Patent Number: 046817280
Section: summary

CROSS-REFERENCE TO RELATED APPLICATIONS Application Ser. No. 490,099 filed Apr. 29, 1983 to Luciano Veronesi et al. for Nuclear Reactor (herein Veronesi) pending and assigned to Westinghouse Electric Corporation is incorporated herein by reference. The reactor disclosed by Veronesi is sometimes referred to in the art as the Mechanical Moderator Controlled Reactor (MMCR). Application Ser. No. 490,097 filed Apr. 29, 1983 to Luciano Veronesi for Nuclear Reactor (herein Veronesi '097) pending and assigned to Westinghouse Electric Corporation is also incorporated herein by reference. BACKGROUND OF THE INVENTION This invention relates to nuclear reactors and has particular relationship to reactors having a large number of different rods moveable in and out of the core to control the neutron flux. In its specific aspects, this invention concerns itself with reactors of the type in which a flow-through screen and disclosed in Veronesi '097 encircles the neutron-absorption rod guides. The flow-through screen has the effect of reducing the radial velocity of the coolant as it flows through the upper internals. The Mechanical Moderator Controlled Reactor disclosed by Veronesi is such a reactor. It this reactor there are control rods, gray rods and water-displacement rods for adjusting the neutron flux. The rods are suspended from spiders in clusters which are moveable by drives to achieve the desired or required neutron flux. The control rods are substantially neutron absorbent and serve for load-follow operations and must be capable of deactivating the reactor during refueling and quickly and effectively during scram. The gray rods have substantially lower neutron-absorbent capability than the control rods and also serve for load follow. They are moveable in and out of the core to "fine tune" the neutron flux. The water-displacement rods are inserted in the core during the earlier part, typically 40%, of the fuel cycle and are removed from the core during the remainder of the fuel cycle. The water-displacement rods when inserted in the core reduce the low energy neutron flux during the early part of the fuel cycle when the flux is high by reducing the moderation by the water coolant. When the water displacement rods are removed from the core, their function of reducing low energy neutron flux is substantially reduced. Each control-rod and gray rod cluster is associated with a fuel assembly and is moveable by its drive in or out of the fuel assembly or relative to a fuel assembly. Each displacement-rod cluster is associated with a plurality of fuel assemblies. Typically, there are in a Mechanical Moderator Controlled Reactor eighty-eight control-rod clusters and gray-rod clusters. Each cluster is of cruciform shape and carries eight rods positioned along the axial member and cross member of the cruciform. Typically, there are ninety-seven water-displacement rod clusters (WDRC's). Each WDRC in a typical MMCR, except those along the periphery of the upper internals, carries forty rods. Those along the periphery carry fewer than forty rods. The water-displacement rods are carried by a plurality of radial arms in the form of crucifixes extending from a central sleeve. The control-rod clusters in their functioning for load follow, experience frequent and rapid movement relative to their associated fuel assemblies. The gray-rod clusters typically may be inserted or fully withdrawn from their associated fuel elements 5600 times during the life of an MMCR. Each WDRC is fully inserted in a number of fuel assemblies symmetrical about its axis during the earlier part of the fuel cycle, and is fully withdrawn from the fuel assemblies and held in the withdrawn position during the remainder, typically 60%, of the fuel cycle. For reliable operation of a nuclear reactor, it is necessary that the neutron-flux adjusting rod clusters be effectively guided as they move in and out of the core. It is indispensable that the clusters be moveable without binding or sticking. It is an object of this invention to provide a nuclear reactor of the type having a large number of neutron-flux adjusting rods, such as the MMCR, including guides for the neutron-absorption rods which shall effectively guide these rods as they move in and out of the core and in whose use the rods shall not bind or stick as they are moved. It is another object of this invention to provide a nuclear reactor of the type that includes a flow-through screen about the guide structure having such guides for the neutron-flux adjusting rods. SUMMARY OF THE INVENTION This invention is applicable to, and can be embodied in, nuclear reactors of all types, those which have a flow-through screen and those which do not have such a screen. The invention, however, has unique applicability to nuclear reactors which have a flow-through screen. In prior art reactors each control rod, when it is retracted, is protected, by a housing, which forms a part of the guide structure, from the forces exerted by the cross-flow or radial flow of the coolant. This invention arises from the realization that in reactors including a flow-through screen, the water-displacement rods, when retracted into the upper internals, can, at least to an extent, be exposed to the coolant because in such reactors the radial velocity of the coolant is low and the drag forces exerted by the coolant on the exposed rods would be low. An additional factor which leads to the conclusion that exposure of the retracted water-displacement rods is feasible is that the time of exposure of these rods is substantially less than the time for control rods or gray rods. In accordance with this invention there is provided a nuclear reactor whose upper internals include a plurality of generally vertical guides. Each guide is formed of a plurality of vertically coextensive guide sections; each section is best described as a can open at the ends. Each can is formed and dimensioned so as to accommodate the neutron-absorber-rod clusters. Specifically, each can may have a cruciform transverse cross-section so as to pass and guide the control-rod and gray rod clusters. A generally horizontal plate is supported on the tops of each array of guide sections which are at the same level. There are thus a plurality of generally horizontal plates in a vertical array. The plates are perforated, the perforations in each plate being shaped and coordinated so as to pass the others of the neutron-flux adjusting rods. Specifically, the perforations in each plate extend radially from a central opening and include cruciform radial slots so as to pass the WDRC's. The plates are oriented in the vertical array with corresponding perforations precisely aligned so that the plates serve as guides for the WDRC's. Typically, in an MMCR, the plates have a diameter of about one hundred seventy-five inches and are composed of stainless steel. Because the plates are perforated as described, they would not, if they were integral structures, be self-supporting so that they could be manipulated unless they were very thick. To facilitate manipulation of the structure during assembly or disassembly of the reactor, each plate is formed of separate plate sections which can be nested in the manner of a "jig-saw puzzle" during assembly. Each plate section is supported on one or a plurality of the vertical guide sections. The water-displacement rods are partially exposed to the cross-flow of the coolant. When the plate sections are nested to form a plate, each plate may be regarded as divided into areas defined by a plurality of cans. The columnar volume under each plate-section area, except those around the periphery of a plate, is bounded to a substantial extent by arms on the cruciforms configuration of the cans. The areas around the periphery of each plate sections are partly bounded by the arms of cans. The water-displacement rods, when retracted, are partially protected by the arms of the surrounding cans from the cross-flow of the coolant. In the case of reactors with flow-through screens, as disclosed in Veronesi '097, the radial velocity of the coolant is low and the water-displacement rods are subjected to relatively low cross-flow forces. The plates formed of the assembled plate sections serve as plate guides for the WDRC's The reduction of velocity achieved with the flow-through screen is particularly important in reactors including WDRC's. There are a large number of water displacement rods, typically forty in each cluster. These rods together with the guides for the control rods and gray rods occupy a substantial portion of the volume of the upper internals. The coolant is driven by powerful pumps and is sucked out of the outlet nozzles at a high velocity, typically 50 feet per second. In the absence of the flow-through screen, the coolant would flow to the outlet nozzles predominantly through the portion of volume of the upper internals which is defined by an imaginary cylinder having the transverse cross-sectional areas of the openings in these nozzles and through the region immediately adjacent these cylinders. The coolant would wash the parts of the control rod and gray-rod guides and the water-displacement rods in this volume. Because the parts of the guides and water displacement rods occupy a large portion of this volume, the coolant velocity in this region of the guides and the water-displacement rods would be very high and the guides and rods would be subjected to high stresses resulting in failures. The flow-through screen distributes the coolant over the whole volume of the upper internals, reducing the flow velocity. Typically the flow velocity is reduced to about 4 ft/sec. Failure of the guides and water-displacement rods is thus precluded. In assembling the reactor, each plate section is assembled into an integrated unit, external to the reactor vessel, with its associated guide section or sections. The guide sections of each plate section are welded to end plates. The guide sections and end plates of the plate section are then stacked and secured to form an integrated columnar unit with the like plate sections interposed between adjacent end plates. The guide sections which extend along the unit are each bolted between end plates properly oriented so that the perforations of these sections are precisely aligned. Each column includes a number of complete guides equal to the number of guide sections which support the corresponding plate sections. Once all of the plate sections are assembled in columns, they are positioned in the pressure vessel on the upper core support with the plate sections at each level nested to complete the array of plates in the manner of a "jig-saw puzzle". The integrated units are pinned to the upper core support properly laterally aligned. Before these integrated units are inserted, the flow-through screen had been inserted in the pressure vessel. The integrated units are centered within the flow-through screen. The neutron-flux adjusting rod clusters are then inserted in the guide structures with their drive rods extending above the upper-internals. The upper-internals top plate is then positioned on top of the assembly of integrated units.