Patent Number: 039322157
Section: summary

The invention relates to a vertical control rod for shutting off nuclear reactors rapidly and safely. More particularly, the control rod is of the type that is removable from the reactor in an upward direction, and is formed of a flexible absorber guided in a tube, an acceleration spring, a drive mechanism for tensioning the spring, and a holding device for the spring. Flexible absorbers are employed to avoid dangerous jamming of the absorbers in the guide therefor, particularly in the case of very long control rods and are known, for example, from German Pat. Nos. 1,092,572 and 1,100,831. In these and other shut-off devices, the flexible absorbers are suspended from a holding device above the reactor core during the operation of the reactor and are dropped into the reactor core, accelerated by gravity in order to shut off the reactor. Shut-off devices of this kind are very reliable, but the shut-off speed thereof and the forces available for overcoming resistances are limited. Any additional force acting upon the suspended, flexible absorbers from above and exceeding the force of gravity, however, increases the danger of jamming in the guide. It is accordingly an object of the present invention to provide a vertical control rod for rapidly and safely shutting off a nuclear reactor which is removable from the reactor in an upward direction, and has a force available for effecting shut-off which exceeds the force of gravity, yet avoids jamming of the absorber in the guide therefor, even when, in the event of damage, deformation of the guide tube might result. With the foregoing and other objects in view, there is provided, in accordance with the invention, vertical control rod for rapidly and safely shutting off a nuclear reactor, the rod being removable from a reactor in upward direction and comprising an elongated tube, a flexible absorber guided in the tube, spring means operatively connected to the absorber for accelerating vertical displacement thereof, a drive mechanism for tensioning the spring, means for holding the spring, a traction device suspending the absorber below the core of the reactor during operation of the reactor and means for disposing the accelerating spring means and the drive mechanism above the reactor core. The flexible absorber is thereby stressed only in tension and jamming thereof in the guide is thereby avoided. In accordance with a further feature of the invention, the absorber is disconnectible from the traction device therefor, and the drive mechanism above the reactor core, while being simultaneously lockable into its shut-off position. Thus, the entire control rod, except for the absorber part, is exchangeable without impairing the shut-off action. In accordance with another feature of the invention, the absorber is moveable into shut-off position by means of the drive mechanism for tensioning the acceleration spring. While sacrificing shut-off speed, a result of this construction is that the absorber is moved, with greater reliability, into the shut-off position if the acceleration spring should fail. It is also advantageous so to control the drive mechanism that it follows the absorber into the shut-off position immediately after the shut-off is effected. In accordance with yet another feature of the invention, the traction device has a hollow construction and contains a wire rope secured at both ends thereof. This construction affords an additional safety precaution and assures a strong connection between the absorber and the drive mechanism even if the traction device itself should break. In accordance with an additional feature of the invention, the absorber is formed of several members that are movable with respect to each other and are guidable in a guide tube which is traversible by a coolant flow, the lowermost absorber being disposed, during the operation of the reactor in a lower end section of the guide tube, substantially sealing the spacing between the absorber and the inner surface of the guide tube from the outside. This construction has the advantage that, in the event of shut-off, the absorber is sufficiently cooled by a flow of coolant between the absorber and the inner surface of the guide tube but that, during the operation of the reactor, no appreciable or substantial quantities of coolant can flow through the guide tube which would contribute only little to the cooling of the reactor core.