1. Field
The present invention relates generally to nuclear reactors, and more particularly, to nuclear fuel assemblies that employ guide thimbles with tube-in-tube dashpots.
2. Related Art
In a nuclear reactor for power generation, such as a pressurized water reactor, heat is generated by fission of a nuclear fuel such as enriched uranium, and transferred into a coolant flowing through a reactor core. The core contains elongated nuclear fuel rods mounted in proximity with one another in a fuel assembly structure, through and over which the coolant flows. The fuel rods are spaced from one another in co-extensive parallel arrays. Some of the neutrons and other atomic particles released during nuclear decay of the fuel atoms in a given fuel rod pass through the spaces between fuel rods and impinge upon fissile material in adjacent fuel rods, contributing to the nuclear reaction and to the heat generated by the core.
Movable control rods are dispersed through the nuclear core to enable control over the overall rate of the fission reaction, by absorbing a portion of the neutrons, which otherwise would contribute to the fission reaction. The control rods generally comprise elongated rods of neutron absorbing material and fit into longitudinal openings or guide thimbles in the fuel assemblies running parallel to and between the fuel rods. Inserting a control rod further into the core causes more neutrons to be absorbed without contributing to the fission process in an adjacent fuel rod; and retracting the control rods reduces the extent of neutron absorption and increases the rate of a nuclear reaction and the power output of the core.
FIG. 1 shows the schematic of a simplified conventional pressurized water nuclear reactor primary system, including a generally cylindrical pressure vessel 10 having a closure head 12 enclosing a nuclear core 14 that supports the fuel rods containing the fissile material. A liquid coolant, such as water or borated water, is pumped into the vessel 10 by pump 16, through the core 14 where heat energy is absorbed and is discharged to a heat exchanger 18 typically referred to as a steam generator, in which heat is transferred to a utilization circuit (not shown) such as a steam driven turbine generator. The reactor coolant is then returned to the pump 16 completing the primary loop. Typically, a plurality of the above described loops are connected to a single reactor vessel 10 by reactor coolant piping 20. One of the loops includes a pressurizer 22 for controlling the pressure in the reactor primary system.
Commercial power plants employing this design are typically on the order of 1,100 megawatts or more. More recently, Westinghouse Electric Company LLC has proposed a small modular reactor in the 200 megawatt class. The small modular reactor is an integral pressurized water reactor with all primary loop components located inside the reactor vessel. The reactor vessel is, in turn, surrounded by a compact, high pressure containment. Due to both the limited space within the containment and the low cost requirement for integral pressurized light water reactors, the overall number of auxiliary systems needs to be minimized without compromising safety or functionality. For that reason, it is desirable to maintain most of the components in fluid communication with the primary loop of the reactor system within the compact, high pressure containment.
In the processes for designing the small modular reactor, one of the fuel/core design requirements for the small modular reactor is that the reactor should have load follow capability while minimizing CVS duty, i.e., Chemical and Volume Shim system duty, which adds and removes Boron to control reactivity. In order to satisfy this requirement, a group of gray rod cluster assemblies will be moved up and down within the fuel assemblies during reactor operation to control reactivity to satisfy load follow requirements. Proper cooling of the gray rods becomes critical to ensure the safe operation of the reactor and protect the gray rods from overheating. One of the design options to keep the control rods from overheating is to form side holes in the control rod guide thimble cladding (also referred to hereafter as the sheath) near the thimble assembly end plugs. This design choice is not a problem for the integral dashpot tube designs. However, the tube-in-tube dashpot designs make it more difficult to implement this design choice because the inside dashpot assembly has to be designed such that there will be through holes existing after assembling the dashpot tube within the guide thimble tube, regardless of the installation orientation of the dashpot tube.
Accordingly, a new tube-in-tube dashpot and thimble tube assembly design is desired that will provide adequate cooling without complicating manufacture.