A neutron absorber apparatus for use in restoring reactivity control to a nuclear fuel rack. The apparatus comprises an elongated tubular insert assembly configured for insertion in a storage cell of the rack. First and second absorber plates, each formed of a boron-containing material, are coupled together by upper and lower stiffening bands at the insert extremities and form a longitudinally-extending cavity configured for receiving a fuel assembly. The absorber plates and stiffening bands may have a rectilinear cross sectional configuration in one embodiment. At least one elastically deformable locking protrusion mounted proximate to the lower end of the absorber plates lockingly engages an available lower edge disposed in the cell sidewall above its bottom end. This fixes the tubular insert axially in the cell, thereby preventing its withdrawal after installing the insert. In one embodiment, the edge may be the bottom of existing absorber sheathing in the cell.