Experimental Study of Thermal Mixing in Flow Mixing Header Assembly of SMART

SMART (System-integrated Modular Advanced ReacTor) is an integral-type nuclear reactor for cogeneration that adopts a flow mixing header assembly (FMHA) to maintain a uniform temperature distribution in the coolant at the core inlet in the case of failure in the steam generator or reactor coolant pump. The SMART FMHA is important for enhancing thermal mixing of the coolant during a transient and even during accidents, so it is essential that the thermal-hydraulic characteristics of flow in the FMHA be understood. Scaling analysis was performed to design the experimental facility for the FMHA test through computational fluid dynamics (CFD) analysis on the SMART prototype and experimental model. The experimental facility was designed by a linear scaling factor 0.18, and the experimental pressure and temperature conditions were 0.1 MPa and 30°C to 60°C, respectively.

The experiment was performed in two ways: using FMHAs with large outlet flow hole sizes and FMHAs with small outlet flow hole sizes. In the cases of failure of one or two steam generators, the maximum temperature deviation on the side of the reactor core inlet was measured to be 1°C to 2°C, which demonstrates excellent thermal mixing through the FMHA. In particular, the FMHA with small outlet flow hole sizes tended to have better thermal mixing than the FMHA with large outlet flow hole sizes. The experimental results were comparable to those from CFD analysis.