Patent Number: 
Section: claims

1. A control rod drive system for a nuclear reactor vessel, the system comprising:a reactor vessel having a top head and an interior cavity;a nuclear fuel core supported in the interior cavity of the reactor vessel;a rod cluster control assembly comprising a plurality of control rods configured for removable insertion into the nuclear fuel core;a control rod drive mechanism mounted externally to the reactor vessel above the top head;a drive rod mechanically coupled to the control rod drive mechanism and extending through the top head of reactor vessel into the interior cavity, 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 inside the interior cavity of the reactor vessel and movable with the drive rod, the grapple assembly including an electromagnet;a drive rod extension extending axially between the rod cluster control assembly and the grapple assembly, the drive rod extension including a bottom end releasably coupled to the rod cluster control assembly in a non-locking manner and a top end releasably coupled to the grapple assembly via the electromagnet; anda longitudinally-extending drive rod extension support structure mounted in the reactor vessel above the nuclear fuel core, the support structure including a plurality of vertically-oriented guide tubes, at least one of the guide tubes which is configured to slideably receive the drive rod extension therein for axial upward and downward movement;wherein the electromagnet is operable to magnetically couple the drive rod extension to the grapple assembly when the electromagnet is energized and uncouple the drive rod extension from the grapple assembly when the electromagnet is de-energized;wherein de-energizing the electromagnet drops and uncouples the drive rod extension from the rod cluster control assembly remotely at the bottom of the drive rod extension. 2. The system of claim 1, wherein the drive rod extension includes:an axially extending actuator shaft having a top end extending above the drive rod extension support structure and including a magnetic block releasably engageable with the electromagnet of the grapple assembly, the actuator shaft further including a bottom end releasably engageable with the rod cluster control assembly; anda lifting head sleeve including a diametrically enlarged lifting head selectively engageable with the grapple assembly, the lifting head sleeve slideably receiving the actuator shaft therethrough for axial upward and downward movement. 3. The system of claim 2, wherein the drive rod extension support structure includes a retaining collar receiving a portion of the lifting head sleeve therein, the retaining collar having radially-acting spring-loaded retaining pins configured and arranged to releasably engage the lifting head sleeve. 4. The system of claim 3, wherein when the lifting head sleeve is engaged with the retaining collar, the actuator shaft is moveable upwards independently of the lifting head sleeve by energizing the electromagnet and raising the grapple assembly. 5. The system of claim 4, wherein when the lifting head sleeve is not engaged with the retaining collar, the actuator shaft is moveable upwards together with the lifting head sleeve by raising the grapple assembly when the electromagnetic is energized. 6. The system of claim 3, wherein the lifting head sleeve includes an outwardly protruding annular stop flange arranged to engage a top surface of the retaining collar to limit an insertion depth of the lifting head sleeve through the retaining collar into the at least one of the guide tubes. 7. The system of claim 1, wherein responsive to a loss of power to the electromagnet, the drive rod extension is released from the grapple assembly and drops vertically to automatically uncouple the rod cluster control assembly from the drive rod extension for full insertion of the control rods into the fuel core while the drive rod and grapple assembly remain stationary in axial position. 8. The system of claim 2, 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 and the lifting head therein. 9. The system of claim 8, wherein the chamber of the grapple assembly further includes a plurality of radially retractable lifting pins engageable with the lifting head of the drive rod extension. 10. The system of claim 9, further comprising a diametrically enlarged bobbin slideably disposed and axially movable on the lifting head sleeve, the bobbin operable to selectively engage the lifting head and enter the downwardly open chamber of the grapple assembly. 11. The system of claim 10, wherein the bobbin is configured and operable to enter a downwardly open cavity of the lifting head in a nested relationship. 12. The system of claim 11, wherein the bobbin is engageable with the retractable lifting pins of the grapple assembly when the bobbin is nested in the cavity of the lifting head. 13. The system of claim 10, wherein the bobbin is engageable with the retractable lifting pins of the grapple assembly. 14. The system of claim 2, wherein the bottom end of the drive rod extension includes a locking mechanism comprising radially movable locking elements releasably engageable with the rod cluster control assembly, the locking elements movable between an outward locked position coupling the drive rod extension to the rod cluster control assembly and an inward unlocked position uncoupled from the rod cluster control assembly. 15. The system of claim 14, wherein the locking elements are locking balls radially movable between the locked and unlocked positions by raising or lowering the drive rod extension with the drive rod and grapple assembly. 16. The system of claim 15, wherein the locking balls are arranged to selectively engage an annular groove formed on the rod cluster control assembly in the locked position thereby coupling the drive rod extension to the rod cluster control assembly. 17. The system of claim 16, wherein the locking balls are mounted in an adapter sleeve coupled to a bottom end of the lifting head sleeve, and the actuator shaft is axially slideable inside the adapter sleeve and configured to selectively engage and move the locking balls laterally outwards and inwards between the locked and unlocked positions. 18. The system of claim 17, further comprising an actuator cap attached to a bottom end of the actuator shaft and including an upper end portion and a diametrically larger lower end portion engageable with the locking balls, the upper end portion having a diameter which does not engage the locking balls, wherein moving the actuator shaft upwards relative to the adapter sleeve engages the lower end portion of the actuator cap with the locking balls to force them outwards to the locked position. 19. The system of claim 1, wherein the guide tubes of the drive rod extension support structure each include an upper guide tube and a lower guide tube having a larger diameter than the upper guide tube, the rod cluster control assembly being configured for upwards and downwards movement within the lower guide tube. 20. The system of claim 19, wherein the nuclear fuel core is disposed inside a tubular core support structure located in a lower portion of the interior cavity of the reactor vessel, the core support structure sitting atop the core support structure in the interior cavity, and wherein the guide tubes of the drive rod extension support structure include a plurality of perforations in fluid communication with the primary coolant flow.