Patent Number: 047132108
Section: summary

This invention relates to a rod grapple and driveline mechanism for placement between a poison containing rod for parasitic neutron absorption required for shutdown of a fission reactor and a rod drive mechanism. More particularly, a rod driveline including a rod release mechanism is shown for operation in a fast neutron breeder reactor where rod release can occur responsive either to loss of electromagnetic rod retention power or alternately thermal rise of the reactor beyond set limits. BACKGROUND OF THE INVENTION Nuclear reactors are shut down by the insertion of rods containing poisons for parasitic neutron absorption. Rod drives are typically connected by drivelines to the rods. A first form of rod driveline includes a mechanical connection, typically threaded, between the driveline and rod. A second type of driveline includes a driveline which can selectively attach and release a rod. This disclosure relates to this second type of driveline. Rod release mechanism from drivelines are known. Moreover, thermally responsive rod release mechanisms are known. A common type of thermally responsive rod release mechanisms includes an electromagnetic coupling between the mechanism and a magnet. When the magnetic portion of the rod adjacent a magnet reaches the Curie point of the metal and becomes non-magnetic, rod release and drop occurs. Rod excursion responsive to reactor overheat is known. See Zebroski U.S. Pat. No. 4,227,967. In this type of device, concentric cylinders of bimetallic differential expansion properties are series connected for maximum thermal excursion. When the reactor is too hot, rod insertion occurs. When the reactor cools and rod drive movement has occurred, rod withdrawal occurs. SUMMARY OF THE INVENTION A control rod driveline and grapple is disclosed for placement between a control rod drive and a nuclear reactor control rod. The control rod is provided with an enlarged cylindrical handle which terminates in an upwardly extending rod to provide a grapple point for the driveline. The grapple mechanism includes a tension rod which receives the upwardly extending handle and is provided with a lower annular flange. A plurality of preferably six grapple segments surround and grip the control rod handle at the flange. Each grapple rod segment grips the flange on the tension rod at an interior upper annular indentation, bears against the enlarged cylindrical handle at an intermediate annulus and captures the upwardly flaring frustum shaped handle at a lower and complementary female segment. The tension rod and grapple segments are surrounded by and encased within a cylinder. The cylinder terminates immediate an outward extending annulus at the lower portion of the grapple segments. Excursion of the tension rod relative to the encasing cylinder causes rod release at the handle. Rod release at the handle occurs by permitting the grapple segments to pivot outwardly and about the annulus on the tension rod so as to open the lower defined frustum shaped annulus and drop the rod. Relative movement between the tension rod and cylinder can occur either due to electromagnetic release of the tension rod within defined limits of travel or differential thermal expansion as between the tension rod and cylinder as where the reactor exceeds design thermal limits. OTHER OBJECTS AND ADVANTAGES An object of this invention is to disclose a simplified mechanical grapple for gripping and releasing a control rod at its handle. According to this aspect of the invention, the rod drive includes an inner tension rod and an outer cylinder. The lower end of the tension rod has an attached annular flange. The grapple includes a plurality of preferable six grapple segments. Each grapple segment pivots at an upper indentation about the flange and receives the rod handle at an lower indentation. Inward and outward movement of the grapple segments with respect to one another is restricted by relative motion between the tension rod and cylinder. The lower end of the cylinder restricts the rod segments towards one another. When the cylinder does not restrict the grapple segments, the handle of the rod can be received at the rod segments. Once the handle is received in the rod segments, the cylinder moves downwardly relative to the tension rod. The cylinder then restricts movement of the grapple segments. With the control rod handle captured, motorized and conventional slow driven movement of the control rod can occur. A further object of this invention is to disclose a control rod grapple that interacts with a rod handle to effect grappling of the handle. According to this aspect, the grapple segments include two bearing surfaces, one for bearing on the top of the control rod handle and the other for bearing on the bottom surface of the flange attached to the tension rod. As the grapple segments come down over the control rod handle, a gathering surface at the bottom of the segments forces the control rod segments apart. Once the handle is fully received within the segments, the two bearing surfaces on the top of the control rod handle and on the bottom of the tension rod flange exert a torque on each rod segment. This torque causes the rod segments to move inwardly too and towards one another. Control rod capture at the handle occurs. An advantage of this interaction between the control rod handle and grapple is that the grapple segments are disposed for capture by the surrounding cylinder. This cylinder maintains control rod handle capture until specific release is triggered, either by thermal excursion above design limits or alternately release of an electromagnetic keeper mechanism. An advantage of the disclosed grapple is that it is actuated by relative movement between the interior tension rod and the exterior cylinder. This relative movement can be mechanical, electromechanical or thermal. A further advantage of the driveline and grapple is that it is ideal for attachment to conventional slow drive units used for reactor control. Specifically, rod grappling can only occur under deliberate motorized drive control of the grapple. First, the driveline must be moved downwardly about the control rod handle. Second, the grapple closes and captures the control rod handle. Third, and only when the control rod handle and grapple are securely attached on the drive move both driveline and rod deliberately upward for reactor energizing. No credible way exists with the disclosed driveline and grapple that will permit rapid rod withdrawal from the reactor. Yet another advantage of the disclosed mechanism is that an electromagnetic connection is required to maintain the control rod within the grapple. Thus, traditional series circuitry for maintaining the electromagnetic grapple closure can be accommodated. Any interruption in the disclosed electromagnetic circuitry will result in the rod being released at the handle by the grapple and correspondent reactor SCRAM. A further object of this invention is to disclose a thermal circuit breaker for use with a sodium cooled fast neutron breeder reactor. According to this aspect of the invention, the tension rod and cylinder are designed for bimetallic differential thermal expansion, the tension rod having a high index of thermal expansion, the cylinder having a low index of thermal expansion. Where a thermal design limit is exceeded in the reactor, the tension rod expands relative to the cylinder causing relative movement beyond a preselected limit at the vicinity of the grapple. With movement of the cylinder with respect to the grapple, the cylinder clears a male annulus at the upper end of the grapple. The grapple segments are free to pivot outwardly. The rod handle is released. Upon completion of the grapple segments pivotal motion, the rod is dropped, typically into a dash pot, and causes correspondent reactor SCRAM. An advantage of this aspect of the invention is that an essentially shock proof thermal circuit breaker is disclosed. Damage to the driveline by extra ordinary causes such as earthquake and the like is highly unlikely; the disclosed thermal circuit breaker will drop rods and cause reactor SCRAM even though connected drives may be disabled. A further advantage of this invention is that it is particularly suited to overhead rod release mechanisms, especially those kinds of overhead rod mechanisms that are commonly prescribed for sodium cooled fast neutron breeder reactors. Yet another advantage of the disclosed grapple is that the grapple is essentially a thermal circuit breaker. SCRAM of a plant can be followed by reconnection of the drive unit at the grapple and resumption of ordinary control rod movement pursuant to drive movement.