Patent Number: 041394143
Section: summary

BACKGROUND OF THE INVENTION The invention relates to shutdown mechanisms for nuclear reactors, and in particular to poison release mechanisms for reactors of the liquid metal type. It is known in the art of nuclear reactor control to use a multiplicity of small neutron absorbing masses, usually stainless steel spheres or the like, to rapidly shutdown (scram) the nuclear chain reaction on the occurrence of a severe accident. Because of the difficulties associated with resetting the masses after a scram, these prior art devices are usually provided in the nuclear reactor as a final safeguard against the most serious kinds of accidents, such as earthquakes or other events that might physically damage the regular shutdown system. Thus these devices have not been disclosed as having means for easily resetting the masses for renewed core operation, since it has been implicitly assumed that significant damage to the reactor has occurred, that repairs will be required, or that a prolonged testing period will be needed before operation can resume. During these prolonged outages, the prior art devices can be repaired or replaced, but they cannot be quickly reset if they scram inadvertently during normal operation or if they are actuated for test purposes. Examples of such prior art devices are described in U.S. Pat. Nos. 3,088,903 issued to A. Firth; 3,147,188 issued to R. H. Cambell; and 3,249,510 issued to A. J. Dohm, Jr. et al. SUMMARY OF THE INVENTION It is an object of the present invention to provide a scram system utilizing a multiplicity of neutron absorbing masses that can be used for routine scram incidents. It is a related object of the invention to provide for simple resetting of the scram device following a scram. It is a further object of the invention to provide a scram system that can be self-actuated from within the safety assembly for dropping a multiplicity of neutron absorbing masses into the reactor core. The present invention is an apparatus for holding, releasing and resetting a multiplicity of neutron absorbing balls within a safety assembly of a liquid metal nuclear reactor. Vertically hinged trap doors rest on the shoulders of a generally cylindrical release valve which is actuated either by the regular or self-actuated scram actuator. The doors and the valve shoulder provide a floor for the multiplicity of balls to be suspended above the reactor core during normal operation. When the actuator displaces the release valve, the doors lose their support and swing downward, permitting the poison balls to drop into the core. In the reset mode of operation, a platform at the bottom of the core is raised to lift the balls and swing the trap doors upward until the balls are above the door hinges. The release valve is reset to support the doors and the platform is lowered to the bottom of the safety assembly. The invention can be used as part of the regular scram system and is adaptable for use in conjunction with a variety of self-actuating scram devices used for backup scram control.