Patent Number: 050948040
Section: description

DESCRIPTION OF THE PREFERRED EMBODIMENTS In describing the preferred embodiments of the invention, reference is first made to FIG. 1 in order to describe the preferred method for making high temperature nuclear fuel elements. Next, the other figures of the drawing are referred to in connection with the disclosure of several preferred high temperature nuclear fuel element structures that are made according to the invention. Nuclear fuel elements made in accordance with the present invention are effective to localize molten fissionable material within the pores of carbon or graphite bodies. In these elements the localized fissionable material is shielded from contact with high temperature moderating gases by providing a coating of diamond or other suitable material over the pores within which the fissionable material is localized. It is recognized that prior art nuclear materials intended for use in gas moderated reactors which characteristically have operating temperatures around 1000.degree. C. or less, have been developed. These known types of porous graphite fuel kernels are used to encapsulated fissionable material such as uranium oxide into so called BISO fuel kernels that are commercially available. It is also understood that such commercially available porous graphite kernels have in some cases been coated with one or more layers of graphite and/or pyrolytic carbon, in order to allow gases generated by fission of the nuclear fuel to be absorbed in the surrounding graphite layers and confined therein against release from the fuel elements. Such prior art fuel elements are not suitable for use in high temperature gas reactors that have a sustained operating temperature in excess of 2000.degree. C., because those elements are not capable of retaining molten fissionable material within the fuel kernel until most of the energy available from the fuel has been transferred to the moderating gases of the reactor. Moreover, such prior art fuel elements do not provide a diamond coating or other suitable means for providing a barrier or shield that protects molten fissionable material within the fuel kernels from contact with the high temperature gases used to moderate the nuclear reaction within such ultra high temperature reactors, i.e. those that operate in excess of 2000.degree. C. In nuclear fuel elements made according to the present invention, fissionable material is melted within the pores of graphite fuel elements, thereby to cause the fissionable material to chemically react with the walls of the pores in a manner that localizes and stabilizes the fissionable material within the pores. Such fuel elements are also provided with one or more coatings of pyrolytic carbon or diamond, thereby to form barriers that further localize and stabilize the fissionable material within the pores of the fuel element and also serve as barriers that shield the fissionable material from exposure to the high temperature moderating gases present in high temperature gas reactors. Referring now to FIG. 1 of the drawing, the preferred steps of the method of the invention will be described. In practicing this method one provides a plurality of porous graphite members, such as the commercially manufactured BISO or TRISO nuclear fuel kernels that are readily available. Next, those graphite members are impregnated with a suitable oxidant, such as air or oxygen, and the members are heated to increase their porosity by creating a controlled reaction between the graphite of the members and the oxidant. Next, the graphite members are impregnated with a conventional solution of fissionable fuel material, such as uranyl nitrate and a suitable solvent, such as water. After the members are thus impregnated, the solvent is evaporated to leave the fissionable fuel material deposited within the pores of the members. A next important step of the method of the invention is to heat the graphite members sufficiently to cause the fissionable material to react and then melt with the graphite walls of the pores in the members, thereby to form uranium carbide and to cause the molten fissionable material to be stabilized and localized within the pores of the graphite members. If desired for given applications, at this point in practicing the method of the invention, one may re-impregnated the graphite members one or more times with a solution of fissionable fuel material and solvent, such as that used for the initial impregnation, then the solvent is evaporated from the graphite members. Another optional step at this point in practicing the method is to again heat the members above the melting point of the fissionable material to cause the fissionable material and the graphite walls of the pores to undergo further chemical reaction, thereby to further localize and stabilize the molten fissionable material within the pores. Once a desired level of loading of fissionable materials within the pores is thus achieved, the pores are covered with a layer of graphite to seal the fissionable material therein. Finally, the graphite members are coated with a layer of pyrolytic carbon or diamond so that the coating layers provide barriers that further localize the fissionable material within the pores and also provides a barrier between the fissionable material and the hot moderating gases used in high temperature gas reactors. In certain applications of the invention, it is desirable to deposit one or more additional layers of pyrolytic carbon or diamond over the outer surface of the graphite members or over the earlier-applied coating layers thereon, in order to more fully localize the molten fissionable material within the pores of the graphite member, and to better shield the fissionable material from the moderating gases of an associated reactor. The desired layers of pyrolytic carbon or diamond are deposited by using a suitable conventional vapor deposition process that is controlled to make the layers as suitable barriers against migration of molten fissionable material from the pores of the members. It should be understood that by practicing the method of the invention it is possible to manufacture high temperature nuclear fuel elements in many different configurations. Some of the more preferred configurations for such high temperature fuel elements will now be described. FIG. 2 is a schematic illustration of a basic porous core component for the nuclear fuel element structure of the present invention. As shown in FIG. 2, this basic component comprises a generally spherical member 1 made of a suitable conventional, commercially available porous graphite or carbon. A plurality of irregularly shaped pores in the member 1 are formed to have a desired fuel-carrying volume, by any suitable commercially available process for manufacturing and shaping the pores in such porous graphite or carbon members. In the preferred embodiment of the invention, each of the members 1 is made to be approximately 500 microns in diameter, but it will be understood that the particular outer surface configuration of the member 1 and its size is not critical in practicing the method of the present invention. It should be noted that it is typical in such a member that some of the pores open at the surface of the member, while other pores are closed relative to exposure to the outer surface. Accordingly, the accessibility of the pores to impregnation with fissionable fuel material is often somewhat restricted by the number of open-ended pores within the members. FIG. 3 illustrates this point by showing a graphite member 1 having its pores 2 in which a suitable fissionable fuel material 3 is disposed only in those pores that open to the surface of the member. Several closed-end pores 2A are not accessible to fuel/solvent at the surface of member 2, so those pores 2A remain empty. As noted above with respect to the description of the preferred method steps of the invention, the fissionable fuel material 3 can be stabilized and localized within the pores 2 of member 1 by heating the member 1 sufficiently to melt the fissionable material and to cause it to chemically react with the walls of the pores 2. After one or more such melting steps have been performed in order to localize the fuel within the member 1, the pores are thus made effective to hold the fuel within the pores, through resultant capillary forces and surface tensions forces with the molten fissionable fuel material and the walls of the pores. After a desired number of fuel-loading steps have been performed on the element 1, as discussed in greater detail relative to the method steps described above, a coating of pyrolytic carbon is formed over substantially the entire outer surface of the member 1, as is illustrated in FIG. 4. The coating of pyrolytic carbon 4 forms a kinetic barrier against migration of molten fissionable material from the pores 2; thus, the coating of pyrolytic carbon 4 is effective to further localize and stabilize the fissionable material 3 within the pores 2 of element 1. It is important to note that the coating of pyrolytic carbon 4 is not a porous pyro-carbon structure, which would permit expansion of the graphite member 1, or which would accomodate the gaseous fission products that are generated as the fissionable fuel material is consumed. Instead, the coating 4 is made of a dense, non-porous pyrolytic carbon so that it is effective to prevent the migration of molten fissionable material 3 from the pores 2. In a modified embodiment of the new fuel element 1 of the invention, a coating of diamond 5 is deposited over the entire outer surface of the pyrolytic carbon 4 to act as a further kinetic barrier to the migration of melted fissionable material from the pores 2 and to further act as a barrier against the reaction of reactor moderating gases, such as hydrogen or helium, with the molten fissionable material 3. Another modification of the fuel element of the invention illustrated in FIG. 4 can be achieved by replacing the coating of pyrolytic carbon 4 with a coating of diamond 5 being deposited directly on the graphite member 1, so that the diamond coating 5 would directly seal the pores 2 and also serve as a barrier against reaction between the reactor moderating gases and the molten fissionable material 3 within the member 1. It has been found preferable to form the diamond coating 5 on either the outer surface of the porous graphite or carbon member 1 or on the coating of pyrolytic carbon 4, by use of a conventional controlled vapor deposition process in which hydrogen is present in a concentration greater than about 95% of the gas ambient for the deposited diamond film during the vapor deposition step. That concentration of hydrogen is effective to minimize the presence of graphite in the deposited diamond coating 5. It has also been found that the nuclear fuel element configuration illustrated in FIG. 4 can be further modified by depositing a layer of porous graphite carbon over the entire outer surface of the coating of pyrolytic carbon 4, between it and the diamond coating 5. A portion (only) of such a layer of a porous graphite carbon 6 is illustrated in FIG. 4. By using such alternate layers of different forms of carbon, the successive layers are made more effective to form a series of kinetic barriers to further localize and stabilize molten fissionable material 3 within the pores 2. In the preferred embodiment of the nuclear fuel element structure illustrated in FIG. 4, the fissionable material 3 comprises a composition of uranium or plutonium carbide or nitrate and the porous graphite member 1 is either selected or suitably modified by the oxidation steps of the method of the invention to assure that at least some of the pores do extend to the outer, generally spherical surface of the member 1 and are greater in length than a radius of member 1, as is clearly illustrated in FIG. 4. The thicknesses of the respective coatings 4 and 5 illustrated in FIG. 4 must be effective to form barriers against migration of the fissionable fuel material from the pores 2, and the outer diamond coating 6 must be effective to shield the fissionable material from the reactor moderating gases; thus, each of the coatings 4 and 5 should be made at least 25 microns thick, up to about 25 mils thick, measured in a radial direction. The diamond coating 6 is preferably made 25 microns to 5 mils thick. Another preferred configuration of nuclear fuel element made according to the invention is one in which the fuel elements are made as relatively thin filaments or fibers that are capable of performing at temperatures up to the sublimation temperatures of graphite, i.e. at temperatures greater than 3300.degree. C. Suitable carbon filaments or fibers for practicing this form of the invention are commercially available in both porous and solid graphite filament form. There is illustrated in FIG. 5, in transverse cross section along a diameter of such a porous carbon or graphite filament member 1A a generally circular (or cylindrical) configuration for the filament, but it will be recognized that such filaments may have other cross section configurations without departing from the scope of the present invention. A plurality of pores 2A that extend into the filament 1A from its outer surface are filled with fissionable material 3A in a manner similar to that used for filling the pores 2 of the generally spherically carbon or graphite fuel elements illustrated in FIGS. 3-4. In addition to the fissionable material that is localized within the pores 2A, a layer of fissionable material 3B is deposited around the graphite or carbon filament element 1A. In practicing the method of the invention to make such a filament type fuel element it has been found that a carbon filament can be suitably heated by passing electric current through it, after it is suitably electrically connected between conventional commercially available terminals, which in turn are operatively connected to a conventional source of electric power. The graphite filaments can thus be either partially or totally converted to a nuclear fuel element by controlling the current, time and pressure of an ambient gas or vapor environment of a suitable fissionable material such as uranium hexalflouride, which is made to surround the filament during the fuel impregnation step. By thus suitably controlling the heat transfer geometry within such a conventional furnace, uranium or other suitable fissionable material is deposited within the pores 2A and if desired the deposition is continued to build up a surrounding coating 3B of fissionable material, as illustrated in FIG. 5. By controlling current passed through the filament thereby to regulate the temperature of the graphite filament 1A, the fissionable material 3A within the pores 2A is melted sufficiently to cause it to react with the graphite walls definining the pores 2A, thereby to localize and stabilize the molten material within the pores. After that reaction, the gas used to deposit the fissionable fuel material in and around the graphite film element 1A is removed as an ambient for the filament and appropriate alternative gases are used to deposit either pyrolytic carbon or diamond, such as the coating layer 4A illustrated in FIG. 5. As was the case with the type of nuclear fuel element illustrated in FIGS. 3-4, a multiple-layer fuel element can be made in a filament configuration, as is shown in FIG. 6. In FIG. 6, a hollow porous carbon or graphite member 1A' having pores 2A' therein is impregnated with a suitable fissionable material 3A' and is surrounded by additional fissionable material 3B' that is coated on both the interior and exterior surfaces of the member 1A'. By suitably raising and lowering the temperature of the filament 1A', for example by selectively regulating the electric current that is passed through it, and by changing the gas composition and the pressure of the furnace ambient surrounding the filament during such heating, predetermined and controlled layers of graphite, pyrolytic carbon, and diamond can be formed on the filament, generally in the manner noted above. Thus, as is shown in FIG. 6, the fissionable fuel coating 3B' on the filament 1A' is coated with a layer of pyrolytic carbon 4A', which in turn is surrounded by another layer of fissionable fuel material 3C', which in turn is coated with another layer of pyrolytic carbon 4B' which acts as a kinetic barrier against migration of the fissionable material 3C' through the pyrolytic carbon coating 4B' according to the present invention. It should understood that although only a portion of a diamond coating 5A' is illustrated, this coating is formed to completely surround the outer surface of the pyrolytic carbon layer 4B', thereby to form a kinetic barrier against migration of fissionable fuel material from the fuel element, and also to form a barrier between the moderating hydrogen or helium gas used in a high temperature gas reactor, and the fissionable fuel material within the fuel element. One advantage of the filament type fuel element illustrated in FIGS. 5 and 6 is that they are sufficiently flexible to enable the individual filaments to be twisted to form a thicker bundle of such elements, which in turn can be deposited in a graphite housing. For example, as is shown in FIG. 7, a plurality of such fissionable fuel elements 10 are schematically shown surrounded by a body of graphite 11, which preferably is pyrolytic carbon that forms yet another barrier against migration of the fissionable material from the elements 10. Of course, the cross section configuration of individual fuel element 10 can be made of any desired multiple layer configuration, such as the configurations shown in FIGS. 5 and 6. A desirable feature of such a multi-filament fuel element is that it permits a number of different fissionable fuel materials to be used in selected combinations within a single multi-fuel element such as the graphite cylinder 11 illustrated in FIG. 7. It should be understood that, according to the present invention, the fissionable fuel materials used in making a multi-fuel element bundle, such as that shown in FIG. 7, can be stabilized and localized within the fuel member 11 by heating it to melt the fissionable material and cause is to react with the graphite that surrounds the fissionable material, thereby to help prevent the fissionable material from migrating out of the fuel element. As noted above, a diamond coating could be provided over the exterior surface of the element 11 to form a further barrier against migration of fissionable fuel material from the combined fuel element 11, as well as to prevent the fissionable material from reacting with the reactor gases. To illustrate such a modification a simpler form of flexible fuel filament is shown in FIG. 8. In this modification a hollow porous graphite filament 12 has a fissionable fuel material 13 impregnated within its pores (not shown) and built up on its inner cylindrical surface. A diamond coating 14 is formed by a conventional vapor deposition process over the outer surface of the filament 12. Referring, again to FIG. 7, it should also be noted that when a plurality of such filaments 10 are positioned adjacent to one another to form a true element bundle, i.e. without added graphite of the member 11 between the filaments (10) as shown in FIG. 7, the juxtaposed surfaces of the respective filaments 10 provide additional surface barriers that further serve to localize and stabilize molten fissionable fuel materials within the respective filaments 10. It will be appreciated that a bundle of filament, fuel-containing elements 10, such as those shown in FIG. 7, may be formed by either twisting individual filaments 10 together, or by pressing a body of graphite material, such as the material 11, around individual filaments to form a larger cylindrical multi-filament fuel element of the type shown in FIG. 7. Alternatively, a body of graphite (11) could be bored to form passageways for accepting the fuel filaments (10). From the foregoing description of the invention it will be apparent to those skilled in the art that various further modifications and alternative embodiments of it may be developed without departing from the scope of the invention; thus, it is my intention to encompass within the following claims the true limits of the invention.