Patent Number: 042279674
Section: summary

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention generally relates to nuclear reactors and, in particular, to devices for moving control rods with respect to reactor cores. 2. Description of the Prior Art One of the most serious accidents that can occur to a nuclear power plant is a loss of coolant followed by the failure of the control system to accomplish a rapid shutdown of the reactor. A loss of coolant flow can occur from either the rupture of piping or the stoppage of one or more of the coolant circulating pumps. This type of accident is especially serious because the heat generated in the reactor cannot be carried off. If the reactor continues to generate heat, then tremendous pressures are built up in the coolant system. In addition, this heat generation, if it is not terminated by a scram, could melt down a majority of the core of the reactor. In the reactors using liquid sodium for primary coolant, there is a special problem caused by a partial or total loss of sodium flow if a reactor scram does not follow promptly. In the present design of liquid metal fast breeder reactors there is a gain in reactivity called a positive sodium void coefficient that occurs when sodium flow is interrupted. The sodium temperature may increase to its boiling point, whereupon "voids" of sodium vapor are formed, resulting in increased reactivity, power, more boiling, and the possibility of serious consequences. This gain in reactivity occurs because although the neutron absorption effect of sodium is small, it is not zero. Any loss of of sodium from the core causes a shift in the neutron absorption spectrum and increases the number of neutrons. This shift, in turn, increases the probability of neutron capture by the fissionable atoms in the fuel. Although the reactor control systems and scram circuits in use today are quite reliable, there is a continuing effort in the nuclear reactor industry to make reactor control systems absolutely foolproof. No one in this industry is entirely satisfied with the present state of the art. There is a continuous emphasis on making reactor shutdown systems as reliable as possible. These efforts include minimizing the number of moving mechanical parts and designing the components to be independent of both electrical failures and sensor failures. SUMMARY OF THE INVENTION The primary object of the present invention is to overcome the limitations and disadvantages of the prior art. An additional object of the present invention is to prevent damage to a reactor and its surroundings during a temperature excursion and especially during a loss of flow accident. A further object of the present invention is to develop a simple system for reducing the reactor power level that has a minimum number of moving mechanical parts and is independent of both electrical power failure and sensor failure. Another object of the present invention is to reduce the power level in a nuclear reactor in a manner that is directly proportional to the increase in temperature of the primary coolant. Still another object of the present invention is to provide a system that produces a negative temperature coefficient of reactivity in a liquid metal fast breeder reactor. Such reactor typically has a temperature coefficient of reactivity that is nearly zero or positive. In addition, a further object of the present invention is to reduce the power level in a nuclear reactor after a loss of flow accident to less than 15 percent of full power without the intervention of any sensing devices or control rod actuators. These and other objects are achieved by an apparatus for reducing the power level in a nuclear reactor that includes a neutron absorber and a thermally elongatable member. The elongatable member is adapted for mounting on a rod drive shaft or on the vessel containing the reactor core. When so attached the elongatable member moves the absorber relative to the core and is sensitive to the temperature of the primary coolant. The elongatable member moves the absorber by its thermal elongation into closer proximity with the core as the temperature of the primary coolant increases. The elongatable member also includes means for amplifying its thermal elongation so that the motion of the absorber into the core is amplified. Additional objects and features of the invention will appear from the foregoing description in which the preferred embodiments have been set forth in detail in conjunction with the accompanying drawings.