[en] This paper presents the results of modeling studies performed by Energy Incorporated for the Electric Power Research Institute using the RELAP thermal-hydraulics code to predict the blowdown response of the General Electric Two-Loop Test Apparatus (TLTA). The TLTA is a scale model test system used to simulate boiling water reactor blowdown transients. Areas investigated in the study included the selection of critical flow multipliers to be applied at the break area, the geometric input modeling of the broken loop jet pump, and the use of alternate assumptions in defining the phase separation in the lower plenum. The results of these studies were compared with experimental data from various break size tests performed in TLTA