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A BWR 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 mount

A BWR 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 BWR fuel assembly includes an improvement in the form of a coolant flow direction control device mounted in the bottom nozzle of the fuel assembly and being operable to open the inlet thereof 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. In an exemplary form, the coolant flow direction control device is 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.

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In a nuclear reactor having a flow of coolant/moderator fluid therein, at least one fuel assembly installed in the fluid flow, said fuel assembly comprising in combination: a bundle of elongated fuel rods disposed in side-by-side relationship so as to form an array of spaced fuel rods; an outer tubu

In a nuclear reactor having a flow of coolant/moderator fluid therein, at least one fuel assembly installed in the fluid flow, said fuel assembly comprising in combination: 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 said fuel rods so as to direct the flow of coolant/moderator fluid along said fuel rods; bottom and top nozzles mounted at opposite ends of said flow channel and having an inlet and outlet respectively for allowing entry and exit of the flow of coolant/moderator fluid into and from said flow channel and along said fuel rods therein, said bottom nozzle having an annular surface defined therein so as to surround said inlet thereof, said annular surface having circumferentially spaced sectors and circumferentially spaced segments which alternate with said spaced sectors; and a coolant flow direction control device operatively disposed in said bottom nozzle so as to open said inlet thereof to the flow of coolant/moderator fluid in an inflow direction into said flow channel through said bottom nozzle inlet but close said inlet to the flow of coolant/moderator fluid from said flow channel through said bottom nozzle inlet upon reversal of coolant/moderator fluid flow from the inflow direction; said coolant flow direction control device being a unidirectional flow check valve positioned across said inlet of said bottom nozzle for sensing the direction of coolant/moderator fluid flow and automatically opening when the flow direction sensed is into said bottom nozzle and closing when the flow direction sensed is out of said bottom nozzle; said flow check valve including a plurality of outer portions mounted to said respective circumferentially spaced sectors of said bottom nozzle annular surface surrounding said inlet thereof, and a plurality of inner portions being pivotably connected to said respective outer portions for pivotal movement toward and away from one another between lowered and closed and raised open positions; said inner valve portions, when in their raised positions, extending toward said bottom nozzle in the direction of coolant flow into said bottom nozzle, said inner valve portions being configured to extend in close fitting relationship adjacent to one another and coplanarly across said inlet so as to close said inlet when disposed in their respective lowered positions and to extend in generally parallel relationship to the direction of coolant flow and be located remote from one another so as to open said inlet when disposed in their respective raised positions; each of said inner valve portions, when in its lowered position, has opposite lateral edge portions that seat on respective surface portions of the two segments located on opposite ends of said sector to which the respective outer portion of said valve is mounted, said inner valve portions in seating on said spaced segments of said bottom nozzle annular surface being stopped by said surface from pivoting past their lowered positions in which they would extend away from said bottom nozzle and allow reverse flow of coolant therefrom; said inner valve portions being arranged in first and second pairs which are angularly displaced about ninety degrees from one another wherein said inner valve portions of each pair are placed in opposing relation to one another such that one is a mirror image of the other, said inner valve portions located directly opposite to one another in said respective pairs thereof extending generally parallel to one another when in said raised positions.