The properties of sealing materials are important for the performance and reliability of solid oxide fuel cells (SOFC). Even if the properties of a sealing material can be studied separately, it remains difficult to quantify the effect of an imperfect seal on the repeat-element behavior. In this case simulation is used to study the effects of imperfect seal behavior on the performance and reliability of SOFC's. Diffusion through the sealing material and inherent local combustion of fuel is added to the CFD repeat-element model, which also allows to compute the flow field,the electrochemical reactions and the energy equations. The results are in good agreement with experiments. The location of parasitic combustion and local overheating are well reproduced. Furthermore, the model predicts a risk of re-oxidation under polarization that is well observed. The model also shows the necessity to take into account the diffusion transport for the development of compressive seal materials, hence verifying the hypotheses made by other groups. The novel modeling approach presented here, which includes the imperfections of components, allows to reproduce experiments with good accuracy, and gives a better understanding of degradation processes. With its reasonable computational cost, it is a powerful tool for a design of SOFC based on reliability. This paper has been presented at the First European Fuel Cell Technology and Applications Conference.