Patent Number: 
Section: claims

1. A control rod mechanism for use in a nuclear reactor, the control rod mechanism comprising:a control rod configured for insertion in a reactor core to absorb neutrons;a lifting rod configured to support the weight of the control rod;a hollow lead screw through which the lifting rod passes;a drive mechanism including (i) a rotating nut engaged with the lead screw and (ii) a motor engaged with the nut to rotate the nut, the drive mechanism configured to operate by rotating the nut engaged with the lead screw to linearly translate the lead screw; anda latch mounted to the lead screw and configured to latch with the lifting rod wherein the latch and the latched lifting rod and the control rod translate together with the lead screw, the latch mounted to the lead screw further configured to open responsive to a loss or removal of electrical power to unlatch the lifting rod wherein the unlatched lifting rod and the control rod fall under the influence of gravity to scram the control rod when the latch is released;wherein the latch does not include the rotating nut of the drive mechanism. 2. The control rod mechanism as set forth in claim 1, wherein the control rod comprises a plurality of control rods and the lifting rod is configured to support the weight of the plurality of control rods by a spider assembly connecting the lifting rod with the plurality of control rods. 3. The control rod mechanism as set forth in claim 1, further comprising:an electromagnet configured to magnetically control the latch, wherein loss or removal of electrical power to the electromagnet causes the latch to open to unlatch the lifting rod. 4. The control rod mechanism as set forth in claim 3, wherein the electromagnet has a length greater than or equal to the length of a linear translation stroke over which the latch can be linearly translated by the drive mechanism. 5. The control rod mechanism as set forth in claim 3, wherein the latch includes permanent magnets that are attracted to the electromagnet when electrical power is supplied to the electromagnet. 6. The control rod mechanism as set forth in claim 3, wherein the latch is mechanically biased open and electrical power to the electromagnet operates to magnetically close the latch to latch with the lifting rod. 7. The control rod mechanism as set forth in claim 6, wherein the latch is mechanically biased open by a spring. 8. The control rod mechanism as set forth in claim 3, wherein the electromagnet is configured to not move with the lead screw when the drive mechanism operates to linearly translate the lead screw. 9. The control rod mechanism as set forth in claim 8, further comprising:an electromagnet housing that houses the electromagnet, the latch also being disposed inside the electromagnet housing. 10. the control rod mechanism of claim 1 wherein the lifting rod passes through the lead screw and the latch is mounted to an upper end of the lead screw and is configured to latch with an upper end of the lifting rod. 11. A nuclear reactor comprising:a reactor core;a pressure vessel including a lower vessel section containing the reactor core, an upper vessel section disposed above the reactor core and above the lower vessel section, and a mid-flange located above a reactor core and disposed between the lower vessel section and the upper vessel section; anda control rod mechanism as set forth in claim 1 disposed inside the pressure vessel and supported by the mid-flange. 12. The nuclear reactor as set forth in claim 11, further comprising:an integral steam generator disposed in the pressure vessel above the mid-flange. 13. The nuclear reactor as set forth in claim 11, further comprising:an internals basket assembly supporting the control rod mechanism and secured to the mid-flange. 14. The nuclear reactor as set forth in claim 11, further comprising:control rod guide frames supported by the mid-flange. 15. The nuclear reactor as set forth in claim 11, further comprising:a hydraulic shutdown control rod drive mechanism supported by the mid-flange. 16. The nuclear reactor as set forth in claim 11, wherein the mid-flange provides electrical and hydraulic inputs to the drive mechanism of the control rod mechanism. 17. A control rod drive mechanism (CRDM) for use in a nuclear reactor, the CRDM comprising:a drive motor including a rotor;a hollow lead screw through which the lifting rod passes and engaged with the drive motor, the lead screw configured to translate but not rotate in response to rotation of the rotor of the drive motor;a latch assembly mounted to the lead screw; anda lifting rod configured to support the weight of a control rod;wherein the latch assembly mounted to the lead screw is configured to have:(i) a latched state in which the lifting rod is latched to the lead screw by the latch assembly mounted to the lead screw, and(ii) an unlatched state in which the latch assembly mounted to the lead screw unlatches responsive to a loss of power initiating scram to unlatch the lifting rod from the lead screw. 18. the control rod mechanism of claim 17 wherein:the lifting rod passes through the lead screw and the latch assembly is mounted to an upper end of the lead screw, andin the latched state the upper end of the lifting rod is latched to the lead screw by the latch assembly mounted to the upper end of the lead screw. 19. A control rod mechanism for use in a nuclear reactor, the control rod mechanism comprising:a control rod configured for insertion in a reactor core to absorb neutrons;a lifting rod having a bottom end connected with the control rod;a hollow lead screw through which the lifting rod passes;a motor-driven nut configured such that rotating the nut causes linear translation of the hollow lead screw; anda latch mounted on a top end of the hollow lead screw, a top end of the lifting rod being latched by the latch to the top end of the hollow lead screw to move together with the hollow lead screw in response to rotating the nut, the latch configured to open in response to a loss or removal of electrical power to unlatch the top end of the lifting rod from the top end of the hollow lead screw. 20. The control rod mechanism as set forth in claim 19, wherein the control rod comprises a plurality of control rods connected with the bottom end of the lifting rod via a spider assembly. 21. The control rod mechanism as set forth in claim 19, further comprising:an electromagnet magnetically controlling the latch, wherein loss or removal of electrical power to the electromagnet causes the latch to open to unlatch the top end of the lifting rod from the top end of the hollow lead screw.