Patent Number: 
Section: claims

1. A boiling water reactor, comprising: a coolant system; and  a reactor core cooled by water circulated by said coolant system, wherein said coolant system and said reactor core are configured so as to cool said reactor core during rated power operation primarily by natural-circulation of said water and wherein said reactor core has a void reactivity coefficient more than xe2x88x920.079% xcex94k/k/% void fraction and less than xe2x88x920.03% xcex94k/k/% void fraction during rated power operation. 2. A boiling water reactor, comprising: a pressure vessel;  a reactor core in said pressure vessel, and having a plurality of fuel assemblies; wherein a fuel assembly includes a plurality of fuel rods arranged with spaces between said fuel rods and a water-rod having a path for coolant, surrounded by a channel box; said reactor core having a by-pass portion for said coolant between said fuel assemblies; and wherein said reactor core is cooled by natural-circulation of water during rated power operation and has a void reactivity coefficient during rated power operation that is more than xe2x88x920.07% xcex94k/k/% void fraction and is less than xe2x88x920.03% xcex94k/k/% void fraction;  a main steam pipe for carrying steam generated in said pressure vessel by said reactor core; and  a turbine for performing expansion work with said steam. 3. A boiling water reactor according to  claim 2 , wherein a fuel rod includes uranium  235  and uranium  238 , and is divided into a plurality of parts in which enrichment of uranium  235  differs in a vertical direction, and wherein an enrichment difference between said parts is more than 0.3 wt % of uranium  235 . claim 2 4. A boiling water reactor according to  claim 2 , wherein a fuel rod includes uranium  235  and uranium  238 , and is divided into a plurality of parts in which concentration of uranium  235  differs in a vertical direction, said fuel rod having an upper blanket area and a lower blanket area including at least one of natural uranium and depleted uranium, and having enriched uranium in between said upper blanket area and said lower blanket area, and wherein a length of said lower blanket area is longer than a length of said upper blanket area. claim 2 5. A boiling water reactor according to  claim 2 , wherein a fuel rod includes uranium  235  and uranium  238 , and is divided into a plurality of parts in which enrichment of uranium  235  differs in a vertical direction, said fuel rod having a top area and a bottom area comprising a blanket area which includes at least one of natural uranium and depleted uranium. claim 2 6. A boiling water reactor according to  claim 2 , wherein a ratio of a width of said by-pass portion to a width of said channel box is greater than or equal to 12%. claim 2 7. A boiling water reactor according to  claim 2 , further comprising four water rods arranged at four corners in a fuel assembly. claim 2 8. A boiling water reactor according to  claim 2 , further comprising four boiling water stream spaces, the same size as a fuel rod, at four corners of a fuel assembly. claim 2 9. A boiling water reactor according to  claim 2 , further comprising: claim 2 a turbine governor valve controlling an amount of steam sent from said pressure vessel to said turbine; and  a plurality of relief safety valves to reduce steam pressure of said steam, located upstream of said turbine governor valve, wherein at least one of said plurality of relief safety valves is opened upon receipt of a signal to close said turbine governor valve. 10. A boiling water reactor according to  claim 2 , further comprising: claim 2 a circulating pump for making coolant circulate in said pressure vessel. 11. A boiling water reactor according to  claim 10 , wherein a flow rate due to natural circulation when said circulating pump stops is at least 70% of total flow rate. claim 10 12. A boiling water reactor according to  claim 10 , further comprising a circuit to stop said circulating pump when a turbine governor valve is closed. claim 10 13. A boiling water reactor according to  claim 2 , further comprising: claim 2 a control rod to be inserted into said by-pass portion;  a control rod guide to guide said control rod when said control rod is inserted into said by-pass portion; and  a control rod drive to insert said control rod into a lower part of said pressure vessel from an upper part of said pressure vessel. 14. A method for operating a boiling water reactor, comprising: a pressure vessel;  a reactor core in said pressure vessel, and having a plurality of fuel assemblies, at least one fuel assembly including a plurality of fuel rods arranged with spaces between said fuel rods, and a water-rod having a path for coolant, surrounded by a channel box; at least one fuel rod having a blanket area having at least one of natural uranium and depleted uranium in at least one of an upper part and a lower part of said fuel rod; said reactor core having a by-pass portion for said coolant between said fuel assemblies, and said reactor core being cooled by natural-circulation during rated power operations;  a control rod to be inserted into said by-pass portion;  a control rod guide to guide said control rod when said control rod is inserted into said by-pass portion;  a control rod drive to insert said control rod into a lower part of said pressure vessel from an upper part of said pressure vessel;  a main steam pipe for carrying steam generated in said pressure vessel; and  a turbine for performing expansion work with said steam, comprising the step of:  operating said boiling water reactor at or near rated power so as to maintain a void reactivity coefficient of xe2x88x920.07% xcex94k/k/% void fractionxe2x89xa6void reactivity coefficient xe2x89xa6xe2x88x920.03% xcex94k/k/% void fraction. 15. A method according to  claim 14 , further comprising operating said boiling water reactor such that at least one control rod is inserted into said reactor core to a border position between a blanket area and an enriched area during a last stage of an operational cycle. claim 14 16. A boiling water reactor according to  claim 13 , wherein said boiling water reactor is operated such that at least one control rod is inserted into said reactor core into a border position between a blanket area and an enriched area during a last stage of an operational cycle. claim 13