Thermionics Quo Vadis?: An Assessment of the DTRA's Advanced Thermionics Research and Development Program (2001)

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 The major thrust of the new combined program was a cooperative U.S.-Russian project called the TOPAZ International Program (TO-PAZ is a Russian acronym meaning thermionic power from the active zone). The TOPAZ program was terminated in 1996 in response to (1) findings 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 National Aeronautics and Space Administration (NASA) need for near-term space nuclear reactor power systems, and (3) pressure to prioritize resources. Most of the remaining thermionic technology projects being conducted by BMDO and the Air Force Research Laboratory were terminated or phased out shortly thereafter.

Congress subsequently directed DTRA to establish a modest, technology-focused thermionic program. The DTRA program incorporated a variety of projects being performed by industry, universities, several Russian institutes, and a Department of Energy (DOE) laboratory. In 1999, after 3 full years of planning and management, DTRA sought an independent assessment of its stewardship of the advanced thermionics research and development program and the technical progress of the program. The NRC accepted the charge of performing this assessment.

The statement of task for this study, which appears in Appendix A, 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 meetings and workshop included participants from nongovernmental organizations, industry, and academia. The results of the workshop are incorporated into this report.

To accomplish these tasks, the NRC’s Aeronautics and Space Engineering Board established the Committee on Thermionic Research and Technology, consisting of 11 members. Brief biographies of the committee members are presented in Appendix B.

The committee first met with DTRA representatives in May 2000 to clarify the objectives and purposes of the study. DTRA representatives attended and participated in all subsequent open meeting activities. The

study was conducted independently, in keeping with NRC procedures and government contracting regulations.

At the second meeting in June 2000, the committee met with all of the government thermionic research and development organizations and potential technology user organizations, including NASA, DOE, and DoD. The committee was also briefed on current and potential NASA and DoD mission and system applications for thermionic technology, including envisioned power requirements. Earth-based (terrestrial) applications and commercial power technology development activities were assessed based on discussions with commercial power industry representatives and a recent NRC study on the DOE’s renewable energy program (NRC 2000).

During the information gathering phase of the study, the committee received technical briefings from all of the researchers in the United States currently sponsored by the DTRA program. The committee also sponsored a 2-day thermionic technology workshop in La Jolla, California, in August 2000. At that workshop, the committee presented an overview of the major tasks to representatives of the thermionics community. In turn, the committee received additional technical briefings and suggestions for recommendations from the thermionics community, some of which the committee ultimately adopted.

All written materials presented to the committee during the course of this study, including materials presented at the workshop, are maintained on file as a matter of public record at the NRC.

The information gathering phase of this study also included a complete review of three earlier NRC studies related specifically to thermionics, Advanced Nuclear Power Sources for Portable Power in Space (NRC 1983), Advanced Power Sources for Space Missions (NRC, 1989), and Assessment of the TOPAZ International Program (NRC 1996).

A related report, Renewable Power Pathways: A Review of the United States Department of Energy’s Renewable Energy Programs (NRC 2000), and discussions with commercial power industry representatives, aided the committee in evaluating terrestrial applications and national commercial power technology development activities.

The seven recommendations in this report are prioritized as presented in the executive summary. However, in the main body of the report, they are placed with the relevant subject matter topics and discussion, rather than in prioritized order.

The committee found that many of the technology program elements that the DTRA is currently funding should be discontinued. For the purpose of this study, the remaining program elements fall into three broad categories discussed in Chapters 4, 5, and 6, respectively:

1. Space solar power applications,

2. Space nuclear power applications, and

3. Terrestrial applications.

Chapter 2 of this report presents the conclusions of the study. Thermionic systems offer the potential to satisfy many future power system needs. However, thermionics is but one candidate in a field of many, several of which are also in as austere funding situation as thermionics. The committee believes that in relation to these other technologies, thermionic technology has worth and should continue to be developed. However, the committee acknowledges that preserving, continuing, and advancing this technology in the near term will be extremely challenging.

The committee praises the technical quality and accomplishments of the cooperation between U.S. and Russian researchers under the auspices of the DTRA program. At the same time, the committee is concerned that there is a possibility of undesired transfer of technology from the United States to China through the Russian researchers. It has been reported that China is engaging in thermionic research and development.

The committee believes that a firm understanding of the technical and programmatic history of past thermionic activities, of the technology’s successes and failures, and of programmatic and national policy issues is essential for planning the future direction of the program. Accordingly, Chapter 3 briefly reviews thermionic energy conversion principles and history and discusses thermionic system attributes as they relate to potential applications in future missions. Although it found no firm requirements for thermionics for any DoD- or NASA-approved missions, the committee believes that the system performance advantages offered by thermionic energy conversion could be utilized in future high power space missions employing a solar-concentrator or nuclear reactor heat source. In some cases, fully developed thermionic technology may be mission enabling. The committee also acknowledges

that the technical risks in developing a functional thermionic system are high. The technical risk and uncertainty are especially high for power systems that use thermionic converters powered by nuclear reactors. Given the tremendous cost of developing and deploying space nuclear reactors, the committee does not recommend pursuing either short-term thermionic technology solely for use with nuclear power sources or system development activities until a mission is identified that will require such a power source.

Should a high power mission, one requiring a nuclear reactor in space, be identified, the demonstrated capabilities of thermionic systems, coupled with their intrinsic low mass and compactness, could satisfy future space power requirements in the low to mid tens of kilowatts to megawatts.

Chapter 3 also summarizes the demonstrated state of the art of thermionics technology as related to space and terrestrial applications. Much of the existing technology base supporting the feasibility of system application has already been demonstrated, particularly for solar applications as demonstrated by NASA’s Jet Propulsion Laboratory (JPL) Solar Energy Technology (SET) program. The remaining development issues within the arena of solar thermionics are significant, but those problems have been clearly defined as a result of past efforts.

The most challenging and expensive technology feasibility issues are those that are related to integration of the converter into the nuclear reactor core and that are mostly dependent on structural deformation induced by nuclear fuel swelling. The structural deformation (or creep) results in electrical shorting in the converter and radiation damage to converter insulator materials. Both problems raise questions about the suitability of a thermionic system for an extended space mission life of 10 years or more.

Chapter 4 reviews the potential use of thermionics in conjunction with power systems that use concentrated solar energy. First considered in the 1960s, development of solar thermionics was curtailed in the early 1970s owing to the competitive advantages of solar photovoltaic battery systems and their ability to satisfy the prevalent need at that time for hundreds of watts up to a few kilowatts of electrical power. As potential power requirements grow into the 30-plus kilowatt range, solar thermionic systems appear to offer stowed payload volume advantages, competitive specific power capabilities, and the ability to operate in higher natural radiation orbital environments than most other energy conversion systems.¹ The feasibility of such solar thermionic system concepts is based in part on the demonstrated JPL planar converter and thermionic generator technology of the 1960s, especially those technologies generated under the JPL SET program. Under that program, converters operated at 25 watts per square centimeter and 0.7 volts with a demonstrated life of 15,000 hours. Progress in large, oriented space structures, particularly inflatable structures, has also contributed greatly to solar thermionic feasibility.

Chapter 5 presents a review of thermionic technology as it relates to space nuclear reactor power systems. The demonstrated performance of the short-life Russian TOPAZ thermionic space reactor system is discussed, as are the accomplishments of the Thermionic Fuel Element Verification program sponsored by the Strategic Defense Initiative during the mid 1990s. The key remaining technology issues are described, as are arguments for nuclear in-core thermionics versus nuclear out-of-core conversion systems.

Chapter 6 covers terrestrial applications of thermionics. Even though these applications have received little attention in the past two decades, the committee was specifically tasked with identifying them. In response, the committee has included a brief summary of past terrestrial efforts; however, the committee found no current interest for terrestrial military thermionic systems or commercial fossil-fueled thermionic systems. The desire to increase power conversion efficiency and decrease pollution motivated past system concepts, but there is currently no market incentive to develop terrestrial thermionic systems in spite of rising fuel costs, significant power shortages, and environmental pollution. The committee believes that this lack of interest is a result of the high cost of thermionic systems and the fact that neither long-term reliability nor the systems themselves have been proven. By contrast, combined cycle gas turbine systems that have a proven long life, high efficiency, and reliability are being used.

In Chapter 7, the committee assesses progress made under the current DTRA program in certain key areas. The committee believes that the DTRA program has made good progress, especially in light of the limited funding since the program’s redirection toward technology development from the previous TOPAZ Inter-

national Program system-level approach. In general, industry and university participants in the present program have performed admirably given the uncertainties surrounding funding.

Appendix A contains the DTRA statement of task, and brief committee member biographies are presented in Appendix B. Appendixes C and D contain supporting material on electric propulsion and list the acronyms used in the report, respectively.

NRC (National Research Council). 1983. Advanced Nuclear Power Sources for Portable Power in Space. National Academy Press, Washington, D.C.

NRC (National Research Council). 1989. Advanced Power Sources for Space Missions. National Academy Press, Washington, D.C.

NRC (National Research Council). 1996. Assessment of the TOPAZ International Program. National Academy Press, Washington, D.C.

NRC (National Research Council). 2000. Renewable Power Pathways: A Review of the United States Department of Energy’s Renewable Energy Programs. National Academy Press, Washington, D.C.