Our study is focused on the behavior of grain boundaries in uranium dioxide system under irradiation conditions. The research can be seen as two parts: the study of interaction of the grain boundary and the damage cascade, and the calculation of Kapitza resistance of grain boundaries. The connection between these two parts lies in that damage cascades bring in changes in the structure and other properties of grain boundaries, and inevitably the Kapitza resistance of the grain boundary changes as well. For the first part, we studied interactions of grain boundaries and damage cascades in uranium dioxide system by simulating two types of bombardments: one direct bombardment into a grain boundary leading to ballistic-collision-mediated interface mixing; the other bombardment is in the close vicinity of a grain boundary causing interface biased defect migration. We found that more defects are trapped by the grain boundary followed by the first type of bombardment, resulting in enhanced grain boundary energy. By comparing with the second type of bombardment, we are able to reveal the mechanisms of the interaction between defects and grain boundaries. For the second part, we employed the non-equilibrium molecular dynamics method to calculate the Kapitza resistance of different coincident site lattice boundaries with or without defects loaded, and later we found that a universal positive correlation between the Kapitza resistance and the grain boundary energy can be well established, regardless of the cause of boundary energy changes. Our study provides a deeper understanding of the Kapitza resistance of the grain boundary and its evolutions under irradiation, which benefits multi-scale modeling of uranium dioxide thermal properties under extreme radiation conditions as well as experimental studies of fuel material thermal properties.