Reactor pressure vessel steels are well-known to harden and embrittle under neutron irradiation. The primary mechanism of radiation embrittlement for these bainitic steels is the obstruction of dislocation motion, mainly due to clusters or precipitates of solute atoms such as Cu, Ni, Mn, Si and P. Microstructural examinations reveal that these clusters or precipitates are often preferentially formed at dislocation lines, which are sometimes completely surrounded by segregated solute clusters. Evidence of this is provided in this work, too, which extends a previous one dedicated to edge dislocations, by studying the effect of this segregation around screw dislocations (Burgers vector b = 1/2 ) on the critical stress for dislocation motion. A Monte Carlo algorithm in a variance-constrained semi-grand canonical (VC-SGC) ensemble is applied to study the decoration of atoms around dislocations, by minimizing the free energy. Next, the critical stress for dislocation unpinning from the clusters is evaluated by standard molecular dynamics to analyze the effect of Cu, Ni, Mn, and P segregation in the Fe matrix. Consistently with expectations and in agreement with previous work, our results highlight that the required stress for triggering dislocation motion drastically increases due to the presence of segregated solutes. Our finding is that solute-decorated screw dislocations may be considered as practically immobile because of the strong segregation around them.