Annular burnable absorber rod

This patent describes a neutron irradiation environment of a water cooled and moderated reactor core a fuel assembly having a discrete annular burnable absorber rod, the rod being free of nuclear fuel material the rod comprising a first elongated hollow tube having opposed ends, a second elongated hollow tube having opposed ends. The first and second tubes are constructed of a zirconium material, the first and second tubes concentrically arranged about a common axis to provide a coextensive annular space therebetween and a path for the water through the first tube along the common axis. A sealing means seals the annular space at common opposed ends of the first and second tube and a hollow niobium tubular support is arranged within the annular space. The tubular support has an inner and outer surface, a layer of burnable neutron absorber material provided on at least a portion of one of the surfaces of the support the layer of the burnable neutron absorber material selected from the group of boron compounds consisting of boron carbide, boron nitride and zirconium diboride; and the layer of the burnable neutron absorber material having a thickness between 0.002 to 0.010 inches