Fuel assembly for BWR type reactor

The present invention provides a BWR type fuel assembly, capable of lowering an infinite multiplication factor effectively for a long period of time in a less loading amount of burnable poisons even at a high infinite multiplication factor. Namely, the fuel assembly of the present invention comprises a large diameter water rod and a plurality of fuel rods bundled surrounding the water rod substantially at the center, with a portion of the fuel rods being burnable poison-incorporated fuel rods. When defining the outermost circumferential region of the bundle as (A), the inner circumferential region comprising a plurality of fuels including the water rod as (B) and an intermediate region excluding these regions as (C), the burnable poison-incorporated fuel rods are disposed by the greater number to the region (C). The average uranium enrichment degree of a plurality of fuels disposed to the region (B) is made higher than that of the fuel rods disposed in the region (C). Such an arrangement of the fuel rods can lower the infinite multiplication factor in the middle burning stage than that by other arrangement of other fuels containing the same amount of burnable poisons. (I.S.)