Patent Number: 
Section: claims

1. A natural circulation boiling water reactor in operation having a chimney comprising:a cylindrical chimney shell disposed above a core in a reactor pressure vessel;a plurality of square tubes, each square tube having a lower end and an upper end, disposed so as to extend in a vertical direction in said chimney shell; anda first grid support plate, disposed in said reactor pressure vessel above said core, arranged to have a plurality of first grid holes and a plurality of second grid holes so that said first grid holes and said second grid holes are arranged alternately in rows and columns;wherein said lower ends of said square tubes are disposed and detachably supported by said second grid holes of said first grid support plate such that said second grid hole faces an opening at said lower end of said square tube, and said square tubes are not disposed at positions of said first grid holes such that said first grid holes are vacant without any of said square tubes being disposed thereat;wherein said square tubes are disposed with respect to said second grid holes in the rows and columns so as to be spaced from each other at an interval which exceeds a width of one of said square tubes;wherein a gap is formed between edges of said square tubes of said adjacent rows which face each other at least along a diagonal of a cross section of the square tubes; andwherein a two-phase flow path communicated with said core includes said first grid holes of said first grid support plate where said square tubes are not disposed and outer side surfaces of said square tubes extending upwardly from said second grid holes of said first grid support plate where said square tubes are disposed. 2. The natural circulation boiling water reactor according to claim 1,wherein each of said square tubes is a bent plate member having a welded seam which extends in a longitudinal direction of said square tube. 3. The natural circulation boiling water reactor according to claim 2, wherein a welding line for said welded seam is formed at one of the outer side surfaces of said square tube, and is positioned between ⅛ and ⅜ of the width of the one of the outer side surfaces. 4. The natural circulation boiling water reactor according to claim 1, wherein said two-phase flow path communicated with said core further includes an inside of said square tubes. 5. The natural circulation boiling water reactor according to claim 1, further comprising:a second grid support plate in said reactor pressure vessel, disposed above said first grid support plate, having a plurality of third grid holes and a plurality of fourth grid holes so that said third grid holes and said fourth grid holes are arranged alternately in rows and columns corresponding to the alternate arrangement of said first grid holes and said second grid holes of said first grid support plate, respectively;wherein said upper ends of said square tubes are detachably supported by said fourth grid holes such that said fourth grid hole faces an opening at said upper end of said square tubes. 6. The natural circulation boiling water reactor according to claim 1, further comprising a plurality of fuel assemblies disposed in the core, wherein said fuel assembles are configured to be inserted through said first and second grid holes. 7. The natural circulation boiling water reactor according to claim 1, wherein said square tubes are spaced from each other at an interval which exceeds a width of one of said square tubes in each row such that an arrangement of said square tubes disposed in one row are offset with respect to an arrangement of said square tubes disposed in an adjacent row. 8. The natural circulation boiling water reactor according to claim 1, wherein said first grid holes and said second grid holes are substantially of a same size enabling said lower ends of said square tubes to be disposed and detachably supported thereby.