Ferritic/martensitic steels considered as candidate first-wall materials for fusion reactors experience significant radiation hardening at temperatures below similar to400 degreesC. A number of experimental studies in ferritic alloys, performed at 1/2(111) and (100) in the higher temperatures, have shown the existence of large interstitial loops with Burgers vector 2 bulk, which may provide a significant contribution to the hardening caused during irradiation at lower temperatures. Hardening arises from a high number density of loops, voids and small precipitates, which pin system dislocations, impeding their free glide. In this work, we review the nature of the different interstitial dislocation loops observed in alpha-Fe and ferritic materials, assess the effect of substitutional impurities on migrating 1/2 (111) clusters, and apply atomistic modeling to investigate the mechanisms of formation and growth of (100) loops from smaller cascade-produced 1/2 (111) clusters. The proposed mechanism reconciles experimental observations with continuum elasticity theory and recent MD modeling of defect production in displacement cascades. In addition, the interaction of screw dislocations, known to control the low-temperature plastic response of b.c.c. materials to external stress, with (10 0) dislocation loops is investigated with MD, where the main physical mechanisms are identified, cutting angles estimated and a first-order estimation of the induced hardening is provided. (C) 2003 Elsevier B.V. All rights reserved.