Patent Number: 
Section: claims

1. An anti-rotation device for installation on a gate valve in situ in a nuclear reactor power plant to prevent separation of a valve stem having a top and valve disc(s) in the gate valve which includes an actuator for controlling the valve and a connection means threadedly engaged by the valve stem to actuate the valve comprisingan extension secured in a hole machined into the top of the valve stem;an anti-rotation housing secured to the actuator and carrying an anti-rotation abutment structure; andan anti-rotation coupling carried by the extension and having a shape mating with said abutment structure to prevent rotation of the valve stem;said anti-rotation coupling including a transversely extending anti-rotation bar, said abutment structure including spaced rails defining longitudinal channels slidably receiving said anti-rotation bar and said extension carrying an anti-rotation cup having a slot shape to mount said anti-rotation bar to assure alignment of said anti-rotation bar with said spaced rails. 2. An anti-rotation device as recited in claim 1 wherein said extension is secured to the valve stem using a splined joint. 3. A maintenance method for a gate valve utilized in a nuclear reactor power plant where the gate valve includes an actuator and is actuated via a threaded stem connection comprising the steps ofmachining a hole in a top of the stem;connecting an extension to the stem using the machined hole;bolting an anti-rotation housing to the actuator, the housing having an anti-rotation abutment structure;clamping an anti-rotation coupling onto the extension; andpositioning the coupling in the housing to mate with the abutment structure whereby rotation of the stem and application of torque to the stem connection is prevented. 4. A maintenance method for a gate valve as recited in claim 3 wherein disassembly of the valve is not required. 5. A maintenance method for a gate valve as recited in claim 4 wherein the actuator is not required to be removed or disassembled. 6. A maintenance method for a gate valve as recited in claim 3 wherein the valve is a double disc gate valve and the stem connection is a wedge.