Dynamics of CANDU fuel bundles

One of the most important parts of a nuclear reactor is the fuel bundle in which the nuclear reaction takes place and heat is generated. The heat is transported through the flow of the coolant to the steam generator. During such process, the fuel bundles are subjected to severe operation conditions, such as highly turbulent coolant flow, high temperatures and excessive irradiation doses. These severe conditions significantly affect the integrity of the fuel bundle in terms of flow-induced vibrations (FIV). The FIV are produced by various excitation mechanisms such as, the turbulence buffeting, the fluidelastic forces (Motion-dependent forces) and the acoustic pressure pulsations coming from the primary heat pump. In the current study, a numerical approach is presented to characterize the motion-dependent forces. The model was used to predict these forces in flexible fuel kernel. In addition, an analytical model was developed utilizing the force model to predict the dynamic response of the fuel bundle. The dynamic response was compared with the available experimental data. In the second part, a fully-flexible fuel bundle structural model was developed to investigate the dynamic response of the fuel bundle under various excitation mechanisms. The model is capable of predicting the vibration response of fuel bundles with a large number of elements and various end conditions, such as flexible endplates. The fuel bundle and the supporting structure were modelled utilizing finite beam and plate elements. The contacts between the system components were modelled using the single point contact method (SPC). Fluid excitations, such as turbulence, pressure pulsation, and motion-dependent forces, were included in the model. Finally, the irradiation effects on the mechanical behavior of the fuel bundle is investigated using the same structural model. This was accomplished by utilizing a constitutive model that describes the thermal and irradiation effects on the mechanical properties. The current work represents a major advancement step towards a realistic modelling of the complex dynamics of fuel bundles.