Patent Number: 044118570
Section: summary

BACKGROUND OF THE INVENTION Complete shutdown of the operation of a nuclear reactor, commonly referred to as a scram, is required under any condition wherein continued operation could cause damage to the reactor. A scram is generally effected by releasing vertically disposed "safety" rods carrying neutron-absorbing material at their lower ends, thus allowing the rods to drop to a level wherein they position the absorber material within the reactor core. The inertia of long safety rods limits the speed at which they can be moved to scram position. Furthermore, it is possible that seismic shocks can interfere with the release of absorber carrying rods by bending them so that they cannot slide in their bearings. All scram control mechanisms must, of course, provide a means for returning safety rods to their raised position when an adverse operating condition has been eliminated. As shown in the description of scram mechanisms presented in the text titled "Fast Reactor Technology: Plant Design", published by M.I.T. Press in 1966, electromagnets have previously been used to hold reactor safety rods in a raised position until an unsafe reactor operating condition occurs, at which time the electromagnets are de-energized to release the rods. An electromagnet is incorporated in some latch mechanisms of embodiments of the invention disclosed herein, but the construction of safety rod release apparatus in accordance with the invention differs from that of known devices. Furthermore, known safety rod release devices use only one release means, whereas in preferred embodiments of this invention a plurality of release latches provide for two types of scram. SUMMARY OF THE INVENTION It is an object of this invention to provide an improved means for rapidly inserting a neutron absorber into a nuclear reactor core to control fission therein. Another object of the invention is to provide a nuclear reactor control apparatus having (1) plural latch assemblies for quickly releasing, under different conditions, a shaft which carries a neutron absorber, and (2) an additional lead screw drive means for moving the absorber into and out of a reactor core. A major difference between preferred embodiments of this invention and known apparatus for shutting down a nuclear reactor is that in the latter there is a single release mechanism that is actuated to permit a safety rod to move downwardly and thereby position a neutron absorber within a reactor core, whereas the invention provides (1) a first release mechanism that operates to drop an entire safety rod, and (2) a second release mechanism that operates to separate from the lower end of the safety rod a small segment thereof that holds the neutron absorber. The entire safety rod is released when a condition occurs that cannot affect the lowering of the rod, such as, for example, the failure of a coolant pump. However, when a condition occurs that may affect the movement of the entire safety rod (such as an earthquake), the absorber-holding segment is released from the lower end of the safety rod and allowed to move within the reactor core. The long safety rod could be bent by seismic shock and thus jammed in a position wherein the neutron absorber is held outside the reactor core. The small absorber holding segment of the rod will not be affected by seismic shock and, in addition, it can be moved more rapidly within the reactor core than the entire safety rod because its inertia is much less than that of the rod. Another advantage of using a plurality of release mechanisms is that the position of the safety rod can be detected during a scram of the entire safety rod (normal scram) using known types of position sensors such as reed switches and magnets.