Analysis and understanding the depressurization response of the HTGR reactor is essential to the understanding of accident scenarios and design of these reactors. The 350MWe GA-MHTR was used as reference reactor for the current design of the test facility. The design of the test facility was based on a well-established dynamic scaling analysis for integral system transient. The scaling accounts for integral system response, boundary mass and energy flow, system pressure, and local phenomena such as choking, convective and diffusive mixing, stratification, etc. The key phenomena determining the gas flow paths and distribution in the reactor building are depressurization, gas discharge, choked flow, venting, and mixing. Two scaled systems were developed, one 1/20 scale of the prototype and another 1/28 scale of the prototype. Based on sizes, the scale of 1/28 was chosen for the facility that would simulate all local and global phenomena in the blowdown process. The scaled 1/28 model design was then translated to engineering drawings to manufacture vessels and tanks for the test facility. The important aspect of the scaling/sizing the compartment lies in correctly scaling flow losses in the compartments, flow paths, and vent paths, gas hide-out spaces, and thermal stratification and mixing. Heat loss to the cavity walls is scaled via matching the thermal penetration depth/inertia of the model vessel (cavity) insulation material with that of the prototype concrete. The Engineering design considered the geometrical similarity to simulate the flow path and the hideout of oxygen in the reactor cavity.