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

A 1998 report published by the National Research Council’s Committee on Advanced Space Technology (NRC 1998) stated as follows:

For some space propulsion missions that require high power, or where nuclear power is a critical requirement, the potential performance advantages of a nuclear thermionic system are compelling. The demonstrated state of the art of thermionic systems in terms of lifetime and device-level power output, coupled with their low mass and compactness, make this technology attractive and suggest that it could satisfy future space power requirements in the low to mid tens of kilowatts to megawatts.

In some cases, fully developed thermionic technology may be mission enabling. However, the committee acknowledges that the technical risks in developing a functional thermionic system are high. The technical uncertainty in developing an operational system that could achieve the desired performance is especially high for power systems that use thermionic converters powered by nuclear reactors.

There is no capability in the United States to test nuclear thermionic fuel materials for fuel swelling issues because those fast-flux test facilities were deactivated. A possible alternative to reestablishing test facilities in this country is to coordinate with Russia in future thermionic materials testing.

As discussed in Chapter 4, the committee recommends orienting the near-term thermionics research and development program toward a solar thermionics conversion technology aimed at competing with other energy conversion technologies available today, such as solar photovoltaics. Basic research and long-term planning, however, should be oriented to establish a technology base that could be used by a future space mission requiring nuclear power. This chapter details the current state of nuclear thermionic research and the path that should be followed to establish a long-term nuclear thermionic capability.

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.

The history of the TOPAZ International Program work is recounted briefly at the end of Chapter 3. The statement of task required the committee to review the work conducted under the joint U.S.-Russian TOPAZ International Program. A previous National Research Council report on the TOPAZ program reviewed the work conducted under that program (NRC 1996). No further work has been conducted under the program since it was canceled.

In 1995, the Defense Nuclear Agency (DNA), which has since then become a part of the Defense Threat

Reduction Agency (DTRA), requested that the National Research Council examine and assess the TOPAZ International Program that the DNA was then conducting. The DNA asked for an assessment of the following:

• The status of the program at the time;

• The value of continuing the ongoing activities as they related to developing operational space nuclear power systems;

• The possible effects of discontinuing certain elements of the program;

• The state of the TOPAZ reactor technology in relation to the equivalent U.S. technology;

• The value of establishing revised goals for the program; and

• Steps DNA could take to serve the national interest more effectively, including continuing, modifying, expanding, or discontinuing the program.

Representatives of various organizations in DoD, DOE, and NASA, together with Russian interests, U.S. industrial companies, and private individuals, participated in the 1996 deliberations on the TOPAZ International Program. Since the TOPAZ International Program was devoted to thermionic system advancement, the 1996 report’s assessment is relevant in many respects to the current examination requested by DTRA. Although no work has been conducted in the United States related to the TOPAZ International Program since the program was discontinued in 1996 and the TOPAZ hardware was returned to Russia, many of the factors considered by the previous committee and the conclusions it reached are still valid. The conclusions of the 1996 report, and the current committee’s observations, are summarized below.

• Support of high power, long lifetime nuclear systems. The 1996 TOPAZ committee stated that one objective of a U.S. thermionics program should be to advance critical technologies that could support potential future high power, long lifetime space nuclear reactor systems. However, at that time the program did not have sufficient funds to advance the critical technologies required for such a nuclear system. Those funds are still not available.

• User applications. The 1996 TOPAZ committee emphasized throughout its report that there were no dedicated users or then-current applications for thermionic or nuclear technology. The 1996 committee also found that there were no planned or confirmed mission-directed activities within the TOPAZ International Program. While the Solar Orbital Transfer Vehicle program may address this issue with respect to solar thermionic systems, there is still no confirmed mission for a nuclear thermionic system.

• Knowledge capture. The 1996 TOPAZ committee found that the TOPAZ International Program had no formal mechanisms to record and archive the technical knowledge gained from the space power nuclear reactor technology efforts so that it would be accessible for future efforts. There is still no formal archival method being used today in the DTRA thermionics program.

• Collaboration. The TOPAZ committee found that an integrated and collaborative interagency approach that existed in earlier nuclear power development programs, such as SP-100, had broken down.

• Role of government. The 1996 TOPAZ committee found that there is a role for the government in research and development for thermionics because U.S. companies often do not have sufficient economic motivation to maintain the scientific and engineering staffs or facilities to support continued work on high risk, long-term projects such as thermionics. The situation has not changed since 1996.

All of the points made by the TOPAZ committee in 1996 are relevant to the current situation in the DTRA thermionics program. In contrast to the previous committee, the current committee, as already stated, recommends that emphasis be placed on solar thermionics research as a near-term goal and that nuclear thermionics research, as advocated by the 1996 NRC committee, be viewed as a long-term goal.

The current funding situation is analogous to the funding that existed for the TOPAZ International Program in 1995 and 1996: that is, year-to-year funding mandated by Congress limits the efforts. The conclusions listed above should help guide future thermionics work aimed at space nuclear power.

The 1996 TOPAZ committee considered six program options that ranged from terminating the TOPAZ program to revisiting the possibility of conducting a flight test and revamping the overall program. The TOPAZ International Program was canceled shortly after the committee’s report was released, and no further work has been conducted.

The following is a paraphrased account of some of the recommendations from the 1996 report that would be relevant to a future nuclear thermionics program in the United States:

• The overriding objective of thermionic research should be to advance the critical technologies for high-power, long-lifetime U.S. space nuclear reactor power systems.

• The U.S. government should support a single, comprehensive, integrated thermionics program rather than a collection of uncoordinated programs. Funding at the level of $ 15 million to $20 million per year would be required to develop a space nuclear reactor program.

• The nuclear portion of an integrated thermionics program should focus on fast-spectrum reactors using in-core multicell thermionics aimed at high-power, long-lifetime systems rather than thermal-spectrum, single-cell systems. An integrated thermionics program should also include other power conversion approaches, including thermoelectrics and out-of-core thermionics.

• An integrated thermionics program should cooperate with Russian institutes involved in thermionic development to benefit from their experience and testing facilities. In-core lifetime testing is not readily available in the United States owing to the declining availability of domestic irradiation test facilities.

• A thermionics program should include the participation of U.S. industry to help establish and benefit from a strong, long-term knowledge base.

• A thermionics program should initiate cooperation among DoD, NASA, and DOE and should include multiagency funding to provide continuing support.

In Recommendation 3 (see Chapter 4), the committee states that the thermionics research and development program should be directed in the near term toward the development of a solar thermionic system. Long-term thermionic program goals, however, should be directed toward establishing and maintaining an option for a nuclear thermionic system, a position stated in Recommendation 4 above in this chapter. Balancing two sets of requirements to meet these short-and long-term goals will not be easy. It will not, for example, be possible for the sponsoring agency to design a solar thermionic system that simultaneously addresses issues such as the radiation damage to materials and mechanical stress caused by nuclear fuel swelling. However, for a viable nuclear thermionic system to be built, those are exactly the issues that must be addressed.

Despite the difficulty, the committee believes that the sponsoring agency should work to develop a technology base that can advance systems that will meet both sets of requirements. However, the area where the two technologies overlap is difficult to define, so the sponsoring agency needs to carefully decide which specific technologies or systems will be developed.

The committee feels that the cylindrical inverted multicell (CIM) thermionic converter, proposed by General Atomics in conjunction with the HPALM system (described in Chapter 4), and the conductively coupled multicell (CC/MC) thermionic fuel element, also proposed by General Atomics, may be specific examples of technologies that can be used for solar thermionic applications and adapted to nuclear thermionic applications in the future. The following discussions of the CC/MC and CIM are not meant to indicate that these technologies are the perfect (or the only) technologies that can be developed for use in both nuclear and solar conversion systems. Rather, these technologies should be considered indicative of the type of devices that offer promise of being compatible with both heat source systems.

A traditional multicell thermionic fuel element (TFE) consists of thermionic converters connected in series. Each element is loaded with nuclear fuel and both ends of the element are sealed. The nuclear fueled “flashlight” TFE, which was the baseline for most in-core reactor concepts in the United States as well as the former Soviet Union, is a stack of these individual thermionic converters connected in series to form a thermionic generator. Each fueled thermionic converter gives the impression of a standard D-cell sized battery, hence the term “flashlight configuration.” General Atomics has performed most of the work on flashlight TFEs in the United States.

By the early 1990s, program planners and researchers realized that the ability to conduct nonnuclear ground testing of flight units prior to launch would be useful for acceptance or flight qualification testing. Unfortunately, conventional sealed thermionic cells used in the thermionic flashlight generator are difficult to heat-test electrically because of the mechanical design of the TFE.

Moreover, nuclear heating is not practical for flight system verification on the ground. When units are

tested for flight qualification on the ground, radioactive fission products are produced in the nuclear core. Having these products in the nuclear core during a rocket launch creates additional complications for launch safety assurance. A predecessor of the only U.S. nuclear reactor to fly in space, SNAP-10A, was ground tested. The second, untested unit was then flown on the experimental space mission.

Radiative coupling is one way around the dilemma of not being able to test a TFE individually prior to combining it with the nuclear heat source. With a TFE designed for radiative heating, a vacuum gap electrically isolates the nuclear fuel from the thermionic converters. An electrical heat source is then used to mimic the radiative heat properties of a nuclear heat source. In this way, each individual TFE and the entire energy conversion subsystem can be tested before loading the reactor with nuclear fuel. The reactor can then be fueled relatively late in the checkout process before launch.

The major disadvantages of using radiative electrical heat testing are that the radiative heating introduces an additional temperature gradient between the heat source and thermionic emitter due to the vacuum gap between the two. This situation requires that the fuel maintain a temperature roughly 200 K higher than the emitter surface to compensate for the gap.

An alternative approach for a design that keeps the thermionic converter separate from the nuclear fuel is conduction coupling. There, the heat from the nuclear fuel is transferred conductivly using a ceramic insulator that electrically isolates the fuel from the emitter. As is the case for radiatively coupled converters, the conduction coupled converter can be heated electrically by placing a heating element inside the hollow center of the cylindrical thermionic converter to replicate the heating properties of nuclear fuel. This is the method that General Atomics proposes to use with its CC/MC concept.

Although conceptually simple, using a heat-conducting medium between the nuclear fuel heat source and the thermionic emitter is a challenge because of the combination of high temperature, voltage gradient, and nuclear radiation, which cause electrolytic dissociation of most ceramic insulators.

Russian research offers a possible solution. It has shown that scandia (Sc₂O₃) insulators offer extraordinary high temperature capability. An emitter trilayer may be fabricated with a fuel cladding layer, followed by a scandia insulator layer and then the thermionic emitter. However, the lifetime for a device such as a CC/MC has not been conclusively demonstrated, especially under the combined influence of temperature, voltage gradient, and irradiation (Streckert et al. 2000a, b).

Since the CC/MC was designed to be heated by sources other than nuclear fuel, it could be used with some other method of transporting heat into the CC/ MC cylinder, such as with a heat pipe assembly. However, the addition of a heat pipe system to a solar concentrator, for example, may negate some of the potential weight advantages of the solar thermionic system. So, although such a system is possible, the system tradeoffs to make such a system viable need to be examined before any final conclusions can be made.

Finding: The conductively coupled multicell (CC/MC) thermionic fuel element, as proposed by General Atomics, needs to be evaluated for nuclear in-core use by resolving radiation-induced fuel swelling and insulator degradation issues before the concept can be declared viable.

The cylindrical inverted multicell (CIM) thermionic converter is essentially an inverted version of the CC/ MC device and was proposed specifically for use with the HPALM concept (see Figure 5.1). So while one challenge with the CC/MC device may be to find a suitable solar candidate mission, a challenge with the CIM device is to find a suitable nuclear candidate application.

In the CIM thermionic converter, the heat is applied externally, where the emitter is located, and the waste heat is removed from the hollow interior of the cylinder. This conceptual device is intended for use with the HPALM system and is, therefore, inherently capable of being tested using an electrical heater. The CIM converter would be immersed in the heat receiver of the HPALM system, so a comparable nuclear system could be the space thermionic advanced reactor compact (Star-C) concept as was proposed by General Atomics. The STAR-C concept is a nuclear reactor in which the heat from the uranium carbide nuclear fuel is captured by a graphite block. The CIM thermionic converter could be placed into this graphite block so that the CIM is heated radially inward. One potential advantage of this design is that, since the nuclear fuel is not in contact with the CIM, fuel swelling issues may not be as

FIGURE 5.1 Cylindrical inverted multicell cross section.

SOURCE: L.Begg, General Atomics, presentation to the Committee on Thermionic Research and Technology, August 2000.

large a concern for the thermionic converter portion of the device. However, the CIM would still be inside the nuclear core, and so any development would have to contend with radiation damage to materials just as would the CC/MC development. This configuration could introduce the added complexity of carbon diffusion into the thermionic device.

Finding: Both the inverted thermionic element and the planar thermionic converter that could be used the HPALM concept are compelling, but the system has not been built or tested.

Finding: The cylindrical inverted multicell (CIM) thermionic fuel element, as proposed by General Atomics, needs to be proven for solar conversion applications. The same technology, if chosen for use with a nuclear in-core system, needs to be evaluated for nuclear in-core use by resolving radiation-induced insulator degradation issues before the concept can be considered viable.

Nuclear heat conversion is an alternative to solar heat conversion for providing power to a spacecraft. Nuclear power applications can be divided roughly into those with low and high power requirements. Low power requirements can be satisfied by radioisotope power systems that generate a few kilowatts of power at most. High power requirements, near or exceeding 100 kilowatts, may require the use of nuclear reactors, depending on the specific mission.

In general, thermionic converters would not be used with radioisotope heat sources because other conversion devices are better suited to operate at the lower temperatures typical of radioisotope heat sources.

The decision to use a nuclear power source on a spacecraft will generally be driven by compelling mission requirements. To date, the United States has only flown one reactor in space. There have been other missions that use nonreactor-based radioisotope power. All of the spacecraft that have flown beyond the orbit of Mars have been powered by radioisotope power sources because there is simply not enough sunlight for photovoltaic arrays. Figure 5.2 illustrates that the solar energy available decreases rapidly as an interplanetary spacecraft flies to the outer planets of the solar system.

Currently, there are no planned or approved space missions that use a nuclear reactor. Smaller spacecraft, such as NASA’s planned Europa orbiter and the Pluto/ Kuiper Express, meet the criteria for needing a nuclear power source. However, both spacecraft require a fairly

FIGURE 5.2 Solar energy flux as a function of distance from the Sun.

small amount of power and could therefore make use of the smaller radioisotope power systems. In addition, the Pluto/Kuiper Express program at NASA has been canceled at the time of this report’s publication. A number of potential future missions have been postulated that would use nuclear reactors based on the following conditions:

• High power. Nuclear power is the only practical source of continuous, high power levels in space (more than 100 kilowatts), especially where solar energy is not adequately available. This is due to an economy of scale effect: little size or mass is added as power levels increase.

• Self-sufficiency. Nuclear power sources make the spacecraft more independent of potentially unreliable external solar or chemical heat sources. For example, a nuclear power source can be used for missions to the outer planets where there is not sufficient solar energy for a solar-powered system. Or, for missions on the Martian surface, a nuclear power system would not be affected as much by dust storms that reduce the available sunlight.

• Survivability. Nuclear power sources are generally less vulnerable to external radiation (e.g., the radiation belts around Earth and Jupiter) and to other potentially hostile environments, such as meteoroids, Martian dust storms, space weapons, and extreme temperatures such as those experienced on the lunar surface.

Examples of missions that are considered for nuclear reactor use include human and cargo missions to Mars, human lunar or planetary bases in harsh conditions, electric propulsion missions to the outer planets, and missions to the outer planets with high power science instruments or high information data rates.

Some of these missions combine a reactor power system with a high power electric propulsion system for enhanced deep space travel. The dual mode systems are described briefly in Chapter 4 for a solar thermionic power system, and Appendix D discusses the potentially significant benefits of combining a lightweight power system with emerging electric propulsion technology.

Some future military missions may have a need for nuclear reactor systems. These include high power ra-

dar systems and space-based electric weapons. However, current studies of these types of missions indicate that they can be accomplished with nonnuclear power systems. Most space-based radar concepts being studied use a combination of low orbits and low duty cycles to reduce the continuous power level required by the vehicle to between 4 and 30 kilowatts. This requirement can currently be met by state-of-the-art solar power systems.

None of the approved NASA far-term missions seem to require power that cannot be provided by solar arrays or advanced radioisotope power sources. The exception is the establishment of a lunar base and a human mission to Mars, mentioned above, which have not been approved but are being considered.

Rationalizing the development of nuclear power supplies for spacecraft is difficult, especially in the near term, because of the absence of current missions and the effects of other factors associated with nuclear development, such as cost, development risk, and potential nuclear safety issues. These risks (or perceived risks) have halted the development of space-based nuclear reactors. However, most studies that explore the concept of space bases and their power requirements assume the future availability of nuclear power.

The advanced conversion technologies that are currently being pursued by various government agencies, as discussed in other sections of this report, are aimed primarily at solar energy and isotope heat sources with heat to electric conversion via:

• Free piston Stirling engine,

• Brayton engine,

• Alkali metal thermal to electric converter (AMTEC),

• Thermophotovoltaics, and

• Advanced thermoelectric generators.

Should a permanent human mission to the Moon or Mars be authorized and a nuclear heat source for the power be selected, thermionic conversion might well be able to compete with these conversion technologies. While NASA and DOE are sponsoring development work on the other conversion systems, the DTRA program is the only U.S. program working on thermionic converters in relation to space nuclear power. The committee believes that the research on thermionic converters should continue so that there will be a technology base on which future nuclear power reactor system development programs can draw.

Finding: The current thermionic research and development program sponsored by the DTRA is the only thermionics work being conducted today in the United States related to space nuclear power. However, the program does not include efforts to address nuclear issues related to incorporating the technology into a reactor system, namely radiation-induced fuel swelling or radiation damage to converter materials for use with nuclear in-core systems.

While the DTRA program is important because it is the only funded effort in this area, there are limits to what the program can realistically accomplish given the relatively meager funding. Also, if the sponsoring agency follows this committee’s recommendation to establish a U.S. thermionic research program that focuses on near-term solar thermionic applications, the program will be limited in its ability to achieve a nuclear thermionic capability.

Previous thermionic technology programs have identified as a problem the fuel swelling that occurs over time in an in-core thermionic fuel element. Not only is DTRA not examining fuel swelling effects, but there is also no materials or device testing being conducted in nuclear environments, presumably due to cost and the lack of fast-flux test facility availability in the United States. The absence of nuclear in-core testing could invalidate any nuclear thermionic design that is developed by the current program.

Degaltsev, Yu.G., et al. 1987. “Behavior of High Temperature Nuclear Fuel Under Irradiation,” Atomizdat.

NRC (National Research Council). 1998. Space Technology for the New Century. National Academy Press, Washington, D.C.

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

Streckert, Holger H., et al. 2000a. “Development and Testing of Conductively Coupled Multi-Cell TFE Components,” American Institute of Physics Conference Program 504, Space Technology and Applications International Forum-2000, Albuquerque, N.Mex., Jan. 30-Feb. 3.

Streckert, Holger H., et al. 2000b. “Development of a Cylindrical Inverted Thermionic Converter for Solar Power Systems,” American Institute of Physics Conference Program 504, Space Technology and Applications International Forum-2000, Albuquerque, N.Mex., Jan. 30-Feb. 3.