Patent ID: 7907695

Claim:
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; and a 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; and wherein 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.