The following relates to the nuclear power reactor arts, fuel assembly construction arts, and related arts.
With reference to FIG. 1, a nuclear reactor of the pressurized water reactor (PWR) variety includes a pressure vessel 10 containing primary coolant, such as primary coolant water. The illustrative pressure vessel 10 is a cylindrical pressure vessel (where “cylindrical” is intended to encompass deviations from a mathematically perfect cylinder such as the illustrative non-uniform diameter of the illustrative cylindrical pressure vessel 10, the inclusion of vessel penetrations, support structures, or so forth). A nuclear reactor core 12 is disposed at or near the bottom of the pressure vessel 10. (Note that in diagrammatic FIG. 1 the pressure vessel 10 is partially cut away as indicated by a dashed “opening” in order to reveal the reactor core 12 disposed inside. Moreover, diagrammatic FIG. 1 omits mounting features such as a core basket that typically are provided to secure the reactor core 12 inside the pressure vessel 10). Although a PWR is shown in FIG. 1 by way of illustrative example, it is to be understood that the spacer grids disclosed herein are suitably used in nuclear reactors of various varieties, such as PWR, boiling water reactor (BWR), and so forth.
The nuclear reactor core 12 typically comprises a plurality of fuel assemblies arranged in a closely-packed array. The fuel assembly includes a bundle of vertically oriented fuel rods each comprising a fissile material such as 235U. For example, each fuel rod may contain enriched uranium dioxide (UO2) or mixed UO2/gadolinium oxide (UO2—Gd2O3) pellets. Interspersed amongst the fuel rods are guide tubes that provide conduits for control rods, instrumentation, or so forth. The top of the fuel assembly is terminated by an upper end fitting or nozzle and the bottom of the fuel assembly is terminated by a lower end fitting or nozzle. The fuel assembly is held together by a plurality of spacer grids including end grids disposed at the top and bottom of the fuel assembly and one or (typically) more mid-grids disposed at spaced apart positions between the top and bottom of the fuel assembly.
Conventional spacer grids are formed by interlocking orthogonally oriented metal straps made of sheet metal to define a two-dimensional grid of square or rectangular spaces, also called grid “cells”, with each cell being delineated by four straps. Alternatively, a hexagonal arrangement can be employed in which each cell is generally hexagonal and is delineated by six straps. In one suitable approach employing square cells for receiving fuel rods, the strap portions defining each cell have two dimples formed from the grid straps that form two adjacent walls of the cell. One dimple in each pair is located near the top of the grid strap and the other is located near the bottom of the grid strap. The opposite walls of the cell each contain a single spring which may be formed from the strap that makes that cell wall, or may be an insert made of a different material that is mechanically trapped or restrained by features formed from the strap that make up that cell wall. The springs are located at or near the mid-plane of the spacer grid, and are sized such that an interference condition exists when a fuel rod is inserted into the grid cell. This interference causes the springs to deflect backwards towards the cell walls on which they are located, preloading the fuel rod in two orthogonal directions against the opposing dimple pair and clamping it in position. The axial offset between the plane of action of the springs and the plane of action of the dimples creates restoring moments that cause the local vertical orientation of the fuel rod at the spacer grids to remain relatively fixed should lateral forces be applied to the fuel rod between any two axially adjacent spacer grids. In some approaches, each spring contacts its fuel rod at two locations along the length of horizontal or vertical folds in the spring convolutes. Sometimes local flats, and/or secondary arches, are also provided to spread out any wear should the fuel rod oscillate in service due to flow-induced vibration. The straps in a conventional spacer grid are typically oriented such that the springs in a given cell are on the outboard walls of the cell and the dimples are on the inboard walls of the cell. This arrangement has the advantage of providing a rigid foundation to resist any inward-acting forces that may be applied to the outer row of fuel rods should the fuel assembly contact a neighboring fuel assembly or other adjacent structure during handling.