The question of how loops nucleate and grow in α-Fe under irradiation is addressed using object kinetic Monte Carlo with parameters from molecular dynamics and density functional theory calculations. Two models are considered for the formation of <100> loops, both based on recent atomistic simulations. In one model <100> loops are formed by the interaction between ½<111> loops. In a second model small interstitial clusters, nucleated in the collision cascade, can grow as <100> or ½<111> loops. Comparing results from the calculations to experimental measurements of loop densities, ratios and sizes produced by Fe+ 100 keV irradiation of UHP Fe thin films at room temperature, the validity of the models is assessed. For these experimental conditions, the reaction model does not seem to be very efficient in the production of <100> loops due to the fast recombination of ½<111> loops to surfaces. Therefore, in our thin film simulations (at very low carbon concentrations) most <100> loops are a result of the nucleation model. In bulk simulations this effect could change since the probability of interactions between ½<111> loops would increase. Moreover, simulations show that total visible cluster concentration depends strongly on sample thickness and carbon content, while crystal orientation does not seem to have a significant role. Finally, the ratio of <100> to ½<111> visible clusters changes with increased carbon concentration.