Numerical study for CANDU moderator temperature prediction by using the two-phase flow analysis code, CUPID

• Single- and two-phase flow in the CANDU moderator tank was simulated by the CUPID code.
• Local maximum temperature in the Calandria vessel versus injection flow was obtained.
• The CUPID code is applicable to the flow analyses of the Calandria vessel.

KAERI has been developing a component-scale thermal–hydraulics code, CUPID. The code adopts a three-dimensional, transient, three-field model for two-phase flow. In this study, we investigated the thermal hydraulic behavior of the moderator inside the Calandria tank of a CANDU reactor by using the CUPID code. At first, we have validated the CUPID code using the experiments that were performed at Stern Laboratories Inc. To avoid the complexity to generate computational geometry around the Calandria tube bundles, a porous media approach was applied for that region and the flow resistance inside the porous media zone was modeled by an empirical correlation. An open media is applied to generate the outer fluid layer including the inlet nozzles. Computational grids near the inlet nozzles should be well-generated because the flow field is very sensitive to the momentum flux from the nozzle. Since the axial flow can be assumed to be invariant for this experiment, a two-dimensional approach was adopted. The mixed flow pattern of forced and natural convection inside the Calandria vessel has been successfully predicted by the CUPID code. The analysis has been further extended to two-phase flow conditions and, then, a map of the local maximum moderator temperature in the Calandria vessel versus the injection flow rate was derived, which can be used to predict the local subcooled margin in the vessel.