Patent Number: 
Section: claims

1. A control rod mechanism for use in a nuclear reactor, the control rod mechanism comprising:at least one control rod comprising neutron-absorbing material and configured for insertion in a reactor core;a hollow lead screw;a motor including a rotor, the motor operatively coupled with the hollow lead screw by a threaded connection, the motor configured to rotate the rotor to drive the hollow lead screw via the threaded connection linearly toward or away from the reactor core;a lifting rod configured to support the weight of the at least one control rod, the lifting rod disposed partially inside the hollow lead screw; anda latch assembly mounted on the hollow lead screw and having latches configured to be closed by applied power to latch the lifting rod to the hollow lead screw, the latches of the latch assembly being further configured to open responsive to removal of the applied power to unlatch the lifting rod from the hollow lead screw,wherein the unlatched lifting rod and the at least one control rod are configured to fall toward the reactor core; andwherein the hollow lead screw is configured to remain operatively coupled with the motor by the threaded connection and to not fall toward the reactor core when the latches of the latch assembly open. 2. The control rod mechanism as set forth in claim 1, wherein the at least one control rod comprises a plurality of control rods and the lifting rod is configured to support the weight of the plurality of control rods via at least one of a spider and a yoke. 3. The control rod mechanism as set forth in claim 1, wherein the applied power is applied hydraulic power. 4. The control rod mechanism as set forth in claim 1, wherein the latch assembly comprises:a lifting assembly configured to receive the applied power;cam bars arranged parallel with the hollow lead screw and connected with the lifting assembly, the lifting assembly configured to operate under the applied power to lift the cam bars; anda four-bar mechanical linkage configured to move the cam bars inward while keeping the cam bars arranged parallel with the hollow lead screw in response to the cam bars being lifted by the lifting assembly to cam the cam bars against the latches to close the latches. 5. The control rod mechanism as set forth in claim 4, wherein the length of the cam bars in the direction parallel with the hollow lead screw is sufficient for the cam bars to cam against the latches of the latching assembly for any position of the hollow lead screw obtainable by driving the hollow lead screw using the motor. 6. A control rod drive mechanism (CRDM) comprising:a hollow lead screw having an inner diameter;a lifting rod configured to support the weight of a control rod comprising neutron-absorbing material and configured for insertion in a reactor core, a portion of the lifting rod being disposed inside the hollow lead screw;a motor threadedly coupled with the hollow lead screw and configured to linearly drive the hollow lead screw in an insertion direction or an opposite withdrawal direction;a latch assembly mounted on the hollow lead screw and configured to (i) latch the lifting rod to the hollow lead screw in response to applied power and to (ii) unlatch the lifting rod from the hollow lead screw while the latch assembly remains mounted on the hollow lead screw in response to removal of the applied power, the lifting rod being configured to be free to fall in the insertion direction when unlatched;wherein the motor is configured to remain threadedly coupled with the lead screw to prevent the lead screw from falling when the latch assembly is either latched or unlatched. 7. The CRDM as set forth in claim 6, wherein the latch assembly is hollow and is configured to allow the lifting rod to be removed from the CRDM with the latch assembly unlatched by withdrawing the lifting rod in the withdrawal direction through the CRDM and through the hollow latch assembly. 8. The CRDM as set forth in claim 6, wherein the applied power is hydraulic power and the latch assembly includes a hydraulic lift assembly configured to latch the latch assembly when pressurized. 9. The CRDM as set forth in claim 6, wherein the latch assembly comprises:latches configured to latch to the lifting rod;elongated members arranged parallel with the hollow lead screw;a lift mechanism configured to lift the elongated members when powered by the applied power; andfour-bar linkages configured to respond to the lifting of the elongated members by moving the elongated members toward the latches while keeping the elongated members arranged parallel with the hollow lead screw to cam the elongated members against the latches to latch the latch assembly. 10. The CRDM as set forth in claim 9, wherein the lift mechanism is selected from a group consisting of (i) a hydraulic piston and (ii) an electric solenoid. 11. The CRDM as set forth in claim 6, further comprising:at least one ball nut assembly providing the threaded coupling between the motor and the hollow lead screw. 12. A control rod drive mechanism (CRDM) comprising:a hollow lead screw;a drive assembly including a non-separable ball nut coupled with the outside of the hollow lead screw, the drive assembly configured to linearly drive the hollow lead screw in an insertion or opposite withdrawal direction by rotating the non-separable ball nut coupling with the outside of the lead screw, the non-separable ball nut configured to not move in the insertion or opposite withdrawal direction;a lifting rod configured to support the weight of at least one control rod and disposed partially inside the hollow lead screw; anda latch assembly with mounted to the hollow lead screw and having (i) a latched state in which the latch assembly is latched to the lifting rod and (ii) an unlatched state in which the latch assembly is not latched to the lifting rod; anda latch closing mechanism configured to latch the latch assembly to the lifting rod and to unlatch the latch assembly from the lifting rod in response to removal of applied power to the latch closing mechanism. 13. The CRDM as set forth in claim 12, wherein the latch closing mechanism includes one of (i) a hydraulic piston and (ii) an electric solenoid. 14. The CRDM as set forth in claim 12, wherein the latch closing mechanism includes a four-bar mechanical linkage having a length parallel with the lead screw that is at least as long as a stroke of the drive assembly. 15. An apparatus comprising:at least one control rod;a lifting rod configured to support the weight of the at least one control rod at a lower end of the lifting rod; anda control rod drive mechanism (CRDM) including:a latch assembly configured to have (i) a latched state in which the latch assembly is latched to an upper end of the lifting rod and (ii) an unlatched state in which the latch assembly is not latched to the upper end of the lifting rod, anda linear drive mechanism configured to drive the latch assembly linearly toward or away from a nuclear reactor core. 16. The apparatus as set forth in claim 15, wherein the CRDM is hollow and configured to allow the lifting rod to be removed with the latch assembly in the unlatched state by drawing the lifting rod away from the nuclear reactor core through the CRDM. 17. The apparatus as set forth in claim 15, wherein:the linear drive mechanism of the CRDM includes a hollow lead screw through which the lifting rod is free to move and a motor threadedly coupled with the hollow lead screw to linearly drive the hollow lead screw toward or away from the nuclear reactor core; andthe latch assembly includes latches mounted on an upper end of the hollow lead screw, the latches being configured to latch to the upper end of the lifting rod in the latched state. 18. The apparatus as set forth in claim 17, wherein, in the unlatched state of the latch assembly, the motor of the linear drive mechanism of the CRDM remains threadedly coupled with the hollow lead screw. 19. The apparatus as set forth in claim 15, wherein:the unlatched lifting rod is configured to fall toward the nuclear reactor core, andno portion of the CRDM is configured to fall toward the nuclear reactor core in either the latched state or the unlatched state of the latch assembly.