Abstract:
A control rod drive mechanism (CRDM) comprises a lead screw, a motor threadedly coupled with the lead screw to linearly drive the lead screw in an insertion direction or an opposite withdrawal direction, a latch assembly secured with the lead screw and configured to (i) latch to a connecting rod and to (ii) unlatch from the connecting rod, the connecting rod being free to move in the insertion direction when unlatched, and a release mechanism configured to selectively unlatch the latch assembly from the connecting rod.

Description:
BACKGROUND 
       [0001]    In a pressurized water reactor (PWR) or other type of nuclear reactor, movable control rods are used to control the nuclear reaction. The control rods include a neutron absorbing material, and are arranged to be inserted into the reactor core. In general, the further the control rods are inserted into the core, the more neutrons are absorbed and the more the nuclear reaction rate is slowed. Precise control of the amount of insertion, and accurate measurement of same, is useful in order to precisely control the reactivity. The control rods drive mechanism (CRDM) provides this control. 
         [0002]    In an emergency, the control rods can be fully inserted in order to quickly quench the nuclear reaction. In such a “scram”, it is useful to have an alternative fast mechanism for inserting the control rods. Additionally or alternatively, it is known to have dedicated control rods that are either fully inserted (thus turning the nuclear reaction “off”) or fully withdrawn (thus making the reactor operational). In such systems, the “on/off” rods are sometimes referred to as “shutdown rods” while the continuously adjustable control rods are sometimes referred to as “gray rods”. 
         [0003]    Given these considerations, it is known to construct a CRDM employing a lead screw that is engaged by a separable roller-nut assembly. During normal operation, the roller-nut assembly is clamped onto the lead screw by an affirmative magnetic force acting against biasing springs. By turning the roller nut the lead screw, and hence the attached control rods, are moved in precisely controllable fashion toward or away from the reactor core. In a scram, the electrical current is cut thus cutting the magnetic force, the biasing springs open the separable roller nut, and the gray rod including the lead screw scrams. An example of such a configuration is disclosed, for example, in Domingo Ricardo Giorsetti, “Analysis of the Technological Differences Between Stationary &amp; Maritime Nuclear Power Plants”, M.S.N.E. Thesis, Massachusetts Institute of Technology (MIT) Department of Nuclear Engineering (1977) which is incorporated herein by reference in its entirety. 
         [0004]    For an integral pressurized water reactor (integral PWR), it is known to mount the CRDM externally and to couple with the control rods inside the pressure vessel by suitable feedthroughs. To reduce the extent of feedthroughs, it has also been proposed to integrate the CRDM within the pressure vessel. See, for example, Ishizaka et al., “Development of a Built-In Type Control Rod Drive Mechanism (CRDM) For Advanced Marine Reactor X (MRX)”, Proceedings of the International Conference on Design and Safety of Advanced Nuclear Power Reactors (ANP &#39;92), Oct. 25-29, 1992 (Tokyo Japan) published by the Atomic Energy Society of Japan in October 1992, which is incorporated herein by reference in its entirety. 
         [0005]    Existing CRDM designs have certain disadvantages. These disadvantages are enhanced when an internal CRDM design is chosen in which the complex electro-mechanical CDRM is internal to the high pressure and high temperature environment within the pressure vessel. Placement of the CRDM internally within the pressure vessel also imposes difficult structural challenges. 
         [0006]    The separable roller-nut creates a complex linkage with the lead screw that can adversely impact gray rod insertion precision during normal operation. Reattachment of the roller-nut to the lead screw can be complex, and it may not be immediately apparent when contact is reestablished, thus introducing a positional offset after recovery from the scram event. Scramming the lead screw also has the potential to cause irrecoverable damage to the threading or structural integrity of the lead screw. Still further, wear over time can be a problem for the complex separable roller-nut. 
         [0007]    Another consideration is reliability. Because rod scramming is a safety-critical feature, it must operate reliably, even in a loss of coolant accident (LOCA) or other failure mode that may include interruption of electrical power, large pressure changes, or so forth. 
         [0008]    The control rod position detector is also typically a complex device. In some systems, an external position detector is employed, which requires feedthroughs across the pressure vessel wall. For the internal CRDM of the MRX reactor, a complex position detector was designed in which a transducer generates a torsional strain pulse that passes through a magnetoresistive waveguide, and magnetic field interactions are measured to adduce the rod position. In general, an internal position detector operating on an electrical resistance basis is prone to error due to temperature-induced changes in material resistivity. 
       BRIEF SUMMARY 
       [0009]    In one aspect of the disclosure, a control rod mechanism for use in a nuclear reactor comprises: at least one control rod configured for insertion in a reactor core to absorb neutrons; a hollow lead screw; a motor operatively coupled with the hollow lead screw to drive the hollow lead screw linearly toward or away from the reactor core; a connecting rod connected with the aforementioned at least one control rod and disposed partially inside the hollow lead screw; a latch assembly having latches that when closed operatively connect the connecting rod and the lead screw so that when the latches are closed the connecting rod and the aforementioned at least one control rod move together with the lead screw when the lead screw is driven by the motor; and a release mechanism configured to cause the latches of the latch assembly to open responsive to a scram condition to detach the connecting rod from the lead screw such that the connecting rod and the aforementioned at least one control rod scram but the lead screw remains operatively coupled with the motor and does not scram. 
         [0010]    In another aspect of the disclosure, a control rod drive mechanism (CRDM) comprises: a lead screw; a motor threadedly coupled with the lead screw to linearly drive the lead screw in an insertion direction or an opposite withdrawal direction; a latch assembly secured with the lead screw and configured to (i) latch to a connecting rod and to (ii) unlatch from the connecting rod, the connecting rod being free to move in the insertion direction when unlatched; and a release mechanism configured to selectively unlatch the latch assembly from the connecting rod. 
         [0011]    In another aspect of the disclosure, a control rod drive mechanism (CRDM) comprises: a plurality of CRDM units each comprising a lead screw and a motor configured to drive the lead screw; and a support mounting the plurality of CRDM units in a nuclear reactor vessel with the motors of adjacent CRDM units arranged at different heights respective to a reactor core of the nuclear reactor vessel. Each CRDM unit is connected with one or more control rods such that the motor driving the lead screw moves the connected one or more control rods toward, away from, or within the reactor core. 
         [0012]    In another aspect of the disclosure, a control rod drive mechanism (CRDM) comprises: a lead screw; a drive assembly configured to linearly drive the lead screw in an insertion or opposite withdrawal direction, the drive assembly including a motor and at least one non-separable ball nut coupling with the lead screw; and a latch assembly connected with the lead screw and having (i) a latched state in which the latch assembly is latched to a connecting rod and (ii) an unlatch state in which the latch assembly is not latched to the connecting rod. 
         [0013]    In another aspect of the disclosure, a control rod mechanism for use in a nuclear reactor comprises: at least one control rod; a connecting rod connected with the aforementioned at least one control rod at a lower end of the connecting rod; and a control rod drive mechanism (CRDM) including a latch assembly having (i) a latched state in which the latch assembly is latched to an upper end of the connecting rod and (ii) an unlatched state in which the latch assembly is not latched to the upper end of the connecting rod, and a linear drive mechanism configured to drive the latch assembly linearly toward or away from a nuclear reactor core. 
         [0014]    In another aspect of the disclosure, in a control rod mechanism as set forth in the immediately preceding paragraph the CRDM is configured to allow the connecting rod to be removed by placing the latch assembly in the unlatched state and drawing the connecting rod away from the nuclear reactor core through the CRDM. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0015]    The invention may take form in various components and arrangements of components, and in various process operations and arrangements of process operations. The drawings are only for purposes of illustrating preferred embodiments and are not to be construed as limiting the invention. 
           [0016]      FIG. 1  diagrammatically shows an illustrative nuclear reactor vessel of the pressurized water reactor (PWR) type. 
           [0017]      FIG. 2  diagrammatically shows the upper internals section of the illustrative nuclear reactor vessel of  FIG. 1 . 
           [0018]      FIGS. 3-5  diagrammatically show aspects of a shutdown control rod system employing hydraulic lift. 
           [0019]      FIGS. 6-15  diagrammatically show aspects of a control rod system with electromagnetic gray rod functionality and a magnetic latch system for scram functionality. 
           [0020]      FIGS. 16-21  diagrammatically show aspects of a control rod system with electromagnetic gray rod functionality and a latch system driven by a hydraulic lift for scram functionality. 
           [0021]      FIGS. 22 and 23  show perspective and perspective partial sectional views, respectively, of a suitable array of CDRM employing a staggered vertical motor arrangement. 
           [0022]      FIGS. 24 and 25  show perspective and exploded perspective views, respectively, of a “J”-groove coupling between the lower end of a connecting rod and a rod cluster assembly. 
       
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
       [0023]    With reference to  FIG. 1 , an illustrative nuclear reactor vessel of the pressurized water reactor (PWR) type is diagrammatically depicted. An illustrated primary vessel  10  contains a reactor core  12 , internal helical steam generators  14 , and internal control rods  20 . The illustrative reactor vessel includes four major components, namely: 1) a lower vessel  22 , 2) upper internals  24 , 3) an upper vessel  26   
         [0024]    The lower vessel  22  of the illustrative reactor vessel  10  of  FIG. 1  contains the reactor core  12 , which can have substantially any suitable configuration. One suitable configuration includes a stainless steel core former structure that contains the fuel assemblies and is replaceable in order to refuel the reactor, and which is supported by the lower vessel. The illustrative upper vessel  26  houses the steam generators  14  for this illustrative PWR which has an internal steam generator design (sometimes referred to as an integral PWR design). In  FIG. 1 , the steam generator  14  is diagrammatically shown. A cylindrical inner shell or upper flow shroud  30  separates a central riser region  32  from an annular downcomer region  34  in which the helical steam generators  14  are located. The illustrative steam generator  14  is a helical coil design, although other designs are contemplated. Primary reactor coolant flows across the outside of tubes of the steam generator  14  and secondary coolant flows inside the tubes of the steam generator  14 . In a typical circulation pattern the primary coolant is heated by the reactor core  12  and rises through the central riser region  32  to exit the top of the shroud  30  whereupon the primary coolant flows back down via the downcomer region  34  and across the steam generators  14 . Such primary coolant flow may be driven by natural convection, by internal or external primary coolant pumps (not illustrated), or by a combination of pump-assisted natural convection. Although an integral PWR design is illustrated, it is also contemplated for the reactor vessel to have an external steam generator (not illustrated), in which case pressure vessel penetrations allow for transfer of primary coolant to and from the external steam generator. The illustrative upper vessel head  28  is a separate component. It is also contemplated for the vessel head to be integral with the upper vessel  26 , in which case the steam generator  14  and upper shroud  30  are optionally supported by lugs on the inside of the vessel head. 
         [0025]    The illustrative embodiment is an integral PWR in that it includes the internal steam generators  14 , which in general may have various geometric configurations such as helical, vertical, slanted, or so forth. For the purpose of redundancy, it is generally advantageous to have more than one steam generator, whose pipes or tubes are typically interleaved within the downcomer region  34  to facilitate thermal uniformity; however, it is contemplated to include only a single steam generator. Although the illustrative steam generators  14  are shown disposed or wrapped proximate to the shroud  30 , in general the steam generators may fill a substantial volume of the downcomer region  34 , and in some embodiments the steam generators may substantially fill the annular volume between the outer surface of the shroud  30  and the inside surface of the pressure vessel  10 . It is also contemplated for the internal steam generators or portions thereof to be disposed in whole or in part in the riser region  32 , above the shroud  30 , or elsewhere within the pressure vessel  10 . On the other hand, in some embodiments the PWR may not be an integral PWR, that is, in some embodiments the illustrated internal steam generators may be omitted in favor of one or more external steam generators. Still further, the illustrative PWR is an example, and in other embodiments a boiling water reactor (BWR) or other reactor design may be employed, with either internal or external steam generators. 
         [0026]    With reference to  FIG. 2 , the upper internals section  24  in greater detail. In the illustrative design the upper internals section  24  provides support for control rod drives or drive mechanisms  40 ,  42  and control rod guide frames  44  and is also the structure through which control rod drive power and control instrumentation signals pass. This allows the upper vessel  26  and integral steam generator  14  to be removed independently of the control rod drives and associated structure. However, a more integrated design is also contemplated, such as using a common section for both the CRDM support and the integral steam generator support. 
         [0027]    With particular reference to the illustrative embodiment of  FIG. 2 , the upper internals structure  24  includes an upper internals basket  46 , a CRDM support structure  48 , control rod guide frames  44 , and the control rod drive mechanisms  40 ,  42  themselves. The upper internals basket  46  is suitably a welded structure that includes the mid-flange  29  and the support structure for the control rod guide frames  44 . In one suitable embodiment, the control rod guide frames  44  are separate 304L stainless steel welded structures that are bolted in place, the mid-flange  29  is a SA508 Gr 4N Cl 2 carbon steel forging, and the balance of the structure is 304L stainless steel. The CRDM support structure  48  includes support lattices for the control rod drives  40 ,  42  and guide structure for the in-core instruments. All of these are suitably 304L stainless steel. The CRDM support structure  48  is bolted to the upper internals basket  46 . These are merely illustrative materials and construction, and other configurations and/or reactor-compatible materials are also contemplated. 
         [0028]    The illustrative example of  FIG. 2  employs two types of control rod drives  40 ,  42 : a hydraulic control rod drive type  42  that operates the shutdown rods which are either fully withdrawn or fully inserted into the core; and an electrical control rod drive type  40  that operates the gray rods which are inserted various amounts throughout the life of the core to control the nuclear reaction rate during normal reactor operation. The gray rods are also configured to scram, that is, to be rapidly inserted into the reactor core  12 , during certain emergency conditions. In other embodiments, it is contemplated to omit the shutdown rods entirely in which case the gray rods also provide shutdown operation. 
         [0029]    With continuing reference to  FIG. 2  and with further reference to  FIGS. 3-5 , aspects of the shutdown rods are illustrated. The shutdown rods are suitably arranged in clusters mounted on spiders or the like that are all operated in single bank and are all moved by a single shutdown rod drive  42 .  FIGS. 3-5  show only the single shutdown rod drive  42 , but not the spiders and individual shutdown rods. This configuration is cognizant of the fact that the shutdown rods are used in a binary “on/off” manner, and are either all wholly inserted into the reactor core  12  in order to shut down the reaction, or are all wholly withdrawn from the reactor core  12  in order to allow normal reactor operation. 
         [0030]    With particular reference to  FIG. 3 , the shutdown rod drive  42  includes a cylinder housing  50 , a cylinder cap  52 , a cylinder base plate  54 , and a connecting rod  56  providing connection to the shutdown rod lattice (not shown). The illustrative shutdown rod drive  42  of  FIGS. 3-5  is a hydraulically actuated drive using reactor coolant inventory clean-up return fluid from high pressure injection pumps at approximately 500° F. (260° C.) and 1600 psi to hold the shutdown rod bank out of the reactor core  12 . 
         [0031]    With particular reference to  FIG. 4 , a sectional view of the piston region with the rod in the withdrawn position is shown. In an enlarged portion of  FIG. 4  a vent port  60  of the cylinder cap  52  is shown, together with a lift piston  62 , piston rings  64  (which in some embodiments are metallic), a scram buffer  66 , and a buffer cocking spring  68 . The withdrawn position shown in  FIG. 4  corresponds to the shutdown control drive cylinder  42  being pressurized. 
         [0032]    With particular reference to  FIG. 5 , a sectional view of the piston region with the rod in the inserted position is shown. An enlarged portion of  FIG. 5  shows the lift piston  62 , the piston rings  64 , the scram buffer or scram buffer piston  66 , a rod guide bushing  70 , and rod sealing rings  72  (which in some embodiments are metallic). The cylinder base plate  54  is seen in the enlarged portion to include a pressure port or inlet port  74 . The inserted position shown in  FIG. 5  corresponds to the shutdown control drive cylinder  42  being unpressurized. 
         [0033]    In some embodiments, the coolant is allowed to bleed past the piston and shaft seals  64 ,  72  and becomes part of the inventory returned to the reactor vessel  10 . The shutdown rod drive cylinder  42  is mounted above the reactor core  12 . A hydraulic line (not shown) to actuate the cylinder  42  is routed through the flange  29  and instrument lines are routed through pressure tight conduit to common connectors that are also optionally used for the gray rod drives  40 . The extension rods that connect the control rod spiders to the shutdown rod lattice are optionally designed so that they will slide through the lattice so that a single stuck cluster will not prevent the other sets of control rods from dropping. Additionally, the extension rods are designed to be disengaged from the control rod spider so that the shutdown rods remain in the core when the upper internals  24  are removed. Disengagement and reengagement is done using remote tooling at during refueling operations. 
         [0034]    During normal reactor operation, the shutdown rods are suspended completely out of the reactor core (that is, in the withdrawn position) by pressurization of the shutdown rod hydraulic cylinder  42 . For example, in one suitable embodiment coolant inventory clean-up return fluid from the high pressure injection pumps is supplied at 500° F. (260° C.) and 1600 psi to the underside of the lift cylinder piston  62 , via the inlet port  74  of the cylinder base  54 . In this example, the fluid present in the cylinder  50  above the piston  62  is supplied from the reactor vessel  10  through the cylinder cap vent port  60 , and is therefore at the reactor vessel conditions of 600° F. (315° C.) and 1500 psi, resulting in a net  100  psi pressure differential across the piston  62 . Piston sizing is selected such that the developed pressure differential is sufficient to support the specified load of the shutdown rods and supporting spiders and other associated components and lift the shutdown rod bank through the cylinder stroke to the top stop of the piston  62 . 
         [0035]    In the event of a vessel-pressurized scram, the shutdown rod bank is abruptly released by ceasing the supply of pressurized coolant to the bottom side of the lift piston  62  and venting the supply line to atmospheric pressure. In the aforementioned example the vessel pressure at the top surface of the lift piston  62  is expected to create an initial  1500  psig pressure differential across the lift piston, which acts along with the influence of gravity to propel the translating assembly (including the lift piston  62 , scram buffer piston  66 , cocking spring  68 , connecting rod  56 , and shutdown rod lattice (not shown) downward toward the full insertion position illustrated in  FIG. 5 . During the descent of the translating assembly, the force of the buffer cocking spring  68  holds the buffer piston  66  out of the bore of the lift piston  62 , preserving a fluid-filled buffer cavity between the two pistons  62 ,  66 . When the bottom surface of the buffer piston  66  impacts the fixed base plate  54  of the cylinder assembly, the continued travel of the lift piston  62  expels the trapped fluid through controlled flow restrictions, thereby dissipating the kinetic energy of the translating assembly. Additionally, kinetic energy is dissipated through elastic deformation of the translating assembly components, especially the long, relatively slender, connecting rod  56 . Other kinetic energy dissipation mechanisms are also contemplated. When the fluid is expelled from the cavity, the lift piston  62  impacts the buffer piston  66 , bringing the translating assembly to rest. 
         [0036]    With continuing reference to  FIGS. 1 and 2  and with further reference to  FIGS. 6-14 , an illustrative embodiment of the gray rods and associated drive mechanisms  40  is described. As seen in  FIG. 6 , in the illustrative embodiment there are two different gray rod configurations (Type  1  and Type  2 ). The gray rods  80  are arranged as gray rod clusters, which in turn are yoked together in groups of two or four and supported by connecting rods  82  as shown in  FIG. 6 . The configuration Type  1  also includes a counterweight  84  in place of one connecting rod/cluster unit. More particularly, a yoke  86  connects two connecting rods  82  and the counterweight  84  to form a configuration of Type  1 . A yoke  88  connects three connecting rods  84  to form a configuration of Type  2 . The gray rod drives  40  are mounted above the reactor core  12 .  FIG. 7  shows a plan view of the locations of the gray rod drives  40  and of the shutdown rods lift cylinder  50 , respective to the CRDM support structure  48 . The shutdown rods lift cylinder  50  is centrally located. Four outboard gray rod drives  40 , each moving two rod configurations of Type  1  including yokes  86 , move simultaneously. Two inboard drives  40 , each moving four rod configurations of Type  2  including yokes  88 , move simultaneously. These different sets of drives  40  optionally move together or independently. Power and signal connections are suitably routed through a pressure tight conduit or in-core instrumentation guide  90  to connectors on the mid-flange  29  (not shown in  FIG. 7 ). 
         [0037]    As with the shutdown rods, the extension rods that connect the control rod spiders to the rod lattice are optionally designed so that they will slide through the lattice so that a single stuck cluster will not prevent the other sets of control rods from dropping. Additionally, the extension rods are optionally designed to be disengaged from the control rod spider so that the gray rods can remain in the core when the upper internals are removed or be removed while the upper internals are on their support stand. Two suitable design styles for the gray rod control mechanism include the “magnetic jack” type and the “power screw” type. Of these, the power screw type is expected to provide more precise position control for the gray rod clusters, and accordingly the illustrated embodiment employs the power screw type control mechanism. 
         [0038]    With reference to  FIG. 8 , in one illustrated embodiment the gray rod control mechanism  40  employs a ball nut lifting rod configuration.  FIG. 8  shows both the fully inserted state (left-side drawing) and fully withdrawn state (right-side drawing). The drawings of  FIG. 8  show the yoke  88  of the Type  2  configuration; for the Type  1  arrangement the yoke  88  is replaced by the yoke  86 . In the embodiment shown in  FIG. 8 , a bottom stop/buffer assembly  100  is mounted on a reactor support  101 , optionally with additional lateral support provided for the electromagnet coil assembly. Lower and upper support tubes  102 ,  104 , which mount to the top of the bottom stop  100 , provide the guidance for the lead screw/torque taker assembly. A ball nut/motor assembly  106  mounts on top of the upper support tube  104  and an electromagnet coil assembly  108  mounts to the top of the motor. Within the electromagnet coil assembly  108  resides a lifting rod-to-lead screw latching assembly  110  that (when latched) supports a lifting/connection rod assembly  112  (seen extended in the inserted state, i.e. left-side drawing). 
         [0039]    A position indicator assembly is mounted to the support tubes  102 ,  104  between the ball nut/motor assembly  106  and the bottom stop assembly  100 . In some embodiments, the position indicator is a string potentiometer suitably mounted below the latching assembly  110 , although other mounting locations are contemplated. The illustrated string potentiometer includes a tensioned spool  120  mounted on the support tube  102  and a “string” or cable or the like  122  having an end attached to the lifting/connection rod assembly  112  such that the string or cable  122  is drawn off the spool  120  against the tension as the lifting/connection rod assembly  112  (and, hence the attached gray rod clusters) move toward the reactor core  12  (not shown in  FIG. 8 ). When the motion is reversed, the tension in the tensioned spool  120  causes the string or cable  122  to roll back onto the spool  120 . A rotational sensor  124  measures the rotation of the tensioned spool  120  using an encoder that counts passage of fiducial markers or another rotational metric. The mounting of the string potentiometer can be otherwise than that illustrated, so long as the tensioned spool  120  is mounted at a location that does not move with the gray rods and the string or cable  122  is secured to move with the gray rods. It is also contemplated to integrate the rotational sensor  124  with the tensioned spool  120 . The string potentiometer provides an electrical output signal consistent with the location of the connecting rod or other component  112  that moves with the gray control rod, thus providing positional information for the gray control rods within the reactor core  12 . The electrical position indication signal is conveyed out of the reactor vessel  10  through an electrical feedthrough (not shown), which can be made small and/or integrated with other electrical feedthroughs. The position indicator device is configured and calibrated for operation at reactor vessel temperature and radiation level. 
         [0040]    With continuing reference to  FIG. 8  and with further reference to  FIGS. 9-14 , in the illustrated embodiment the translating assembly of the gray rod CRDM  40  includes three elements: a lead screw/torque taker assembly; a lifting rod/connecting rod assembly; and a latching system that operatively connects the lifting rod with the lead screw.  FIG. 9  shows the lead screw/torque taker assembly in perspective (left side) and sectional (right side) views. A motor assembly includes a stator housing  130  housing a stator  132  and a rotor  134 . An upper stator end plate  136  and a radial bearing  138  with adjustable spacer  140  complete an upper portion of the motor assembly, while a lower housing  142  and a thrust bearing  144  complete a lower portion of the motor assembly. A lower ball-nut assembly  150  disposed within the lower housing  142  is threaded to the rotor  134 , and an upper ball nut assembly  152  is also threaded to the rotor  134 . Both ball-nut assemblies  150 ,  152  are coupled in threaded fashion with a lead screw  160  (shown in part in  FIG. 9 ).  FIG. 9  further shows portions of the lifting rod  112  and the upper support tube  104 . 
         [0041]    With reference to  FIG. 10 , the latching system is illustrated, including the lifting rod-to-lead screw latching assembly  110  and a portion of the electromagnet coil assembly  108 . Also shown in  FIG. 10  are an end  111  of the lifting rod  112  and a proximate end of the lead screw  160  terminating at or in the latching assembly  110 . Latches  170  directly connect the top end  111  of the lifting rod  112  to the lead screw  160  for normal operation, and disconnect the lifting rod  112  during scram (see  FIG. 11 ). The bottom of the lifting rod  112  is threaded to the top of the connecting rod  82  (optionally by the intermediary yoke  86  or intermediary yoke  88 ) thereby creating a continuous lifting rod/connecting rod assembly. The bottom of the connecting rod  82  couples directly to the control rod spiders thereby attaching the control rods to the mechanism. Optionally, a magnet  113  is disposed proximate to the top  111  of the lifting rod  112  to provide a magnetic signal for a magnetically-based position indicator (see  FIG. 21 ).  FIG. 10  also shows a portion of the motor including portions of the motor housing  130 , stator  132 , and rotor  134 , which is shown in full in  FIG. 9 . 
         [0042]    The latches  170  are housed in a latch housing  172  that includes a spring guide for a latch spring  174 . Additional components of the illustrated latching system embodiment include an electromagnet housing  176  housing electromagnets  177  forming an electromagnet coil stack, and permanent magnets  178  on the latches  170 . The lead screw  160  is threaded into a latching system base  179  of the latch housing  172 . The latches  170  are arranged to pivot about pivot locations  180  to provide a failsafe scram due to downward rod load. 
         [0043]    In this embodiment, the lead screw  160  is continuously supported by a ball nut motor assembly (best seen in  FIG. 9 ) which allows for very fine control of lead screw position and consequently very fine control of the position of the control rod assembly. In the illustrated embodiment, the motor (e.g., stator  132 , rotor  134 ) is a synchronous motor in which the rotor  134  is a permanent magnet. This design has advantages such as compactness and simplicity; however, other motor configurations are also contemplated. 
         [0044]    The lead screw  160  does not scram. Instead, during a scram the top end of the lifting rod  112  of the lifting rod/connection rod assembly is disconnected from the lead screw  160  by the magnetically activated latching system (see  FIG. 11 ). When power is cut to the electromagnets  177  the failsafe latching system releases the lifting/connection rod assembly (and thus the control rod assembly) from the lead screw  160  thereby initiating a scram. A bottom stop and buffering system (not illustrated, but suitably similar to the bottom stop and buffering system of the illustrative shutdown rods described herein with reference to  FIGS. 4 and 5 ) is incorporated into the base/buffer assembly to dissipate the kinetic energy at the end of the scram stroke and to set the rod bottom elevation. A torque taker (not shown) is attached to the lead screw  160  to react the motor torque thereby providing translation of the lead screw/control rod assembly. 
         [0045]    The normal state, that is, the state prior to scram, is shown in  FIGS. 9 and 10 .  FIG. 9  illustrates the ball nut motor assembly and  FIG. 10  shows the latching system engaged for normal operation. As seen in  FIG. 10 , the permanent magnets  178  on the latches  170  are magnetically attracted toward the powered electromagnets  177  thus pivoting the latches  170  about the pivot locations  180  and engaging the latches  170  with a mating region of the lifting rod  112 . Thus, the latches  170  are secured with the lifting rod  112  in the normal state shown in  FIG. 10 . Further, the latching system base  179  is threaded to or otherwise secured with the lead screw  160 . Accordingly, in the normal state of  FIG. 10  the lifting rod  112  is secured with the lead screw  160  via the latching system, and so as the ball nut motor assembly shown in  FIG. 9  translates the lead screw  160  the lifting rod  112  is translated with the lead screw  160 . 
         [0046]    Scram is described with reference to  FIG. 11 , which shows the lifting rod  112 , and consequently the control rod assembly, during a scram. To initiate scram the power to the electromagnets  177  is cut, that is, turned off. This removes the attractive force on the permanent magnets  178  on the latches  170 , and the latch spring  174  extends to pivot the latches  170  about the pivot locations  180  and away from the mating region of the lifting rod  112 . This disengages the latches  170  from the lifting rod  112 , and the lifting/connection rod assembly (and thus the control rod assembly) falls toward the reactor  12 . The lead screw  160  is seen in  FIG. 11  still at the previous withdrawal height (that is, the lead screw  160  is not scrammed), but power to the electromagnet coils  177  has been cut so that the magnetic field from the coils is removed. 
         [0047]    As further shown in  FIG. 11 , the pivoting of the latches  170  about the pivot locations  180  is stopped by impingement at a location  181  with the spring guide of the latch housing  172 . 
         [0048]    With continuing reference to  FIG. 11  and further reference to  FIGS. 12 and 13 , to re-engage the mechanism after a scram, the lead screw  160  is driven to the fully inserted position via the ball nut motor (see again  FIG. 9 ). A lead screw on-bottom sensor is used to confirm lead screw full insertion. With particular reference to  FIG. 12 , as the lead screw  160  nears the fully inserted position an angled camming surface  182  on the top  111  of the lifting rod  112 , which is scrammed to the bottom, will cam the latches  170  to their near full out position. With particular reference to  FIG. 13 , when power is restored to the electromagnets  177 , the latches  170  will fully re-engage with the mating region of the lifting rod  112  so that the lifting/connection rod assembly is once again connected to the lead screw  160 . Normal operation can then resume as per  FIG. 10 . To reiterate,  FIG. 12  shows the lead screw  160  being driven back down to the fully inserted position in preparation for re-engagement of the lifting rod  112 . Power to the electromagnet coils  177  is still cut and the latches  160  are still disengaged. The angled camming surfaces  182  on the top  111  of the lifting rod  112  are camming the latches  170  back into partial engagement with the top  111  of the lifting rod  112 .  FIG. 13  shows the lead screw  160  still on bottom but with the power restored to the electromagnet coils  177 . The restored magnet field has now re-engaged the latches  170  with the mating region of the lifting rod  112 . 
         [0049]      FIG. 9  diagrammatically shows a suitable embodiment of the ball nut/motor assembly  106 , including lower and upper ball nut assemblies  150 ,  152 . In general, substantially any type of motor can be used, suitably configured for operation in the pressure vessel environment. 
         [0050]    With reference to  FIGS. 14 and 15 , an illustrative embodiment is shown which employs a brushless DC electronically controlled (BLDC) motor  184  with lower ball nut assembly  185 . The assembly  184 ,  185  is an illustrative embodiment of the ball nut/motor assembly  106 . With particular reference to  FIG. 14 , the illustrative BLDC motor  184  includes a wound stator core assembly  186  disposed between a stator outer shell  187  and a stator inner shell  188  and secured by a stator upper housing  189  and stator lower housing  190 . A permanent magnet rotor  191  includes permanent magnets  192 . The BLDC motor  184  produces torque from interaction of magnetic flux of the rotor magnets  192  and the current carrying stator conductors of the stator core assembly  186 . The lower ball nut assembly  185  is analogous to the lower ball-nut assembly  150  of  FIG. 9 ; however, in the illustrative assembly of  FIG. 14  there is no upper ball-nut assembly corresponding to the upper ball nut assembly  152  of  FIG. 9 . The assembly of  FIG. 14  also includes a radial bearing  193 , a thrust bearing  194  secured by a thrust bearing lock nut  195 , and a motor top cap  196 . An insulated and environmentally robust electrical connection to the motor is provided by a lead wire gland  197 . For example, some suitable insulated lead wire glands are available from Conax® Technologies (Buffalo, N.Y., USA). With particular reference to  FIG. 15 , the BLDC motor  184  and lower ball-nut assembly  185  are illustrated in the context of the control rod drive mechanism (CRDM) of  FIGS. 10-13 . The illustrative CRDM of  FIG. 15  also includes the previously described electromagnet coil stack assembly  177 , lifting rod-to-lead screw latching assembly  110 , lead screw  160 , and lifting rod  112 . The ball-nut assembly  185  engages the lead screw  160  so that, as the motor  184  rotates the permanent magnet rotor  191  and the secured ball-nut assembly  185 , the lead screw  160  is driven linearly. 
         [0051]    With returning reference to  FIGS. 1 and 2 , an advantage of the disclosed reactor design is that the middle section includes the internals support flange or “mid-flange”  29 . This section can be made relatively thin, and provides support for the control rod drive mechanism and guides for the in-core instrumentation. This section provides electrical and hydraulic inputs for the control rod drive mechanisms (CRDMs). A reactor coolant drain penetration (not illustrated) is optionally also incorporated in this section. This drain line, if incorporated, is optionally isolated by an internal valve whenever the reactor is pressurized to limit or eliminate its potential as a loss of coolant accident (LOCA) site. 
         [0052]    The illustrated upper internals  24  including the CRDM do not include illustrated thermal insulation. However, it is contemplated to insulate these components using an insulation system capable of withstanding a design temperature of at least about 650° F. (343° C.). By using the insulation system, external cooling water will not be required although may optionally also be used. For example, cooling water can be supplied to the electrical devices to reduce the severity of the heat duty imposed by the operating environment. The insulation system facilitates locating the electrical CRDM within the pressure vessel, which reduces the overall height of the reactor vessel  10 , significantly reduces the number of penetrations into the reactor vessel  10 , and enables a complete reactor module to be shipped as a single unit. Another advantage is reduction of the overall height of the containment structure (not shown). Although the use of insulation is believed to be advantageous, other contemplated solutions include the use of water cooling and/or selecting materials capable of withstanding the high operating temperature without insulation. 
         [0053]    The illustrative reactor embodiment is an integral pressurized water reactor (PWR) configuration. However, one or more of the disclosed techniques, apparatuses, or so forth are also expected to be suitably used in other types of nuclear reactor vessels, such as boiling water reactors (BWRs) that can advantageously incorporate internal CRDM assemblies, efficient control rod position sensors, and so forth. 
         [0054]    The CDRM configuration of  FIGS. 2-15  provides two separate scram mechanisms: a hydraulic scram provided by the shutdown rods described with reference to  FIGS. 3-5 ; and a magnetic latch scram mechanism described with reference to  FIGS. 6-15  with the magnetic latch system described with particular reference to  FIGS. 10-15 . This advantageously provides redundant hydraulic and magnetic scram mechanisms thus reducing likelihood of a complete scram failure in the event of a loss of coolant accident (LOCA) or other safety-related event. 
         [0055]    With reference to  FIGS. 16-20 , in another control rod system embodiment is described, which provides electromagnetic gray rod functionality and a hydraulic latch system providing scram functionality. Like the control rod system of  FIGS. 6-15 , the control rod system of  FIGS. 16-20  allows for failsafe scram of the control rod cluster without scramming the lead screw. 
         [0056]    With particular reference to  FIG. 16 , the motor/ball nut assembly of  FIG. 15  is employed, such that a lead screw  200  is permanently engaged to the ball-nut assembly  185  which provides for axial translation of the lead screw  200  by driving the motor  184 . A control rod cluster (not shown in  FIG. 16 ) is connected to the lead screw  200  via a connecting rod or connecting rod assembly  204  and a latch assembly  206 . The lead screw  200  is substantially hollow, and the connecting rod assembly  204  fits coaxially inside the inner diameter of the lead screw  200  and is free to translate vertically within the lead screw  200 . The latch assembly  206 , with two spring loaded latches, is permanently attached to the top of the lead screw  200 . When the latches are engaged with the connecting rod  204  they couple the connecting rod  204  to the lead screw  200  and when the latches are disengaged they release the connecting rod  204  from the lead screw. In the illustrated embodiment, latch engagements and disengagements are achieved by using a four-bar linkage cam system including two cam bars  208  and at least two (and, in the illustrated embodiment four) cam bar links  209  per cam bar  208 . The additional cam bar links add support for the cam bar. When the cam bars  208  move upward the cam bar links  209  of the four-bar linkage also cams the cam bars  208  inward so as to cause the latches to rotate into engagement with the connecting rod  204 . In the illustrated embodiment, a hydraulic lift assembly  210  is used to raise the cam bar assemblies  208 . In an alternative embodiment (not illustrated), an electric solenoid lift system is used to raise the cam bar assemblies. When a lift force is applied to the cam system, the upward and inwardly-cammed motion of the cam bars  208  rotates the latches into engagement thereby coupling the connecting rod  204  to the lead screw  200 . This causes the control rod cluster to follow lead screw motion. When the lift force is removed, the cam bars  208  swing down and are cammed outward by the cam bar links  209  of the four-bar linkage allowing the latches to rotate out of engagement with the connecting rod  204 . This de-couples the connecting rod  204  from the lead screw  200  which causes the control rod cluster to scram. During a scram, the lead screw  200  remains at its current hold position. After the scram event, the lead screw  200  is driven to the bottom of its stroke via the electric motor  202 . When the lift force is reapplied to the cam system via the hydraulic lift assembly  210 , the latches are re-engaged and the connecting rod is re-coupled to the lead screw  200 , and normal operation can resume. Still further, other latch drive modalities are contemplated, such as a pneumatic latch drive in which pneumatic pressure replaces hydraulic pressure in the illustrated lift assembly  210 . 
         [0057]    With continuing reference to  FIG. 16 , the lead screw  200  is arbitrarily depicted in a partially withdrawn position for illustration purposes. It can be seen in  FIG. 16  that the latching assembly  206  is attached to the top of the lead screw  200 . In  FIG. 16  the latches are engaged the connecting rod  204 , which is coupled to the lead screw, is also at the same partially withdrawn position as the lead screw  200 . The ball nut  185  and motor  184  are at the bottom of the control rod drive mechanism (CDRM), the latch cam bars  208  extend for the full length of mechanism stroke, and the hydraulic lift system  210  is located at the top of the mechanism. In some embodiments, the CRDM of  FIGS. 16-20  is an integral CDRM in which the entire mechanism, including the electric motor  184  and ball nut  185 , the latching system  206 , and a position indicator (not shown in  FIG. 16 ), are located within the reactor pressure vessel  10  at full operating temperature and pressure conditions. 
         [0058]    With reference to  FIGS. 17 and 18 , the lower end of the control rod drive mechanism (CRDM) including the latching assembly  206  is illustrated in additional detail. The latching assembly  206  includes a latch housing  212  and latches  214 . The latch housing  212  provides a frame or support for pivot positions  216  (e.g., pivot pins or rods) about which the latches  214  can pivot. In  FIG. 16 , the connecting rod  204  is withdrawn, that is, latches  214  of the latching assembly  206  are pivoted inwardly into engagement with mating region at an upper end  215  of the connecting rod  204 . In the illustrative embodiment, the top of the connecting rod  204  includes the optional magnet  113  to provide a magnetic signal for a magnetically-based position indicator (see  FIG. 21 ).  FIG. 17  shows the connecting rod  204  scrammed, that is, latches  214  are pivoted outwardly so as to be disengaged from the mating region at the upper end  215  of the connecting rod  204  so that the connecting rod  204  is mechanically decoupled from the lead screw  200  and is free to move within the inner diameter of the lead screw  200 . Thus decoupled as shown in  FIG. 17 , the connecting rod  204  (and hence the control rod bundle or bundles secured to the connecting rod  204 ) fall toward the reactor core  12  under the influence of gravity. In both  FIGS. 16 and 17 , the lead screw  200  is again shown slightly withdrawn to an arbitrarily position—as seen in  FIG. 17  the lead screw  200  does not scram. 
         [0059]    Referring particularly to  FIG. 17 , the two cam bars  208  are shown raised at their maximum inward (that is, engaged) position. The inward movement of the cam bars  208  caused by the cam bar links  209  rotates or pivots the latches  214  inward into full engagement with the mating region at the upper end  215  of the connecting rod  204 . When moved inward to full engagement, the cam bars  208  are supported along their full length by a cam bar housing cover  222  which provides a positive stop for the inward movement of the cam bars  208 . The cam bar housing cover  222  is slotted down its center for the full mechanism stroke length to allow latch fingers  224  or other outward extensions of the latches  214  to pass through the cam bar housing cover  222  and contact the cam bars  208  at any withdrawal position of the lead screw  200 . 
         [0060]    In the illustrative embodiment, there are two latches  214  and two cam bars  208 , one per latch. However, other numbers of latches and cam bars are contemplated—as another example, there can be four latches and four corresponding cam bars spaced at 90° intervals around the central axis of the lead screw  200 /connecting rod  204 . The illustrated two cam bars  208  drive a corresponding two latches  214  in a two-fold rotationally symmetric arrangement respective to the central axis of the lead screw  200 /connecting rod  204 . Again, more generally, it is contemplated for the number of cam bars/latches to be greater than two, with the number of cam bars/latches being selected and arranged to provide balanced latching support for the connecting rod  204 . 
         [0061]    The lower portions of  FIGS. 17 and 18  also show an upper portion of the motor  184 , whose details are described with reference to  FIG. 14  herein. Again, the illustrative motor  184  is merely an illustrative example, and various types of motors can be employed, such as the illustrative brushless DC electronically controlled (BLDC) motor  184  with a wound stator core and a permanent magnet rotor which produces torque from interaction of magnetic flux and the current carrying stator conductors, or a variable reluctance stepper motor (VRS) having a wound stator core and a laminated steel rotor which produces torque from the variation in reluctance as a function of rotor position, or a hybrid stepper motor (HBS) which is a combination of the BLDC and VRS types and utilizes permanent magnets in the rotor and a reluctance component to produce torque, or so forth. In some embodiments it is contemplated to omit the separate the ball nut assemblies and instead or additionally provide engagement with the lead screw directly via the rotor by forming thread engagements on an inner diameter surface of the rotor. Additionally, a torque taker (not shown) is provided to prevent rotation of the lead screw  200  responsive to operation of the motor  184 . In some suitable embodiments, the cam bar housing cover  222  includes guide features (not shown) that engage the latch housing  212  to prevent the latch housing  212  from rotating and thus serve as a torque taker to prevent rotation of the lead screw  200  responsive to operation of the ball nut motor  202 . In this arrangement, the lead screw  200  is suitably secured together with a bottom portion  226  of the latch housing  212  so that preventing rotation of the latch housing  212  also prevents rotation of the lead screw  200 . 
         [0062]    Again with particular reference to  FIG. 17 , the cam bars  208 , when rotated inward, provide a positive full stroke track to guide the engaged latches  214  via camming of the latch fingers  224  against the cam bars  220  as the translating assembly (including the lead screw  200 , latch housing  212  and latches  214 , and latched connecting rod  204 ) is withdrawn (i.e., moved upward) or inserted (i.e., moved downward). The hydraulic lifting of the cam bars  208  instigates a four-bar linkage action via cam bar links  209  that connect the cam bars  208  with a cam bar support housing  232 . Each cam bar link  209  is pivotally pinned to the cam bar support housing  232  via a pivot location  234  and to the cam bar  208  by a pivot location  235 . In this way, two cam bar links  209  together with the portion of the cam bar support housing  232  between the pivot locations  234  of the cam bar links and the portion of the cam bar  208  between the pivot locations  235  of the cam bar links together define a four-bar linkage. Optionally, more than two cam bar links  209  per cam bar  208  can be provided—in the illustrative example four cam bar links  209  per cam bar  208  are provided (see  FIG. 16 ). Hydraulic lifting of the cam bars  208  causes the cam bar links  209  to pivot upward about the pivot locations  234  and thus force the lifting cam bars  208  inward via the pivot locations  235 . When the cam bars  208  are moved to their full inward position, the cam bar links  209  are closest to, but below, horizontal, e.g. at a minimum angle of 20° from the horizontal in some contemplated embodiments, which reduces the likelihood that the four-bar linkage may jam in a horizontal null position. 
         [0063]    With particular reference now to  FIG. 18 , the cam bars  208  are shown lowered at their maximum outward position. Again said briefly, hydraulic lowering of the cam bars  208  (or, gravitational, spring-biased, and/or other lowering of the cam bars  208  responsive to removal of the hydraulic lifting force) causes the cam bar links  209  to pivot downward about the pivot locations  234  and thus force the lifting cam bars  208  outward by a four-bar linkage action. The outward movement of the cam bars  208  allows the latches  214  to freely rotate or pivot outward about the pivot locations  216  and disengage from the connecting rod  204  to initiate scram of the connecting rod  204  and hence of the control rods connected with the connecting rod  204 . The scram action is failsafe in that the weight of the connecting rod  204 , with the assist of latch springs  240 , disengages the latches  214 . More particularly, the latch springs  240  are compressively held between the latch housing  212  and the upper portions of the latches  214  (above the pivot positions  216 ) so that they bias the upper portions of the latches  214  inward and consequently bias outward the lower portions of the latches  214  (below the pivot positions  216 , i.e. including the latch fingers  224 ). 
         [0064]    When moved outward to full disengagement, the cam bars  208  are supported along their full length by the cam bar support housing  232  which provides a positive stop for their outward movement. When the cam bars  208  are moved to their full outward position, the cam bar links  209  are closest to, but not exactly, vertical, for example at a minimum angle of 20° from the vertical in some embodiments, which reduces the likelihood that the four-bar linkage may jam in a vertical null position. 
         [0065]    With reference to  FIGS. 19 and 20 , the upper end of the control rod drive mechanism (CRDM) including the hydraulic lift system  210  is illustrated in additional detail. The hydraulic lift system  210  includes a hydraulic cylinder  250  and hydraulic piston  252 . Cam bar hangers  254  are coupled with the top of the piston  252 , and connection links  256  extend downward from the cam bar hangers  254  to the upper portions of the cam bars  208 . During normal operation ( FIG. 19 ) the hydraulic cylinder  250  is pressurized so as to raise the piston  252  and so raise the cam bars  208   
         [0066]    In  FIGS. 19 and 20 , the lead screw  200  is shown fully withdrawn for illustration purposes, so that the latch system is also visible in the view of  FIGS. 19 and 20 . However, the operation of the hydraulic lift system  210  as described with reference to  FIGS. 19 and 20  is applicable for any position of the lead screw  200 . 
         [0067]    With particular reference to  FIG. 19 , as was described previously with reference to  FIG. 17 , in the normal (latched) state the cam bars  208  are raised and, due to action of the cam bar links  209 , are at their maximum inward position. The inward movement of the cam bars  208  rotates or pivots the latches  214  into full engagement with the top end  215  of the connecting rod  204 . Also, when moved inward to full engagement the cam bars  208  are supported along their full length by the cam bar housing cover  222  which provides a positive stop for their inward movement. 
         [0068]    With continuing reference to  FIG. 19 , in the engaged condition the hydraulic piston  252  is in the fully raised position due to pressurization of the hydraulic cylinder  250 . As the piston is raised the cam bar hanger  254  is lifted by the piston  252  and pulls upward on the pair of connection links  256  which in turn lift the cam bars  208 . The piston  252  also lifts against the downward force produced by the scram assist spring  260 . In some contemplated embodiments, the hydraulic piston lift assembly operates at a differential pressure of only 5.5 psi, although design for higher differential pressure operation is also contemplated. 
         [0069]    With particular reference to  FIG. 20 , as was described previously with reference to  FIG. 18 , in the scrammed (unlatched) state the cam bars  208  are lowered and, due to the four-bar linkage action of the cam bar links  209 , are at their maximum outward position. The outward movement of the cam bars  208  allows the latches  214  to freely pivot or rotate and disengage from the connecting rod  204 . In illustrative  FIG. 20 , the connecting rod  204  has scrammed out of view to the fully inserted position, and hence the connecting rod  204  is not shown in  FIG. 20 . When moved outward to full disengagement, the cam bars  208  are supported along their full length by the cam bar support housing  232  which provides a positive stop for their outward movement. 
         [0070]    With continuing reference to  FIG. 20 , in order to scram the pressure in the hydraulic cylinder  250  at the bottom side of the piston  252  is evacuated to allow the piston  252  to lower. In a suitable approach, the depressurization is accomplished by a double-acting valve (not shown) that simultaneously cuts the supply pressure to the piston  252  while evacuating the piston cavity to the reactor environment. If the valve fails, it fails in an open state to the dump side for scram reliability. A large flow area valve is optionally employed to provide fast evacuation of the (typically small-volume) piston cavity. Once the pressure is dumped, the combined weight of the cam bars  208 , the linkages  254 ,  256 , and the piston  252  gravitationally drive lowering of the cam bars  208  and resultant disengagement of the latches  214 . Optionally, as in the illustrated embodiment the scram assist spring  260  is provided in or with the hydraulic lift assembly to assist in lowering the pistion  252  and cam bars  208 . The scram action is preferably also failsafe in that the connecting rod weight, with the assist of the latch springs, disengages the latches. Camming action by the cam bar links  209  also pushes the cam bars  208  outward toward disengagement. 
         [0071]    Reengagement of the latch assembly  206  with the connecting rod  204  after a scram can be performed similarly to the reengagement process described with particular reference to  FIGS. 12 and 13  for the embodiment of  FIGS. 6-15 . The electric motor  184  is driven to move the latching assembly  206  and lead screw  200  (which, again, are secured together) downward toward the top  215  of the scrammed connecting rod  204 . The hydraulic cylinder  250  remains depressurized and the latches  214  remain in the disengaged position due to bias of the latch springs  240 , as shown in  FIG. 18 . Thus, the latches  214  can be driven downward by the motor  184  to align with the mating region at the upper end  215  of the connecting rod  204 . In the illustrated embodiment, the magnet  113  disposed at or near the top  215  of the connecting rod  204  is magnetically sensed by a position indicator (not shown) in the latching assembly  206  in order to detect when the latches  214  are aligned with the mating region at the upper end  215  of the connecting rod  204 . Once the latches  214  are aligned with the mating region at the upper end  215  of the connecting rod  204 , the hydraulic cylinder  250  is re-pressurized to lift the hydraulic piston  252  and thus raise the cam bars  208  and reengage the latches  214 . Thereafter, the electric motor  184  can be operated to drive the lead screw  200  and re-latched connecting rod  204  upward to a desired operational position. 
         [0072]    In an alternative embodiment, the hydraulic lift system  210  described with illustrative reference to  FIGS. 19 and 20  can be replaced by an electric solenoid lift assembly, for example suitably located at the top of the control rod drive mechanism (CRDM) in place of the illustrative hydraulic lift assembly  210 . Such an electric solenoid lift assembly can be suitably connected with the illustrative four-bar linkage latch cam system, and the latch assembly  206  functions as described herein. In this alternative embodiment, instead of applying pressure to the hydraulic piston  252  to provide the lifting force for engaging the cam bar assemblies, the lifting force is provided by applying electrical power to the solenoid. When electric power is cut the lifting force is immediately lost, the cam bars disengage the latches and the control rod cluster scrams as described herein. 
         [0073]    With reference back to  FIG. 17  and with further reference to  FIG. 21 , a section S indicated in  FIG. 17  is shown in  FIG. 21 . The section S passes through a coupling between each cam bar  208  and one of its coupling cam bar links  209 , and through the finger  224  of each latch  214 , and through the position sensor magnet  113 . The sectional view S shown in  FIG. 21  includes the cam bar support housing  232  and an supporting cam housing assembly  232   a,  and the latch housing  212 , and the top  215  of the connecting rod  204  with sectioning through the position sensor magnet  113 . The sectional view S further includes sectioning through the two cam bars  208  and their latch fingers  224 , and shows cam links  209  and their pivot locations  234  connecting with the cam bar support housing  232 , with sectioning through their pivot locations  235  connecting with the latch housing  212 . As seen in  FIG. 21 , the pivot locations  234 ,  235  are suitably embodied by pins. The sectional view S of  FIG. 21  also shows an illustrative magnetic position indicator assembly  270  that senses the magnet  113  in the top end  215  of the connecting rod  204  based on magnetic coupling between the indicator assembly  270  and the magnet  113 . 
         [0074]    As already mentioned, the connecting rod  204  is connected at its lower end with a control rod bundle. Optionally, this connection is via one or more intermediate linkages, for example the illustrative yokes  86 ,  88  shown in  FIG. 6 . 
         [0075]    With reference to  FIGS. 22 and 23 , the nuclear reactor typically includes an array or other plurality of control rod clusters operated by corresponding control rod drive mechanisms supported by a suitable support frame  274  (for example, as shown in greater detail in  FIG. 2 ). In some embodiments, the electric motor  184  is the bulkiest component of the CDRM. In the illustrative array shown in  FIGS. 22 and 23 , the bulky motors  184  are accommodated in a compact array by vertically staggering the positions of the motors  184  so that the motors  184  of any two adjacent CRDM are not at the same vertical level or height. This enables a more compact array as compared with conventional arrangements in which all the motors are at the same vertical level or height. 
         [0076]    The CRDM embodiments described with reference to  FIGS. 6-21  advantageously provide both “grey rod” incremental control capability and also provide an efficient scram capability and hence can perform the task normally allocated to dedicated shutdown rods (e.g., as described herein with reference to  FIGS. 3-5 ). Accordingly, it is contemplated to omit dedicated shutdown rods and instead rely wholly on control rods of embodiments such as those of  FIGS. 6-21 , for example arranged as shown in  FIGS. 22 and 23 . In a variant embodiment, to provide further redundancy in a LOCA or other emergency event, it is contemplated to employ a configuration including: (i) no dedicated shutdown rods; (ii) a first set of control rods with hydraulic lift as described herein with reference to  FIGS. 16-21  so that in an emergency the rods perform the shutdown function via a hydraulic mechanism; and (iii) a second set of control rods with configured to perform the shutdown function via an electromagnetic mechanism. The latter set (iii) can be embodied, for example, by control rods conforming with the embodiment described herein with reference to  FIGS. 6-15 , or alternatively by control rods conforming with the embodiment described herein with reference to  FIGS. 16-21  but with the hydraulic lift system  210  replaced by a solenoidal lift system. Such an arrangement advantageously uses (or can use) all available control rods for reactivity control while also providing a two-fold redundant (hydraulic and electromagnetic) safety system. 
         [0077]    With reference back to the CRDM embodiments of  FIGS. 6-20 , an advantage of employing a latch to decouple the connecting rod  204  from the lead screw  200  (and, hence, to decouple the connecting rod  204  from the CRDM) is that the CRDM can be configured for removal of the connecting rod  204  through the CRDM without first removing the CRDM. To provide this capability, the CRDM is constructed with a hollow central region providing a pass-through by which the connecting rod  204 , once unlatched from the lead screw  200 , may pass. A cylindrical opening  280  (see  FIGS. 18 and 20 ) through the latch assembly is made large enough for the connecting rod  204  to pass through when the latches  214  are open. In the embodiment of  FIGS. 6-15 , such an opening can be provided by replacing the centrally positioned latch spring  174  with a side-positioned biasing mechanism similar to the latch springs  240  of the embodiment of  FIGS. 16-21 . For the embodiment of  FIGS. 16-21 , a cylindrical opening  282  is also provided through the hydraulic lift system  210  (see  FIGS. 19 and 20 ). Both openings  280 ,  282  are made large enough for the connecting rod  204  to pass through when the latches  214  are open. Regarding the latter opening  282 , the scram assist spring  260  is suitably an annular spring providing for the opening  282 , and the piston  252  is also suitably made hollow with the requisite inner diameter. In the case of an alternative lifting mechanism, such as a solenoidal lift, the solenoid is suitably hollow. 
         [0078]    With reference to  FIGS. 24 and 25 , for the connecting rod  204  to be removable through the CRDM it should be detachably connected with the spider or other mechanical control rod structure in such a way that (i) it can be detached from the spider from above the CRDM and (ii) so that the outer diameter of the connecting rod  204  at the detachable connection is not so large so as to prevent withdrawal of the lower end of the connecting rod  204  through the openings  280 ,  282 .  FIGS. 24 and 25  show one suitable detachable connection, in which a low-profile “J-groove” couples the connecting rod  204  with the control rod bundle. In this illustrative detachable connection, one or more inverted “J” shaped grooves  300  are formed in the lower end of the connecting rod  204 . Since these grooves are recessed into the connecting rod  204 , the J-grooves  300  do not increase the outer diameter of the connecting rod  204  at the lower end. A biasing spring  302  is terminated at the end proximate to the connecting rod  204  by a spring guide/capture element  304 , and the elements  302 ,  304  are disposed inside a generally cylindrical rod cluster threaded cap  306  that secures to the top of a rod cluster  310 . The cluster cap  306  includes mating tabs  312  that are sized and positioned to slide into the inverted J-shaped grooves  300  of the connecting rod  204 . To establish the coupling, the long vertical portions of the inverted J-shaped grooves  300  are aligned with the mating tabs  312  of the cluster cap  306 , and the connecting rod  204  is then pushed downward against the compressive force of the spring  302  such that the tabs  312  enter the long vertical portions of the grooves  300 . When the connecting rod  204  is pushed down far enough for the tabs  312  to reach the horizontal portions of the inverted J-shaped grooves  300 , the connecting rod is rotated by a rotation  314  (which is clockwise in  FIGS. 24 and 25 ) until the tabs  312  align with the short vertical portions of the inverted J-shaped grooves  300 . At that point, removal of the downward force allows the upward spring force generated by the spring  302  to push the connecting rod  204  upward in order to lock the tabs  312  into the short vertical portions of the inverted J-shaped grooves  300 . The process is reversed to decouple the connecting rod  204  from the rod cluster  310 . After removal, the spring  302  and guide/capture element  304  are retained at the rod cluster  310  by the cluster cap  306 . 
         [0079]    Thus, the coupling/decoupling of the connecting rod  204  with the rod cluster  310  advantageously can be performed with the latches  214  disengaged, so that the connecting rod  204  can be installed or removed with the CDRM in place. This is made possible because the lead screw  200  and the connecting rod  204  are not directly connected together, but rather are coupled by the latch assembly  206 . When the latches  214  are disengaged, the connecting rod  204  can move freely inside the substantially hollow lead screw  200 , and if the hydraulic piston  252  (or hollow solenoid, in the case of an electromagnetic lifting mechanism) is also made substantially hollow and the hydraulic cylinder  250  is annular with a sufficiently large inner diameter, then the connecting rod  204  can also pass through the hydraulic lift assembly  210 . Thus, installation of the connecting rod  204  amounts to inserting the connecting rod  204  into the opening of the CDRM and pushing it down until it presses against the spring  302  (see  FIGS. 24 and 25 ) and then rotating the connecting rod  204  as per the illustrated rotation  314  and releasing the connecting rod  204  so that the force of the spring  302  completes locking of the coupling. To remove the connecting rod  204 , the process is reversed. 
         [0080]    The preferred embodiments have been illustrated and described. Obviously, modifications and alterations will occur to others upon reading and understanding the preceding detailed description. It is intended that the invention be construed as including all such modifications and alterations insofar as they come within the scope of the appended claims or the equivalents thereof.