Patent Number: 052672859
Section: summary

BACKGROUND OF THE INVENTION This invention relates generally to cooling systems for nuclear reactors, and, more particularly, to an apparatus and method for suppressing the formation of vortices in coolant fluid circulating within such reactors. Generally, nuclear reactors include a cylindrically reactor vessel having a hemispherical lower end. Within the reactor vessel is the reactor core, supported by a main core support. The main core support is connected to the interior walls of the reactor vessel at or adjacent to the area where the cylindrical and hemispherical portions of the reactor vessel meet. Below the main core support, the hemispherical vessel defines a lower head, or lower plenum. A generally cylindrical downcomer surrounds the reactor core. Coolant fluid, typically water, is pumped into the downcomer. The coolant fluid circulates downwardly into the lower plenum. The hemispherical shape of the lower plenum assists in evenly circulating the coolant fluid therein. A plurality of reactor core coolant inlet openings are located on the underside of the main core support. Coolant flows from the lower plenum, into the core coolant inlet openings and upwardly into the core to cool the fuel assemblies. In order to maintain adequate and uniform cooling throughout the core, it is important that a uniform coolant flow and pressure be maintained across all of the reactor core coolant inlet openings. Non-uniform coolant pressure or flow causes uneven coolant flow into the core, which results in uneven cooling of the fuel assemblies of the core. Uneven fuel assembly cooling may force the entire core to be derated to accommodate "hot assembly" locations. Non-uniform coolant flow and pressure may result if vortices or other flow disruptions form in the coolant fluid circulating in the lower plenum. It is desirable to provide core monitoring instrumentation within the core of a nuclear reactor. Traditionally, the leads connecting such instrumentation to the exterior of the reactor exit the reactor vessel through a central portion of the hemispherical portion of the reactor vessel. A plurality of conduits carry the instrumentation lines through the lower plenum. The presence of the conduits in the lower plenum assists in maintaining even coolant flow within the lower plenum and disrupting the formation of vortices in the circulating coolant fluid. Such vortices disrupt coolant flow and produce low pressure areas at the core coolant inlets which they intersect. In newer reactors, it has become desirable for any instrumentation conduits to exit the reactor vessel other than through the lower plenum. It has been found that the absence of instrumentation conduits from the lower plenum permits vortices to form in the circulating coolant in the lower plenum. Accordingly, there is a need for a simple and inexpensive apparatus and method for effectively suppressing the formation of vortices in the coolant fluid circulating within the lower plenum of a nuclear reactor where instrumentation conduits are not present in the lower plenum. SUMMARY OF THE INVENTION The present invention has met the above-described needs. This invention provides a generally planar plate which is suspended in the lower plenum of a reactor vessel below and generally parallel to the main core support. The plate has a plurality of openings therein which allow coolant fluid to flow therethrough. A plurality of support columns connect the upper surface of the plate with the lower surface of the main core support. The plate is positioned within the lower plenum so as to optimize its vortex suppression effect. The plate's distance below the bottom of the main core support, its distance from the interior side walls of the reactor vessel and its distance above the center of the interior of the hemispherical portion of the reactor vessel may be selected so as to maximize vortex suppression. Preferably, the plate will be positioned so as to intersect any potential vortex at or near the center line of such vortex. Modelling techniques may be utilized to predict the locations where vortices are likely to form. The plate's diameter, its thickness and the size of the openings therein may also be varied to obtain maximum vortex suppression. The vortex suppression plate may also be utilized to brace secondary core support columns which extend generally downwardly from the main core support. A secondary core support plate is typically disposed near the bottom of the center of the lower plenum. It is an object of this invention to provide an apparatus and method for suppressing the formation of vortices in coolant fluid circulating in the lower plenum of a nuclear reactor. It is another object of this invention to provide an apparatus and method for suppressing the formation of such vortices in reactors in which core monitoring instrumentation conduits are absent from the lower plenum thereof. It is a further object of this invention to provide an apparatus and method for suppressing formation of such vortices which assist in maintaining uniform coolant flow and pressure to the reactor coolant inlets. It is yet another object of this invention to provide an apparatus and method for suppressing formation of such vortices which produce minimal contribution to the pressure drop of the coolant fluid circulating within the lower plenum. It is another object of this invention to provide an apparatus and method for suppressing formation of such vortices which are simple and relatively inexpensive. It is a further object of this invention to provide an apparatus and method for suppressing formation of such vortices which are readily realized using existing hardware design and fabrication techniques. These and other objects of the present invention will be more fully understood from the following description of the preferred embodiment of the invention with reference to the drawings appended hereto.