is typically equal to the spacing between the centers of the rods 57 57a along the vane 73 the rods 57c and 57d on each vane 75 are in a crucifix configuration there are rods 57c along each vane 75 one at the end of the vane and another inwardly the centers of rods 57c are typically spaced equally from the center of the intersection of each vane 75 and each cross vane 79 the cross vane 79 typically carries rods 57d at its ends the spacing of the centers of rods 57d from the center of the intersection of the cross vane 79 and the vane 75 is equal to the corresponding spacing for rods 57b the centers of rods 57c and 57d thus define the corners of a square the reactor 31 has upper internals 81 fig1 including guide means for the neutron absorber rod clusters 51 and 55 the guide means includes columnar guides of cruciform transverse cross section for the control rod and gray rod clusters 51 and plates 85 for the water displacement rod clusters 55 each columnar guide is formed of a plurality of coextensive vertical columnar sections generally labelled 87 to or between which the plates 85 are secured in the interest of facilitating the description of.