1. Field of the Invention
This invention generally relates to control rod assemblies for nuclear reactors and, more particularly, to the operative connections between control rods and control rod drive mechanisms.
2. Description of the Prior Art
One of the most serious accidents that can occur to a nuclear power plant is a loss of the flow 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 the 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 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 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.
Many organizations, government agencies, and corporations have studied the problem of minimizing the probability of the loss of low accident. A considerable design effort has been expended over a priod of many years in order to provide maximum assurance to both the public and the various reactor licensing agencies that this accident can be avoided.
Heretofore, the most reliable and simplest system that has been proposed contemplates hydraulically supporting a plurality of tantalum absorber balls in a column above the reactor core by the flow of primary coolant. In the event coolant flow is reduced, these balls which have a large neutron absorption coefficient fall into the high flux region of the core and quickly shut down the reactor.
Although the use of hydraulically supported, absorber balls provides a self-actuated and reliable reactor shut-down system, there are many inherent limitations with this system. For example, the absorber balls are hydraulically supported by the flow of primary coolant in a position substantially above the high flux region of the core. In this position the balls cannot be maneuvered to regulate the amount of neutron flux during normal operations. Secondly, the absorber balls are only controlled by the flow of primary coolant through the column. Therefore, the balls add reactivity to the system whenever the main coolant pumps are started and primary coolant flow is commenced. Further, the hydraulically supported absorber balls only shut down the reactor when the flow of primary coolant decreases. The balls cannot be commanded to scram a reactor by either the reactor operator or one of the other reactor safety systems. Finally, considerable testing will be necessary to demonstrate the exact response of the tantalum balls under actual reactor conditions of flow and wear.