Small angle neutron scattering (SANS) techniques have been employed to study the nature of the radiation damage and its dependence on phosphorus concentration at two different copper levels (0.002 and 0.3%) in a set of laboratory melts of A533-B steel irradiated to ˜2.5 × 1019 n/cm2 at 288°C under test reactor conditions. In all materials, distributions of clusters were found with number densities ˜ 1017 to 1018 cm-3 and diameters ˜ 1 to 2 nm. Cluster volume fractions were approximately an order of magnitude higher in the 0.3% Cu steels, which were associated with greater radiation strengthening and embrittle-ment. Magnetic and nuclear scattering data are best interpreted in terms of mixed clusters containing Cu, Mn, and Ni; increasing P content leads to some refinement of the cluster distributions. In the low-Cu steels embrittlement is also associated with radiation hardening via cluster formation promoted by increasing phosphorus content.