FIGURE 3.2 A cross sectional view of a thermionic fuel element (TFE). The nuclear fuel is located in the middle of the TFE and radiates heat outward.

SOURCE: Oregon State University, Nuclear Engineering and Radiation Health Physics, Department of Nuclear Engineering.

sources for use in space as well as for high temperature terrestrial power systems that generate large amounts of excess heat, such as coal or natural gas fired power plants. The United States, France, Germany, Sweden, Holland, and Russia have pursued thermionic research and development since the early 1960s. More recently China has begun investigating thermionic conversion technologies. Research is still ongoing in Russia, Sweden, and China. The committee does not know the current status of thermionics research in France, Germany, or Holland.

Efficient operation of a thermionic converter requires an emitter surface with a relatively low work function of 3 electron volts or less and an even lower collector work function of 1.5 electron volts or less.1 The converter must also have the right charge transport conditions in the interelectrode gap, the area between the emitter and collector surfaces, to allow electrons to flow from the emitter to the collector.

Historically, emitters with low work functions were made for vacuum tubes by coating a metallic surface with compounds such as barium-calcium-strontium oxide. Such a technique proved to be impractical for thermionic converters because these compounds evaporated, limiting converter life to a few hundred hours.

Another problem relating to the operation of a practical thermionic converter is that the emitted electrons passing through the interelectrode gap create a negative voltage barrier in the gap. When electrons leave the emitter surface, they sense the negative space charge in the interelectrode gap and are forced to return to the emitter surface. In this way, the flow of current is obstructed. The negative space charge of the electron gas in the interelectrode gap must be suppressed to allow sufficient current to flow.

One solution to the negative space charge problem is to make the interelectrode gap small enough to be less than the mean free path of the emitted electrons. One type of thermionic device, the vacuum converter, obtains practical output currents by reducing the interelectrode spacing to a few micrometers (~5). The gap is small enough so that the electrons do not have the opportunity to collide with one another or to accumulate in the gap where they can create a space charge. The electrons are absorbed by the collector before this can happen.

The principal technical challenge in making such devices is maintaining the extremely close spacing required. A mechanically stable electrode support structure also conducts too much heat from the emitter.

1  

The work function of a material is defined as the amount of energy required for an electron with a certain energy to overcome and escape the binding attractive charge, or surface potential, of a material’s surface.



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