The exact nature of the radiation defects causing hardening in reactor vessel pressure steels at high doses is not yet clearly determined. While generally it is attributed to solute-rich clusters (precipitates) and point defects clusters (matrix damage), recent fine-scale experiments and atomistic simulations suggest that solute rich clusters, mainly containing Mn, Ni and Cu, might be the result of the segregation of these elements to small dislocation loops (heterogeneous nucleation), so that the distinction between precipitates and matrix damage becomes blurred. Here, we perform an atomistic study to investigate the interaction of a0/2h111i dislocation loops with moving dislocations and specifically address the effect of solute segregation on the loop’s strength and interaction mechanism, focusing in particular on Mn, alone or with other crucial solute elements such as Cu and Ni. It is found that the enrichment of Mn in the core of dislocation loops causes significant increase of the unpinning stress, especially for small, invisible ones. At the same time, the solute segregation at the dislocation loops enhances their resistance against absorption by moving dislocations.