Patent Number: 
Section: claims

1. A control rod drive mechanism for use in a nuclear reactor including a reactor core disposed in a pressure vessel, comprising:a control rod configured for insertion into the reactor core;a lead screw having a top end and a bottom end, the control rod being secured to the bottom end of the lead screw;a drive mechanism including:a torque tube having a top end and a bottom end,a pair of segment arms, each segment arm including a top end and a bottom end and being pivotably mounted to the torque tube,a pair of roller nuts, each roller nut being rotatably secured to the bottom end of a respective segment arm, anda drive motor including a stator and a rotor, the rotor secured to the top end of the torque tube and including a plurality of permanent magnets embedded therein, wherein the stator defines a central bore and the rotor is disposed within the central bore of the stator; anda latch coil assembly including a latch coil and a hold coil, wherein the latch coil assembly defines a central bore and the top ends of the segment arms are disposed within the central bore of the latch coil assembly radially-inwardly of the latch coil and the hold coil,wherein the latch coil and the hold coil are independently energizable and configured so that when the hold coil is energized and the latch coil is de-energized, the roller nuts are engaged with the lead screw. 2. The control rod drive assembly of claim 1, wherein the hold coil is disposed above the latch coil in the latch coil assembly. 3. The control rod drive assembly of claim 1, further comprising a pressure tube that defines a portion of a pressure boundary of the pressure vessel, wherein the stator of the drive motor and the latch coil assembly are disposed outside of the pressure tube. 4. The control rod drive assembly of claim 3, wherein the torque tube, the rotor of the drive motor, the segment arms and the roller nuts are disposed inside an interior of the pressure tube. 5. The control rod drive assembly of claim 1, wherein the plurality of the permanent magnets are disposed evenly about a perimeter of the rotor of the drive motor. 6. The control rod drive assembly of claim 5, wherein the plurality of permanent magnets further comprise a plurality of Sm2Co17 magnets. 7. The control rod drive assembly of claim 1, wherein the latch coil is a circumferentially wound coil.