An in-depth shielding analysis of a large indoor facility designed for storing low- and intermediate-level radioactive waste generated from nuclear power plant decommissioning was performed. The facility has a storage capacity of 1260 box-type containers, which are stacked in five layers. The characteristics of the radiation field around the facility were investigated comprehensively through efficient ADVANTG/MCNP simulations. The contribution of each source layer to the dose rates exterior to the facility was determined. Furthermore, dose components from various transport pathways (e.g., direct, streaming, and skyshine) were isolated using algebraic manipulation of the outputs obtained from additional simulations. The combined results provided valuable information for shielding optimization during the design phase and could facilitate the future operation of the storage facility.