Patent Number: 
Section: claims

1. A control rod drive mechanism (CRDM) comprising:a hollow lead screw engaged by a CRDM motor;a lifting rod having an upper end disposed in the hollow lead screw, the lifting rod supporting at least one control rod;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;cam bars configured to move to an inward position to cam the latches closed responsive to operation of the latch engagement mechanism; anda latch holding mechanism configured to hold the latches closed, the latch holding mechanism including latch holding elements that engage upper ends of corresponding cam bars to hold the cam bars in the inward position,wherein the latch holding mechanism is separate from the latch engagement mechanism, andwherein the latch holding elements are configured to move in a horizontal plane responsive to a holding force applied to hold the cam bars in the inward position. 2. The CRDM of claim 1 further comprising:a 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. 3. 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. 4. The CRDM of claim 2 wherein the latch holding mechanism is located at a top of the CRDM. 5. The CRDM of claim 4 wherein the latch holding mechanism comprises a magnetic coupling including an electromagnet that when energized magnetically holds the cam bars in the inward position. 6. The CRDM of claim 1 wherein the holding force is magnetic. 7. A control rod drive mechanism (CRDM) comprising:a hollow lead screw engaged by a CRDM motor;a lifting rod having an upper end disposed in the hollow lead screw, the lifting rod supporting at least one control rod;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;a latch holding mechanism configured to hold the latches closed; anda bar linkage including cam bars, the 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 engagement mechanism is not effective to keep the latches closed when the latches are supporting the weight of the lifting rod and supported at least one control rod, andwherein the bar linkage is configured to bias the latches closed under the force of gravity. 8. The CRDM of claim 7 wherein the latch holding mechanism is not effective to close the latches. 9. The CRDM of claim 7 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.