Patent Number: 048805963
Section: summary

The invention relates to control systems for nuclear reactors, and, more particularly, to a self-actuated control system responsive to temperature increase or over-power conditions of a reactor. The use of control systems to regulate the reactivity of a nuclear reactor by varying the location of control (neutron absorber) elements or rods with respect to the reactive core is well known. With a view toward the possibility of an emergency condition arising, as by an unexpected drop in coolant flow, increase in temperature, or rise in reactivity, such control systems include arrangements for "scramming" the control rods, i.e., for rapid insert of the absorber elements into the core to quickly shut down the reactor. With the advent of the liquid metal fast breeder reactor (LMFBR) and the Gas-Cooled Fast Reactor (GCFR), a need for faster, less complex, more reliable control rod scram or shutdown systems has become apparent, whereby the reactivity of the reactor can be quickly shut down. More recent efforts have been directed to the desirability of utilizing secondary or alternate control systems of the self-actuating type which would make an LMFBR or GCFR inherently safe. Such alternate or self-actuating systems provide control without reliance on the primary reactor control system or plant operators, while being capable of actuation by the plant operators. These efforts have resulted in systems which sense the reactor flow rate and actuate when the flow drops below a predetermined level, measure the temperature of the coolant and actuate when the temperature exceeds a specified point, or measure the flux or reactivity level of the reactor and actuate when the reactivity exceeds a specified level. The following exemplifies various operator-actuated and/or self-actuated prior art control systems. U.S. Pat. No. 4,158,602 issued June 19, 1979, to L. E. Minnick discloses a self-actuating scram system triggered by a loss of primary coolant flow which supports the absorber rods above the reactor core region. A loss of primary coolant flow causes a decrease in the supporting pressure on the absorber rods allowing the rods to fall into the core region, thus scramming the reactor. U.S. Pat. No. 3,359,172 issued Dec. 19, 1967, to C. S. Olsson discloses a reactor shutdown system employing an electromagnet-operated valve to terminate coolant flow. Absorber rods, normally suspended above the core, will fall into the core region upon loss of coolant flow. U.S. Pat. No. 2,931,763 issued Apr. 6, 1960, to J. A. Dever discloses a control apparatus incorporating electromagnetically held control rods. The control rods are released upon a signal initiated within an ionization chamber. Electron tubes conduct sufficient current to retain the control rods as long as the neutron flux remains below a predetermined level. U.S. Pat. No. 2,781,308 issued Feb. 12, 1957, to E. C. Creutz et al discloses a neutronic reactor control system in which voltage produced by an ionization chamber effects release of absorber rods from an electromagnetic latching mechanism. U.S. Pat. No. 3,940,309 issued Feb. 24, 1976, to F. Imperiali discloses a self-actuated scram system utilizing electromagnetic means to suspend and release absorber material into the reactor core region. U.S. Pat. No. 2,867,727 issued Jan. 6, 1959, to H. Welker et al discloses a neutron-sensing device in which neutrons, penetrating a semiconductor, create electron-hole pairs which produce a voltage which can be monitored. U.S. Pat. No. 4,085,004 issued Apr. 18, 1978, to J. c. Fletcher et al discloses a control device for a nuclear thermionic power source. Actual neutron flux is compared to a linear function of current supplied by a thermionic converter. U.S. Pat. No. 3,970,007 issued July 20, 1976, to J. R. Klein discloses a neutron detection device utilizing uranium hydride as a neutron sensor. Radiation causes the uranium hydride to fission, releasing heat and hydrogen gas. The gas pressure breaks a normally closed circuit causing activation of a safety device. U.S. Pat. No. 3,177,124 issued Apr. 6, 1965, to D. T. Eggen et al discloses a reactor control device triggered by the melting of a solder joint. Upon experiencing an increase in neutron flux a layer of uranium abutting the solder joint begins to heat the joint until it melts, releasing absorber material. U.S. Pat. No. 2,904,487 issued Sept. 15, 1959, to J. J. Dickson discloses a reactor control system employing a temperature responsive transducer actuated by heat generated from a uranium strip. Neutron flux causes the uranium strip to fission and heat a bimetallic transducer which generates an automatic control signal. Thus, while various approaches have been developed for reactor control, a need still exists for a simple self-actuated control system which is failsafe, reliable, testable in the core at shutdown, resettable and capable of actuating upon sensing either the initiation of a transient coolant temperature increase event or a transient over-power (increased reactivity) event, as well as being capable of actuation by plant operators. The above-cited art fulfills certain of these requirements in various ways, but involves complex apparatus and is not fully responsive to both or either of these reactor conditions by use of simple control apparatus. RELATED APPLICATION The present invention is in the same general field of art as U.S. application Ser. No. 270,672, filed June 4, 1981, and assigned to the assignee of this application. SUMMARY OF THE INVENTION It is an object of the present invention to provide a self-actuated control system for nuclear reactors. It is a further object of the invention to provide a self-actuated shutdown system for a reactor which is responsive to coolant temperature increase and/or over-power (increased reactivity) conditions of the reactor. A further object of the invention is to provide a self-actuating reactor shutdown system, particularly applicable for liquid metal cooled fast breeder reactors (LMFBR) and gas-cooled fast reactors (GCFR). Another object of the invention is to provide a self-actuated shutdown system for a reactor which utilizes a thermionic switched electromagnetic latch arrangement responsive to reactor neutron flux changes and to reactor coolant temperature changes. Another object of the invention is to provide a thermionic switched, electromagnetic latched self-actuating reactor shutdown system which utilizes a thermionic diode for actuating the electromagnetic latch for releasing absorber elements into a reactor core. The self-actuating shutdown system (SASS) of the present invention, which utilizes a thermionic sensing device, acts directly to cause release (scram) of the control rod (absorber element) without a reference or signal from the main reactor plant protection and control systems. The thermionic trigger or switch acts in conjunction with, but independent of, the plant control and protective system and therefore provides separate and redundant reactor shutdown capability for selected off-normal conditions. To optimize both the temperature and neutron flux effects, the invention utilizes two separate detectors which are tailored to their specialized positions and functions, as follows: 1. Self-actuation in response to a temperature increase of the reactor coolant occurs by heating of a thermionic sensor to a selected set trigger point by the coolant as it emerges from fuel assemblies adjacent to the SASS and impinges on the sensor mounted above the reactor fuel assemblies. As the reactor coolant temperature increases, the temperature of the thermionic sensor is raised to a point where it conducts current (changes from a high impedance to a very low impedance) generating the signal used for shutdown. 2. Self-actuation in response to reactor neutron flux increase is achieved by placing the thermionic sensor near the reactor core or flux region. The thermionic sensor is made responsive to the reactor flux by the attachment of uranium, or other material which heats from neutron bombardment, to its emitter or by enclosing the emitter of the sensor inside a blanket of these materials. When the reactor neutron flux is increased, the uranium or other heating material responds by heating the thermionic sensor to the selected trigger point where it conducts current generating a shutdown signal. The present invention broadly encompasses a self-actuated reactor shutdown system wherein an electromagnetically actuated latch mechanism retains the control rod (neutron absorber element) in a ready or cocked position exterior of the reactor core region, and upon an increase in coolant temperature beyond a selected point and/or upon an increase in neutron flux (over-power) beyond a selected point, a thermionic device connected electrically to the electromagnetic latch mechanism is heated so as to conduct current which effects a short-circuit of the electromagnet causing same to lose holding power which releases the control rod to drop by gravitational force into the reactor core causing shutdown of the reactor. The thermionic device may, for example, constitute a thermionic diode connected electrically in parallel with the electromagnet.