Fuel assembly having water passageway in channel wall

A nuclear fuel assembly for a boiling water reactor comprising an assembly base hydraulically connected to a vertical fuel channel with a first pair and a second pair of opposite, mutually parallel channel walls, said first pair being perpendicularly oriented to said second pair, and a plurality of vertical tube elements disposed in said fuel channel, the majority of said tube elements being fuel rods and some of said tube elements being arranged to form at least two first rows and at least two second rows, each of said rows comprising a plurality of said tube elements arranged one after the other in a horizontal direction, said first rows being parallel to said first pair of channel walls and said second rows being parallel to said second pair of channel walls, said tube elements being positioned in said fuel channel in such a way that the vertical middle line of each of said tube elements runs through a corresponding nodal point in an imaginary square lattice, wherein each of said channel walls has an integral, middle wall portion constituting an inwardly directed, hollow projection projecting inwardly beyond all other portions of the wall and at least two of said tube elements, said projection comprising an elongated, vertical internal passageway for providing a flow of moderator water, said vertical passageway constituting more than 50% of the length of said fuel rods, and said middle wall portion keeping the entire flow of moderator water flowing in said passageway separated from said fuel rods and preventing this water from making contact with the fuel rods, and wherein an intermediate row of said tube elements is arranged between the two projections of each pair of said mutually parallel fuel channel walls, said first rows and said second rows having at least four more of said tube elements than said intermediate rows.