Patent Number: 
Section: claims

1. A method of operating a boiling water reactor including fuel assemblies each comprising a plurality of fuel rods, at least one water rod, an upper tie plate for holding upper end portions of said fuel rods and said water rod, a lower tie plate including a fuel holding portion to hold lower end portions of said fuel rods and said water rod, and a channel box surrounding an outer periphery of said fuel rods tied up into a bundle; and fuel support pieces for supporting said lower tie plates of said fuel assemblies, said fuel support piece including a first coolant passage formed therein and having an orifice;  said fuel holding portion having a plurality of through holes for introducing a coolant in said lower tie plate to a second coolant passage defined between said fuel rods above said fuel holding portion, a total cross-sectional area of all said through holes being smaller than a total cross-sectional area of said second coolant passage;  said water rod including a rising passage opened to a space in said lower tie plate below said fuel holding portion and introducing upward the coolant introduced to said rising passage, and a falling passage communicated with said rising passage and introducing downward the coolant introduced through said rising passage, said falling passage having a coolant outlet opened to said second coolant passage above said fuel holding portion, wherein:  said rising passage is filled with the coolant during a period of rated power operation, and a surface of the coolant is formed in said rising passage during a period of non-rated power operation in which a flow rate of the coolant supplied to said fuel assemblies is lower than that during the period of said rated power operation. 2. A method of operating a boiling water reactor including fuel assemblies each comprising a plurality of fuel rods, at least one water rod, an upper tie plate for holding upper end portions of said fuel rods and said water rod, a lower tie plate including a fuel holding portion to hold-lower end portions of said fuel rods and said water rod, and a channel box surrounding an outer periphery of said fuel rods tied up into a bundle; and fuel support pieces for supporting said lower tie plates of said fuel assemblies; said fuel support piece including a first coolant passage formed therein and having an orifice;  said fuel holding portion having a plurality of through holes for introducing a coolant in said lower tie plate to a second coolant passage defined between said fuel rods above said fuel holding portion, a total cross-sectional area of all said through holes being smaller than a total cross-sectional area of said second coolant passage;  said water rod including a rising passage opened to a space in said lower tie plate below said fuel holding portion and introducing upward the coolant introduced to said rising passage, and a falling passage communicated with said rising passage and introducing downward the coolant introduced through said rising passages said falling passage having a coolant outlet opened to said second coolant passage above said fuel holding portion, wherein:  said rising passage is filled with the coolant during a period of rated power operation, and a surface of the coolant is formed in said rising passage during a period of non-rated power operation in which a flow rate of the coolant supplied to said fuel assemblies is lower than that during the period of said rated power operation;  wherein said first coolant passage having said orifice has an inner diameter of at least 5.6 cm,  said total cross-sectional area of all said through holes is represented by S 1  and said total cross-sectional area of said second coolant passage is represented by S 2 , the relationship of 0.2xe2x89xa6r0.4 holds on an assumption that a ratio S 1 /S 2  of said total cross-sectional area S 1  to said total cross-sectional area S 2  is r, and a height h from an upper surface of said fuel holding portion to said coolant outlet in relation to a fuel effective length L of the plurality of fuel rods is set to satisfy at least one of relationship (a), (b), and (c) of:  (a) xe2x88x922.1r 2 +2.2rxe2x88x920.3xe2x89xa6(h/L) less than xe2x88x922.2r 2 +1.8r+0.04;  (b) xe2x88x924.2r 2 +3.4rxe2x88x920.4xe2x89xa6(h/L) less than xe2x88x920.53r 2 +0.5r+0.46; and  (c) xe2x88x924.2r 2 +3.4rxe2x88x920.4xe2x89xa6(h/L) less than xe2x88x922.2r 2 +1.8r+0.04. 3. A method according to  claim 2 , wherein said first coolant passage having said orifice has said inner diameter of not more than 6.2 cm. claim 2 4. A method according to  claim 3 , wherein said first coolant passage having said orifice has said inner diameter of about 6.2 cm. claim 3 5. A method according to  claim 4 , wherein the relationship (a) of: claim 4 xe2x88x922.1r 2 +2.2rxe2x88x920.3xe2x89xa6(h/L) less than xe2x88x922.2r 2 +1.8r+0.04 is satisfied. 6. A method according to  claim 1 , wherein the total cross-sectional area of all said through holes is represented by S 1  and the total cross-sectional area of said second coolant passage is represented by S 2 , a relationship of 0.2xe2x89xa6rxe2x89xa60.4 holes on an assumption that a ratio S 1 /S 2  of the total cross-sectional area S 1  to the total cross-sectional area S 2  is r, and a height h from an upper surface of said fuel holding portion to said coolant outlet in relation to a fuel effective length L of the plurality of fuel rods is set to satisfy a predetermined relationship. claim 1 7. A method according to  claim 6 , wherein the predetermined relationship is at least one of relationships (a), (b) and (c) of: claim 6 (a) xe2x88x922.1r 2 +2.2rxe2x88x920.3xe2x89xa6(h/L) less than xe2x88x922.2r 2 +1.8r+0.04;  (b) xe2x88x924.2r 2 +3.4rxe2x88x920.4xe2x89xa6(h/L) less than xe2x88x920.53r 2 +0.5r+0.46; and  (c) xe2x88x924.2r 2 +3.4rxe2x88x920.4xe2x89xa6(h/L) less than xe2x88x922.2r 2 +1.8r+0.04. 8. A method according to  claim 1 , wherein the surface of the coolant formed in said rising passage during at least a portion of one of the period of rated power operation and the period of non-rated power operation corresponds to a fuel effective length of the plurality of fuel rods. claim 1 9. A method according to  claim 1 , wherein the surface of the coolant formed in said rising passage during at least a portion of one of the period of rated power operation and the period of non-rated power operation corresponds to a fuel effective length of the plurality of fuel rods, whereby influences of a transient event occurring during the rated power operation are suppressed and nuclear thermal-hydraulics stability during the non-rated power operation is improved. claim 1