Abstract: A control rod having a lower tip absorber material which exhibits substantially lower irradiation induced swelling than a second absorber material which extends above the lower tip absorber material. The lower tip absorber material having a substantially lower reactivity worth than the second absorber material, extends from a lower end plug of the control rod to an elevation just above a dashpot in a thimble guide tube in a nuclear fuel assembly when the control rod is fully inserted within the thimble guide tube.

Abstract: A safety grade protection system for monitoring parameters indicative of the status of a core of a nuclear reactor, and reducing the criticality of the nuclear reaction when the onset of unsafe conditions are detected. The invention directly detects, in real time, the power at a plurality of axial and radial locations within or about the core and provides a three-dimensional real-time core power output. The system processes those outputs together with a measure of core coolant inlet temperature, pressure and flow rate to calculate, in real time, the fuel centerline temperature, the heat transfer condition at the fuel elements' boundary and the rate of power change within the core.

Abstract: A system for measuring the power distribution in a reactor core includes a plurality of gamma detectors disposed outside of the reactor vessel which houses the reactor core. The gamma detectors correspond in number to the number of instrumentation tubes in the fuel assemblies in the reactor core to be sampled. A first plurality of separate flow branches connect the instrumentation tubes with the gamma detectors for routing the flow of coolant from the instrumentation tubes to the gamma detectors so that the gamma detectors can measure the .sup.16 N decay gamma activity in the respective instrumentation tubes of the fuel assemblies. A second plurality of separate flow branches connect a cold leg of the reactor coolant system with the gamma detectors for routing the flow of coolant from the cold leg to the gamma detectors so that the gamma detectors can measure the background gamma activity in the respective instrumentation tubes of the fuel assemblies.

Abstract: A method and apparatus for removably attaching a top nozzle to a nuclear fuel assembly to permit easy access to the fuel rods. The quick disconnect top nozzle structure includes guide thimbles which are clearance fitted into an adapter plate of the top nozzle. Load collars are attached to the guide thimbles to support compressive loads such as those imposed by holddown springs. Tensive forces, such as lift forces, are transferred through the top nozzle into peripheral straps of a top spacer grid. The peripheral straps include extensions which resiliently latch onto the top nozzle for ease of assembly and disassembly.