Energy Production by Nuclear Radiation

Energy production by nuclear fission and by thermonuclear reactions has been discussed in chapter 11. In this section, energy production in radionuclide batteries by the radiation emitted by radionuclides will be considered. 

The principle of thermionic conversion is that of a diode, in which the cathode (emitter) emits electrons that are collected at the anode (collector). Alloys of W, Re, Mo, Ni or Ta are used as emitters, and the diodes operate at temperatures of about 2200 K. The efficiency varies between about 1 and 10%, depending on the power. In order to take advantage of thermionic conversion, radionuclides of high specific power output (high power per mass unit) are needed, such as Pu, Ac, 

In thermophotovoltaic batteries the heat emitted by the radionuclides is converted to electric energy by means of infrared-sensitive photoelements (e.g. Ge diodes), which must be cooled effectively because the efficiency decreases drastically as the temperature rises. With respect to high emitter temperatures, thermophotovoltaic conversion is of interest for power levels between about 10 W and 1 kW, but the efficiency is relatively low (up to about 5%). 

Radiophotovoltaic (photoelectric) radionuclide batteries operate in two stages. First the radiation energy is converted to light by means of luminescent substances and then to electric energy by means of photoelements. Because of radiative decomposition of luminescent substances, the number of radionuclides applicable is limited. Alpha emitters are unsuitable, and the most suitable f emitter is Pm. The construction of this kind of radionuclide batteries is relatively simple radionuclide and luminophore are mixed in a ratio of about 1 1 and brought between two photoelements (e.g. Cu-Se or Ag-Si) in form of a thin layer. Efficiencies of the order of 0.1 to 0.5% and powers of the order of lOpW per cm are obtained. Because of the low efficiency, this type of radionuclide battery has no technical significance. 

In contrast to the methods described in the preceding paragraphs, radiophotovoltaic conversion is a direet method. In a semiconductor the incident radiation generates free charge carriers, that are separated in the n,p-barrier layer of the semiconductor. Suitable radiation sources are radionuclides emitting p particles with energies below the limits of radiation damage in the semiconductors. These limits are about 145 keV for Si and about 350 keV for Ge. Therefore only Pm, Ni and T are suitable as radiation sources. By use of the combination Pm/Si, efficiencies of about 4% are obtained. 

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