Patent Number: 047708450
Section: summary

The invention relates to control systems for nuclear reactors, particularly to a control system for a liquid metal cooled reactor, and more particularly to a self-actuated control system responsive to low-flow or over-power conditions of the reactor. The use of control systems to regulate the reactivity of a nuclear reactor by varying the location of control (neutron absorber) elements 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 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), 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 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, or measure the neutron 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. 3,462,345 issued Aug. 19, 1969, to F. S. Jabsen discloses a reactor control system utilizing coolant pressure to support control rods above the reactor core region. Upon loss of coolant flow, the control rods fall under the influence of gravity into the core region effecting a self-actuated scram. A scram may also be initiated by de-energizing an electromagnet coupled to a valve disposed in the coolant flow line. Actuation of the valve cuts off coolant flow, terminating the support pressure on the control rod, thus allowing the rod to fall under the influence of gravity into the core region. U.S. Pat. No. 4,187,145 issued Feb. 5, 1980, to R. C. Noyes et al discloses a scram release system in which control rods are supported by coolant pressure acting on a hydraulic latch plug. A reduction in coolant flow rate causes a reduction in pressure across the plug, resulting in release of absorber material into the core under the influence of gravity as well as under the influence of high-pressure fluid applied to the top of the absorber material by a conduit opened by the release of the plug. U.S. Pat. No. 4,138,320 issued Feb. 6, 1979, to A. L. Grantz discloses a fluidic control module which senses reactor coolant flow rate and/or flux changes and varies the flow rate accordingly. The flow of coolant supports absorber materials, and when coolant flow is restricted the absorber material falls into the reactor core region. In addition, a flux sensor is employed to cause the thermal expansion of a metallic orifice upon the sensing of an increase in neutron flux. Expansion of the metallic orifice triggers a scram release mechanism. 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. Thus, while various approaches have been developed for reactor control, a need still exists for a 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 undercooling (loss of flow) event, or a transient over-power (increased reactivity) event, as well as being capable of response to reactor coolant over-temperature and actuation by plant operators. RELATED APPLICATION The present invention is in the same general field of art as U.S. application Ser. No. 270,682, 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-actuating reactor shutdown system responsive to loss of reactor coolant flow or high-neutron flux. Another object of the invention is to provide a reactor shutdown system which utilizes pressure differential of reactor coolant across a neutron absorber element and/or an electromagnetic valve actuated by a thermionic diode in response to neutron-flux level. Other objects of the invention will become apparent from the description of the invention and the accompanying drawings illustrating the invention. The present invention involves a hydrostatic supported absorber/self-actuating reactor shutdown system which is particularly applicable for a liquid metal fast breeder reactor, but is not limited to this specific type of reactor. The system is capable of initiating scram insertion of an absorber element into the reactor core by a signal from the plant protection system, or by independent action by directly sensing reactor conditions of low flow or over-power. By these means, this system achieves a degree of independence from reactor operational instrumentation and control systems. More specifically, upon loss or reduction of reactor coolant flow, a decrease in pressure differential across the neutron absorber element which produces a force less than the weight of the absorber element, allows the element to begin to drop. Equalization of the pressures across the element allows the element to fall under full gravitational force. The reduction in coolant flow for actuating the absorber element may result from an undesirably high neutron flux, or coolant over-temperature, which actuates a thermionic diode to control an electromagnetically actuated valve which, in turn, controls the coolant flow across the absorber element. The thermionic diode, which is uranium blanketed, changes state (becomes conductive) as the uranium blanket is heated by the high-neutron flux or by high coolant temperature, causing the electromagnet to be shorted; and the valve element, formerly held in open condition by magnetic attraction, closes off the flow of coolant across the absorber element. It is thus seen that the invention in its broadest sense involves a shutdown system for a nuclear reactor, including means for retaining neutron absorbing material above a core region of the reactor, the retaining means being responsive to a pressure differential created by the coolant flow across the neutron absorbing material, and means responsive to neutron flux or coolant temperature for controlling the coolant flow.