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[Editor’s Note: The author recently retired from Ingalls Ship building and is carrying forward his studies of thermionics at the University of Tennessee in Knoxville. For more information, he can be contacted via E-mail at]


As we move towards construction of the next submarine class, the New Attack Submarine, it is important to begin considering concepts to incorporate in the submarine after next. Nuclear thermionic propulsion offers elimination of steam plant components, and the associated weight reduction and space benefits can revolutionize modem submarine design.

A nuclear reactor utilizing thermionic fuel elements is capable of producing electrical energy with no turbine generator machinery-free from vibration, noise and wear of moving parts. All that a thermionic reactor requires is space for the reactor, conduction of waste heat, and electrical components to modify and transmit the electricity for the drive motor and ship’s hotel loads. The thermionic fuel elements which produce the electric power are integral to the reactor itself. Without the steam plant, and with the increased reliability of thermionic power, backup emergency systems for propulsion and electrical power can be reduced in size or even eliminated altogether.

The History of Thermionics

In the early 1880s, Thomas Edison encountered a serious problem. His light bulbs burned out prematurely. Quite naturally, short-lived light bulbs disturbed the customers of his fledgling electric company. In the process of solving this problem, he made a significant ancillary discovery, disclosed in this 1884 patent.

“I have discovered that if a conducting substance is inter-posed anywhere in the vacuous space within the globe of an incandescent lamp, and said conducting substance is connected outside the lamp with one terminal, preferably the positive one, a portion of the current will, when the lamp is in operation, pass through the shunt circuit thus formed … This current I have found to be proportional to the degree of incandescence of the conductor, or the candle power of the lamp.”
Edison US Parent 307,301

Edison patented bis newly discovered effect as an “electrical indicator” for detecting and regulating voltage fluctuations in various parts of his electrical distribution system. Twenty-one years later John Ambrose Fleming used the effect to create the radio tube, thus ushering in the electronics age. Fleming’s vacuum electron tube replaced a solid state “coherer” for the detection and amplification of radio waves. By the 1950s the electron tube was rendered obsolete by another solid state device, the transistor.

In the early 1940s, while electron tubes were in wide general use and readily available, Winston Caldwell, Sr. of Nashville, Tennessee, began investigating the use of the heat-driven thermionic emission of electron tubes for electrical power generation. On Sunday, August 9, 1942, he noted in his yearbook: “Made successful test showing that by superimposing a 330 volt DC current on a #80 radio tube a direct conversion of heat to electricity was made.” Of course, he was repeating Thomas Edison’s old experiment, but be saw in the experiment something more than an electrical indicator. He envisioned a new basic source of electrical power.

Winston Caldwell’s study of radio tubes was to provide the answer to a quest be began while studying electrical engineering at Vanderbilt University in 1905. From his early studies he was convinced that there must be a simpler way to generate electricity than building huge dams on the rivers or boiling water to make steam to spin turbines, to tum magnets to move charges, and finally to make electricity flow. Although he did not pursue an engineering career, he experimented with electricity throughout his life. His quest was a long one. He was 70 years old when he was awarded US Patent 2,759,112 for bis Electron Tube Thermoelectric Generator issued August 14, 1956.

During the period 1953-1956, while his patent application was pending, Caldwell solicited the assistance of General Electric Company in obtaining a gas or vapor filled electron tube with close cathode-to-anode spacing for his experiments. In May 1956 Dr. V.C. Wilson at GE’s Schenectady Laboratory began experiments with filament-type electron tubes containing cesium vapor. Cesium vapor diodes proved to be the ideal candidate for thermionic converter development. The Caldwell and Wilson inventions paved the way for extensive thermionic converter research that followed in the period from 1960 to 1973.

While Caldwell and Wilson were investigating gaseous diodes, Dr. George N. Hatsopoulos at MIT applied for a patent for a device that accomplishes thermionic power generation by the magnetic triode concept. Dr. Hatsopoulos founded Thermo Electron Corporation which became a principal investigator of cesium thermionic converters under government R&D contracts. Other principal contractors were General Electric Company, General Atomic_ Division of General Dynamics Corporation and ·RCA. From 1960 forward, NASA and the Atomic Energy Commission (AEC) (predecessor to the Department of Energy) pursued the development of thermionic converters for space nuclear power.

The nuclear thermionic reactor program made continued progress. As early as 1964, the AEC reported that General Atomic had reproducibly accomplished the continuous generation of over 75 watts of electrical power with small cylindrical thermionic cells only one inch long and five-eighths inch in diameter. The thermionic reactors under development by the AEC were being designed for operation in the 100 to 300 kilowatt range. The planned reactors were physically quite small-on the order of three feet tall by two feet in diameter.

In 1970 the AEC-NASA nuclear thermionic reactor program showed dramatic progress, but the U.S. had by this time landed men on the moon and effectively won the space war with Russia. NASA had placed a thermoelectric (i.e., working on thermocouple rather than thermionic principles) nuclear generator on the moon. Although the moon device only produced 65 watts of electricity, rather than up to 300,000 watts expected from a thermionic reactor of the same size, it met NASA’s radio and TV signal power requirements. Because NASA had no foreseeable near term missions that required the amount of power that thermionic reactors would provide, the thermionics program was put on the chopping block.

The thermionic program’s demise was detailed in hearings before the Joint Committee on Atomic Energy, Congress of the United States, March 20 and 22, 1973: Chairman Price: Now under this cutback, is thermionics out entirely?

Mr. Gabriel: Yes, sir; if no additional funding is provided our work on thermionic conversion will be terminated by the end of June this year.

Chairman Price: As director of this program, do you personally feel that there are commercial and even military applications for thermionic conversation that would compel us to continue in this area?

Mr. Gabriel: Mr. Chairman, I am not aware of any military requirements for power plants of this size …

JCAE Hearings 3120173, page 2394

In the period 1960 to 1973, more than $100,000,000 was expended on government sponsored thermionic conversion research. Fortunately for modem researchers, the extensive technical reports and data produced in the course of the research provide an excellent database from which to move forward toward practical thermionic applications. Over 100 now-expired U.S. and foreign patents that followed the Winston Caldwell patent add to that base of knowledge.

Back to the Future

The problems inhibiting thermionic converter development up to now are essentially practical ones. Dr. Robert W. Pidd’s 1965 testimony is particularly enlightening

Dr. Pidd: Thermionics is not all that tough. The first device we put together eight years ago produced over five watts per square centimeter. Three years later, we understood why we were getting it. The fact is, we didn’t struggle to get that. It happened when we turned it on. The thermionic device is a very practical system. It is self-stable. You don’t have to fight to make it work

Chairman Holifield: What is your problem-metallurgy?

Dr. Pidd: No. Certainly, when we started, the basic problem

was materials because reactors had not operated at 1, 800 centigrade before. The closest precedents we had were the Rover reactor and the HTGR (High Temperature Gas Reactor).

Since then, we have had enough radiation data to operate at 1800 centigrade for thousands of hours and we have got full radiation now for a 50 kilowat system. I am willing to say now that the temperature materials problems at that power level are over, at 50 electrical [kilo]watts. We certainly need much more data for 500 and 1,000.

Chairman Holifield: Tell me how you construct these thermionic cells?

Dr. Pidd: The way we construct them is as follows … We make a cell which is-first of all, you want a hot surface. We simply make that identical with the fuel element. That is a tungsten cup and we put uranium carbide in it. You have the fissioning material, the source of beat and the thing that wants to get bot all in one. That boils off the current.

Dr. Tape: Give the approximate dimensions

Dr Pidd: It turns out that the practical dimensions are that it will be greater or no less than two inches long.

Chairman Holifield: Each one?… Then you would have thousands of those.

Dr. Pidd: It depends on the system. For a 50 kilowatt system, 180 … For as megawatt system, 54,000.

I was completing the construction of the cell for you. It is this bot fuel element, two inches long, about a half inch in diameter. You surround it, very closely spaced, with a collector to collect the current and that is it.

On our test cells in the laboratory today, we are at 7,000 hours .. In our in-pile test, we are at 2,000 hours … We have not encountered any fuel trouble yet. In fact, that is why I am willing to say that the fuel problem is over. Most of our trouble now is making equipment last that long. Most laboratory equipment does not last more that 1,000 hours. We are having trouble with our environment. We have to purify that more.

Chairman Holifield: What do you mean, purifying the environment? Operating in a vacuum?

Dr. Pidd: Preferably in a vacuum or in a highly purified gas … I had to bring up such mundane things, sir. we need better pumps and they cost $200 apiece.

JCAE Hearings, August 6, 1965

The Importance of Inventiveness

In the 1960s thermionic research, rather than taking the free ranging approach of Edison, researchers adopted a theory of thermionic conversion which precluded full exploration of potential thermionic converter materials. In contrast to the many cases in which scientific theory has led to the development of new and better products, thermionic converter development has been inhibited by the adopted scientific theory.

Because of the strictures of prior radio tube theory, particularly Richardson’s 1901 equation as modified by Dushman in 1923, it has been assumed that the voltage output of thermionic converters derives from the difference in work function of the cathode and the anode materials. Accordingly only high work function materials, such as bare molybdenum or tungsten, have been used for the cathode, even though the high work function reduces the flow of electrical current in the converter.

In the 1960s thermionic converter programs, we were back to Edison’s problem of keeping electricity flowing in the sometimes unfriendly space of an evacuated enclosure. The inventive mind of Edison kept working at his problem by experimenting with thousands of potential materials until he found a reliable carbon filament derived from a particular type of bamboo. No scientific theory would ever have led Edison to bamboo to solve the problem of the light bulb.

The Politics of Energy

Another problem that may have inhibited development of thermionic conversion in the 1970s is the energy politics. In the mid 1960s Gulf Oil Company bought the General Atomic Division of General Dynamics Corporation and became the major competitor to General Electric Company on the thermionic reactor program. In 1970 the AEC decided to select a single prime contractor for the program. Gulf General Atomic won the competition, and (as detailed in the 1973 hearings of the Joint Committee on Atomic Energy) soon thereafter brought the program to an end.

From Russia, Thermionics for Sale

In spite of the obstacles of oil politics and limiting theory, thermionic power generation research did not die. Throughout the Cold War, there was cooperation between U.S. and Russian scientists in many areas of research, including thermionics. Our avowed enemy used U.S. thermionic research to carry the work forward in the Topaz thermionic reactor project.

In post Cold War technology exchange the Defense Department acquired a Russian built Topaz II thermionic reactor for test and evaluation. The September 1993 issue of Mechanical Engineering Magazine reported on four U.S. thermionic programs: the Thermionic Fuel Element Verification Program (TFEVP), the Advanced Thermionic Initiative (A Tl), the Thermionic System Evaluation Test (TSET), and the Thermionic Space Nuclear Power System Design and Technical Demonstration Program.

Currently, the demise of Star Wars missile defense programs and other continuing cuts in space and defense spending again leave this high powered, mighty midget wanting for a space limited customer that needs the power that it can deliver. When the Navy is ready for a silent generator that needs only heat-no turbines, no spinning dynamos, no moving parts-thermionic conversion is ready and waiting.


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