Evaluation of the structural safety of reactors often involves the analysis of various types of fluid-structural components interacting in three-dimensional space. For example, in the design of a pool-type reactor several vital in-tank components such as the primary pumps and the intermediate heat exchangers are contained within the primary tank. Typically, these components are suspended from the deck structure and largely submersed in the sodium pool. Because of this positioning these components are vulnerable to structural damage due to pressure wave propagation in the tank during a hypothetical core disruptive accident. To assess the transient response of these components, it is necessary to perform a dynamic analysis in three-dimensional space that accounts for the fluid-structure coupling. A formulation for a three-dimensional Lagrangian hydrodynamic element was applied to the above safety problem. A model that has many of the salient features of this fluid-structural component system was developed and then analyzed using the NEPTUNE computer code. The primary tank and the in-tank component were modeled as deformable elastoplastic structures, the sodium pool as an inviscid, compressible fluid, while the deck was taken to be rigid and fixed in space. The transient response of the model showed that although the pressure waves loaded the in-tank component so that it moved toward the primary tank, they also loaded the primary tank and moved it away from the component preventing structural damage due to impact between the component and tank.