Prediction of transient behaviour of heavy water and helium gas in reactivity mechanism thimbles during an in-core break in candu reactor

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.