A neutron absorber member including a tube having a split extending radially completely through a sidewall of said tube throughout the entire axial length thereof such that the absorber member has a C-shaped geometry formed by the split. The sidewall of the tube is resiliently biased radially outwards and comprised of a metal matrix composite that is configured to absorb neutrons emanation from a spent fuel assembly in a sufficient amount to thereby maintain subcriticality of said spent fuel assembly.