Supercritical () is a promising working fluid for future high-efficiency power conversion cycles. In order to develop these cycles, it is necessary to understand supercritical and two-phase flow. This paper presents a methodology for the computational fluid dynamic (CFD) simulation of flowing through a restriction under a wide range of flow conditions. Under an accidental situation, such as a pipe break, the inventory of leaks out through a small restriction. In this research, we use circular and annular orifices to mimic the behavior of restrictions. As the atmospheric pressure is much smaller than the operating pressure in the pipe, a two-phase choked flow will happen. Such behavior is considered in the simulation. The homogeneous equilibrium model (HEM) is employed to model the two-phase state. To correctly simulate the behavior of the power cycle under this accidental scenario, the inventory leakage rate should be calculated precisely. Therefore, at the current state, this study only focuses on the prediction of mass flow rate through orifices.