Copper concentrations as high as ^0.4atom% were found in early reactor pressure steels, largely due to both steel recycling and the use of copper as a corrosion-resistant coating on steel welding rods. Research that began in the 1970s demonstrated that this minor impurity was responsible for a signifi­cant fraction of the observed vessel embrittlement due to its segregation into a high density of very small (a few nanometer diameter) copper-rich solute clusters (Becquart and coworkers,126 Chapter 4.05, Radiation Damage of Reactor Pressure Vessel Steels). Becquart and coworkers employed MD cas­cade simulations to determine whether displacement cascades could play a role in the Cu-segregation process, for example, by coalescing with vacancies in the cascade core during the cooling phase. The set of interatomic potentials used is described in Becquart and coworkers.126 Cascade energies of 5, 10, and 20 keV were employed in simulations at 600 K, with copper concentrations of 0, 0.2, and 2.0 atom%. Similar to the case for Fe-C, no effect of copper was found on either stable defect formation or point defect clustering. The tendency for copper to be found bound with either a vacancy or an interstitial in solute-defect complex was observed. The copper-vacancy com­plexes may play a role in the formation of copper — rich clusters over longer times, but no evidence for copper clustering was observed in the cascade debris. Similar results were found in an earlier study by Calder and Bacon.127 Overall, the results of the Fe-Cu studies completed to date are consistent with the fact that Fe and Cu have similar masses and do not strongly interact.