As illustrated in Fig. 1c, a low-velocity region develops near the flow collector edge, indicating flow separation. The results of fin gap velocity measurements (Fig. 1d) indicate a decrease in velocity downstream.

A passive reactor shutdown system, known as the self-actuated shutdown system (SASS), is designed for advanced sodium-cooled fast reactors to ensure safe reactor shutdown during an accident. The SASS employs a Curie-point electromagnet (CPEM)^*1, which loses its magnetism at the Curie temperature. When the core temperature of the reactor rises significantly, the control rod of the SASS descends automatically into the core under gravity without requiring plant operation^*2. As depicted in Fig. 1a, the CPEM features a complex structure with multiple fins for generating rapid responses to temperature surges. Understanding the flow field surrounding the CPEM is essential for evaluating SASS performance. Thus, in this study, water experiments were conducted to examine the flow field surrounding the CPEM, including the narrow regions between its fins. The test section illustrated in Fig. 1b replicated the full-scale structure from the core outlet to the CPEM.

Velocity measurements using a laser-light technique (Fig. 1c) indicated that the velocity surrounding the CPEM increased owing to flow separation at the edge of the flow collector^*3. Additionally, as depicted in Fig. 1d, velocity measurements using an electric probe sensor in the fin gaps revealed that the fin gap velocity exceeded that at the upstream of the CPEM and gradually decreased downstream. These measurements elucidated the flow characteristics between the fins. Overall, the experimental results provide validation data for computational-fluid-dynamics-based assessments of SASS feasibility.

 

^*1 The CPEM is an electromagnet that loses its magnetism when the ambient temperature reaches the Curie point (~660 ℃).
^*2 During normal operation, the SASS uses a CPEM to hold the control rod in place. If the coolant temperature rises abnormally, the CPEM loses its magnetism, causing the control rod to descend automatically into the core.
^*3 A flow collector may be installed around the CPEM to efficiently direct the high-temperature fluid from the core outlets to the CPEM.

Yamasaki, R. et al., Study on Performance Evaluation of Self-Actuated Shutdown System for Sodium-Cooled Fast Reactor; Flow Field Measurement Around a Curie Point Electromagnet, Proceedings of 13th Korea-Japan Symposium on Nuclear Thermal Hydraulics and Safety (NTHAS13), Seoul, Korea, 2024, 1182, 7p.

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