The radiation-induced primary dislocation loops in thin foils of ultra-high purity Fe and Fe(Cr) model alloys were investigated using ion irradiation in situ in a TEM. In the '111 mechanism' the induced nanometric loops of type a(0) <100> stem from mutual interaction of 1/2 a(0) <111> loops following their thermal diffusion. In the present work the conditions for this mechanism to occur are scrutinized. The effect of He, irradiation dose, dose rate, temperature and Cr content on the production of loops is assessed. Fe, Fe-5, -10 and -14Cr were irradiated with 500 keV Fe+ and 10 keV He+ ions up to 1 dpa and 1000 appm He at room and liquid nitrogen temperature. The initial loop population consists of 1/2 a(0) <111> and a(0) <100> loops, with no visible 1/2 a(0) <110>'s. Helium appears to stabilize 1/2 a(0) <111>'s by impeding their motion, as in its presence they are more numerous relative to a(0) <100>'s. At 1 dpa Cr plays a similar role. This is supported by (i) irradiations of Fe at three different dose rates, as only 1/2 a(0) <111>'s are observed after the fastest irradiation, and (ii) irradiation at liquid nitrogen temperature. The later leads to a majority of 1/2 a(0) <111>'s, while upon warming up to RT a(0) <100>'s become more numerous. All this supports the idea of the '111 mechanism'. However, surprisingly, at 0.05 dpa Cr actually favours the formation of a(0) <100> loops, irrespective of its influence on the mobility of the 1/2 a(0) <111>'s. (C) 2017 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.