A fuel assembly for use in a core of a nuclear power reactor. The assembly includes a frame shaped and configured to fit within the nuclear reactor internal core structure; and a plurality of helically twisted fuel elements supported by the frame in a fuel rod bundle. Each of the fuel elements includes fissile material. When viewed in a cross-section that is perpendicular to an axial direction of the fuel assembly, the outermost fuel elements of the fuel rod bundle define a substantially circular perimeter. Other features, and a nuclear reactor, are also described.