Water Experiments on Thermal Striping at the Bottom of Upper Internal Structure of Japan Sodium-cooled Fast Reactor

A design study of an advanced sodium-cooled fast reactor (Advanced-SFR) has been conducted in Japan Atomic Energy Agency. Hot sodium from the fuel subassembly can mix with the cold sodium from the control rod (CR) channel and the blanket assemblies at the bottom of Upper Internal Structure (UIS). Temperature fluctuation due to mixing of the fluids at different temperature between the core outlet and the bottom of the UIS may cause high cycle thermal fatigue on the structure around the bottom of the UIS. A water experiment using a 1/3 scale 60°-sector model simulating the upper plenum of the Advanced-SFR has been conducted to examine countermeasures for the significant temperature fluctuation generated around the bottom of UIS. As a result of temperature distribution measurement around the radial blanket fuel subassemblies, a large temperature fluctuation was observed. In order to reduce this temperature fluctuation, the shape of Core Instrumentation Support Plate (CIP) edge was improved. The countermeasure to mitigate the temperature fluctuation intensity was applied and its effectiveness was confirmed.