CFD Analysis for a Natural Convection in the Annulus Gap under Post-Blowdown

A CFD benchmark calculation for a post-blowdown test of the CANDU 28-element fuel bundle has been performed to develop the CFD analysis methodology and support the verification work of the CATHENA code for the post-blowdown event. The comparison of the CFD results with the test data showed that the temperature difference of about 40 .deg. C at the PT(Pressure Tube) was found, where the CFD result is higher than that of test. This difference may be caused by the fact that the CFD analysis could not exactly calculate the fraction of the radiation and the convection heat transfer between the PT and the CT (Calandria Tube). The larger heat transfer by the radiation may give the higher temperature distribution at the PT because the radiation heat transfer is generally proportional to a fourth of the temperature. To understand the heat transfer phenomena and find a more accurate fraction of both the radiation and the convection heat transfer inside the annulus gap, we investigated the previous experimental correlation of a natural circulation in the concentric horizontal cylinder based on the test data and perform a separate CFD calculation for the annulus gap and compared it with the estimation results by the correlation