In this study, applicability of the TRAC/BF1-ENTRÉE code to regional instability was demonstrated in two parts. In Part I, fidelity of numerical models was studied with regard to the density-wave oscillation. Based on the FRIGG-4 loop test, predictability of the code has been demonstrated. An appropriate time integration scheme was explored, and it was found that the numerical viscosity can be minimized by applying the semi-implicit method and specifying the material Courant number close to unity. Applicability of the code was studied for simulating parallel channel configurations. The bypass channel effect was quantified as a function of its flow area and power level. The influence of flow mixing at the upper and lower plena was studied.