Design and Performance Evaluation of a Combined DHX unit for SFR Design Application

Based on a higher operating temperature with excellent thermal conductivity and larger thermal inertia of liquid sodium coolant, the SFR system has employed passive safety systems to ensure reliable decay heat removal (DHR) and consequential plant safety enhancement. Although a passive type DHR system has many advantages over an active one, designing a well coordinated passive system is usually more difficult than designing an effective active system. This is mainly because a cooling flow control is made directly by the system designer in an active system, while it is determined automatically by an intricate balance between the flow head loss and natural circulation head generation obtained from the density difference through the whole thermal flow system. To this end, securing a sufficient natural-circulation flow becomes one of the primary challenges for designing a reliable and successful Dh system in passive. In a current pool-type Sf design, an internal cooling flow path from the hot sodium pool to the cold pool is somewhat ambiguous owing to the split flow ratio formed in parallel paths between the intermediate heat exchangers (IHXs) and decay heat exchangers (DHXs), which results in a large uncertainty in the DHX shell-side flowrate and corresponding heat transfer to the DHR sodium loops. To improve passive the DHR performance, we proposed a new design concept with a simplified flow path from the hot pool to the cold pool through a unified flow path serially passing the DHX and IHX units. The present study aims at introducing the innovative design concept of the combined IHX-DHX unit and evaluating its design features in view of the heat transfer capability. From a comparison of the CHX performance designed by a one-dimensional approach with that made by a CFD analysis, it was quantitatively obtained that the difference in heat transfer rate is about 5.7%. It was also found that unexpected bypass flow in the shell-side CHX unit gave rise to a discrepancy. Since this feature cannot be fully considered in any one-dimensional design approach, we need to improve the physical models for CHX thermal sizing to get a more reasonable design