Control rod drive shaft latch

In a nuclear reactor, in combination, a reactor vessel having a relatively high internal pressure, a longitudinally movable control rod drive shaft, a control rod drive mechanism disposed externally of the reactor vessel, sealed housing means for the mechanism having substantially the entire interior thereof open to the interior of said vessel so as to be subjected to the internal pressure in the reactor vessel, motive means for moving the drive shaft longitudinally and incrementally, a latch device positioned adjacent to and movable into and out of engagement with said drive shaft and mounted on a support to prevent movement of said control rod shaft when in engagement therewith, said latch device being positioned out of engagement with the control rod drive shaft when said housing means is sealed, cam surfaces fixedly positioned with respect to the housing and complementary cam surfaces on said latch device movable into engagement with said fixed cam surfaces to move said latch device into engagement with said drive shaft, and movable differential pressure responsive means having opposed sides and located in said housing means adjacent the area of joinder of said housing and said vessel, said last-mentioned differential pressure means being coupled to said latch device and exposed to said internal pressure, said differential pressure means being positioned within said housing with said pressure on opposed sides of said differential pressure means being balanced when said housing means is sealed such that said latch device is out of engagement with said drive shaft yet becomes unbalanced in the event of a break in said housing means at a location on the side of said differential pressure means remote from said vessel and moves in response to said unbalance to engage said fixed and complementary cam surfaces and thereby engage the latch device with the drive shaft.