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A theoretical method to predict the transient behaviour of helium gas and heavy water in the reactivity mechanism thimbles due to pressure pulse in the moderator of calandria vessel of CANDU reactor is developed. The maximum helium gas pressure during an in-core break is used as a design basis for the mechanical design of reactivity mechanism thimbles. Two approaches, static and dynamic model, are applied and compared. The application of static model at the peak moderator pressure is simple, however, it gives unrealistically high helium gas pressure. Dynamic model can simulate the actual phenomena and provides reasonable value for maximum helium gas pressure. The approach derived in this paper could be applied to other similar engineering problems.

The gap conductance between the fuel and the sheath depends strongly on the gap width and has a significant influence on the amount of initial stored energy. The modified Ross and Stoute gap conductance model in ELESTRES is based on a simplified thermal deformation model for steady-state fuel temperature calculations. A review on a series of experiments reveals that fuel pellets crack, relocate, and are eccentrically positioned within the sheath rather than solid concentric cylinders. In this paper, the two recently-proposed gap conductance models (offset gap model and relocated gap model) are described and are applied to calculate the fuel-sheath gap conductances under experimental conditions and normal operating conditions in CANDU reactors. The good agreement between the experimentally-inferred and calculated gap conductance values demonstrates that the modified Ross and Stoute model was implemented correctly in ELESTRES. The predictions of the modified Ross and Stoute model provide conservative values for gap heat transfer and fuel surface temperature compared to the offset gap and relocated gap models for a limiting power envelope.

The loss of cooling flow to the fuel channel in CANDU reactor can result in high fuel temperatures and low steam flowrates over extended periods of time. Under these conditions, high temperatures can increase the quantity of fission products released from the fuel, and the production of large quantities of hydrogen gas, and eventually may cause failure of pressure tube. The CHAN-IIA code has been designed to simulate the behaviour of the CANDU fuel channel under these extreme conditions and validated against the CS28-1 experiments. However, the code over-estimates fuel temperature transient and hydrogen production. It is observed that subchannel model and simple radiation model in fuel rings would contribute to under-estimation of heat removal in fuel channel. The CATHENA code, which-has been developed for the analysis of two-phase flow and heat transfer in piping network and fuel channel, shows the good agreement with the CS28-1 experimental data.