Study on performance evaluation of self-actuated shutdown system for sodium-cooled fast reactor; Flow field measurement around a curie point electromagnet

Yamasaki, Ryota; Aizawa, Kosuke ; Kobayashi, Jun ; Kurihara, Akikazu 

A self-actuated shutdown system (SASS) is one of the innovative technologies for a sodium-cooled fast reactor. The SASS is a passive reactor-shutdown system that utilizes a Curie point electromagnet (CPEM), which features the characteristic of losing magnetism when the magnet temperature reaches the Curie point. The control rod with the CPEM is inserted into the core by gravity without recourse to any active shutdown system. To allow the effective function of the SASS, it is important ensure a quick response of the CPEM following the coolant temperature increase. Therefore, the CPEM is designed to have a radial fin-like structure to improve temperature response, and a fluid is guided into the narrow gaps between fins. Accordingly, it is important to understand the flow field around the CPEM, including the gaps between fins. In this study, a water experiment was performed to understand the characteristic flow field around the CPEM and to develop the SASS performance evaluation method. The test section modeled the structure from the outlets of fuel subassemblies to the CPEM. The velocity conditions at the fuel subassembly outlet were varied between 0.26m/s (Re=25,000) and 1.55m/s (Re=150,000). The velocity in the gaps between fins was directly measured using a hot-wire anemometer. In addition, the flow field around the CPEM was measured using particle image velocimetry (PIV). As the results, the velocity distributions around the CPEM and inside the gaps between fins were quantitatively obtained. It was shown that the velocity in the gaps between fins was higher at the upstream side than at the downstream side, indicating that the fluid flowed out from the gap between fins flowing to the downstream. Furthermore, these experimental results will be used as validation data of the SASS performance evaluation method in the future.