Moderator Circulation Analysis for the Modified CANDU 6 Design Using the CFD Code MODTURC-CLAS

Abstract

Steady-state moderator behaviour based on a full three-dimensional representation of the CANDU 6 calandria geometry is investigated using the CFD (Computational Fluid Dynamics) code MODTURC-CLAS for a proposed modified design. This modified CANDU 6 moderator design is geometrically identical to the standard CANDU 6 design,but with a different inlet nozzle/outlet port configuration. In fact, this modified design is similar to the CANDU 9 moderator design.Under nominal operating conditions, analysis shows that the maximum temperature is reduced by 7 degrees C compared to the maximum temperature observed for the standard CANDU 6 design. The effectiveness (stability) of this modified design is demonstrated by considering several case studies. It has been demonstrated that with an 18% reduction in the inlet mass flow rate and a mass flow imbalance into the two inlet headers (based on the nominal conditions), a minimum subcooling margin above 30 degrees C is maintained.Solution dependence (in particular, the maximum temperature) on nodalization was investigated by performing a preliminary grid sensitivity study. It has been demonstrated that steady-state solutions for the modified design are relatively insensitive to grid refinement. Also, sensitivity to inlet mass flow rates for the modified design shows that the flow structure within the moderator is relatively insensitive to moderate design changes to input parameters.