Patent Number: 050948040
Section: summary

The invention relates to nuclear fuel element construction and composition, and to a method of manufacture for making fuel elements that are particularly useful in high temperature gas reactors. More particularly, the invention relates to fuel elements that are useful in high temperature reactors in which the normal operating temperature of the reactor is in excess of 2,000 degrees centigrade and exceeds the melting temperature of fissionable material that is localized and stabilized within the reactor fuel elements by a combination of capillary and surface tension forces that result from the unique structure and composition of the elements. In recent years attempts have been made to design small nuclear power reactors that can be used as the source of driving power on space ships or other vehicles that impose stringent space and weight limitations on the reactor design. In such designs, it has been proposed that particle bed reactors be used, in which relatively small diameter fuel particles in the range of 500 microns in diameter are provided to supply high power densities within the reactor. It has also been proposed that the fuel charge in such reactors be changed or reloaded on a relatively short-cycle basis. For example, such reloading could occur every several hours or perhaps once per day. The normal sustained operating temperature of the moderating gas in such prior art high temperature gas reactors is in a range that does not exceed about 1,000 degrees centigrade and is not in excess of the melting point temperature of the nuclear fuel compositions used in the fuel elements for the reactors. In order to prevent the nuclear fuels from being diffused or evaporated from the reactor prematurely, that is before the fuels serve their intended function of transmitting a major proportion of their power content to the hot gases within the reactor, it has been necessary to design the fuel elements with a multi-layered structure that enables its outer layers to prevent premature diffusion of the fissionable material from the fuel elements into the reactor moderating gases. So far as the inventor knows, prior to the present invention there was not known or available any nuclear fuel element or reactor concept in which nuclear fuels could be used at high temperatures that substantially exceed the melting point temperature of the fissionable materials in such fuels by several hundred degrees. By permitting the use of molten fissionable fuels, the present invention increases the operating temperature and resultant performance of high temperature gas reactors by many hundreds of degrees above the known maximum sustained operating temperatures heretofore achieved by gas moderated reactors. An example of the type of composite nuclear fuel elements that were developed for use in prior art high temperature gas reactors is shown in U.S. Pat. No. 3,212,989. It describes the use of nuclear fuel elements that include at least two independent sealing zones around a core material for retaining fission products within a sealing jacket. Such prior art nuclear fuel elements and related systems have limitations that are imposed by their inherent graphite reactions, and they have two classes of temperature limitations. First, the outlet gas temperature from the associated reactor is limited to about 1,000 degrees centigrade by their primary circuit metal properties and, second, the maximum fuel temperature is limited by melting, with resultant diffusion or evaporation of the fuel before its power has been used to heat the gases in the reactor. As noted in that prior art patent, the type of "high temperature" reactors contemplated for use with the disclosed, jacketed fuel elements has an operating temperature in the range of 700.degree. C. to about 1,000.degree. C. It should also be noted that in such prior art "high temperature" gas reactors there exists geometry problems that arise from the need to separate the reactor structural components from the nuclear fuel. Such limitations have, prior to the present invention, prevented the development of high temperature gas cooled reactors that have an outlet temperature substantially above 1000.degree. C., and have maximum short-term operating fuel temperatures of only about 2000.degree. C. In general, the operating temperatures in present day nuclear fuel reactors are limited by the melting points of the nuclear fuels used to power the reactors. The present invention discloses a process that is used to form predetermined microscopically localized liquid nuclear fuel concentrations within a confining graphite or carbon fuel element, in a manner such that the fuels are capable of performing at temperatures up to the sublimation temperatures of the confining graphite or carbon, i.e. is in the range of about 3300.degree. C. The invention also discloses novel structures of a variety of useful nuclear fuel elements that are made to include predetermined and controllable porosity configurations for localizing and confining fissionable nuclear fuel within the fuel elements. Common causes of failure of known prior art nuclear fuel elements at the elevated operating temperatures of high temperature gas reactors result from either the reaction and decomposition of the carbide coating of the element surrounding the fissionable fuel, or from the migration of molten fuel and fission products through the pores of the surrounding carbon or graphite element into the moderating gases before the fuel has discharged its energy to heat the gases. SUMMARY OF THE INVENTION A primary object of the present invention is to provide a nuclear fuel element having a distribution of fissionable material within the pores of a confining graphite or carbon member so that the fissionable material is thermodynamically stable with respect to its migration beyond the pores of the confining graphite or carbon member. A further object of the invention is to provide a method for making nuclear fuel elements in a commercially feasible manner such that the porosity of carbon or graphite fuel members within such elements is controlled to a predetermined degree, so that fissionable material can be effectively deposited within the pores of the carbon or graphite members, which are then sealed, and the members are heat treated to melt the fissionable materials causing them to react with the carbon walls of the members to increase the fuel localizing and stabilizing porosity of the members. Yet another object of the invention is to provide a nuclear fuel element having fissionable material confined within pores of a carbon or graphite member in a manner such that when the nuclear fuel is heated to above the melting point of the fissionable material the molten fuel material is held by capillary forces and surface tension forces within the pores of the surrounding graphite or carbon member. A still further object of the invention is to provide a nuclear fuel element in which a porous graphite or carbon member is impregnated with fissionable material, which is localized within the pores of the member, even after the fissionable material melts during operation of the reactor. Both capillary and surface tension forces hold the molten fuel within the pores and the fuel is also stabilized within the pores by coating the member with one or more layers of pyrolytic carbon or diamond. Still another object of the invention is to provide nuclear fuel elements in the form of flexible filaments having predetermined and controlled porosity such that fissionable material disposed within the pores of the element is localized therein by capillary and surface tension forces when the fissionable material is melted during its use within a reactor. To assure further localization of the molten fissionable material within the flexible filament fuel elements, one or more layers of pyrolytic carbon or diamond are formed over the exterior surface of the carbon or graphite member to afford additional surface barriers to the migration or diffusion of fissionable material from the pores of the carbon or graphite member. Additional objects and advantages of the invention will become apparent to those skilled in the art from the description presented herein, considered in conjunction with the accompanying drawings. In the preferred practice of the method of the invention conventional high temperature gas reactor type carbon or graphite fuel particle members are impregnated with oxidants and subjected to a high temperature reaction to develop a predetermined and controlled degree of porosity within the members. The porous members are then impregnated with a solution of fissionable material, such as uranyl nitrate in a suitable solvent, such as water, methanol, etc., to fill the pores of the members with the solution. The solvent is then removed by heat treatment of the members and further heat treatment at higher temperatures reacts the fissionable material with the graphite of the members thereby to further increase the porosity of the members. Additional fuel impregnations may be made to achieve desired, controllable levels of fuel loading of the porous members. The matrix of carbon or graphite members is then soaked in a solution of sugar or other organics and is again heated, thereby to form a layer of carbon that plugs the pores at the surface of the graphite members. The fuel elements are then heated above the melting point temperature of the fissionable material in order to react the fissionable material with the walls of the pores. The molten fuel is held by capillary and surface tension forces within the pores of the graphite members. In a most preferred embodiment of the invention, additional assurance for localization of the molten fissionable material within the pores of the fuel element is afforded by forming one or more layers of pyrolytic carbon or diamond over the outer surface of the porous graphite members, using conventional fluidized bed or controlled vapor deposition techniques for depositing such layers. The novel nuclear fuel elements formed by the method of the invention may be made as generally spherically shaped pellets, elongated filaments, or other arbitrary configurations. In one preferred form, the fuel elements are formed as flexible filaments of carbon or graphite material having predetermined and controllable porosity and having fissionable material localized within the pores of the carbon members by the capillary and surface tension forces existing between the surface of the pores and the molten fissionable material, when the molten state is achieved by heating the fuel above its melting temperature. Alternate layers of pyrolytic carbon and/or diamond are deposit on the outer surface of the porous carbon or graphite members to provide further assurance against migration or diffusion of molten fissionable material from the pores of the graphite or carbon members.