Patent Number: 052415708
Section: summary

This invention relates to nuclear reactors and, more particularly, to a core-control assembly for a dual-phase nuclear reactor. A major objective of the present invention is to provide for more convenient removal of a control rod in a nuclear reactor. Nuclear reactors generate heat in a fissionable reactor core and transfer this heat by flowing coolant through the core. In dual-phase reactors, the flowing coolant stores energy in the form of a phase change, e.g., from liquid to vapor. Since dual-phase reactors are primarily boiling-water reactors, the phase change is from water to steam. Hereinbelow, the operation of dual-phase reactors generally can be extrapolated from the description of the following typical boiling-water reactor. In a typical boiling-water reactor, a water-steam mixture rises from the core. The water is to be recirculated down a downcomer and then back up through the core. The steam is to be directed from the reactor for utilization; for example, the steam can drive a turbine, which, in turn, can drive a generator to produce electricity. Steam condenses as it gives up energy to the turbine; the condensate can be returned to the reactor vessel to merge with the recirculating water. The core of this typical reactor comprises an array of fuel bundles with square cross section. The fuel bundles are supported from below by a fuel support. Each fuel support supports a group of four fuel bundles. The heat generated in the core can be decreased by inserting control rods into the core; the generated heat can be increased by retracting control rods from the core. The control rods have a cruciform cross section with blades that can be inserted between the fuel bundles of a group of four. To the extent that the control rods are inserted, some of the neutrons that would otherwise travel from one bundle to another to promote fission are absorbed by the control rod. The control rods are driven by control-rod drives that extend through the bottom of the reactor vessel. The control-rod drives are supported by control-rod-drive (CRD) housings, which are tubes that extend through the vessel bottom. The CRD housings support control-rod guide tubes (CRGTs) that help guide the vertical movement of the control rods. The fuel supports are seated on the CRGTs. Control rods have a limited lifetime and must undergo scheduled replacement. Control-rod removal is a time-consuming and expensive procedure. Generally, the top of the reactor vessel and intermediate components are removed. Before all the fuel bundles are removed, blade guides are inserted into the core to provide lateral support that would otherwise be lost once the fuel bundles are removed. The last fuel bundles are then removed, followed by the fuel supports. The control rods are fully withdrawn. The control rods are rotated to effect decoupling from the control-rod drives. Once decoupled, the control rods can be removed. The use of blade guides is undesirable because of the extra handling required. Blade guides are contaminated during control rod replacement. Since they are not used during reactor operation, they present a storage problem. In some reactors, the need for blade guides is obviated by using control rods that extend to the top guide at the top of the core region. The top guide provides lateral support to the top of the fuel and guides the insertion and removal of fuel bundles. The same guides can be used to provide lateral support for control rods. However, designing the control rods to reach the top guide for lateral support can conflict with other design constraints. In any event, whether the top guide or blade guides provide lateral support, control rod removal is more cumbersome than is desired. What is needed is a core-control assembly that provides for more convenient removal of control rods. Preferably, such an assembly would obviate the need for blade guides. SUMMARY OF THE INVENTION In accordance with the present invention, a core-control assembly has a control-rod drive that can insert a control rod far enough up into a core that it clears a fuel support. The fuel support has an aperture that conforms to the star-shaped cross section of the control rod, thus the control rod cannot be rotated when it extends through the fuel support. Once the control rod clears the fuel support, it can be rotated for disengagement and removal. Since the fuel support need not be removed to replace a control rod, it can be welded to a CRGT below. To reduce the amount of insertion required to clear the fuel support, the blades of the control rod can be beveled or notched. Each point of the star-shaped cross section corresponds to a control rod blade. The preferred cross section has four points, and thus is referred to as a cruciform cross section. These are conventionally used with fuel bundles having square cross sections. Alternatively, three-pointed star-shaped apertures conform to three-bladed control rods that can be used with hexagonal fuel bundles. Also, six-pointed star-shaped apertures can conform to six-bladed control rods that can be used with fuel bundles with triangular cross sections. The core-control assembly comprises the control rod, the control-rod drive, a CRD housing, the CRGT, the fuel support, and a coupling mechanism between the control-rod drive and the control rod. Removal of the control rod involves removing a vessel top head, removing other reactor components such as a steam separator and a dryer, removing fuel bundles, insertion of the control rod to a decoupling position, rotation of the control rod to effect decoupling from the control rod drive, and lifting the control rod out of the reactor vessel. Preferably, the decoupling position is the same as a minimum power position of the control rod. Alternatively, the decoupling position can involve overtravel beyond the minimum power position. One advantage of the present invention is that the fuel support piece does not need to be removed when a control rod is removed. This saves a costly and time-consuming step, and relieves the reactor plant of the logistics of transferring, storing, and reinstalling a fuel support. Because it does not need to be removed, the fuel support can be welded securely to the CRGT, forming an integral unit, decreasing fabrication costs and increasing the stability and reliability of the core-control assembly. Where overtravel is required for the control rod to reach the decoupling position, the fuel support can provide lateral support while the control rod is at the minimum power position. Further provisions can be made for lateral support beyond the minimum power position. Since lateral support is provided by operational components at the minimum power position, blade guides need not be used for lateral support. This saves considerable time and expense, and relieves storage and disposal logistics. These and other features and advantages of the present invention are apparent in the following description with references to the drawings below.