The Advanced Neutron Source Reactor (ANSR) is being designed to become the world’s most advanced reactor for neutron scattering research. It is both moderated and cooled by heavy water (D2O) and uses a core of highly enriched uranium silicide fuel in a plate geometry. During the refueling process of the ANSR, the spent fuel must be moved from within the primary coolant loop (containing D2O), through a pool of D2O, and finally into a light water spent-fuel storage area. This is accomplished by using a refueling tunnel and fuel transfer cask or lock. Some means of cooling the core, using either natural circulation or forced convection, must be accommodated during this process. Several thermal-hydraulic aspects of this refueling process have been studied. A modified version of the NATCON computer code developed at Argonne National Laboratory was used for the analysis. The NATCON code was revised and modified to incorporate improved friction and heat transfer correlations, and routines for the physical properties of D2O were added. The revised code can also accommodate a two-dimensional power density distribution. The results for the refueling process design show that ∼48 h is required to ensure the no-boiling condition. The addition of a short chimney reduces that time to ∼12 h.