KAERI is building the Thermal Energy Storage Experimental verification Test loop (TESET) to verify the performance of a thermal energy storage system, one of the core technologies for the realization of the Nuclear-Renewable Hybrid Energy System (NRHES). The high-temperature thermal energy storage tank, one of the main components of TESET, plays a role in storing high-temperature thermal energy, which is very important to secure its structural integrity at high temperatures. For this purpose, high-temperature material behavior characterization and 3D-based high temperature structural integrity evaluation have been conducted. In order to evaluate the effect of sloshing in the thermal energy storage tank where the free surface of working fluid fluctuates freely by an external dynamic load, the acceleration data measured in the El Centro, Gyungju, and Pohang earthquakes were used to calculate the pressure acting on the thermal storage tank wall surface, and the relationship between the seismic load and sloshing response component was analyzed. The results show that El Centro, which has the largest intensity and duration of seismic waves, has the largest energy of seismic waves compared to other cases, while Gyungju has higher energy in some high-frequency regions. It is also found in all the cases that the absolute value of pressure fluctuation increases as the convective pressure rises near the free surface