Patent Number: 
Section: claims

1. A method for coupling a control rod drive mechanism to a rod cluster control assembly in a nuclear reactor vessel, the method comprising:providing: a reactor vessel having a top head and an interior cavity; a nuclear fuel core supported in the interior cavity; a rod cluster control assembly positioned at a top of the fuel core and comprising a plurality of control rods configured for removable insertion the fuel core; a control rod drive mechanism mounted externally above the reactor vessel; a drive rod assembly including a drive rod mechanically coupled to the control rod drive mechanism and extending into the interior cavity of the reactor vessel, and a grapple assembly disposed on an end of the drive rod and including an electromagnet;lowering the drive rod assembly;contacting the drive rod assembly with a top end of a drive rod extension extending vertically between the rod cluster control assembly and the top head of the reactor vessel, a bottom end of the drive rod extension contacting the rod cluster control assembly in a non-locking manner;engergizing the electromagnet to magnetically couple the drive rod assembly with the drive rod extension;raising the drive rod assembly by a first vertical distance;locking the bottom end of the drive rod extension with the rod cluster control assembly, wherein raising and lowering the drive rod assembly with the control rod drive mechanism raises and lowers the rod cluster control assembly for controlling the reactivity within the fuel core. 2. The method of claim 1, wherein the lowering step includes:engaging a plurality of radially biased lifting pins movably disposed in the grapple assembly with a lifting head disposed on the drive rod extension;retracting the lifting pins at least partially outwards into the grapple assembly;moving the grapple assembly downwards over the lifting head; andprojecting the lifting pins back inwards from a grapple body. 3. The method of claim 2, further comprising:engaging the lifting pins with a bottom of the lifting head;raising the drive rod assembly by a second vertical distance; anduncoupling a lifting head sleeve on the drive rod extension from a retaining collar in the reactor vessel. 4. The method of claim 1, further comprising:de-energizing the electromagnet;dropping the drive rod extension wherein the drive rod assembly remains stationary; anduncoupling the bottom end of the drive rod extension from the rod cluster control assembly. 5. The method of claim 2, further comprising:lowering the drive rod assembly while contacting the top end of the drive rod extension with the drive rod assembly;compressing a spring against the lifting head with the drive rod assembly;engaging the lifting pins with a vertically movable annular bobbin on the drive rod extension;lifting the bobbin into engagement with the lifting head;retracting the lifting pins at least partially outwards into the grapple assembly;moving the grapple assembly upwards over the lifting head; andprojecting the lifting pins back inwards from the grapple body, wherein the drive rod assembly is uncoupled from the drive rod extension. 6. The method of claim 1, wherein the locking step includes driving a plurality of locking elements radially outwards from the drive rod extension to engage a groove formed in the rod cluster control assembly, thereby locking the rod cluster control assembly to the drive rod extension. 7. The method of claim 6, wherein the locking elements are movably disposed in an adapter sleeve mounted on the bottom end of the drive rod extension, the adapter sleeve being partially inserted into an upward standing tubular mounting extension disposed on the rod cluster control assembly. 8. The method of claim 1, wherein the drive rod extension is slidably disposed in a guide tube of a drive rod extension support structure mounted between the fuel core and the top head of the reactor vessel. 9. The method of claim 8, further comprising raising and lowering the rod cluster control assembly inside the guide tube with the control rod drive mechanism.