On-power fuelling is an important feature of the CANDU® reactor. Fuelling is a routine operation with a large number of channel fuelling visits made each year with the fuelling machines acted as the key system. Hence, safety analysis must consider fuelling machine events typically when the fuelling machine is in transit toward the spent fuel port. This paper presents a model of fuelling machine containing spent fuel with complex configuration and multi-process mechanisms. This fuelling machine model is developed for simulations of the fuelling machine coolant thermalhydraulics behavior, the spent fuel behavior and potential fission product release during postulated loss of heat removal accidents. An example of its application is also presented in this paper.