Patent Number: 
Section: claims

1. A control rod drive system for a nuclear reactor vessel, the system comprising:a control rod drive mechanism mounted externally to the reactor vessel;a drive rod mechanically coupled to the control rod drive mechanism and extending through the reactor vessel into an interior cavity of the reactor vessel holding a nuclear fuel core, the control rod drive mechanism operable to raise and lower the drive rod through a plurality of vertical axial positions;a grapple assembly connected to the drive rod in the interior cavity of the reactor vessel and movable with the drive rod;an electromagnet mounted in the grapple assembly;a rod cluster control assembly comprising a plurality of control rods configured for removable insertion into the nuclear fuel core; anda drive rod extension extending axially between the rod cluster control assembly and the grapple assembly, the drive rod extension comprising:an axially extending actuator shaft having a top end including a magnetic block configured to releasably engage the electromagnet of the grapple assembly and a bottom end configured to releasably engage the rod cluster control assembly; anda lifting head sleeve including a diametrically enlarged lifting head, the lifting head sleeve slideably receiving the actuator shaft therethrough for axial upward and downward movement;wherein the electromagnet is operable to magnetically couple the actuating shaft to the grapple assembly at the top of the drive rod extension when the electromagnet is energized and uncouple the actuator shaft from the rod cluster control assembly at the bottom of the drive rod extension when the electromagnet is de-energized;wherein raising the actuator shaft when the electromagnet is energized couples the actuator shaft to the rod cluster control assembly and de-energizing the electromagnet lowers and uncouples the actuating shaft from the rod cluster control assembly. 2. The system of claim 1, wherein the bottom end of the actuator shaft includes a locking mechanism comprising radially movable locking elements releasably engageable with the rod cluster control assembly, the locking elements movable between a locked position coupling the actuator shaft to the rod cluster control assembly and an unlocked position uncoupled from the rod cluster control assembly. 3. The system of claim 2, wherein the locking elements are radially movable between the locked and unlocked positions by raising or lowering the actuator shaft with the drive rod. 4. The system of claim 3, wherein the locking elements are locking balls which engage an annular groove formed on the rod cluster control assembly in the locked position. 5. The system of claim 4, wherein the locking balls are movably retained in an adapter sleeve mounted on a bottom portion of the actuator shaft. 6. The system of claim 2, further comprising an actuator cap disposed on the bottom end of the actuator shaft, the actuator cap including a reduced diameter portion and larger diameter actuating portion operable to drive the locking elements radially outwards to engage the rod cluster control assembly. 7. The system of claim 1, wherein the actuator shaft is moveable independently of the lifting head sleeve for a first range of upward motion and moveable in unison with the lifting head sleeve for a second range of upward motion. 8. The system of claim 7, wherein the grapple assembly engages the lifting head in the second range of upward motion and does not engage the lifting head in the first range of upward motion. 9. The system of claim 1, further comprising a lifting head spring mounted on the lifting head, the magnetic block on the actuator shaft being arranged and operable to compress the lifting head spring against the lifting head by lowering the actuator shaft. 10. The system of claim 1, further comprising an annular bobbin concentrically mounted on the lifting head sleeve, wherein the grapple assembly is configured to engage and raise the bobbin into engagement with the lifting head such that the bobbin is at least partially nested inside the lifting head. 11. The system of claim 1, further comprising a longitudinally-extending actuator tube coupled between the lifting head sleeve and an adapter sleeve mounted on a bottom portion of the actuator shaft, the actuator shaft extending axially through the adapter sleeve. 12. The system of claim 1, wherein the control rod drive mechanism is configured and operable to maintain the drive rod and grapple assembly in axial position when the actuator shaft is uncoupled from the grapple assembly via de-energizing the electromagnet. 13. The system of claim 1, wherein responsive to a loss of power to the electromagnet, the actuator shaft is released from the grapple assembly and drops vertically to automatically uncouple the rod cluster control assembly from the actuator shaft for full insertion of the control rods into the fuel core while the drive rod and grapple assembly remain stationary in axial position. 14. 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. 15. The system of claim 1, wherein the grapple assembly includes a plurality of radially retractable lifting pins engageable with the lifting head of the drive rod extension. 16. The system of claim 15, wherein the lifting head is configured and operable to engage and retract the lifting pins via lowering the grapple assembly over the lifting head. 17. The system of claim 1, wherein the lifting head has a larger diameter than the lifting head sleeve. 18. The system of claim 1, further comprising a retaining collar supported in a fixed position in the interior cavity of the reactor vessel, a portion of the lifting head sleeve slideably received in the retaining collar, wherein the retaining collar includes spring-loaded retaining pins configured and arranged to releasably engage the lifting head sleeve. 19. The system of claim 18, wherein raising the lifting head sleeve with the grapple assembly releases the lifting head sleeve from the retaining collar. 20. The system of claim 18, wherein the retaining pins of the retaining collar are radially engageable with an annular seating surface of the lifting head sleeve arranged between a radially protruding stop flange at a top end of the sleeve and a bottom end of the sleeve.