Patent Number: 039792579
Section: summary

BACKGROUND OF THE INVENTION The invention relates to a boiling-water reactor comprising a reactor pressure vessel which encloses the reactor core and contains a steam space containing water separators, above the reactor core, and having control rods arranged so they can be adjusted within the reactor core by a drive located outside of the reactor pressure vessel. In prior art boiling-water reactors, the control rods have been run into the pressure vessel by special drives positioned below the vessel with the control rods requiring to be lifted against the force of gravity. These control rods position neutron-absorbing elements within the core to control its fission. Such drives which must move the control rods against the force of gravity, must be very reliable and quick-acting because of the possibility that it may be necessary to scram the reactor. This makes such drives undesirably expensive. The object of the present invention is to provide a better solution for the control of the core of a boiling-water reactor. SUMMARY OF THE INVENTION According to the invention, this is done by positioning the drive above the reactor pressure vessel and providing a control rod guide arrangement which goes down through the steam space and is structurally connected with the water separators in each instance. With this arrangement it is possible to run the control rods to the reactor core through the top of a boiling-water reactor at a justifiable cost and, particularly, of space. With this arrangement the control rods can be dropped by gravity into the reactor core when necessary, as is done in the case of pressurized-water reactors. Also, the reactor vessel bottom need not be weakened by the numerous holes previously required for control rods working through the bottom of the pressure vessel of a boiling-water reactor. It is possible for the guide arrangement to be used with control rod assemblies of the cluster type comprising a cluster of depending elements of neutron-absorbing material. A cluster type control rod assembly includes operating rods and having its inner end provided with a plurality of radiating arms from which the active elements depend. The length of the neutron-absorbing element of any control rod used is preferably shorter than the length of the control rod guide arrangement so that the element can be removed completely from the core. In the event of a scram the invention provides a hydraulic shock absorber arrangement within the core and using the feed water. This is done by providing what is in effect a cylinder of downwardly decreasing diameter or cross-sectional area in which the absorbing element drops, displacement of the water at velocities which increase as the cylinder diameter decreases providing a gradual retardance to the dropping part.