Patent Number: 050858238
Section: claims

1. A control rod drive for positioning a control rod in a nuclear reactor vessel comprising: a shaft;  a ballnut positioned over said shaft and being translatable upon rotation of said shaft;  a piston disposed coaxially with said shaft and on said ballnut for positioning said control rod;  means for selectively rotating said shaft in a first direction and in a second direction, opposite to said first direction;  a housing surrounding a portion of said shaft;  a gear fixedly joined to said shaft, said gear having a plurality of circumferentially spaced gear teeth;  a latch arm pivotally joined to said housing, and having at least one latch tooth facing said gear teeth; and  means for selectively positioning said latch arm in an engaged position to abut said latch tooth against a first one of said gear teeth for preventing rotation of said shaft in said first direction, and in a disengaged position to spaced said latch tooth away from said gear teeth for allowing said shaft to rotate without obstruction between said gear teeth and latch tooth for translating said ballnut and in turn said piston for inserting and withdrawing said control rod in said reactor vessel.  a first end including said latch tooth;  an intermediate portion pivotally joined to said housing; and  a second end, opposite to said first end, joined to said positioning means.  each of said gear teeth has a first contact surface and a second, opposite, contact surface;  said latch tooth has a first contact surface and a second, opposite, contact surface; and  said latch tooth and said gear teeth being complementary in configuration so that said first gear tooth first contact surface abuts said latch tooth first contact surface in said latch arm engaged position.  said latch arm including an aft surface at said first end;  a stop member fixedly joined to said housing and positioned adjacent to said latch arm aft surface so that rotation of said shaft in said first direction is opposed by wedging of said latch arm aft surface against said stop in said latch arm engaged position. 2. A control rod drive according to claim 1 wherein said positioning means are effective for allowing rotation of said shaft in said second direction by said rotating means to intermittently disengage said latch tooth from said gear teeth while said latch arm is in said engaged position. 3. A control rod drive according to claim 2 wherein said positioning means include an electromagnet having a solenoid, plunger, and spring, said solenoid being fixedly joined to said housing, said plunger extending from said solenoid and pivotally joined to said latch arm, and said spring being disposed in said solenoid to resiliently bias said plunger to an extended position when said solenoid is deenergized for positioning said latch arm in said engaged position, and said solenoid being energizable to draw said plunger to a withdrawn position for positioning said latch arm in said disengaged position. 4. A control rod drive according to claim 2 wherein said latch arm includes: 5. A control rod assembly according to claim 4 wherein said latch arm is arcuate and includes a concave inner surface facing said gear, and said latch tooth is disposed on said inner surface. 6. A control rod assembly according to claim 5 wherein: 7. A control rod drive according to claim 6 wherein said latch arm intermediate portion and said latch tooth are positioned relative to said first gear tooth so that said latch tooth first contact surface opposes rotation of said shaft in said first direction when said latch arm is in said engaged position due to tensile loads imposed by said first gear tooth on said latch tooth affecting elastic extension of said latch arm. 8. A control rod drive according to claim 6 wherein said latch tooth second contact surface is positioned relative to a second contact surface of a second gear tooth disposed adjacent to said first gear tooth in said latch arm engaged position so that rotation of said shaft in said second direction by said rotating means causes said second gear tooth second contact surface to push said latch tooth radially away from said gear to intermittently disengage said latch tooth from said gear teeth. 9. A control rod drive according to claim 6 further including: 10. A control rod drive according to claim 6 wherein said arcuate latch arm includes a convex outer surface having a stop portion at said latch arm second end disposed adjacent to said housing in said latch arm engaged position for contacting said housing upon predetermined wear of said latch arm intermediate portion for maintaining engagement between said latch tooth and said first gear tooth.