Numerical study on thermal stress fluctuation caused by coaxial-jet flow in lower head of central measurement column

In the region affected by thermal striping, the temperature fluctuation will cause alternating thermal stress in the components of nuclear reactor systems. It may lead to fatigue damage accumulation and final failure of the structure. In order to obtain the thermal stress, a simplified model of lower head of the central measurement column is built up based on the coaxial-jet flow model, and a unidirectional fluid-structure interaction simulation is conducted in the present study. The characteristics of transient thermal stress on the surface and cross-sections of the metal plate at different depths along the normal direction are numerically studied. In addition, the effects of the inlet velocity and temperature difference on the thermal stress are analyzed. The feasibility and reliability of the proposed calculation model and method are further verified by experimental data in literatures. The distribution feature of the thermal stress is similar to that of temperature. There is more than one principal frequency of thermal stress and the maximum frequency is 8 Hz. Along the depth direction, the distribution of thermal stress becomes uniform, and the thermal stress fluctuation decreases fast at first and then slows down. From the surface to the depth of 1.5 mm, peak-to-peak value decreases by 81.34% and fluctuation frequency drops to less than 5 Hz. The average thermal stress decreases in the form of slow-fast-slow with the increase of flow velocity and increases linearly with the increase of temperature difference. A high flow velocity will significantly increase the principal frequency of thermal stress fluctuation, which rises from 8 Hz to 25 Hz. A large temperature difference will significantly increase the intensity of thermal stress fluctuation. • Fluid-structure interaction (FSI) simulation based on coaxial-jet flow model. • Thermal stress fluctuation of the surface is within 10 Hz. • The intensity of thermal stress fluctuation weakens quickly from the surface to interior. • The principal frequency of thermal stress fluctuation increases with increasing inlet velocity. • The intensity of thermal stress increases with increasing temperature difference.