Good progress has also been made on the reactor design. The preliminary design has been completed and we are in a position to proceed with detailed design and fabrication of test components and the first NERVA ground test reactor. Except for a few critical components, however, such as fuel elements, the axial support structure, and the periphery of the reactor core, this work will be deferred to a later time.

In support of reactor design, extensive work has been done on materials and fabrication processes. During the past year, over 5000 material and sub-component tests were performed to provide a sound statistical basis for detailed reactor component design. Included in these tests were fuel element corrosion and physical property determinations, environmental effects on physical properties, load and creep tests of fuel assemblies, radiation effects on physical properties, and exploration of the properties of several new refractory insulators for use in the very high-temperature regions of the reactor.

With the completion of the overall design we are now in a position to intensify efforts on detailed design of the components and to release long lead time material procurements and fabrication actions. As discussed last year, fiscal year 1971 funding has limited our progress with respect to component development. Testing is being limited, therefore, to such items as turbopump bearings, critical radiation effects tests on materials and various components of the reactor including the reactor fuel elements, the endurance and performance limiting factor in the engine. The extent to which we can proceed in FY 1972 will again be limited by available funding. Within these limitations, progress will be made on some of the most critical of the development hardware items, including the turbopump, the nozzle extension, the reactor fuel elements and support system and the evaluation and characterization of vital materials in radiation and other applicable environments. These components will be fabricated and test programs to evaluate some of them will be started in FY 1972. In this manner, some technical progress will be made and a capability will be retained which will permit resumption of the full development program when that becomes possible.

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

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

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).