Numerical simulation of transients in two-phase flow is crucial to simulate accident-like and aberrant conditions of nuclear reactors for safety analysis. A considerable number of such problems can be treated as one-dimensional with significant reduction in complexity without much loss in applicability. Most commercial thermal hydraulic codes are based on the two-fluid model, which solves balance equations for each phase and also accounts for thermodynamic non-equilibrium between the phases. However, the homogenous equilibrium mixture (HEM) model of two-phase flow can be employed to develop simple and efficient codes for transient simulations, using which extensive parametric studies can be carried out. In the present work, a code for numerical computation of unsteady one-dimensional two-phase flow has been developed and reactor transients have been simulated. The governing equations were obtained by the HEM model and were decoupled and approximated using the sectionalized compressible flow (SC) model and the momentum integral (MI) model. The equations of state used in the code are based on IAPWS Industrial Formulation-97. Pressure and heat flux transients for PWR and BWR were simulated with the code and compared with those reported in the literature. Further numerical simulations with the code were carried out to predict the transient response of nuclear reactors to various perturbations.

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