Patent Number: 
Section: claims

1. A control rod drive system for a nuclear reactor vessel, the system comprising:a vertically oriented drive rod mechanically coupled to a control rod drive mechanism operable to raise and lower the drive rod through a plurality of axial positions;a rod cluster control assembly comprising a plurality of control rods supported by a common support plate having an upwardly extending mounting tube extension comprising an annular groove, the control rods axially movable as a unit with the support plate and configured for removable insertion into a nuclear fuel core;a drive rod extension extending axially between the rod cluster control assembly and the drive rod, the drive rod extension having a bottom end releasably coupled to the rod cluster control assembly;a drive rod extension grapple assembly connected to the drive rod, the grapple assembly releasably coupled to a top end of the drive rod extension, the grapple assembly comprising cylindrical sidewalls defining an interior chamber and a downwardly open bottom leading to the chamber;the drive rod extension including a longitudinally-extending actuating tube and an actuating shaft slidably disposed in the tube, the actuating shaft extending through a top of the actuating tube and releasably operably coupled to the grapple assembly for upward and downward movement with the drive rod;an annular adapter sleeve affixed to a bottom end of the actuating tube;a plurality of circumferentially spaced apart locking balls disposed in the adapter sleeve and radially movable;an elongated actuator cap affixed to a bottom end of the actuating shaft and axially movable with the actuating shaft upward and downward inside the adapter sleeve, the actuator cap comprising a diametrically enlarged actuating portion defining an annular bearing surface engageable with the locking balls;wherein the grapple assembly includes a plurality of radially retractable spring-biased lifting pins engageable with the drive rod extension and slideably disposed in the cylindrical sidewalls of the grapple assembly for inward and outward linear movement, and the drive rod extension includes a lifting head configured to engage and retract the lifting pins when the lifting head is inserted into the grapple assembly;wherein the actuator cap and actuating shaft are axially movable between an unlocked position in which the actuating portion of the actuator cap disengages the locking balls in the adapter sleeve, and a locked position in which the actuating portion engages and forces the locking balls radially outwards to engage the annular groove in the rod cluster control assembly;wherein raising and lowering the drive rod raises and lowers the rod cluster control assembly. 2. The system of claim 1, wherein the grapple assembly includes an electromagnet which magnetically couples the drive rod extension to the grapple assembly when the electromagnet is energized and uncouples the drive rod extension from the grapple assembly when the electromagnet is de-energized. 3. The system of claim 2, wherein the drive rod extension includes a magnetic block engageable with the electromagnet of the grapple assembly. 4. The system of claim 2, wherein the control rod drive mechanism is configured and operable to maintain the drive rod in axial position when the drive rod extension is uncoupled from the drive rod via de-energizing the electromagnet. 5. The system of claim 2, wherein responsive to a loss of power to the electromagnet, the drive rod extension automatically uncouples from the drive rod extension grapple assembly and drops vertically while the drive rod remains stationary. 6. The system of claim 1, wherein the grapple assembly includes a cylindrically shaped body defining a downwardly open chamber configured to movably receive the top end of the drive rod extension therein. 7. The system of claim 1, wherein the locking balls are radially movable between the locked and unlocked positions by raising or lowering the actuator shaft with the drive rod. 8. The system of claim 1, wherein the control rod drive mechanism is mounted above a top head of the reactor vessel outside the reactor vessel and the grapple assembly is mounted below the top head inside the reactor vessel. 9. The system of claim 8, wherein the control rod drive mechanism is mounted to a flanged nozzle extending upwards from the top head of the reactor vessel. 10. The system of claim 1, wherein the drive rod extension extends axially from a location proximate to the top of a fuel core in the reactor vessel to a location spaced below a top head of the reactor vessel. 11. The system of claim 1, wherein the lifting head comprises an annular inward-downward facing angled lower bearing surface defining a downwardly open cavity configured to receive a mating complementary configured upper bearing surface of a bobbin slidably disposed on the drive rod extension for upward and downward movement thereon, the angled lower bearing surface of the lifting head engageable with the upper bearing surface of the bobbin. 12. The system of claim 11, wherein the bobbin is moveable from an upper position nested in the cavity of the lifting head and a second lower position spaced vertically apart from and below the lifting head. 13. The system of claim 12, wherein the bobbin is insertable into the interior chamber of the grapple assembly. 14. The system of claim 13, wherein the bobbin is configured to engage and radially retract the lifting pins of the grapple assembly when the bobbin is inserted into the interior chamber of the grapple assembly. 15. The system of claim 14, wherein the bobbin when in the upper nested position and engaged with the lifting pins is operable to raise the lifting head via raising the grapple assembly.