US3288572A - Process for making carbides of fissionable and fertile materials - Google Patents

Description

United States Patent 3,288,572 Patented Nov. 29, 1966 Fice 3 288,572 PROCESS FOR MAKIfNG CARBIDES F FISSION- ABLE AND FERTILE MATERIALS Mario H. Fontana, Burlington, Mass, assignor to Avco Corporation, Cincinnati, Ohio, a corporation of Delaware N0 Drawing. Filed Aug. 6. 1964, Ser. No. 387,999

14 Claims. (Cl. 23-349) This invention relates generally to a process for making carbides of fissionable and fertile metals, and in particular to a process wherein hexafluorids of uranium and volatile gaseous compounds of plutonium and thorium are converted directly, in a single-step process, into carbides of these fissionable and fertile materials by the direct reaction with carbon. For the purpose of this discusion, the term carbon shall be understood to mean carbonaceous materials in any form such as graphite, carbon black, coal, tar, hydrocarbon and fluorocarbon.

References herein to carbides shall be understood to mean either the monocarbide (UC), the dicarbide (UC or sesquicarbide (U C and amorphous compounds of uranium and carbon. The amorphous compounds proceed to UC, UC or U C on further heating.

Because of the widespread interest in the use of carbides in nuclear applications and particularly in the use of uranium carbide as a fuel for high-temperature reactors, a great many processes have been evolved for converting uranium ore (predominantly U 0 to uranium carbide.

Under current practices, uranium ore is first converted to a hexafiuoride (UF which is then processed in gaseous difiusion plants to obtain the enriched uranium. The conversion from hexafluoride into a carbide is a multi-step process which usually requires a conversion of the uranium into an oxide and reducing the oxide through several steps into the carbide. At each step of the overall process, care must be taken to account for the uranium, and because of the high value of the raw materials, unreacted elements and compounds must be recovered, separated and reused.

For nuclear fuel requiring uranium enriched in the isotope U uranium hexafiuoride is the most desirable starting material, since this is the form leaving the diffusion plants. At least one patent, 3,089,785, describes a two-step process for converting UP to a carbide of uranium. It is to be noted that the process described in the Lewis patent is a batch process. Each step of the process takes several hours to complete and, additionally, there are different temperatures and pressures specified for each step. From an examination of thermal equilibrium data, it Would appear that the second step does not operate unless the gaseous reaction products are actively withdrawn.

In contrast, the process described below is a one-step direct conversion of uranium hexafluoride, and as will be demonstrated, the conversion takes place continuously and at rates of at least four grams of carbide per second using bench scale equipment. Higher rates can be achieved with appropriately larger apparatus.

It is an object of the invention to provide a process for converting gaseous products of fissionable and fertile compounds to carbides of fissio-nable and fertile material which avoids the limitations and disadvantages of prior art processes.

Another object of the invention is to provide a process for producing uranium carbides by the direct reaction of UP and C.

Still another object of the invention is to provide a process for producing uranium carbide by the direct reaction of UF and carbon vapor.

Other objects of the invention are to provide a process for producing carbides of fissionable and fertile materials,

particularly uranium carbide by: (l) a continuous process, preferably using an electric arc; (2) dissociating UP and C in more elementary highlyreactive forms which react to form uranium carbide; (3) optimizing conditions for bringing about a substantially instantaneous reaction of F and C, particularly by increasing the incidence of contact betwen UP and C; and (4) processes yielding high rates of production.

In accordance with the invention, a proces for making carbides of fissionable and fertile metals comprises heating a mixture of gaseous compounds of the fissionable and fertile material and carbon vapor to' a reaction temperature required to produce a carbide, directly.

In another aspect of the invention, a process for making uranium carbide comprises heating a mixture of uranium hexafiuoride and carbon, preferably in a vapor state to a reaction temperature required to produce carbides of uranium.

The novel features that are considered characteristic of the invention are set forth in the appended claims; the invention itself, however, both as to its organization and method of operation, together with additional objects and advantages thereof, will best be understood from the following description of a specific embodiment.

Broadly, a process for making uranium carbide comprises heating a mixture of uranium hexafluoride and carbon to a reaction temperature at which a direct conversion to uranium carbide results.

It is important to emphasize what is meant by a direct conversion. When UP and C are reacted directly, the reaction products recovered are uranium carbide and fiuorocarbons, generally designate-d C F The overall reactions are as follows:

UF (g) +C(g)- UC (s) +Fluorocarbons (1) UF (g) +C(g) UC (s) +Fluorocarbons (2) At the high temperatures being considered, a large variety of reactions take place simultaneously and instantaneously. Compounds are created and destroyed continuously. In particular, uranium tetrafiuoride (UF reacts readily, and appears in the reaction products in negligible amounts, if at all.

The speed and vigor with which the carbides are formed is also a function of the number of contacts made between U1 and C. It is reasoned that by providing carbon in an extremely fine state, preferably as vapor or an easily vaporizable solid such as colloids, the reaction time is considerably reduced and that the formation of the carbide is favored, overwhelmingly.

A preferable procedure for bringing about a direct reaction between UF and C for producing uranium carbide involves the use of an electric arc.

It is widely recognized that the temperatures of the electric arc and the regions immediately adjacent to the are are regions of extremely high temperature, in the order of 5,000 to 10,000 C. Reactions can be made to take place at these extremely high temperatures by causing the arc to pass through one or more of the reagents. A high-intensity, high-pressure arc (operating at at least one atmosphere) is preferred.

Typically, an electric arc is formed by passing a highintensity current through a gas flowing between two electrodes. The arc current passing through the gas raises the temperature of the gas above the temperature at which many of the molecules of the gas dissociate into free electrons and ions. A dissociated gas is referred to as a plasma, and the plasma is electrically neutral, having an equal number of negative and positive particles. It is clear, however, that these particles occur as separate and distinct states such as molecules, electrons and ions, in highly excited and reactive states.

A simple, efiicient and high-volume production process involves passing UP in combination with carbon, and at times with an inert, diluent gas, between two electric arc electrodes. Under the influence of the arc, the UF is dissociated into more elementary forms including but not limited to UF UF UF U, U U++, F and 1 Similarly, carbon converts to several states such as C, C and C+. A highly complex plasma of highly reactive constituents results. The temperature of the plasma is well above the vaporization temperature of carbon. The extremely high temperature, as well as the kinetic effects widely recognized as existing in plasma due to the dissociation and recombination atoms, ions and molecules all contribute to the reaction of UP and C to uranium carbides and fiuorocarbons.

The time during which the reagents pass through the arc and react is extremely short. A typical bench scale run produced approximately 100 grams of material in 14 seconds, with arc voltage of 2040 volts, curent of 300-500 amperes, and UP fiow rates of approximately grams per second. The product was identified by means of chemical analysis, X-ray difiraction, and gas chromatography of reaction products of the material with water. A very low level of impurities was determined by X-ray fluorescence and emision spectroscopy. A production of 100 grams of carbides in 14 seconds is common.

An extremely eflicient source of carbon used in the reaction described above is obtained by constructing one of the arc electrodes, preferably the anode, of carbon (graphite). The electric arc is operated as a high-intensity arc. As is well known, in the absence of cooling, the anode of a high-intensity are system vaporizes very rapidly. Thus, a further refinement of the process is to strike a high-intensity are through U1 using at least an anode carbon electrode. The carbon vaporized from the electrode mixes with the UP in or adjacent to the are, forming a plasma.

It is quite clear that in a plasma state the chemical reactions take place directly between atoms and molecules of the reacting materials. The contact between plasma constitutents is on the most elemental basis. These are substantially ideal conditions for causing the reaction of two elements such as uranium and carbon. Fortunately, of all the probable reactions that can take place, the re action of uranium and carbon is the most stable at these high temperatures, and virtually all of the uranium is reacted with the carbon to form uranium carbide. Since the uranium carbides are solids, they are easily separated from the fluorocarbon gases also produced. The fluorocarbons may be recycled as a source of carbon if desired.

Although the foregoing discussion has been limited to uranium, it is quite obvious that the process can be practiced with volatile gaseous compounds of other fissionable and fertile material such as plutonium and thorium.

The process described also lends itself to forming mixed carbides. Gaseous compounds of fissionable and fertile materials can be fed concurrently to a carbon arc and reacted. The ratio of the fissionable and fertile material in the product is controlled by controlling the flow rate of the gaseous feed material.

It is important to emphasize the following advantages of the process described above. The process is a continuous direct conversion and the production rates are extremely high. Under the high temperature conditions described, essentialy all of the fissionable and/ or fertile material (uranium) is reacted to form their respective carbides. A complete utilization of the fissionable and fertile materials is very significant because it facilitates accountability procedures and eliminates recovery of unreacted material, in itself a costly process.

The various features and advantages of the invention are thought to be clear from the foregoing description. Various other features and advantages not specifically enumerated will undoubtedly occur to those versed in the 4. art, as likewise will many variations and modifications of the preferred embodiment illustrated, all of which may be achieved without departing from the spirit and scope of the invention as defined -by the following claims.

I claim:

1. A process of making uranium carbide comprising mixing uranium hexafluoride and carbon, and raising the temperature of said mixture to a reaction temperature at which the uranium hexafluoride and carbon are dissociated to react and form uranium carbide.

2. A process for making uranium carbide comprising passing a mixture of uranium hexafluoride and carbon between the electrodes of an electric arc furnace and striking an arc through said mixture for raising the temperature of said mixture above the reaction temperature required to react the uranium hexafluoride and carbon to form uranium carbide.

3. A process for making uranium carbide comprising passing a mixture of uranium hexafiuoride and carbon between the electrodes of an electric arc furnace and striking an are through said mixture for raising the temperature of said mixture above the temperature at which at least one of the reagents, uranium hexafluoride and carbon dissociates and reacts with the other to form uranium carbide.

4. A process as described in claim 3 which includes in addition the step of purging the reaction of residual gaseous products.

5. A process for making uranium carbide comprising providing an electric arc furnace for at least two spaced electrodes, one of which is a carbon electrode, passing uranium hexafluoride between said electrodes, striking an are, said are being of sufiicient intensity to vaporize carbon from said carbon electrode and to react the uranium hexafluoride with the carbon substantially instantaneously for producing uranium carbide.

6. A process as described in claim 5 in which said carbon electrode is an anode electrode.

7. A process as described in claim 5 in which said arc generated is a high-pressure, high-intensity arc.

8. A process as described in claim 5 in which said arc dissociates the uranium hexafiuoride and carbon for forming uranium carbide.

9. A process for making uranium carbide comprising heating a mixture of uranium hexafluoride and carbon to a reaction temperature required to produce uranium carbide substantially instantaneously.

10. A process for making uranium carbide comprising reacting uranium hexafluoride and carbon vapor, said reaction taking place at temperatures above the vaporization temperature of carbon.

11. A process as described in claim 10 in which the uranium hexafluoride is supplied together with an inert diluent gas.

12. A process for making uranium carbide comprising heating a mixture of a gasous halide uranium and carbon to a reaction temperature above the vaporization temperature of carbon to produce uranium carbide.

13. A process for making carbides of fissionable material comprising reacting a gaseous halide of fissionable material selected from the group consisting of uranium, plutonium, and thorium with carbon vapor.

14. A process for making carbides of fertile material comprising reacting a gaseous halide of fertile material selected from the group consisting of uranium, plutonium, and thorium with carbon vapor.

References Cited by the Examiner UNITED STATES PATENTS 3,070,420 12/1962 White et al. 23-l4.5 3,089,785 5/1963 Lewis et a1 2314.5 X

BENJAMIN R. PADGETT, Primary Examiner.

LEON D. ROSDOL, Examiner.

M. J. SCOLNICK, Assistant Examiner.

Claims (2)

1. A PROCESS OF MAKING URANIUM CARBIDE COMPRISING MIXING URANIUM HEXAFLUORIDE AND CARBON, AND RAISING THE TEMPERATURE OF SAID MIXTURE TO A REACTION TEMPERATURE AT WHICH THE URANIUM HEXAFLUORIDE AND CARBON ARE DISSOCIATED TO REACT AND FORM URANIUM CARBIDE.

13. A PROCESS FOR MAKING CARBIDES OF FISSIONABLE MATERIAL COMPRISING REACTING A GASEOUS HALIDE OF FISSIONABLE MATERIAL SELECTED FROM THE GROUP CONSISTING OF URANIUM, PLUTONIUM, AND THORIUM WITH CARBON VAPOR.