CFD simulation of core exit temperature behavior during LSTF small-break LOCA experiment

LSTF小破断LOCA実験時の炉心出口温度挙動に関するCFD解析

岡垣 百合亜  ; 竹田 武司 ; 和田 裕貴   ; 安部 諭  ; 市原 京子*; 塩谷 仁*

Okagaki, Yuria; Takeda, Takeshi; Wada, Yuki; Abe, Satoshi; Ichihara, Kyoko*; Shiotani, Hitoshi*

Core Exit Temperature (CET), measured by core exit thermocouples (T/Cs), is utilized worldwide as a crucial parameter to start Accident Management (AM) operator action by detecting core temperature excursion during accidents in Pressurized Water Reactors (PWRs). The CET is used to switch accident response procedures from preventing core damage to preventing containment failure. Various thermal-hydraulic phenomena in the core influence the behavior of CET during an accident. Previous studies have indicated that CET may rise more slowly than the fuel cladding temperature. This study aimed to deepen the understanding of the relationship between CET and fuel cladding temperature during representative accident progressions by employing Computational Fluid Dynamics (CFD) simulations. It sought to complement experimental findings by evaluating the effects of three-dimensional core thermo-hydraulic behaviors, such as secondary flows in the upper core region. The CFD simulation was performed for a 1% vessel upper head small-break Loss-Of-Coolant Accident (LOCA) experiment, which was conducted at the Large-Scale Test Facility (LSTF) at Japan Atomic Energy Agency (JAEA) in 2023. The LSTF experiment assumed that the high-pressure injection system of the emergency core cooling system had totally failed. This study focused on the representative period when a significant rise in core temperature appeared during core uncovering. The transient solver for compressible fluids in OpenFOAM was employed for the CFD simulation. Boundary conditions, such as mass flow rate, temperature, and pressure at the core's top position, were applied. The turbulence model used was the Shear Stress Transport (SST) model. The CET distributions were compared with the experimental data, which had a total of 20 points. The velocity and temperature distributions in the mainstream and cross-sectional directions were visualized to elucidate thermal-hydraulic phenomena. This study provided valuable insights into CET behavior and related thermo-fluid dynamics.