD @F MAKING A FIBROUS FISSIONABLE MEMBER Harold N. Barr, Baltimore, Md, assignor to Martin- Marietta Corporation, a corporation of Maryland No Drawing. Filed Mar. 28, 1961, Ser. No. 98,760

. 4 Claims. (Cl. 29-474.3)

This invention relates to a novel method of manufacture and is a continuation-in-part of my copending application Serial No. 723,095, filed Mar. 24, 1958, entitled Fissionable Member and Method of Making Same, now abandoned. More particularly, it is concerned with the manufacture of a fibrous fissionable member which can be used in nuclear reactors.

In the reactor field, rods or tubes containing fissionable material may be suitably spaced in a nuclear reactor for emission of energy which is absorbed by a liquid or gas medium in contact with such members. The density of the fissionable material in the rod or tubular member is important because at the high temperature levels of operation voids between the fuel core material and the metal clad which normally encases this material gives rise to poor heat transfer properties causing premature breakdown. in the manufacture of these members the aim is to eliminate void spaces betwen the core and the clad and to enhance thermal conductivity of the core to prevent formation of hot spots.

Present day fuel elements are for the most part formed of powder materials using conventional powder metallurgy methods. These techniques are well established and include a cold press-sinter technique or a hot pressing technique each of which may be followed by a convent-ional hot working operation. It is emphasized that such operations are available to those skilled in the art only with regard to powder materials. Many restrictions are imposed upon techniques for providing fuel elements using powder materials in conjunction with cold press forming techniques. Using the cold pressing method, it. is found that at the temperatures of operation, separation of the binding agent, whether it be metal or other material, from the ceramic or fissionable material, occurs. This results in a reduction of thermal conductivity and the life of the radioactive member is shorter than desired.

When such an element is made by hot pressing, the product is less objectionable than that obtained by the cold pressing technique. However, by the hot pressing technique the size and the thermal properties of the member are limited and for that reason it often appears desirable to resort to other practices for its manufacture.

The present invention is concerned with a novel method of manufacturing a ceramic-metal fiber nuclear fuel element having exceptional characteristics over those produced by convention techniques.

To date, attempts have been made a-t'manufacturing fuel elements using a metal fiber in lieu of metallic powder. Because of unique ditficulties of manufacture, the methods for forming fibrous fuel elements have been restricted to hot pressing arrangements.

Particular restrictions present in cold pressing methods for manufacturing fibrous compacts include the necessity of providing low density compacts because of the poor sinterability between the ceramic-metal fiber combination. Presently, only hot pressing techniques have been found acceptable to attain high bulk density compacts. To the present time the cold pressing of fibrous elements such as UO -rnetal fiber has proved unsatisfactory. The metal fibers, being relatively rigid, did not permit the thoria powder to contract uniformly. As a consequence, specimens are distorted, have numerous cracks, and have bulk densities not much higher than in the green state.

Hot pressing has been employed to eliminate these features. With this technique, the fiber structure was compressed, forcing the oxide powder into voids, thus developing very high bulk densities.

Although hot pressing techniques are workable with fibrous elements only those metallic materials which will not be affected by the temperature at which the hot press ing will be accomplished may be utilized. Obviously, a multitude of low melting point materials as well as those reactive with ceramics are precluded by a hot presing operation.

It is, therefore, an object of this invention to provide method of producing a ceramic powder-metal fibre fuel element using cold working techniques while still achieving a high fuel density with the element.

Another object of this invention is to provide a novel article of manufacture which is especially useful as a high temperature thermal conducting means.

Another object of this invention is to provide a novel method of producing a body from a ceramic powdermetal fibre mixture which has especially good properties in regard to thermal conductivity, heat transfer and resistance to thermal stress.

Another object of this invention is to provide a novel method of producing a fuel element having a core containing a high weight percentage of fissionable material, and which possesses outstanding thermal properties.

Other objects and advantages of this invention will become apparent from the following description and explanation thereof.

In accordance with this invention, the article of manufacture comprises a metallic cylindrical member having encased therein a core comprising a ceramic material with metal fibres distributed therethrough, said core having a density up to about 90% of theoretical. The term metal as used in this application is meant to include elemental metals and metal alloys.

More particularly, this invention is concerned With a method of making the ceramic-metal fiber body by combining a finely-divided ceramic material having an average particle size not greater than 100 microns with metal fibers having a cross-sectional area of not more than about 7 to 8 square mils and a length not greater than Mr inch. The mixture of ceramic material and fiber is placed in a suitable metal tube or the annular space or spaces formed by two or more concentrically disposed tubes and packed substantially to eliminate void space, but especially to maintain a proper alignment of the fibers within the tube. Particularly, the fibers are positioned in a direction normal to the longitudinal axis of the said tube.

The ends of the tube or tubes are sealed by suitable means such as welding, and the sealed tube or tubes are swaged to a cross-sectional area of about 60 to of the original size.

By maintaining the reduction by swaging within the aforementioned limits, the alignment of fibers within the element is not significantly altered. This stability of configuration gives license to the use of cold working techniques contrary to a priori prediction.

Optionally, the swaged member may be sintered at an elevated temperature to enhance bonding of the ceramic particles and/or bonding of the core of the clad.

The ceramic material is used in a finely-divided state in order that the metal fiber can be readily distributed substantially uniformly throughout its entire body. The average particle size is preferably not greater than about 100 microns and it can be as small as possible, such as, for example, about 0.1 micron. In general, the average particle size of the ceramic material may be from about 30 to microns. The ceramic can be any refractory which is chemically compatible with the clad and fiber materials at operating temperature. That is to say, no reaction should occur between the metal or metals and the ceramic which would render the assembly structurally unstable. Fissionable materials in ceramic form are useful for the purpose of this invention, as well as other ceramic materials, such as, for example, alumina, zirconia, titania, ceria, lanthanum oxide, etc. Fissionable ceramic materials include, for example, uranium dioxide, thorium oxide, plutonium oxide, uranium carbide, etc.

The ceramic material is mixed with a metal fiber which has a cross-sectional area of not more than about 7 square mils. The length of the fiber is not greater than about inch, and usually about A3 to inch. The metal fiber can be of any cross-sectional shape, namely, round, square, rectangular, or any other polygonal shape, the only limitation being that the cross-sectional area is not greater than about 7 square mil-s. When the cross-sec tional area is greater than about 7 square mils, it is found that more fiber is required to impart the same thermal characteristics to a ceramic-metal fiber member of a given size. In nuclear fuel elements, the use of metal fiber with an average cross sectional area greater than about 7 square mils serves to dilute the fissionable material out of proportion to gain in thermal conductivity or attainable power output.

A variety of metals can be used as clad or as metal fiber. For thermal reactors, it is desirable that the metal have a low thermal neutron absorption cross-section. The metal has a thermal neutron cross-section of about 0.009 to 5.0 barns and belongs to Groups IIIA, IVB having an atomic number of at least 22 and not greater than 40, VIB or VIII of the Periodic Chart. It is also preferred that the metal have a melting point above about 650 C. A variety of metals may be used for the cladding and for the fiber, according to the specific conditions of reactor operation. Examples of metals which are useful for this invention are Al, Fe, Ti, Zr, Ni, Mo, and alloys of these elements. If it is required that the ceramic be sintered, metals structurally stable at the sintering temperature must be used for the cladding and the fiber. Some suitable refractory metals are Mo and Cr. Fuel elements in which the ceramic need not be sintered may fabricate from stainless steel, Al and Ni. It is understood in all these instances that appropriate alloys may also be used instead of elemental metals.

The bond between the metal fiber-ceramic powder section, hereinafter referred to as the core, and the metal clad may be either metallurgical or mechanical in nature. If the fiber and/ or the clad may be sintered, a metallurgical bond is possible. Otherwise, the core and clad will be bonded mechanically by means of the swaging operation. Sintering temperatures are well known by those skilled in the art and require no further elaboration here except to say that in fabricating fuel elements sintering should be performed at temperatures at which dimensional stability may be maintained.

The clad member is usually cylindrical in shape and can consist of a single tube or at least two concentrically disposed tubes forming an annular region into which the ceramic powder-metal fiber mixture is placed. The clad material may be of the same metal a the metal fiber or it can be a different metal. The selection of the metal for the clad member will depend, among other things, upon the use of the final product as well as whether a metallurgical or mechanical bond is desired between the clad member and the core. For the fabrication of fuel elements, it is possible that zirconium fibers be used with aluminum clad; nickel or molybdenum fibers be used with stainless steel clad; stainless steel, molybdenum or aluminum fibers be used with nickel clad; or that the fiber and clad be the same metal. It is preferred, when other considerations allow it, that the metals for the clad member and fibers be selected to procure a metallurgical bond in the final product, whereby heat transfer from the core to the clad is enhanced.

By reason of the method of preparing the final product there is no limit on the size of the cladded member which can be processed in accordance with this invention. The clad member can be of any length, although usually the diameter is up to about 2 inches.

In the manufacture of the final product, the metal fibers constitute about 5% or more by volume of the core which is placed in a clad member. The core is tamped or agitated by means of a vibratory compactor to increase the density of the mixture. At this stage the density of the mixture is about 60 to of theoretical. Compacting may be effected by any suitable means and this means is readily known by those skilled in the art. It is understood, however, that the fibers must be aligned as beforedescribed so as to provide a maximum of heat transfer from the core to the clad. Plugs of the same metal as the clad member or of a different metal, if desired, are placed in the ends of the tube for holding the core in position. The ends of the tube are crimped or sealed by any suitable means, such as by welding. The sealed tube is then swaged by suitable mechanical means, such as a rotary swager, to a cross-sectional area of about 60 to of the original value. Thi technique makes possible the procuremement of ceramic powder-metal fiber bodies having a density of from 88 to 90% theoretical. The high density imparts to the finished product unusually high thermal conductivity and high resistance to thermal stress.

Moreover, it has been demonstrated that a swaged member containing a given amount of metal fiber possesses etter thermal conductivity than one into which a like amount of metal powder has been incorporated. Apparently, the metal fiber, when utilized as set forth above, forms a more conductive net-work in the ceramic.

Significant and obvious financial savings may be realized in using a cold working technique versus a hot pressing technique with fibrous elements.

Fuel elements may be fabricated with cores containing high concentrations of fissionable material in ceramic form, for example, as U0 In certain instances this high concentration of fissionable material will permit the use of less enriched fuel. More significantly, however, is the fact that, because of the excellent thermal properties of a ceramic metal fiber fuel element as described herein, greater power outputs may be realized for the same amount of fissionable material present in the core, that is, by operation at higher reactor temperatures. By the same token, more highly enriched fissionable material may be utilized than otherwise practicable. It is also possible by this technique to obtain long compacts of high density throughout the length of the material, a result not possible by prior practices. This technique eliminate the need for expensive dies and permits greater latitude in the size of articles which can be processed.

The only previously accepted method by which high density powder-fiber bodies can the manufactured is hot pressing at elevated temperatures, but, as already mentioned, this method of fabrication limits the size and shape of the final product and is too expensive for large quantity production. Furthermore, even when hot-pressing relatively short cladded tubular fuel elements the outer diameter cannot be closely controlled and mold-marks, such as fins, are produced. The product of this invention differentiates from such products by having a clad of substantially uniform thickness. This difference is particularly important in fuel elements where prediction of nuclear characteristics is important. As a result, the hot-pressed element must be further worked to achieve proper dimensions and to remove mold-marks. Cold pressing followed by sintering results in a low density product and, in general, has the inherent difiiculty that the ceramic shrinks from the metal fiber. In another fabrication method, preformed pellets are slipped into a cylindrical clad and the assembly sealed. Because the brittleness of the core precludes swaging,

a high thermal gradient exists across the core-clad interface. The present method, however, provides for a good mechanical or metallurgical bond at the core-clad interface which results in longer life for the element and permits use at higher temperatures.

In the event that the final product must be tubular, the technique described above is varied by placing an inner tube over a mandrel which has been suitably painted with graphite or other suitable lubricant. The outer tube is placed in concentric relationship with the inner tube and spaced therefrom by means of washers or the like. As in the case of the single tube, the ends are sealed and the sealed cylinder is swaged in the same way as described above.

By proper selection of the metals for the clad member and fiber, a metallurgical bond may be formed. G61 erally, the conditions for producing such a bond involve sintering, and those skilled in the art would understand the conditions which are necessary for this result. In the case of using a stainless steel clad member with either nickel, molybdenum or niobium metal fibers, sintering may be conducted at a temperature of about 1150 to 1300 C. When aluminum clad and fiber are used, sintering takes place at a temperature of about 550 to 660 C. Other examples of combinations of metals have been mentioned above. It should be noted that the ceramic powder may be sintered if suitable metals are used. For example, a fuel element consisting of a molybdenum fiber-uranium dioxide core cladded with molybdenum may be completely sintered.

In order to provide a better understanding of the present invention, reference will be had to specific examples.

Example I The core comprises 90% by volume of U and 10% by volume of stainless steel round wire, the stainless steel fiber having an average length of A; and a diameter of .002 inch. The U0 particles comprise 80% of 40-80 microns size and 20% from 5 to microns size. The mixture is placed in a stainless steel tube of 0.50 inch diameter, one end of which has been previously crimped or sealed. The material is tamped with a vibratory packer and the remaining open end sealed by welding. Before sealing, plugs are placed at each end of the cermet to hold the cermet in place. The member is swaged by a rotary swager to a cross-section of 60% the original value. The final diameter of the member is 0.35 inch. The final density is 90% Example I] The procedure in Example I is followed except that the original cermet contains 85% U0 and stainless steel. The density of the swaged member is 88%.

Example III The procedure of Example I is followed except that aluminum is used in place of stainless steel for the clad as well as the fiber. The diameter of the clad member is 1 inch. The cross-section is reduced by of the original value to give a member having a diameter of 0.78 inch. The density of the swaged member is 89%.

Example I V The same method as Example HI, except that the aluminum fiber constitutes 15% by volume of the cermet. The final density is 90%.

Having thus provided a Written description along with specific examples of my invention, no undue limitations or restrictions are to be imposed by reason thereof, the present invention being defined by the appended claims.

I claim:

1. In the method for fabricating a nuclear fuel element by reducing the cross-sectional area of an assembly constituted by a cylindrical core encased in a metal sheath, said core consisting of particles of an oxide selected from the group consisting of uranium dioxide, thorium oxide, plutonium oxide and uranium oxide, said oxide particles admixed with metal fibers present in amount equal to not less than about 5% by volume of said core, said metal fibers having an average cross-sectional area not greater than about 7 square mils and a length of between about inch and A1 inch, said sheath and said fibers being made of a metal selected from the group consisting of aluminum, iron, titanium, Zirconium, nickel and molybdenum, the improvement comprising the steps of aligning said fibers generally normal to the longitudinal axis of said core, and reducing the cross-sectional area of said assembly by cold swaging same so as to effect a reduction of about 30% to 40%.

2. The improvement of claim 1 wherein said assembly is sintered after swaging.

3. The improvement of claim 1 wherein said oxide is uranium dioxide, and said sheath and said fibers are made of aluminum.

4. The improvement of claim 1 wherein said oxide is uranium dioxide, and said sheath and said fibers are made of stainless steel.

References Cited by the Examiner UNITED STATES PATENTS 9/ 57 Handwerk et al.

OTHER REFERENCES CARL D. QUARFORTH, Primary Examiner. OSCAR R. VERTIZ, LEON D. ROSDOL, Examiners.

Claims (1)

1. IN THE METHOD FOR FABRICATING A NUDLEAR FUEL ELEMENT BY REDUCING THE CROSS-SECTIONAL AREA OF AN ASSEMBLY CONSTITUTED BY A CYLINDRICAL CORE ENDASED IN A METAL SHEATH, SAID CORE CONSISTING OF PARTICLES OF AN OXIDE SELECTED FROM THE GROUP CONSISTING OF URANIUM DIOXIDE, THORIUM OXIDE, PLUTONIUM OXIDE AND URANIUM OXIDE, SAID OXIDE PARTICLES ADMIXED WITH METAL FIBERS PRESENT IN AMOUNT EQUAL TO NOT LESS THAN ABOUT 5% BY VOLUME OF SAID CORE, SAID METAL FIBERS HAVING AN AVERAGE CROSS-SECTIONAL AREA NOT GREATER THAN ABOUT 7 SQUARE MILS AND A ENGTH OF BETWEEN ABOUT 1/8 INCH AND 1/4 INCH, SAID SHEATH AND SAID FIBERS BEING MADE OF A METAL SELECTED FROM THE GROUP CONSISTING OF ALUMINUM, IRON, TITANIUM, ZIRCONIUM, NICKEL AND MOLYBDENUM, THE IMPROVEMENT COMPRISNG THE STEPS OF ALIGNING SAID FIBERS GENERALLY NORMAL TO THE ONGITUDINAL AXIS OF SAID CORE, AND REDUCING THE CROSS-SECTIONAL AREA OF SAID ASSEMBLY BY COLD SWAGING SAME SO AS TO EFFECT A REDUCTION OF ABOUT 30% TO 40%.