Patent Number: 047479981
Section: summary

The invention relates to temperature activated switches, particularly to thermionic switches, and more particularly to a thermally actuated thermionic switch. Temperature responsive switches which involve a change of state condition or expansion of an operative material when temperature is applied thereto are well known in the art. These prior switches are exemplified by U.S. Pat. Nos. 3,791,298 issued Feb. 12, 1974 to P. Amberny, and 3,815,816 issued June 11, 1974 to D. Scarelli. Thermionic devices have been in use for many years as lights, vacuum tubes, power converters, and as electrically driven switches. These prior thermionic devices use control grids, temperature difference between the emitter and collector, or voltage changes to cause their actions. More recently, thermionic devices have been utilized in nuclear reactor control systems, particularly in self-actuated control systems responsive to low-flow, high temperature, or over-power conditions of the reactor. In such control systems, the control rods are rapidly inserted into the reactor core for quick shut down of the reactor. Such self-actuated reactor control systems are, for example, described and claimed in copending U.S. Patent Application Ser. Nos. 270,672 and 270,682, each filed June 4, 1981 in the name of D. M. Barrus et al, and assigned to the assignee of this application. While various thermionic switches have been developed, a need has existed for a simple, yet effective thermionic switch responsive to temperature particularly in the field of reactor control systems. Such an improved switch would find use in numerous applications for temperature control and limiting functions, aside from their use in reactor systems. SUMMARY OF THE INVENTION The instant invention satisfies the above-mentioned need by providing a simply constructed, yet effective thermally actuated thermionic switch. The thermally actuated thermionic switch of this invention is a diode differing from those mentioned above in that it operates in an isothermal condition (not depending on temperature differences between the electrodes as in a power converter), nor does it use a control grid as in a vacuum tube. Therefore, it is an object of this invention to provide a thermionic switch. A further object of the invention is to provide a thermally actuated thermionic switch. Another object of the invention is to provide a thermionic switch which responds electrically to an increase in temperature by changing from a high impedance to a low impedance at a predictable temperature set point. Another object of this invention is to provide a thermally actuated thermionic switch which is particularly suitable for use in nuclear reactor control systems. Another object of the invention is to provide a thermally actuated thermionic switch which can be utilized for temperature control and limiting functions. Other objects of the invention will become readily apparent to those skilled in the art from the following description and accompanying drawings. The above objects are accomplished by a thermally actuated thermionic switch which operates in an isothermal condition, and is responsive to temperatures above a pre-set minimum. The thermionic switch of this invention responds to an increase of temperature by changing from a high impedance to a low impedance. Such thermionic switches are particularly useful in reactor control or shutdown systems, wherein the switch is responsive to the temperature of the coolant flowing through the reactor core. More specifically, the thermally actuated thermionic switch of this invention comprises two electrodes, an emitter and a collector, which are separated mechanically and electrically isolated from one another, and provided with a quantity of thermionic material in matrix in a graphite block reservoir, and an electrical circuit connected to said electrodes, such that heating of said thermionic material causes the switch to trigger for activating a desired mechanism, such as an associated electromagnetic apparatus of a reactor control system.