Patent Number: 
Section: claims

1. A pressurized water reactor nuclear steam supply system steam generator pressure vessel having a secondary side for separating steam from condensate comprising:a generally horizontal deck plate within the secondary side of the pressure vessel defining an access opening extending between an upper surface of the deck plate and an undersurface;a removable plate for covering the access opening;a pin holder attached to either one of the deck plate or the removable plate;an elongated mechanical pin retractably attached to the pin holder for moving in a generally horizontal plane between a closed position and an open position;a receiving socket attached to the other one of the deck plate or the removable plate for engaging the mechanical pin in its closed position;a locking mechanism for securely locking the pin when the pin is in the closed position, throughout steam generator operation, the locking mechanism including a lock nut; andwherein the pin has a first peripheral end that engages the receiving socket and a second peripheral end wherein at least a portion of a surface of the pin between the first peripheral end and the second peripheral end is threaded to engage the lock nut. 2. The pressure vessel of claim 1 wherein the pin holder has a slot with opposing, parallel wall portions through which the pin slides, the pin having longitudinally extending, diametrically opposing, relatively flat lands, along a length of the pin that is threaded to engage the lock nut, that are closely received within the opposing wall portions so that the pin can not rotate in the slot. 3. The pressure vessel of claim 1 wherein the thread is a conical thread. 4. The pressure vessel of claim 1 wherein the means for locking the pin includes a moveable cleat that is supported from the pin holder in-line with a transverse slot in the mechanical pin, the cleat being moveable into the slot when the mechanical pin is in its closed position to lock the mechanical pin. 5. The pressure vessel of claim 4 wherein the slot is in the surface of the mechanical pin and oriented in the vertical direction so that the cleat can drop into the slot when the mechanical pin is in its closed position. 6. The pressure vessel of claim 5 wherein the cleat is supported from the pin holder so it can drop into the slot under the force of gravity. 7. The pressure vessel of claim 6 wherein the cleat at one end is hingedly attached to the pin holder. 8. The pressure vessel of claim 1 wherein the means for locking the pin further includes a moveable cleat that is supported from the pin holder in-line with a transverse slot in the mechanical pin, the cleat being moveable into the slot when the mechanical pin is in its closed position to lock the mechanical pin. 9. The pressure vessel of claim 1 including a reaction mount attached to the one of the deck plate or the movable plate and spaced from the pin holder between the pin holder and the receiving socket, the reaction mount having an opening in-line with an engagement opening in the receiving socket, the pin in its closed position extending between and into a slot in the pin holder, the opening in the reaction mount and the opening in the receiving socket. 10. The pressure vessel of claim 9 wherein the mechanical pin includes a circumferential collar that retains the lock nut.