Ferritic/martensitic steels considered as first-wall candidate materials for fusion reactors experience significant radiation hardening at temperatures below similar to400 degreesC. Experimental evidence suggests that the observed defects are large interstitial dislocation loops with Burgers vector b = <100> and b = (1)/(2)<111>. In this work, the atomic character of <100> and (1)/(2)<111> loops is investigated with molecular dynamics simulations and the atomic configurations are used to calculate the defect image contrast through direct simulation of TEM images. The simulated images are subsequently compared with actual TEM micrographs of irradiated ferritic materials and are in very good qualitative agreement, providing strong indication that the observed b = <100> loops are interstitial in nature. (C) 2002 Elsevier Science B.V. All rights reserved.