Abstract: The invention disclosed herein provides nozzles for vessels such as a pressurizer vessel, reactor vessel or piping in a nuclear power facility, and methods for attaching and sealing the nozzles to the vessels without any welding at all, i.e., without using a structural or seal weld. The nozzles are clamped or bolted to the vessel or attached by an interference fit, and a mechanical seal is obtained using gasket material and/or contacting, preferably polished, metal surfaces. Some embodiments of clamping the nozzle to the vessel comprise tightening a nut on the threaded end of the nozzle projecting from the bore in the vessel against the vessel, and structure which prevents the nozzle from being withdrawn through the bore, thereby clamping the nozzle against the vessel. Another embodiment of clamping a nozzle to the vessel comprises a sleeve threaded in the bore and forcing an interfering portion of the nozzle body into engagement with the bore.

Abstract: A control system for avoiding coupled neutronic-thermal hydraulic instabilities during maneuvering of a boiling water nuclear reactor that determines the average elevation in the reactor core of the coolant boiling boundary, and avoids instabilities by maintaining the core average coolant boiling boundary elevation above a target value, which is dependent on reactor design specifics but may be about four feet, in core power-flow operating regions which are subject to instabilities. The system may be implemented without physical modification of existing reactors or reactor instrumentation and control systems through use of the reactor core monitoring computer, which typically has available all core parameters necessary for the computations required in the invention.