a fuel bundle having spacers optimally deployed is disclosed wherein the distance between the spacers varies along the length of the fuel bundle . the fuel bundle has a dense fuel rod array . it includes at least one part length rod extending from the lower tie plate toward but not to the upper tie plate , which part length rod terminates at a spacer and defines under the upper tie plate overlying the end of the spacer a vent volume for the preferential venting of vapor moderator from the fuel bundle . a vertical spatial distribution of the spacers is disclosed in which the bottom spacers in the single phase region of the fuel bundle are relatively closely vertically spaced and the upper spacers are relatively more distantly vertically spaced one from another . the design enables optimum resistance to flow induce vibration and rod bow , minimum interference of the spacers with pressure drop in the sensitive upper two phase region of the fuel bundle , and the required bracing of part length rods adjacent their ends .