Abstract: The invention provides a tool for closing off the jet nozzles of jet pumps located in an almost inaccessible location inside the shell of a boiling water reactor. The tool suspended on a cord is lowered down through a narrow gap extending down along the major part of the shell of the reactor, a distance of approximately 30 feet. The tool is lowered onto a stationary projection--namely, a lug or ear--which provides a gauge point for height and lateral position and an anchorage for torque in a horizontal plane. With these two controls for the tool of the invention, a closure for the jet nozzle may be applied to each of the jet nozzles to close them off against outflow of the water contained in the shell of the reactor thereby avoiding the bleak alternative of taking the reactor completely out of service, which includes removing the fuel bundles and draining the shell.