Patent Number: 047160110
Section: summary

CROSS REFERENCE TO RELATED APPLICATIONS Reference is hereby made to the following copending applications dealing with related subject matter and assigned to the assignee of the present invention: 1. "Nuclear Fuel Assembly" by Robert F. Barry et al, assigned U.S. Ser. No. 368,555, and filed Apr. 15, 1982 (W.E. 50,013). 2. "Coolant Flow Paths Within a Nuclear Fuel Assembly" by Pratap K. Doshi, assigned U.S. Ser. No. 602,089 and filed Apr. 19, 1984, a continuation of U.S. Ser. No. 368,552, filed Apr. 15, 1982 and now abandoned (W.E. 50,105C). 3. "Water Tubes Arranged In Cross-Like Pattern In A Fuel Assembly" by Carl A. Olson et al, assigned U.S. Ser. No. 642,844 and filed Aug. 20, 1984 (W.E. 51,464). 4. "Cross Brace For Stiffening A Water Cross In A Fuel Assembly" by C. K. Lui, assigned U.S. Ser. No. 672,042 and filed Nov. 16, 1984 (W.E. 52,237). 5. "Improved Boiling Water Nuclear Reactor Fuel Assembly" by Rusi Taleyarkhan, assigned U.S. Ser. No. 726,602 and filed May 2, 1985 (W.E. 52,509). 6. "BWR Fuel Assembly With Water Flow Mixing Chamber At FuelBundle/Water Cross Entrance" by Rusi Taleyarkhan, assigned U.S. Ser. No. 746,619 and filed June 19, 1985 (W.E. 52,755). 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 a boiling water reactor (BWR) fuel assembly having a one-way coolant flow valve at its bottom nozzle inlet which allows inflow of coolant but automatically closes upon flow reversal to prevent rapid depletion of fuel assembly coolant inventory in the event of a loss of coolant accident (LOCA). 2. Description of the Prior Art Typically, large amounts of energy are released through nuclear fission in a nuclear reactor with the energy being dissipated as heat in the elongated fuel elements or rods of the reactor. The heat is commonly removed by passing a coolant in heat exchange relation to the fuel rods so that the heat can be extracted from the coolant to perform useful work. In a nuclear reactor generally, a plurality of the fuel rods are grouped together to form a fuel assembly. A number of such fuel assemblies are typically arranged in a matrix to form a nuclear reactor core capable of a self-sustained, nuclear fission reaction. The core is submersed in a flowing liquid, such as light water, that serves as the coolant for removing heat from the fuel rods and as a neutron moderator. Specifically, in a BWR the fuel assemblies are typically grouped in clusters of four with one control rod associated with each four assemblies. The control rod is insertable within the fuel assemblies for controlling the reactivity of the core. Each such cluster of four fuel assembies surrounding a control rod is commonly referred to as a fuel cell of the reactor core. A typical BWR fuel assembly in the cluster is ordinarily formed by a N by N array of the elongated fuel rods. The bundle of fuel rods are supported in laterally spaced-apart relation and encircled by an outer tubular channel having a generally rectangular cross-section. The outer flow channel extends along substantially the entire length of the fuel assembly and interconnects a top nozzle with a bottom nozzle. The bottom nozzle fits into the reactor core support plate and serves as an inlet for coolant flow into the outer channel of the fuel assembly. Coolant enters through the bottom nozzle and thereafter flows along the fuel rods removing energy from their heated surfaces. During LOCA incident (e.g. a recirculation line break), a break in the primary circuit causes coolant depletion from the reactor core. Coolant in each fuel assembly then reverses its upward flow direction and exits downward through the inlet of the fuel assembly bottom nozzle into the lower plenum of the core and then out of the primary circuit. This loss of coolant causes the fuel in the assembly to overheat (after occurrence of critical heat flux (CHF)) and possibly start to melt until the emergency core cooling systems (ECCS) get activated and provide sufficient heat transfer. In plants subject to such shortcoming in LOCA performance, NCR guidelines restrict operation of plants to remain at or below a certain power level. Such plants are often referred to as "LOCA limited." Consequently, it is readily apparent that a need exists for some means to prevent short-term depletion of coolant from the fuel assemblies due to occurrence of a LOCA event so that efficient heat transfer can still proceed until activation of the ECCS. The implementation of such means would allow LOCA limited plants to be operated at higher power levels, resulting in significant gains in economy and safety. SUMMARY OF THE INVENTION The present invention provides a coolant flow direction control device in the fuel assembly bottom nozzle which is designed to satisfy the aforementioned needs. Underlying the present invention is a recognition that the provision of a simple one-way or unidirectional flow (check) valve at the bottom nozzle inlet would allow coolant to enter but not leave the fuel assembly through the bottom nozzle. The valve automatically (by fluid action) closes with flow reversal to contain the coolant inventory of the fuel assembly within it and thereby significantly improve LOCA cooling. Thus, as external recirculation of coolant flow decays to zero upon a loss of coolant and as coolant flow reversal through the fuel assembly begins, the unidirectional valve shuts off coolant depletion from the bottom nozzle. This allows for "pool boiling" heat transfer to occur. Pool boiling in the fuel assemblies produces a pressure gradient which causes coolant flow from the bypass into the fuel assembly (via the bypass flow holes in the bottom nozzle), restricting temperature rise in the assembly and preventing fuel rod overheating before the start of the ECCS. The present invention also provides for improved performance (cooling) during all three modes of ECCS: core spray, interstitial injection and in-vessel injection. Further, the impact of the valve on thermal hydraulics, nuclear and structural performance characteristics are largely beneficial in nature. In summary, the present invention provides for significant improvements in LOCA performance and reduced negligible fuel rod temperature rise. These improvements are felt to be substantial enough to overcome or minimize any regulatory LOCA constraints on maximum linear heat generation rate (LHGR) and average planar heat generation rates (APLHGR), respectively. This results in high power operation, and, therefore, improved fuel cycle economics and safety. Accordingly, the present invention sets forth an improved feature in a BWR fuel assembly. The fuel assembly includes a bundle of elongated fuel rods disposed in side-by-side relationship so as to form an array of spaced fuel rods, an outer tubular flow channel surrounding the fuel rods so as to direct flow of coolant/moderator fluid along the fuel rods, and bottom and top nozzles mounted at opposite ends of the flow channel and having an inlet and outlet respectively for allowing entry and exit of coolant fluid into and from the flow channel and along the fuel rods contained therein. The improved feature of the fuel assembly comprises a coolant flow direction control device operatively disposed in the bottom nozzle of the fuel assembly so as to open the inlet to flow of coolant fluid in an inflow direction into the flow channel through the bottom nozzle inlet but close the inlet to flow of coolant fluid from the channel through the bottom nozzle inlet upon reversal of coolant liquid flow from the inflow direction. More particularly, the coolant flow direction control device is in the form of a one-way or unidirectional flow check valve positioned across the inlet of the fuel assembly bottom nozzle for sensing the direction of coolant flow and automatically opening when the flow direction sensed is into the bottom nozzle and closing when the flow direction sensed is out of the bottom nozzle. In an exemplary embodiment, the valve has a plurality of outer portions mounted to the bottom nozzle adjacent to the inlet and a plurality of inner portions being connected to the respective outer portions for pivotal movement toward and away from one another between lowered and raised positions. The inner valve portions are configured to extend in close fitting relationship adjacent to one another and coplanarly across the inlet so as to close it when disposed in their respective lowered positions and to extend in generally parallel relationship remote from one another so as to open the inlet when disposed in their respective raised positions. The outer valve portions, in being mounted to the bottom nozzle, are configured for attachment on respective circumferentially spaced sectors of an annular surface being defined in the bottom nozzle and concentrically surrounding the inlet thereof. The inner valve portions, when in their respective lowered positions, are configured for seating on respective circumferentially spaced segments of the annular surface which alternate with the spaced sectors of the annular surface and constitute the remainder thereof. When in their raised positions, the inner valve portions extend toward the bottom nozzle in the direction of coolant flow into the bottom nozzle. Their seating on the annular surface when in their lowered positions stops them from pivoting past the lowered position so as to extend away from the bottom nozzle and allow reverse flow of coolant therefrom. These and other advantages and attainments of the present invention will become apparent to those skilled in the art upon a reading of the following detailed description when taken in conjunction with the drawings wherein there is shown and described an illustrative embodiment of the invention.