[en] An Advanced Research Reactor(ARR) is being designed by KAERI since 2002. The final goal of the project is to develop a new and unique research reactor model which is superior in safety and economical aspects. Because many structures of a reactor are surrounded by fluid, we should take fluid effect into consideration when dynamic behavior of a submerged structure is analyzed. In this work, the hydrodynamic mass approach is chosen to analyze structural behavior in fluid and its basic theory is reviewed. We had developed a software of hydrodynamic mass evaluation in the last study. Convergence characteristic of the software was checked with respect to the number of elements. The software was verified to give a reliable result although elements were not so fine. Based on the results, the hydrodynamic mass was estimated to analyze dynamic behavior of the standard and the reduced fuel assembly submerged in water. Despite of occurrence of fluid-induced vibration or earthquake loading, a tubular fuel assembly as well as a fuel channel must keep their structural integrity. Through analysis of the submerged behavior, probable problems have to be identified and their solutions should be reflected on the improved design. Therefore for the first step, hydrodynamic masses were estimated for characteristic sections of the standard and the reduced fuel assemblies. In near future, we will apply the hydrodynamic masses to the dynamic analysis of the submerged fuel assembly