Patent Number: 
Section: claims

1. A control rod drive mechanism (CRDM) comprising:a lifting rod configured to support a control rod configured to be inserted into a nuclear reactor core to quench a nuclear reaction in the nuclear reactor core;a holding mechanism comprising an electromagnetic circuit with magnetic poles configured to be drawn together when the electromagnetic circuit is energized to hold the lifting rod and to release the hold on the lifting rod upon de-energizing the electromagnetic circuit, wherein the magnetic poles include a fixed magnetic pole and a movable magnetic pole configured to apply an upward holding force on the cam bars when the electromagnetic circuit is energized and to release the upward holding force upon de-energizing the electromagnetic circuit; anda translation mechanism configured to linearly translate the lifting rod held by the holding mechanism toward and away from the nuclear reactor core;wherein the holding mechanism includes a non-magnetic spacer between the magnetic poles that defines an air gap between the drawn-together magnetic poles, the spacer comprising an inner annular ring and an outer annular ring, and wherein the spacer is disposed around a perimeter of the drawn-together magnetic poles. 2. The CRDM of claim 1 wherein the spacer is attached to one of the magnetic poles of the electromagnetic circuit. 3. The CRDM of claim 1 wherein the spacer is effective to define the gap between the drawn-together magnetic poles of at least 0.010 cm. 4. The CRDM of claim 1 wherein the spacer is effective to define the gap between the drawn-together magnetic poles of at least 0.025 cm. 5. The CRDM of claim 1 wherein the holding mechanism includes one of:(i) a plurality of latches, a latch engagement mechanism configured to engage the latches to an upper end of the lifting rod, and a latch holding mechanism separate from the latch engagement mechanism, wherein the electromagnetic circuit with the magnetic poles is part of the latch holding mechanism and is not part of the latch engagement mechanism;(ii) a screw on which the lifting rod is mounted and a separable ball-nut including the electromagnetic circuit with magnetic pole pieces, wherein the electromagnetic circuit is configured to magnetically hold the separable ball-nut together engaged with the screw and to allow the separable ball-nut to separate upon de-energizing the electromagnetic circuit. 6. The CRDM of claim 1 wherein:the translation mechanism comprises a screw and a motor configured to linearly translate the screw; andthe CRDM further comprises a plurality of latches mounted on the screw and configured to engage an upper end of the lifting rod;wherein the holding mechanism is configured to draw the magnetic poles together when the electromagnetic circuit is energized to hold the latches engaged with the upper end of the lifting rod and to release the hold on the latches upon de-energizing the electromagnetic circuit to disengage the latches from the upper end of the lifting rod. 7. The CRDM of claim 6 further comprising:a latch engagement mechanism that is separate from the holding mechanism, the latch engagement mechanism configured to close the latches to engage the upper end of the lifting rod. 8. The CRDM of claim 6 further comprising:a cam assembly comprising cam bars and cam bar links interconnected to define a four-bar linkage that urges the cam bars laterally inward to cam the latches closed in response to a vertical actuation force applied to the cam bars;wherein the holding mechanism is configured to draw the magnetic poles together when the electromagnetic circuit is energized to hold the cam bars in their inward position and to release the hold on the cam bars upon de-energizing the electromagnetic circuit. 9. The CRDM of claim 8 wherein the four-bar linkage is configured to urge the cam bars laterally inward to cam the latches closed in response to gravitational actuation force comprising the weight of the cam bars. 10. The CRDM of claim 9 wherein the upper end of the lifting rod includes:a cam surface configured to cam the latches open in response to the translation mechanism lowering the latches over the upper end of the lifting rod; anda recess with which the latches engage in response to the translation mechanism further lowering the latches past the cam surface and in response to the four-bar linkage urging the cam bars laterally inward to cam the latches closed in response to the gravitational actuation force comprising the weight of the cam bars. 11. The CRDM of claim 9 wherein the magnetic poles of the electromagnetic circuit of the holding mechanism include:a fixed magnetic pole; anda movable magnetic pole connected with the cam bars and arranged above the fixed magnetic pole so as to be drawn downward when the electromagnetic circuit is energized and to release the downward hold on the connected cam bars upon de-energizing the electromagnetic circuit. 12. The CRDM of claim 8 wherein the four-bar linkage is configured to urge the cam bars laterally inward to cam the latches closed in response to an upward actuation force applied to the cam bars. 13. The CRDM of claim 12 wherein the magnetic poles of the electromagnetic circuit of the holding mechanism include:a fixed magnetic pole; anda movable magnetic pole configured to apply an upward holding force on the cam bars when the electromagnetic circuit is energized and to release the upward holding force upon de-energizing the electromagnetic circuit. 14. A control rod drive mechanism (CRDM) comprising:a lifting rod configured to support a control rod configured to be inserted into a nuclear reactor core to quench a nuclear reaction in the nuclear reactor core;a translation mechanism comprising a translating element including latches configured to engage an upper end of the lifting rod and a motor configured to linearly translate the translating element so as to linearly translate the lifting rod engaged by the latches;a cam assembly comprising cam bars and cam bar links interconnected to define a four-bar linkage that urges the cam bars laterally inward to cam the latches closed in response to a vertical actuation force applied to the cam bars; anda holding mechanism comprising an electromagnetic circuit with a fixed magnetic pole, a movable magnetic pole, and a non-magnetic spacer between the magnetic poles, wherein the movable magnetic pole is connected with the cam bars, the holding mechanism being configured to draw the movable magnetic pole to the fixed magnetic pole when the electromagnetic circuit is energized to hold the cam bars connected with the movable magnetic pole in their inward position and to release the hold on the cam bars upon de-energizing the electromagnetic circuit, and the non-magnetic spacer defines an air gap between the drawn-together magnetic poles, the spacer comprising an inner annular ring and an outer annular ring, and wherein the spacer is disposed around a perimeter of the drawn-together magnetic poles. 15. The CRDM of claim 14 wherein the four-bar linkage is configured to urge the cam bars laterally inward to cam the latches closed in response to gravitational actuation force comprising the weight of the cam bars. 16. The CRDM of claim 15 wherein the upper end of the lifting rod includes:a cam surface configured to cam the latches open in response to the translation mechanism lowering the latches of the translating element over the upper end of the lifting rod; anda recess with which the latches engage in response to the translation mechanism further lowering the latches past the cam surface and in response to the four-bar linkage urging the cam bars laterally inward to cam the latches closed in response to the gravitational actuation force comprising the weight of the cam bars. 17. The CRDM of claim 15 wherein the movable magnetic pole is arranged above the fixed magnetic pole so as to be drawn downward when the electromagnetic circuit is energized and to release the downward hold on the connected cam bars upon de-energizing the electromagnetic circuit. 18. The CRDM of claim 14 wherein the translating element comprises a hollow screw, the latches are mounted at an upper end of the hollow screw, and the lifting rod passes through the hollow screw with the upper end of the lifting rod protruding from the upper end of the hollow screw. 19. 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 14 disposed in the pressure vessel.