A method for removing and replacing a bottom-mounted instrumentation nozzle on a nuclear reactor pressure vessel. The method (i) caps or plugs the existing bottom-mounted instrumentation nozzle; (ii) cuts the nozzle at or near the nozzle to in-core instrument tube weld; (iii) installs a water-tight sealing enclosure outside the vessel over the bottom of the bottom-mounted instrumentation nozzle creating a water-tight seal with the underside of the reactor vessel; (iv) cuts to sever the existing bottom nozzle from the reactor vessel; (v) extracts the existing nozzle; (vi) installs a replacement alloy 690 nozzle or plug from inside the vessel; and (vii) welds the replacement nozzle or plug in place. The replacement bottom-mounted instrumentation nozzle incorporates an integral shoulder that prevents ejection during operation and facilitates installation, and the entire method is performed while the reactor pressure vessel is filled with water.