Segregation of fission products and their effect on E3/(111) UO2 grain boundary cohesive energy were analyzed via first principles calculations. The most stable segregation sites were investigated for zirconium, molybdenum, cesium, and xenon in UO2. Molybdenum, cesium, and xenon showed segregation tendencies on the E3/(111) UO2 grain boundary but zirconium did not. The grain boundary cohesive energy was analyzed for the structure in which each segregation element was located at the most preferred segregation site. The results showed that zirconium and molybdenum strengthened the grain boundary cohesion, whereas cesium and xenon weakened it. This study shows which fission products cause the weakening of the irradiated UO2 grain boundary, which could help model irradiated fuel performance. (c) 2022 Elsevier B.V. All rights reserved.