Patent Number: 047626690
Section: summary

CROSS REFERENCE TO RELATED APPLICATIONS Reference is hereby made to the following U.S. patent applications dealing with related subject matter and assigned to the assignee of the present invention: 1. "A Partial Grid For A Nuclear Reactor Fuel Assembly" by Edmund E. DeMario et al, assigned U.S. Ser. No. 564,049 and filed Dec. 21, 1983, now U.S. Pat. No. 4,576,786. 2. "A Low Pressure Drop Grid For A Nuclear Reactor Fuel Assembly" by Edmund E. DeMario et al, assigned U.S. Ser. No. 567,448 and filed Dec. 30, 1983, now abandoned. 3. "A Coolant Flow Mixer Grid For A Nuclear Reactor Fuel Assembly" by Edmund E. DeMario et al, assigned U.S. Ser. No. 567,450, filed Dec. 30, 1983, now U.S. Pat. No. 4,692,302. BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates generally to fuel assemblies for a nuclear reactor and, more particularly, is concerned with fuel assemblies positioned adjacent the baffle structure about the periphery of the reactor core which employ annular anti-vibration grids. 2. Description of the Prior Art A typical nuclear power reactor includes a reactor vessel housing a nuclear reactor core. Spaced radially inwardly from the reactor vessel is a generally cylindrical core barrel and within the barrel is a former and baffle system (hereinafter called a baffle structure) which permits transition from the cylindrical barrel to a squared off periphery of the reactor core formed by the fuel assemblies arrayed therein. The reactor core is composed of a large number of elongated fuel assemblies. Each fuel assembly includes a plurality of fuel rods containing the fissile material which reacts to produce heat. The fuel rods of each fuel assembly are held in an organized array by a plurality of grids spaced axially along the fuel assembly length and attached to a plurality of elongated control rod guide thimbles of the fuel assembly. During operation of the reactor, a coolant fluid such as water, is typically pumped into the reactor vessel through a plurality of inlet nozzles. The coolant fluid passes downward through an annular region defined between the reactor vessel and core barrel, turns in a lower plenum defined in the reactor vessel, then passes upwardly through the fuel assemblies of the reactor core, and exits from the vessel through a plurality of outlet nozzles extending through the core barrel. Heat energy which the fuel rods of the fuel assemblies impart to the coolant fluid is carried off by the fluid from the vessel. Due to the existence of holes in the core barrel, coolant fluid is also present between the barrel and baffle structure and at a higher pressure than within the core. However, the baffle structure together with the core barrel do separate the coolant fluid from the fuel assemblies as the fluid flows downwardly through the annular region between the reactor vessel and core barrel. As mentioned above, the baffle structure surrounds the fuel assemblies of the reactor core. Typically, the baffle structure is made of plates joined together by bolts. These bolts sometimes become loose thereby developing a small gap between the baffle structure plates. When this happens, a coolant fluid jetting action takes place through the baffle structure in a radially inward direction from the exterior to the interior thereof due to the greater fluid pressure existing outside of the baffle than within the core. The fluid jetting, when it impinges on the outer row of fuel assemblies in the core, makes their outer fuel rods vibrate, eventually causing them to fail. Consequently, the need exists for a way to deal effectively with fluid jetting through loosened portions of the baffle structure so as to avoid its deleterious effects on the fuel rods of the fuel assemblies positioned adjacent the baffle structure. SUMMARY OF THE INVENTION The present invention provides fuel assemblies with annular anti-vibration grids which are designed to satisfy the aforementioned needs. In particular, the fuel assemblies positioned adjacent the baffle structure about the periphery of the reactor core employ annular anti-vibration grids inserted between the regular grids and anchored to the guide thimbles like the regular grids. The annular grids will only occupy the space about the outer three rows of fuel rods in the fuel assemblies positioned at the core periphery. The grids will retard or dampen vibration of the fuel rods in these outer rows. Specifically, the presence of the annular grid reduces the length of the span of the fuel rods being unsupported between the regular grids, while also changing their vibrational frequency, thereby preventing the fuel rod damage experienced heretofore from vibrations caused by the coolant fluid jetting action through the baffle structure. In such manner, the deleterious effects of fluid jetting through the baffle structure will be substantially avoided. The fuel assemblies having the annular grids are only placed in core locations most susceptible to coolant fluid jetting action through the baffle structure. Since the number of such fuel assemblies will be small, the impact of the presence of the annular grids on overall core pressure drop will be small. Also, since these outside peripheral locations typically have small relative power, the fuel assemblies using these grids will not become DNB limiting. In instances where intermediate flow mixer grids, such as disclosed in the third application cross-referenced above, are already present between the regular grids in the top half of the fuel assemblies, the annular anti-vibration grids will only be used in the bottom half of the fuel assemblies. Accordingly, the present invention is directed to an improvement in a nuclear reactor fuel assembly. The fuel assembly includes a top nozzle, a bottom nozzle, and a plurality of guide thimbles extending longitudinally between and connected at their opposite ends to the top and bottom nozzles. The fuel assembly also has a multiplicity of elongated fuel rods and a plurality of support grids axially spaced along and supported by the guide thimbles. Each of the support grids defines a multiplicity of cells at least equal in number to the multiplicity of fuel rods for receiving respective ones of the fuel rods therethrough and supporting the fuel rods in a side-by-side array with respect to one another and to the guide thimbles. The improvement comprises a plurality of annular anti-vibration grids axially spaced along and connected to the guide thimbles between at least some of the support grids. The annular grids are separate from and unconnected to the support grids. Each of the annular grids defines a plurality of cells being less in number than the multiplicity of fuel rods but at least equal in number to a plurality of the fuel rods positioned along the periphery of the multiplicity of fuel rods. The annular grid cells receive therethrough respective ones of the fuel rods in the plurality thereof and engage these fuel rods so as to dampen any coolant fluid cross flow vibration induced therein. Furthermore, each of the annular grids is composed of a plurality of interleaved members arranged in an egg-crate configuration to define the plurality of cells and a central void region of a size to receive therethrough the rest of the fuel rods in the multiplicity thereof. Each of the annular grids includes a plurality of protrusions formed on the members and projecting into each of the cells in the plurality thereof through a sufficient distance to contact opposing sides of the fuel rod received through each cell. Also, a number of sleeves adapted to receive a like number of the guide thimbles therethrough are connected to the members. The number of the guide thimbles is less than the plurality thereof. The sleeves are unconnected with, and shorter in length than the distance between, the ones of the support grids disposed adjacent to each annular grid. Also, the present invention is directed to the combination in a nuclear reactor, comprising: (a) a reactor core composed of a plurality of fuel assemblies disposed in side-by-side spaced relationships, a first group of the fuel assemblies defining the periphery of the core and a second group of the fuel assemblies positioned inwardly of the first group thereof: (b) each of the fuel assemblies having a plurality of elongated guide thimbles, a multiplicity of elongated fuel rods, a plurality of support grids axially spaced along and supported by the guide thimbles, each of the support grids defining a multiplicity of cells at least equal in number to the multiplicity of fuel rods for receiving respective ones of the fuel rods therethrough and supporting the fuel rods in a side-by-side array with respect to one another and to the guide thimbles; (c) a baffle structure extending about the reactor core adjacent to the fuel assemblies in the first group thereof, the baffle structure having components being subject to unpredictable loosening with respect to one another so as to permit jetting of coolant fluid from the exterior to the interior of the baffle structure and impingement upon fuel rods in the fuel assemblies of the first group thereof so as to cause vibration of the fuel rods; and (d) a plurality of annular anti-vibration grids axially spaced along and connected to the guide thimbles of the fuel assemblies in the first group thereof between at least some of the support grids of the fuel assemblies in the first group thereof, the annular grids being separate from and unconnected to the support grids. Each of the annular grids defines a plurality of cells being less in number than the multiplicity of fuel rods of each of the fuel assemblies in the first group thereof but at least equal in number to the plurality of the fuel rods positioned along the periphery of the each fuel assembly in the first group thereof for receiving respective ones of said fuel rods therethrough and engaging the fuel rods so as to dampen vibration thereof due to impingement by coolant fluid jetting from the baffle structure.