FUEL ELEMENT TECHNOLOGY In our testimony for the last few years, we paid special attention to progress in fuel element technology because the limits of nuclear rocket engine performance are directly related to fuel element characteristics. In a nuclear rocket reactor, fuel elements must withstand extremely severe conditions imposed by the hightemperature, high-power densities, and multiple thermal cycles required for the engine. The high reactor power density (kilowatts per cubic inch) leads to high thermal stresses in the fuel elements. Also, at high temperatures, hydrogen corrodes the graphite materials of uncoated fuel elements at a rate that approximately doubles for every 200°F increase in temperature. Major efforts are being continued to attain fuel element duration and cycling capability with the requisite end-of-life integrity to satisfy system reliability requirements. The fuel element program in the past year has made considerable progress toward our goal of 10-hour endurance. The progress is illustrated in figure 6 (NASA NSO 71-3470). Last year we reported that the then current state-of-theart provided an endurance of 3 to 4 hours at NERVA conditions. This year the advancements are such that laboratory tests indicate that an endurance of about 5 to 6 hours is in hand. In the coming year, research will be continued along several promising lines with the goal of progressing further toward a 10-hour endurance. Last year we described the approaches being taken to achieve the required fuel element characteristics: Westinghouse Nuclear Core Operations and the Y-12 Plant of Union Carbide Nuclear Company at Oak Ridge, Tennessee, were developing graphite elements containing particles of uranium dicarbide coated with pyrolytic carbon. At the same time, Los Alamos Scientific Laboratory was working on "composite" fuel elements where the major constituent is graphite containing a considerable amount of zirconium carbide. Uranium fuel is dissolved in the zirconium carbide. The major aim of the graphite fuel element effort has been to explore the use of high coefficient of thermal expansion (high CTE) graphite flours in manu LIFETIME OF NERVA FUEL ELEMENTS facturing the matrix. The objective was to more closely match the thermal expansion of the matrix with that of the coating, thereby, reducing mechanical stresses and flaws in the coating. Composite fuel elements also exhibit this high expansion feature. In addition, however, the dispersed phase of UC-ZrC improves the coating adherence to composite elements and also intrinsically reduces the matrix corrosion even if a coating flaw occurs. During the course of fuel element development and evaluation the results of electrically heated corrosion tests of composite fuel elements have been very promising. Some elements have been run at the required gas temperatures for the NERVA engine (about 3800°F) as many as 30 cycles and a total of five hours, and other elements at a lower temperature (3600°F) for 60 cycles and a total of ten hours operation with only modest corrosion. One recent discovery of importance was the high resistance to thermal shock for some composite fuel element materials. Originally, it was believed that composite materials would not withstand thermal stresses as well as some graphite materials. Recent tests conducted at LASL have shown composite materials to be capable of exceeding the requirements for the NERVA engine. While not completely understood as yet, the improvement in thermal stress resistance appears to be related to the choice of graphite flours used in making the composite material. Research to understand the stress resistance mechanism is underway. It is planned that continued evaluation will be made of these two fuel element concepts. The Pewee-2 test was intended to assist in this evaluation, subjecting both composite and graphite fuel elements to a large number of 10-minute cycles at rated NERVA operating conditions. The activities involved in fuel element development are indicated in figure 7 (NASA NPO 71-15480) which describes the fuel element development sequence. The current approach is to fabricate and test fuel elements in an electrically heated corrosion furnace in which a direct current of thousands of amperes is used to simulate nuclear heating. Hydrogen is passed through the fuel element as it would be in a reactor at rated conditions of pressure and flow rate. The next step in this sequence is the Pewee test bed reactor which would be scheduled whenever significant advancements in fuel technology are established through the laboratory developments including electrically heated tests. The Pewee-2 reactor (fig. 8, NASA NSO 71-3458), which is now at Test Cell C FIGURE 8 awaiting final approval for testing, contains 234 graphite fuel elements and 186composite fuel elements. It now appears that the budget reductions will require cancellation of this test. LASL has designed a Nuclear Furnace, an assembly of approximately 50 fuel elements, to supplement the electrically heated corrosion furnace testing. The Nuclear Furnace eliminates some of the major difficulties we have faced in utilizing electrical heating for evaluation of fuel elements. The grips, or electrodes, in the electrical furnace are an area of particular difficulty. With the high currents and the high temperatures and long durations, failures due to electrical effects occur in the region of the grips and cause valuable tests to be terminated prematurely. As yet, we have not been able to devise a way to test fuel elements electrically at the limits of their performance potential. It is hoped that the Nuclear Furnace concept will permit tests to be conducted economically and with a short turn-around time. The first Nuclear Furnace had been scheduled for test later this year; the impact of the FY 1972 budget on this test has not been determined. Should this Nuclear Furnace concept prove to be feasible, then the fuel element development sequence will involve a combination of electrically heated and Nuclear Furnace testing in the initial phase followed by tests, as possible, in the Pewee reactor. SUPPORTING AND ADVANCED TECHNOLOGY The nuclear rocket program continues to fund research and advanced technology activities aimed at realizing the full potential of nuclear energy for space propulsion. Some of this work extends the technology of solid-core nuclear rocket engines to higher levels of specific impulse and power density. Other activities are providing a base of technology for development of a nuclear stage, and studies are conducted to define characteristics and capabilities of nuclear stages. A program of applied research and engineering studies is conducted to evaluate the feasibility of advanced nuclear propulsion concepts including both fission and fusion reactions as sources of energy. The major supporting research and technology areas are shown in figure 9 (NASA NSO 71-3460). FIGURE 9 SOLID-CORE REACTOR TECHNOLOGY The nuclear rocket program continues activities with AEC funding to improve the technology of solid-core reactors utilizing some of the resources of the Los Alamos Scientific Laboratory. Throughout the years, the LASL program has pioneered many of the concepts employed in the NERVA reactors and continues to support the flight engine program with basic design information, materials data, and fuel element know-how. As discussed earlier, the most significant recent contribution made by LASL has been the development of graphite-carbide composite fuel elements which have very high promise for the NERVA engine. LASL will continue to make improvements in the composite materials while Westinghouse Astronuclear Laboratory will work on applying the existing composite fuel technology to the NERVA reactors. LASL has also made excellent progress in developing carbide fuel materials for possible use in advanced solid-core reactors. The carbides referred to here are solid solutions of uranium and zirconium carbide. If the percentage of uranium present is maintained at low values, say less than 10 atom percent, these carbide materials will have a high melting point provided there is no excess carbon present. Figure 10 (NASA NPO 70-16277) shows that the melting point of UC-ZrC varies with uranium percentage. Based on these data, operation of solid-core reactors with material temperatures as high as 5400° F may be possible yielding specific impulses of 950-970 seconds. SUPPORTING ENGINE TECHNOLOGY Advanced Nozzle Studies-In the nuclear engine, unlike the chemical rocket, heat rejected from the exhaust to the nozzle cooling fluid is a specific impulse loss, since peak temperature in the nuclear rocket is fixed by reactor capability. This, along with the potential requirement for higher reactor temperature, has led to two NASA-funded advanced nozzle studies. One study investigates an uncooled graphic nozzle that allows system performance gains from reduced nozzle weight and lower pump discharge pressure requirements, as well as a maximum specific impulse. The other study investigates nozzles with limited cooling of a thin composite wall and regeneratively cooled nozzles with redundant design features. These concepts offer higher reliability, improved performance and growth potential. The aggregate of these requirements and goals require considerable advancement of current material, design and fabrication technology. This effort is continuing in the exploration of materials and fabrication techniques for higher performance nozzles. Turbopump Technology-NASA work in this area involves the development of hybrid-hydrostatic bearings for testing with a turbopump available for operation in Test Cell C at NRDS. Hydrostatic bearings are expected to have essentially no wear since the bearing rests on a gas film and no contact is made between moving solid materials when in operation. The hybrid concept, in which the startup loads are carried by ball bearings and the operationing loads carried by hydrostatic bearings, will undergo experimental testing at NRDS during this spring. If this concept is successful, future long-life turbopump operation would be available for nuclear or chemical rockets. Other NASA turbopump technology efforts consists of the investigation of splitter vanes in axial flow turbopumps to increase stall margin, and large diameter pump inducers. The larger inlet diameter may permit the use of a lower inlet design flow coefficient (flat blade angle of the leading edge of the inducer) to increase the ability of the inducer to ingest large quantities of vapor (30 to 40 percent) and still provide acceptable overall pump performance. Further, the test data should provide insight into resolution of the relative contribution of high blade tip speed vs. changing flow incidence angle to enhance vapor ingestion capability previously observed. This testing is also scheduled for this spirng. Other Supporting Engine Technology—An important and potentially very significant long-range technology effort in the nuclear rocket program is the de |