US 3161786 A
Description (OCR text may contain errors)
Dec. 15, 1964 R. GUNTHER SYSTEM FOR THE DIRECT PRODUCTION OF ELECTRICITY IN ATOMIC REACTORS Filed March 20, 1961 INVENTOR w?,Z M ffd/ ATT NEY6 United States Patent 3 Claims. (or. 310-4 The invention relates to a system for the direct production of electricity in atomic reactors.
Arrangements are known for converting energy pro duced by fission of atomic materials directly into electricity, according to the thermionic conversion process. This method depends on the eifect of electron vaporization from heated surfaces. A heated electrode, the emitter, is located in a vacuumopposite a cold electrode, the collector. Electrons vaporize from the emitter wtih consumption of heat, pass to the collector and continuously charge itnegatively. Thus a voltage of the order of 1 volt causes a currentbetween the emitter and the collector, which, in an outer electric circuit, can perform Work. This electron transfer can be prevented by the space charge efiect. In order to compensate for the space charge, positive ions, such as caesium ions, can be introduced into the space between the emitter and the collector.
The special advantage of the thermionic conversion process of obtaining electricity from atomic energy, aside from its simplicity, lies in the fact that higher efficiency can be attained than with a reactor which operates with the conventional thermal power machines. But, for this, it is necessary, according to the known laws of thermodynamics, to maintain the emitter at a high temperature while the temperature of the collector is kept as low as possible.
The fulfillment of these technical requirements, because of the conduction of current from the hot emitter, leads to difficulties since each current conductor also conducts off heat. Hereby, the ratio of the harmful conduction of heat to the desired electrical conductivity increases with the operating temperature of the emitter according to the Wiedemann-Franz law, so that, with increase in temperature, the theoretically expected increase in the efiiciency of the thermionic reactor is nullified by the heat fiOWing along the current conductor into the side circuit.
It is likewise important to have the heat source, that is, the fissionable material such as uranium, in as close contact as possible with the emitting surface. For obtaining good efliciency, it is further desirable to keep the density of the emitted electron stream, at the available voltage of about 1 volt, of the order of a few amperes per square centimeter. In order to keep the internal voltage drop of the ararngement within reasonable limits, the length of the current path through the highly heated (and therefore highly resistant) portion of the apparatus should be kept as low as possible.
The primary object of the present invention is to provide a system which avoids the disadvantages discussed above, thus increasing substantially the eificiency of a thermionic reactor.
In general, theinvention contemplates a base carrier in contact with the emitter for holding fissionable material, this base beingformed of a material which has good electrical conductivity at high temperatures and serving as a current conductor. The base is provided on the side opposite the transmitter with current conductors, which may be formed of stacks of pieces separated transversely to i 4, the direction of current flow, the layers of the stacks which may, for example, be U0 uranium dioxide.
rial of the best possible electrical conductivity and the lowest possible thermal conductivity.
Further objects and advantages of the invention will appear more fully from the following description especially when taken in conjunction with the accompanying drawings, which form a part thereof.
FIG. 1 shows, in vertical section, a reactor embodying the invention on the line 11 of FIG. 2; and
FIG. 2 is a horizontal section through the device of FIG. 1.
The emitter 1, which can be of tungsten or zirconium carbide, is secured on a base carrier 2 of graphite, molybdenum 92 or tungsten 184. These substances have high melting points and low neutron absorption cross-section as well as a relatively good electrical conductivity at high temperatures. The base 2 is provided with openings for the radio-active fissionable material (atomic fuel) 3, which The crosspieces between the bodies 3 serve to conduct the electric current from the emitter 1 to the conductor 5. The conductors 5, as shown in FIG. 2, lie directly below the cross-pieces, so that the current path through the hot carrier 2 is kept short.
Each conductor is formed of several layers divided transversely to the direction of current flow. In 'the drawings, three layers, 6, 7 and 8, are shown. The layer 6 next to the hot base 2 is of a material which is quite heat-resistant, for instance one of those named above for the base 2, such as, graphite. For the materials of the other layers 7 and 8, because of their lower operating temperature, substances of high melting and boiling points are not necessarily needed. They can therefore be of a material which is desirable because of a satisfactory Wiedemann-Franz number. The material of layer 7, for example, can be tungsten and that of layer 8, molybdenum. In this way, by suitable choice of the layer thick nesses and materials at particular temperatures the smallest possible ratio of thermal to electrical conductivity of the whole conductor 5 can be attained. It is advantageous to avoid the use of isotopes with unfavorable neutron-physical characteristics, especially those with large capture cross-section. These matters should be considered in the choice of materials for the individual current conductor layers.
The conductors 5 are connected with a plate 9, for example, of aluminum, which has, on the surface facing the base 2, a high optical reflection. Thus heat loss from the hot base is reduced. The surface can be of polished metal, such as aluminum, or of a white material such as magnesium oxide. Because of its substantially lower operating temperature, its current conducting crosssection can be substantially less than that of the conductor 5. It can be joined directly to the lead wire 10.
The emission plate 1 is arranged opposite the collector plate 12, for example, of aluminum. This latter and the base 2 are enclosed in an annular ceramic body 13 of generally C-shaped cross-section and form with it a vacuum housing. The conductor 14 of the collector 12. may, if desired, also be formed of layers of different materials and constructed as thermal elements which also generate current. The different layers of conductor 14 can for example be p-conducting and n-conducting bismuth-telluride. The conduction to lead 15 is by plate 16. The additional therm'o-electric current generation is present if for some reason a temperature drop from the collector plate 12 to the plate 16, for example of aluminum, must exist, for example if the surface of the collector plate is of a material used for oxide cathodes.
Several of these arrangements can be connected in series the other. The spaces between the then superposed to produce a higher voltage, being positioned one above' plates 9 and 16, as Well as the space between plates 12 and 16, can then be filled with a moderator or a flowing cooling medium.
All the plates, especially plates 1 and 12, can be arched or bent.
The thermionic reactor according to the invention has the following advantages:
(1) The electric current produced has to traverse only a short path in a hot material of relatively poor electrical conductivity. This is very important considering that the operating temperatures are above 1000 C. and the current density around 1 to 100 amperes per square centimeter of surface.
(2) Loss through the thermal and electrical conductors, which at high temperatures can reach quite large values, are kept small by the layering of the current conductor of the emitter as Well as by the reflecting surface of plate 9.
(3) The layer construction of the current conductors of the collector, in a Way similar to that of the emitter, also makes it possible to construct these as thermoelements for additional current generation.
While I have described herein some embodiments of my invention, I wish it to be understood that I do not intend to limit myself thereby except within the scope of the claims hereto or hereinafter appended.
I claim: I
l. A thermionic reactor comprising an emitting electrode heated by fission and a cold collecting electrode opposite to and electrically insulated from the emitting electrode, a base of a material which is a good conductor of electricity at high temperatures enclosing radio-active fissionable material and in electrical contact with the emitting electrode on the side thereof opposite from the collecting electrodes, a current conductor in contact With the side of the base opposite to the emitting electrode, said current conductor being formed of at least two contacting layers separated transversely to the direction of current flow, the layer next to the base being formed of a material highly resistant to heat and the other layer of a less heat-resistant material having a higher ratio of electrical to thermal conductivity than the layer next to the base.
2. In a device as claimed in claim 1, a plate of conductive material engaging the layer remote from the base, said plate having a reflective surface directed towards the base.
3. A device as claimed in claim 1, consisting of materials purified from atomic nuclei with high neutron capture cross sections by isotopic separation.
References Cited by the Examiner UNITED STATES PATENTS 2,527,945 10/ 50 Linder 310-3 2,552,050 5/51 Linder 3l03 2,728,867 12/55 Wilson 3103 2,859,361 11/58 Coleman 3103 3,005,766 10/61 Bartnofi? 3103 X FOREIGN PATENTS 37,5 38 5/ 27 Denmark. 797,872 7/58 Great Britain.
CARL D. QUARFORTH, Primary Examiner.
LEON D. ROSDOL, REUBEN EPSTEIN, Examiners.