1. Field of the Invention
This invention pertains in general to nuclear reactor fuel assemblies and more particularly to a pressurized water reactor nuclear fuel assembly instrumentation thimble.
2. Background
A typical pressurized water reactor includes a reactor vessel which contains nuclear fuel, a coolant, typically a water based solution, which is heated by the nuclear fuel, and means for monitoring and controlling the nuclear reaction. The reactor vessel is cylindrical, and is provided with a hemispherical bottom and a hemispherical top which is removable. The hot water coolant solution is conveyed from and returned to the vessel by a reactor coolant system which includes one or more reactor coolant loops (usually three or four loops, depending upon the power generating capacity of the reactor). Each loop includes a pipeline to convey hot water from the reactor vessel to a steam generator, a pipeline to convey the water from the steam generator back to the reactor vessel, and a pump. The steam generator is essentially a heat exchanger which transfers heat from the reactant coolant system to water from a source that is isolated from the reactor coolant system; the resulting steam is conveyed to a turbine to generate electricity. During operation of the reactor, the water in the vessel and coolant system is maintained at a high pressure to keep it from boiling as it is heated by the nuclear fuel.
Nuclear fuel is supplied to the reactor in the form of a number of fuel assemblies, that are supported within a reactor core by upper and lower traversely extending core support plates. Conventional designs of fuel assemblies include a plurality of fuel rods and control rod guide thimbles which are hollowed tubes held in an organized array by grids spaced along the fuel assembly length and attached to the control rod guide thimbles. The guide thimbles are structural members which also provide channels for neutron absorber rods, burnable poison rods or neutron source assemblies which are all vehicles for controlling the reactivity of the reactor. Top and bottom nozzles on opposite ends thereof are secured to the guide thimbles; thereby forming an integral fuel assembly.
The grids, as is known in the relevant art, are used to precisely maintain the spacing between the fuel rods in the reactor core, resist rod vibration, provide lateral support for the fuel rods and, to some extent, vertically restrain the rods against longitudinal movement. One type of conventional grid design includes a plurality of interleaved straps that together form an egg-crate configuration having a plurality of roughly square cells which individually accept the fuel rods therein. Depending upon the configuration of the control rod guide thimbles, the guide thimbles can either be received in cells that are either sized the same as those that receive the fuel rods therein, or can be received in relatively larger thimble cells defined in the interleaved straps. Typically at least one instrumentation tube is provided that extends through at least one cell, typically the center cell, in each strap and is captured between the top and bottom nozzles. The instrumentation tube, like the control rod guide thimbles, is attached to each of the grid cells through which it passes by a mechanical connection formed by bulging or welding. A number of measuring instruments are employed within the reactor core to promote safety and to permit proper control of the nuclear reaction. Among other instruments, neutron flux detectors are stationarily positioned within the instrumentation tubes within the core for that purpose. For a proper flux reading of the neutron activity within the region of the corresponding fuel assembly it is important that the flux detectors be centrally positioned around the longitudinal axis of the instrumentation tube. Centering of the in-core instrumentation is required to ensure the detector responses are consistent from location to location within the core. One existing instrumentation tube design is illustrated in FIG. 1. FIG. 1 shows the instrumentation tube 10 extending between the upper or top nozzle 12 and the bottom nozzle 14. An in-core instrument 16 extends through the interior of the instrument tube 10 spanning between the top nozzle 12 and lower or bottom nozzle 14. Dimples 18 formed by crimping the instrumentation tube at a number of diametrically opposed points around its circumference, center the in-core instrumentation 16 within the tube 10. Typically the dimples are provided at a number of elevations along the instrumentation tube 10, with subsequent dimples being rotated 90 degrees as shown in the top section of the instrumentation tube 10 shown in FIG. 2. However the dimples preclude the bulging of the instrumentation tube to a spacer grid at the dimple elevations and also are limited in their ability to center smaller outside diameter in-core instrumentation within the instrumentation tube.
Accordingly, a new instrumentation tube design is desired that will center the in-core instrumentation while providing a smooth wall, non dimpled, outside circumference that may be either welded or bulged to the spacer grids.
Furthermore, it is an object of this invention to provide such an in-core instrumentation tube that can center any size in-core instrumentation within the instrumentation tube.