Owing to its attractive material properties, tungsten (W) and its alloys are considered to be the leading solid material candidates for plasma-facing components in future fusion devices such as the Tokamak reactors in the international thermonuclear experimental reactor (ITER) as well as the planned future demonstration fusion reactor (DEMO). However, a fundamental concern of using W for such applications is the high ductile to brittle transition temperature (DBTT) and its dependence on the microstructure and impurity concentrations. Experiments have revealed that the DBTT for single-crystalline tungsten can be as low as -196 °C, whereas poly-crystalline samples can remain brittle up to about 800 °C. For the application of fusion reactor components, this is a major concern in light of the fact that the temperature at the armour material of the first wall and diverter under operating conditions typically lie in the range 600–900 °C. This suggests that the first wall armour material is at risk of rupturing because of brittle grain boundary fracture. This situation becomes especially severe when it contains impurities that reduce the grain boundary strength the material. In the present work we study the impact of embritteling impurities on the grain boundary strength of W by means of first principles quantum mechanical modelling based on density functional theory. The purpose is to model the grain boundary (GB) strength for different impurity concentrations and to investigate how the coverage varies during the gradual separation. In particular, we aim to investigate how the peak stress associated with decohesion and the Griffith work of fracture along with impurity transport influences the cohesive strength during the mode I opening based on a thermodynamic description of the equilibrium impurity coverage. This provides qualitative insight to the influence of impurities on the cohesive strength of grain boundaries and provide traction-separation data that can be used for macroscopic cohesive zone modelling of intergranular fracture in tungsten. As model GB we consider the Σ5(310)[001] high angle configuration with varying degrees of phosphorus impurities, which are notorious for having a detrimental effect on the strength of tungsten GB:s. The results show that by the introduction of a clean (i.e. impurity free) grain boundary in the bulk, the strength and peak stress of the cohesive zone are reduced and they are further reduced by the introduction of impurities. This effect can be attributed to the formation of polar bonds between W and P, which leads to a weakening of the interface. Based on a thermodynamic analysis of the cohesive zone during the straining we find that diffusion of impurities may occur to retain thermodynamic equilibrium for constant chemical potential. This contributes to the gradual reduction of the peak stress related to fracture, which can contribute to diffusion driven delayed cracking, even when subjected to static loads. For further details the reader is referred to [1]. [1] P. A. T. Olsson and J. Blomqvist, Intergranular fracture of tungsten containing phosphorus impurities: A first principles investigation, Computational Materials Science 139, 368-378 (2017).