US3867489A - Method of producing an oxidation-resistant UO{HD 2 {B Fuel Body - Google Patents

Abstract

1. A method of forming a UO2 fuel body resistive to oxidative attack, which comprises mixing UO2 and a ceramic oxide of relatively low thermal neutron absorption cross section, adding to the resulting mixture a small amount of a compound selected from the class consisting of alkali metal halides, and alkaline earth metal halides, and then forming a fuel body of the resulting composition.

Description

1111 3,867,489 [451 Feb. 18,1975

[ METHOD OF PRODUCING AN OXIDATION-RESISTANT U0 FUEL BODY [75] Inventor: Jack A. Rubin, Encino, Calif.

[73] Assignee: The United States of America as represented by the United States Atomic Energy Commission, Washington, DC.

[22] Filed: July 17, 1962 [21] Appl. No.: 210,880

[52] US. Cl. 264/0.5, 252/301.l R [51] Int. Cl G2lc 20/00 [58] Field of Search 204/154.2, 193.2; 264/05; 252/301.1 R

[56] References Cited UNITED STATES PATENTS 2,816,042 12/1957. Hamilton- 106/565 2,818,605 l/1958 Miller 106/55 FOREIGN PATENTS OR APPLICATIONS 874,964 8/1961 Great Britain 106/565 OTHER PUBLICATIONS AEC Report, WAPD-MRP-68, June 1959, pp. 80-82.

AEC Report, WAPD-MRP V78, Aug. 1957, pp. 57 and 63.

J. of American Ceramic Soc., vol. 35, No. 5'May 1952, pp. 107-113.

Nuclear Power, May 1959, pp. 86-88.

AEC Report, TlD-7546 (Book 2), Nov. 1957,, pp. 532 and 533.

I Primary Examiner-Carl D. Quarforth Assistant ExaminerRoger S. Gaither Attorney, Agent, or Firm-John A. Horan; Frederick A. Robertson EXEMPLARY CLAIM 1. A method of forming a U0 fuel body resistive to oxidative attack, which comprises mixing U0 and a ceramic oxide of relatively low thermal neutron absorption cross section, adding to the resulting mixture a small amount of a compound selected from the class consisting of alkali metal halides, and alkaline earth metal halides, and then forming a fuel body of the resulting composition.

9 Claims, No Drawings METHOD OF PRODUCING AN OXIDATION-RESISTANT U FUEL BODY My invention relates to a method of producing an oxidation-resistant U0 fuel body, and more particularly to a method of producing a fuel body in which U0 is contained within the grains of a ceramic matrix.

Uranium dioxide fuel elements have been made in mixtures with ceramic moderator and ceramic structural compositions. U0 is radiation stable and may go to high fuel burnups..However, it is readily oxidized in air and is lost by subsequent volatiliz ation. This presents a serious limitation to the use of unclad ceramic U0 fuel bodies, especially in gas-cooled reactors. It has been observed that U0 compositions prepared by mechanical mixing and hot pressing have U0 positioned at the grain boundaries of the-matrix material; this probably subjects the fuel body to greater oxidative attack than would be experienced by protectively encasing the U0 grains within grains of the matrix material. Such encasement has been accomplished by chemical coprecipitation of U0 and BeO, but this has entailed the handling of large volumes of solution, and there are other inconveniences and costs associated with handling of solutions.

An object of my present invention is, therefore, to provide an improved method of producing an oxidation-resistant U0 fuel body.

Another object is to provide a method of encasing U0 fuel within the grains of a ceramic matrix material to thereby improve its oxidation resistance.

Another object is to provide such a method which does not involve the handling of large volumes of solution.

A further object of my invention is to provide an improved method of forming a UO -BeO fuel body wherein the U0 is contained within the BeO matrix.

A still further object is to provide an additive which will promote the encasement of U0 within a ceramic fuel body matrix.

' Still other objects and advantagesof my invention will become apparent from the following detailed description and the appended claims.

In accordance with my present invention an oxidation-resistant UO -ceramic fuel .body may be prepared by mixing U0 and ceramic powders, and adding thereto a small amountof a subst'ance which forms a low melting liquid phase at or below the sintering temperature of the matrix. Use of the additive overcomes the poor sintering and oxidation characteristics of prior UO -ceramic compositions apparently caused by pinning of the ceramic oxide grains by the U0 grains lodged at the grain boundaries; such pinning prevents any ceramic oxide grain growth around the U0 The mechanism for grain growth may involve the following. As the refractory ceramic oxide powder, comprising the ceramic matrix, grows into grains during the sintering process, the presence of a liquid phase caused by my additives probably acts as a media for material transport of the ceramic matrix allowing the ceramic grains to grow around the U0 grains, thus enclosing them and protecting them against subsequent loss when exposed to an oxidizing environment. Without the presence of this liquid phase-forming additive the U0 grains are pushed into the grain boundaries of the matrix during sintering where they are subject to easy oxidative attack. Upon prolonged sintering, the liquid phase disappears because of volitalization of the liquid phase and/or incorporation of the phase into the matrix as a solid solution.

l have found that halides of the alkali metals (e.g., Na, Li), the alkaline earth metals (e.g., Mg, Sr, Ba), and the rare earth metals (e.g., Y, La) are satisfactory additives for use in my process. CaO is also satisfactory, this oxide forming a low melting eutectic, for example CaOBeO eutectic is inthe liquid phase at approximately l,300 C. The fluoride salts of the foregoing metal halides are preferred, these being stable and low melting. MgF is a particularly satisfactory composition; BeO may dissolve in molten MgF at or below the sintering temperature of the ceramic matrix, which al lows material transport of BeO around the grains of U0 unpinning boundaries and allowing growth to continue by mass transfer of BeO.

Although the concentration of the additive may satisfactorily vary, generally only a small amount need be added to secure effective results. For example, approximately 0.1-1 wt. is suitable, while about 0.25 wt. is preferred. The powders may be mechanically mixed in weight concentrations and then blended by conventional means, such as ball-milling, mulling, wet mixing, and the like. After homogenization and dispersion the powders are formed into a ceramic body by methods known to the art, for example hot pressing, sintering, and the like.

The term ceramic matrix material" is used herein to cover the refractory metal oxides which are customarily used in the nuclear reactor art with U0 in fuel bodies. For example, fuel-moderator bodies are formed with BeO, and non-moderating fuel bodies are formed with A1 0 and MgO as structural materials, these latter being refractory oxides of relatively low thermal neutron absorption cross section. UO -BeO fuel bodies have also been formed containing a small amount (e.g., 1 wt. of a third oxide such as MgO or Y O to enhance the high temperature ductility of BeO and to improve the grain characteristics of BeO. The ceramic oxide is the major constituent in such fuel and fuelmoderator compositions. The concentration of uranium in the body may satisfactorily vary over a wide range;-it will be determined by reactor design considerations and by enrichment of the uranium with respect to the fissionable U-235' isotope. For example, in a UO BeO fuel body using fully enriched uranium (approx. 93% U-235 the U0 will vary between about 5 to 15 Wt.

The U0 does not have to be added as such, but may be added as any uranium compound which can be decomposed to produce U0 for example uranyl nitrate. Similarly, the matrix ceramic may be added in the form of a chemical compound which can be readily decomposed to the oxide, for example beryllium nitrate which can be decomposed to BeO during any sintering or hot pressing steps. The additive also can be formed by suitable chemical reaction rather than the direct addition of the necessary compound. For example, Na BeF reacts with MgO in a BeO-l% MgO fuel composition thereby producing the desired MlgF liquid. lf fluorine were present as an impurity in BeO, it would react with MgO forming MgF liquid.

It is generally preferred to form. the fuel body in a hot pressing operation. The mixed powders are placed in a die of selected fuel body shape, and the assembly placed in a chamber having a non-oxidizing atmosphere. The term non-oxidizing is intended to em- 'The followirrg'examples are iofferedto illustrate my invention in greater detail.

EXAMPLE I 125 grams of a mixture of finely divided powders consisting of 94.25 wt.% beryllium oxide, wt.% uranium dioxide (approx. 93% U-235), and 0.75 wt.% of calcium fluoride are thoroughly mixed together. To this mixture is added 100 ml. distilled water to produce a suspension which has a paint-like consistency. This suspension is then placed in a rubber lined ball-mill and homogenized for a period of 4 hours. The suspension is dried in a conventional spray dryer producing a homogeneous, granular, free-flowing powder.

This powder is then placed in a high strength graphite die. The die with its contents isplaced in an argon atmosphere, and heated at theapproximate rate of 500 .C per hour beginning at the ambient temperature.

When a temperature of 800 C is reached, a pressure of 2,000 psi is applied to the die. The heating is continued at the same rate until a temperature of l,600 C is reached, meanwhile holding the pressure constant. The temperature is'then held constant at l,600 C for 4 hours, during which time the'pressure remains at 2,000

i psi. At the end of this time the pressure is released and the die is allowed to. slowly cool to room temperature.

The resulting ceramic body is approximately 99% of theoretical density, and possesses a minimum transverse breaking strength of 20,000 psi at room temperature. This body loses no more than 0.1% of its weight when exposed to air flowing at 7.5 cfh for 4 hours at l,650 C, which attests to its excellent oxidation resistance.

EXAMPLE ll 1 The same as Example I except'that the ceramic matrix is A1 0 and the salt is'MgF This body also possesses excellent resistance to oxidative attack.

The above examples are only illustrative rather than restrictive of my invention, which should be understood to be limited only as is indicated in the appended claims.

I claim:

1. A method of forming a U0 fuel body resistive to oxidative attack, which comprises mixing U0 and a ceramic oxide of relatively low thermal neutron absorption cross section, adding to the resulting mixture a small amount of a compound selected from the class consisting of alkali metal halides and alkaline earth metal halides, and then forming a fuel body of the resulting composition.

2. A method of forming a fuel body which comprises mixing U0 and a refractory oxide of relatively low thermal neutron absorption cross section selected from the class consisting of BeO, BeO- MgO,"MgO, and Al- 7 0 adding to the resulting mixture 21 small amount of a compound selected from the class consisting of alkali metal halides and alkaline earth metal halides, and then forming a ceramic fuel body of the resulting composition.

3. A method of forming a U0 fuel body which comprises mixing U0 and BeO powders, adding to said mixture a small mount of a compound selected from the class consisting of alkali metal halides and alkaline earth metal halides, and then forming a UO2BeO- fuel body of the resulting mixture. Wm,

The method of claim' 3 wherein approximately 0.l-l wt. of the last-named additive is added to said UO BeO fuel composition.

5. The method of claim 3 wherein said UO BeO fuel body is formed by hot pressing said resulting mixture in a non-oxidizing atmosphere.

6. A method of forming a UO -BeO fuel body, which comprises mixing U0 and BeO powders, adding to the resulting mixture approximately 0.l-l wt. of an alkaline earth metal fluoride, and then hot pressing the resulting composition in a non-oxidizing atmosphere to produce the UO -BeO fuel body.

7. The method of claim 6 wherein said hot pressing is, conducted under the approximate conditions of l,400l,800 C, 2,000-6,000 psi, and 2 to 8 hours in an inert gas atmosphere.

8. The method of claim 6 wherein said U0 comprises approximately 5-l5 wt. of the fuel body composition. i t

9. A method of forming a UO BeO fuel body which comprises forming a powder mixture consisting essentially of 5-15 wt. U0 approximately 0.25 wt. alkaline earth metal fluoride, and the remainder BeO, forming an aqueous suspension of theresulting mixture, drying the resulting-suspension, placing the dried powders in a die in an inert gas atmosphere, and then hot pressing said powders under the approximate conditionsof l,600 C and 2,000 psi for 4 hours.

Claims (9)

1. A METHOD OF FORMING A UO2 FUEL BODY RESISTIVE TO OXIDATIVE ATTACK, WHICH COMPRISES MIXING UO2 AND A CERAMIC OXIDE OF RELATIVELY LOW THERMAL NEUTRON ABSORPTION CROSS SECTION, ADDING TO THE RESULTING MIXTURE A SMALL AMOUNT OF A COMPOUND SELECTED FROM THE CLASS CONSISTING OF ALKALI METAL HALIDES AND ALKALINE EARTH METAL HALIDES, AND THEN FORMING A FUEL BODY OF THE RESULTING COMPOSITION.

2. A method of forming a fuel body which comprises mixing UO2 and a refractory oxide of relatively low thermal neutrOn absorption cross section selected from the class consisting of BeO, BeO-MgO, MgO, and Al2O3, adding to the resulting mixture a small amount of a compound selected from the class consisting of alkali metal halides and alkaline earth metal halides, and then forming a ceramic fuel body of the resulting composition.

3. A method of forming a UO2 fuel body which comprises mixing UO2 and BeO powders, adding to said mixture a small mount of a compound selected from the class consisting of alkali metal halides and alkaline earth metal halides, and then forming a UO2-BeO fuel body of the resulting mixture.

4. The method of claim 3 wherein approximately 0.1-1 wt. % of the last-named additive is added to said UO2-BeO fuel composition.

5. The method of claim 3 wherein said UO2-BeO fuel body is formed by hot pressing said resulting mixture in a non-oxidizing atmosphere.

6. A method of forming a UO2-BeO fuel body, which comprises mixing UO2 and BeO powders, adding to the resulting mixture approximately 0.1-1 wt. % of an alkaline earth metal fluoride, and then hot pressing the resulting composition in a non-oxidizing atmosphere to produce the UO2-BeO fuel body.

7. The method of claim 6 wherein said hot pressing is conducted under the approximate conditions of 1,400*-1,800* C, 2,000-6,000 psi, and 2 to 8 hours in an inert gas atmosphere.

8. The method of claim 6 wherein said UO2 comprises approximately 5-15 wt. % of the fuel body composition.

9. A method of forming a UO2-BeO fuel body which comprises forming a powder mixture consisting essentially of 5-15 wt. % UO2, approximately 0.25 wt. % alkaline earth metal fluoride, and the remainder BeO, forming an aqueous suspension of the resulting mixture, drying the resulting suspension, placing the dried powders in a die in an inert gas atmosphere, and then hot pressing said powders under the approximate conditions of 1,600* C and 2,000 psi for 4 hours.