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Thermionics Quo Vadis?: An Assessment of the DTRA’s Advanced Thermionics Research and Development Program Executive Summary In 1995, the Defense Nuclear Agency, now a part of the Defense Threat Reduction Agency (DTRA), was assigned management responsibility for the remnants of the thermionics research and development programs of the Ballistic Missile Defense Organization (BMDO) and the U.S. Air Force (USAF). The main thrust of the combined program was a cooperative U.S.-Russian project called the TOPAZ International Program, which was based on the Russian TOPAZ nuclear thermionic power system. (TOPAZ is a Russian acronym meaning thermionic power from the active zone.) The TOPAZ International Program was terminated in 1996 in response to (1) findings made by the General Accounting Office and a study by the National Research Council (NRC 1996) questioning the relevance of the unfueled TOPAZ system testing, (2) the absence of a Department of Defense (DoD) and NASA requirement for near-term space nuclear power systems, and (3) a pressing need to prioritize resources. Most of the remaining thermionic technology projects being conducted by BMDO and the Air Force Research Laboratory (AFRL) were terminated or phased out shortly thereafter. Congress subsequently directed DTRA to establish a modest, technology-focused thermionics program. The DTRA program incorporated a variety of projects performed by industry, universities, two Russian institutes, and a Department of Energy (DOE) laboratory. In 1999, after 3 full years, DTRA sought an independent assessment of its stewardship of the advanced thermionics research and development program and of the technical progress of the program. The NRC accepted the charge of performing this assessment. The statement of task for this study required the NRC to perform the following tasks: Evaluate DTRA’s prior and present sponsored efforts. Assess the present state of the art in thermionic energy conversion systems. Assess the technical challenges to the development of viable thermionic energy conversion systems for both space and terrestrial applications. Recommend a prioritized set of objectives for a future research and development program for advanced thermionic systems for space and terrestrial applications. An additional task was to conduct a workshop for the interim discussion of technical challenges and a strategy for meeting those challenges. The results of the workshop are incorporated into this report. PROGRESS IN THERMIONIC RESEARCH Despite being limited by modest funding, DTRA has made good progress since its redirection to a technology program in 1996. Given the funding limitations and uncertainties, the industry and university participants generally have performed admirably. The committee was especially impressed by the technical accomplishments in the cooperative work conducted by U.S. and Russian researchers on single-crystal refractory metal alloys research under the auspices of the DTRA program. Nevertheless the committee believes that, despite these accomplishments, the overall goals of the present
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Thermionics Quo Vadis?: An Assessment of the DTRA’s Advanced Thermionics Research and Development Program program are too broad and diverse to be accomplished given the projected budget constraints. The committee also notes that the thermionic technology program is not encompassed by the primary mission statement of the DTRA organization. This being so, the committee believes that the program could be more effectively planned, managed, coordinated, and conducted by the AFRL. OVERVIEW AND ASSESSMENT OF THE DTRA THERMIONICS PROGRAM The present DTRA thermionics program consists of three major elements, namely the nuclear power in-core thermionic technology element, performed primarily by General Atomics and several subcontractors; the microminiature thermionic converter element performed by DOE’s Sandia National Laboratories; and the theory and theory model validation element, performed by the DTRA staff and consultants. Table ES-1 summarizes the tasks conducted under the DTRA thermionics program. From fiscal year 1996 to 1999, DTRA also sponsored a portion of the thermionic generator testing conducted under the USAF’s Solar Orbital Transfer Vehicle program. The DTRA thermionics program includes both basic and applied research as well as engineering development and demonstration efforts. TABLE ES.1 Major Elements of the DTRA Thermionics Program Major Thermionic Program Element Subelement Subtask Responsible Research Group Nuclear power in-core Conductively coupled/multi-cell thermionic fuel element (TFE) Trilayer insulation design, development, and device testing General Atomics in collaboration with Russian research facilities Oxygenated thermionic converters Oxygenated electrode testing General Atomics in collaboration with Russian research facilities Oxygen mass transport Russian research facilities High-creep strength fuel clad development Single-crystal alloy domestic fabrication and creep testing; closed chemical vapor deposition process Auburn University in collaboration with Russian research facilities Advanced thermionic converter: close-spaced converter Device development and testing Russian research facilities Advanced thermionic converter: low emissivity converter development Design and proof of concept Russian research facilities Microminiature thermionic converter (MTC) Proof of performance and theory validation Low work function coating development device testing Sandia National Laboratories with New Mexico Engineering Research Institute test support Thermionic theory and model validation Thermionic space reactor system mass model RSMASS-T system model upgrade DTRA staff Thermionic theory and theory validation Vacuum converter theory development and surface effects modeling DTRA staff and consultants
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Thermionics Quo Vadis?: An Assessment of the DTRA’s Advanced Thermionics Research and Development Program Of the three major elements that make up the DTRA thermionics program, the committee recommends that the microminiature thermionic converter (MTC) effort and the theory and validation efforts be discontinued. While the MTC effort can be appreciated for its innovation and its attempts to eventually provide some potential technology spin-off to other fields in the future, the committee does not believe that the promise of the MTC concept can ever be realized without unreasonable amounts of funding. Likewise, in the committee’s opinion, the theory and validation task has a relatively low probability of additional success, and the potential end results do not warrant further expenditures at this time in light of limited available funding. By contrast, many of the tasks under the nuclear incore portion of DTRA’s thermionic technology program do show promise, and the committee believes that many of those elements in the program should be continued. However, the activities associated with the oxygenated thermionic converter subtask should not be continued. Although they are categorized under the nuclear in-core portion of DTRA’s thermionic technology program, the remaining tasks in the thermionics program can be broken down into two broad application areas: Space applications Solar power Nuclear power Terrestrial applications. The committee found no firm requirements or need for thermionic systems within DoD or NASA, and thermionic system-level technology is not developed to the point that it is available for mission commitment at this time. However, potential applications may be defined beyond the next decade. The committee believes that the system performance advantages offered by thermionic energy conversion are attractive for future high power space missions employing solar concentrating heat sources or, in the longer term, nuclear reactor heat sources. Because of the unique nature of thermionic systems, the committee believes that a thermionic program should continue to be supported. Key Finding: Thermionic systems are unique for three reasons: (1) the inherently high power density of the conversion mechanism itself, (2) the systems’ high heat rejection temperature, typically 1000 K, which allows thermionic systems to use compact radiators with relatively low masses, and (3) the systems’ potential to operate in a higher power “surge mode” for sustained periods over a small fraction of their programmed life. The combination of these three advantages could allow for potentially significant advances in system power level density (kilowatts per kilogram). SOLAR THERMIONIC SYSTEMS Although solar thermionic development was explored briefly by NASA in the 1960s, research was curtailed in the early 1970s in favor of solar photovoltaic battery systems. However, standard power requirements for satellites have since increased from several kilowatts to tens of kilowatts. In this range, a solar thermionic system appears to offer advantages in terms of stowed payload volume and mass. Space-based solar thermionic systems, such as the high-power, advanced, low-mass (HPALM) solar thermionic converter proposed by General Atomics, potentially offer competitive specific power.1 It should be noted that no such system exists at present. The HPALM concept is an energy conversion system for use with spacecraft operating where solar energy is available. The concept involves the use of an inflatable solar concentrator to focus solar energy onto a thermionic converter to supply power to a spacecraft. The feasibility of solar thermionic systems is based in part on the demonstrated NASA planar converter and generator technology of the 1960s, namely the solar electric converter used under the Solar Energy Technology (SET) program. Under that program, converters operating at 25 watts per square centimeter and 0.7 volts demonstrated 15,000 hours of life through several hundred thermal eclipse cycles. The individual generators developed under the SET program provided 150 watts of electrical power. Since then, substantial progress on large, oriented space structures, particularly inflatable structures not related to thermionics research and development, has raised the possibility of using large solar concentrators in space. The committee recommends that the sponsoring agency2 direct the near-term thermionics research 1 Specific power is defined as the power per unit mass, or kilowatts per kilogram. 2 The term “sponsoring agency” is used to reflect the recommendation that the program be transferred from the DTRA to the AFRL.
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Thermionics Quo Vadis?: An Assessment of the DTRA’s Advanced Thermionics Research and Development Program and development toward a solar thermionic application that could provide mid to high tens of kilowatts (roughly 30 to 70 kilowatts) of electrical power to a client spacecraft. In particular, the program should be aligned with the HPALM concept. The committee conducted a detailed review of the relatively unsuccessful New Mexico Engineering Research Institute (NMERI) string thermionic assembly research testbed (START) efforts. The tests consisted of connecting strings of electrically connected thermionic converters, forming thermionic generators, to validate a system-level power conversion concept for the AFRL Solar Orbital Transfer Vehicle (SOTV) program. The committee decided that the tests should be reviewed because of the conclusions apparently drawn from the inconclusive tests. The testing team concluded that the poor test results indicated problems with the converter technology. Based on available documentation, however, the committee believes that serious test procedural problems may have been to blame and that no conclusions should be made about thermionic converter performance based on those tests. NUCLEAR POWER THERMIONIC SYSTEMS A 1998 report of the National Research Council’s Committee on Advanced Space Technology (NRC 1998) stated as follows: Advanced space nuclear power systems will probably be required to support deep space missions, lunar and planetary bases, extended human exploration missions, and high-thrust, high-efficiency propulsion systems. A major investment will eventually be needed to develop advanced space nuclear power sources…. Unless NASA supports R&T in areas such as innovative conversion methodologies or innovative packaging and integration, future space nuclear power systems will probably be more expensive and less efficient. For some missions that will require high power and long life, or where nuclear power is a critical requirement, the potential performance advantages of nuclear thermionic space power are compelling for electric propulsion missions. In terms of lifetime and device-level power output, coupled with their low mass, compactness, and surge mode capability, thermionic systems are attractive, and the nearly unique features of this technology could satisfy future space power requirements for 20 kilowatts up to megawatts of electric power. In some cases, fully developed thermionic technology may be mission enabling. However, the committee also acknowledges that the technical risks in developing a functional thermionic system are high. The technical uncertainty surrounding an operational system that could achieve the desired performance is especially high for power systems that use thermionic converters powered by in-core nuclear reactors. The most challenging and expensive feasibility issues for nuclear thermionic systems are clearly those related to the integration of the converter into the nuclear reactor core. These issues include nuclear fuel swelling, which causes structural deformation and electrical short circuits in the thermionic converter, and radiation damage to converter insulator materials. At present, any thermionic fuel element using nuclear fuel would be life limited due to nuclear fuel swelling. This limitation currently makes nuclear thermionic systems impractical for missions with a requirement for long operational life. The original Russian TOPAZ reactor program demonstrated a 1 year life operational capability in space. The U.S. thermionic fuel element verification program projected system life to be greater than 3 years; however, no such system has been built. There is no capability in the United States to test nuclear thermionic fuel materials for fuel swelling issues because those fast-spectrum test facilities were deactivated. A possible alternative to reestablishing test facilities in this country is to coordinate with Russia in future thermionic materials testing. Given the very high cost of developing and deploying space nuclear reactors, the committee does not recommend pursuing thermionic technology solely for use with nuclear power sources in the near term. Instead, the thermionics research and technology program should have the development of a thermionic space nuclear capability as a long-term goal. A challenge to balancing near- and long-term plans is to identify technologies that can be adapted to both solar and nuclear thermionic applications. TERRESTRIAL THERMIONIC SYSTEMS Terrestrial thermionic applications are specifically mentioned in the statement of task for this study, even though such applications have received little attention from any research organization in the past two decades. The committee found no significant interest in terrestrial military or commercial fossil-fuel-based thermionic systems. Past interest had been motivated by a desire to increase energy conversion efficiency and reduce pollution. The committee believes that this lack of interest is a result of the high cost of thermionic
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Thermionics Quo Vadis?: An Assessment of the DTRA’s Advanced Thermionics Research and Development Program systems and the fact that neither long-term reliability nor the systems themselves have been proven. There is currently no incentive in the marketplace to develop terrestrial thermionic systems in spite of rising fuel costs, significant power shortages, and environmental pollution. SUMMARY AND CONCLUSIONS Although thermionic systems have the potential to satisfy many future power system needs, other power conversion technologies are also being developed. In relation to these other potential technologies, the committee believes that thermionic technology may offer equal or superior merit for specific missions. The future sponsor should continue to evaluate and develop the possibilities of thermionic systems despite the challenge of preserving, continuing, and advancing this technology in the near term. The following recommendations are presented in order of priority. The first recommendation, to move the thermionics program from the DTRA to the Air Force Research Laboratory (AFRL), is listed as the primary recommendation strictly from a programmatic point of view. The committee urges those working within and managing the thermionics program on a daily basis to concentrate on recommendations two through seven, which are offered by the committee in order to strengthen the program on a technology level. Recommendation 1. The United States Congress and the Administration should transfer responsibility for the technical management of the Defense Threat Reduction Agency’s thermionics program to the Air Force Research Laboratory. Doing so would enhance the technical continuity for the technology and place the program in an agency responsible for developing power systems and conversion technologies. As the focal point for thermionic research, the Air Force Research Laboratory should attempt to establish cooperative activities with other government agencies, such as the Department of Energy, the Naval Research Laboratory, NASA, and the Air Force Office of Scientific Research. Recommendation 2. The sponsoring agency should generate a long-term plan to focus activities related to both solar and nuclear applications for thermionic technology. Recommendation 3. The sponsoring agency should concentrate its near-term thermionic development work on a space-based solar thermionic power system, such as the high-power, advanced, low-mass (HPALM) concept. Recommendation 4. The sponsoring agency should concentrate longer-term thermionic development work on those areas of nuclear thermionic power systems related to materials development, converter development, and radiation effects on materials in order to achieve high power and long life for such systems. Recommendation 5. The sponsoring agency should reestablish an adjunct basic research program on electrode surface physics, plasma, and materials processes relevant to thermionic energy conversion. This program should be funded separately from the thermionics research program. Recommendation 6. The sponsoring agency should discontinue the microminiature thermionic converter (MTC) program, the close-spaced vacuum converter tasks, the oxygenation effects research, and all current theory and theory validation work. Recommendation 7. When working on a system-level solar thermionic design, the sponsoring agency should reexamine the string thermionic assembly research testbed (START) tests in order to record lessons learned. The reexamination should begin with a retest of the original, individual converters to differentiate between problems due to the converter design and generator configuration and those due to the test setup. The sponsoring agency should gather an independent group of experts to devise testing methodologies so as not to repeat past mistakes. REFERENCES NRC (National Research Council). 1996. Assessment of the TOPAZ International Program. National Academy Press, Washington, D.C. NRC (National Research Council). 1998. Space Technology for the New Century. National Academy Press, Washington, D.C.
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