Aging of Inconel X-750 spacers in Canada Deuterium Uranium (CANDU) reactors is managed through a combined program of in-service inspection, component removal for material surveillance testing, research and development, and fitness-for-service evaluation. Crush tests of ex-service spacer coils have shown that the load carrying capacities of Inconel X-750 spacers decrease with operating time. The spacers also exhibit reduced ductility and inter-granular failure that is typical of helium (He)-embrittlement. Examination of ex-service spacers confirmed that material interfaces (mainly grain boundaries) were perforated by He-bubbles. Assuming that the grain boundary bubble coverage dictates the grain boundary strength and thus the failure load of the polycrystalline component, we have simulated the degradation and inter-granular failure of the spacer coils at low and high operating temperatures.The observed inter-granular fracture has been simulated by a cohesive zone model that governs the grain boundary de-bonding process. The grain boundary strength is related to the perforation by He-stabilised cavities. The model of grain boundary traction separation consists of three parts: (i) a rate-theory calculation to predict grain boundary coverage; (ii) a local material degradation model to quantify helium induced grain boundary strength reduction; (iii) a traction separation model to simulate grain boundary de-cohesion. The results show that the accumulation of He-stabilised cavities on grain boundaries causes material failure at lower stresses and strains that are dependent on the irradiation temperature and the neutron exposure. The load carrying capacity of the X-750 spacers decreases non-linearly with irradiation and the rate of change is decreasing with increasing dose.