Materials within a viable D-T fusion power-plant will be subject to hydrogen isotope permeation through implantation, adsorption and transmutation. Hydrogen is known to have low solubility in iron but irradiation-induced defects such as vacancies can strongly trap multiple hydrogen atoms. Tritium breeding blankets will be required to compensate for tritium captured by plasma-facing components and waste management strategies must account for tritium retention due its radioactivity. It is common place for theoretical models to use protium as a surrogate for all hydrogen isotopes when investigating microstructural evolution and fatigue mechanisms within fusion engineering considerations. In this work we present a systematic investigation into the relative stability of hydrogen isotopes within body-centred-cubic (bcc) Fe using the density functional theory method with zero-point energy and elastic corrections. We focus on hydrogen interactions in the bulk, trappingin mono-vacancies and adsorption on clean low index surfaces. Hydrogen interactions are further considered with Cr in the dilute limit within the Fe matrix. We show that all hydrogen isotopes are highly mobile in the bulk and reside in tetrahedral interstitials but the diffusivity reduces with mass. Protium migrates via the octahedral site whilst tritium rotates into an adjacent tetrahedral site. The activation barrier for both pathways is degenerate for deuterium which becomes the most mobile isotope at fusion operating temperatures. Hydrogen is repelled by Cr which tends to increase migration barriers by up to 20 meV. Trapping energies of deuterium in a mono-vacancy closely match desorption stages from ion-beam implantation-annealing[1, 2]. The binding of the heavier isotopes is more exothermic than protium. However, dissolution energies between isotopes is typically within 10 meV such that we do not expect practical kinetic isotope effects at operating temperatures. Cr substitutions near a vacancy change the relative incremental binding energies and push hydrogen away from near octahedral sites. The binding energy to a Cr-vacancy complex is lower for 1-3 hydrogen relative to a vacancy in the absence of Cr but is increased as the complex becomes saturated. The binding energy of heavier hydrogen isotopes to both (100) and (110) surfaces are lower than protium. Subsurface Cr is shown to change the preferred hollow adsorption site to a bridge site on a (100) surface, increasing the binding energy by as much as 50%. Finally, we present elastic dipole tensors and relaxation volumes of the hydrogen implanted systems which can be used to represent source terms in continuum models of irradiated materials[3].