Steel components within fusion reactors will be subject to high transmutation rates due to high energy neutrons. In iron based alloys such as steels, high amounts of helium accumulate through transmutation. This leads to helium embrittlement through helium accumulating on the grain boundaries of metal. Worst case scenario predictions were made for DEMO, estimating that for a grain size of 5 micro-meters, embrittlement could happen within 2 years of the blanket region of DEMO. This thesis elaborates on previous worst case scenario calculations by including inter-granular tapping mechanisms, within rate theory simulations. A rate theory code was developed for the purpose of this work, tailored towards a fusion environment. Calculations were performed using rate theory that predicted the timescales in which helium embrittlement occurred within a conceptual DEMO design in the first wall region and the blanket region. The calculations used several parameter sets, where preliminary simulations were performed using the parameter sets, that were compared with cluster density data determined using Transmission Electron Microscopy (TEM) and Positron Annihilation Spectroscopy (PAS). The simulations showed that the helium embrittlement time was heavily influenced by the chosen dislocation density, parameter set and grain size. The simulations conducted to represent the blanket region, showed an increase as high as 94% from the 2 years that has previously been predicted under certain scenarios. However results also showed that assuming a certain parameter set with a low dislocation density, showed no significant increase in embrittlement time. This was not a concern since it was concluded that advanced steel concepts would be expected to have a small average grain size, that would dramatically increase the embrittlement time. The work in this thesis also focused on defect interaction with dislocations. A model was constructed that made use of elasticity theory and VASP calculations that produced the interaction energy map for various defects with an edge dislocation. The interaction energy map for helium interstitials with an edge dislocation was compared with molecular dynamics (MD) simulations produced for this work. The model and simulations showed good agreement. Temperature effects were then included in the model that allowed the concentration around a dislocation to be temperature dependent. These temperature dependent interaction energy maps were then implemented into the advection-diffusion equation, that were solved numerically to explore the capture efficiencies and bias towards certain defects within iron. These values were then used within the rate theory simulations to produce temperature effects on the dislocation sink strengths for vacancies, SIA and helium interstitials.