Patent Number: 
Section: claims

1. A control rod drive mechanism (CRDM) comprising:a hollow lead screw;a lifting rod having an upper end disposed in the hollow lead screw;latches secured to the lead screw and configured to latch the upper end of the lifting rod to the lead screw;a latch engagement mechanism configured to close the latches onto the upper end of the lifting rod; anda latch holding mechanism configured to hold the latches closed; anda four-bar linkage including cam bars, the four-bar linkage configured to drive the cam bars inward to cam the latches closed responsive to operation of the latch engagement mechanism, the latch holding mechanism configured to hold the cam bars in the inward position to keep the latches closed;wherein the latch holding mechanism is separate from the latch engagement mechanism and the four-bar linkage is configured to bias the latches closed under force of gravity. 2. The CRDM of claim 1 wherein the latch engagement mechanism operates responsive to lowering the latches over the upper end of the lifting rod and is not effective to keep the latches closed when the latches are raised again after the latch engagement mechanism operates. 3. The CRDM of claim 2 wherein the latch holding mechanism is located at a top of the CRDM and is configured to engage the upper ends of the cam bars to hold the cam bars in the inward position. 4. The CRDM of claim 3 wherein the latch holding mechanism comprises a magnetic coupling including an electromagnet that when energized magnetically holds the cam bars in the inward position. 5. The CRDM of claim 3 wherein the latch holding mechanism comprises elements configured to move in a horizontal plane responsive to a holding force applied by an actuator to hold the cam bars in the inward position. 6. A nuclear reactor comprising:a pressurized water reactor (PWR) including:a pressure vessel;a reactor core disposed in the pressure vessel; anda CRDM as set forth in claim 1 disposed in the pressure vessel. 7. The nuclear reactor of claim 6 wherein the latch holding mechanism is magnetically operated. 8. The nuclear reactor of claim 6 comprising multiple CRDMs, wherein each CRDM has an independent latch holding mechanism. 9. The nuclear reactor of claim 8 wherein the latch holding mechanisms are magnetically operated. 10. The nuclear reactor of claim 9 wherein the latch holding mechanisms operate independent of one another. 11. A control rod drive mechanism (CRDM) including:a CRDM motor;a hollow lead screw translated under control of the CRDM motor;a lifting rod supporting at least one control rod;a latch configured to engage a portion of the lifting rod that is disposed in the hollow lead screw so that the lead screw and the lifting rod are non-rotatably secured to each other;a latch engagement mechanism configured to close the latch onto the lifting rod;a latch holding mechanism, separate from the latch engagement mechanism, configured to hold the latch in its closed position; anda four-bar linkage including cam bars, the four-bar linkage being configured to cam the latches closed responsive to operation of the latch engagement mechanism,wherein the four-bar linkage is configured to cam the latches closed responsive to operation of the latch engagement mechanism. 12. The CRDM of claim 11 wherein the latch holding mechanism is magnetically actuated. 13. A control rod drive mechanism (CRDM) comprising:a lead screw having an upper end;a lifting rod having an upper end;latches secured to the upper end of the lead screw and configured to latch the upper end of the lifting rod to the lead screw;a latch engagement mechanism configured to close the latches onto the upper end of the lifting rod;a latch holding mechanism configured to hold the latches closed; anda four-bar linkage including cam bars, the four-bar linkage configured to drive the earn bars inward to cam the latches closed responsive to operation of the latch engagement mechanism, the latch holding mechanism configured to hold the cam bars in the inward position to keep the latches closed,wherein the latch holding mechanism is separate from the latch engagement mechanism and the four-bar linkage is configured to bias the latches closed under force of gravity. 14. The CRDM of claim 13, wherein the lead screw is hollow and the upper end of the lifting rod is disposed in the lead screw. 15. The CRDM of claim 13 wherein the latch engagement mechanism operates responsive to lowering the latches over the upper end of the lifting rod and is not effective to keep the latches closed when the latches are raised again after the latch engagement mechanism operates.