Steady State Modeling of a Regenerative Rankine Cycle for the Natrium System

Abstract With increasing penetration of renewable energy and the challenges that come with it, advanced nuclear energy systems will play an instrumental role for society to reach the goal of carbon-free power in the coming decades. Recently, an advanced nuclear power plant design (Natrium™), a sodium fast reactor with thermal energy storage, is slated to be deployed towards the end of this decade. With 5.5 hours of molten salt energy storage, the Natrium system will complement renewables and ease the intermittency challenges associated with solar and wind energy. In this work, a steady-state model of the Natrium system is modeled and optimized for efficiency. This model not only provides the state points within the regenerative Rankine cycle but also provides initial values for the variables of a dynamic system model in parallel development. With the molten salt storage temperature set by the sodium fast reactor (550 °C), the degree of superheat of the steam and its pressure for the regenerative Rankine cycle are determined. The states and mass fractions (turbine bleeding ratios) of the three regenerators are determined through optimization for maximum thermal efficiency. With the optimized cycle design, the model predicts 40% thermal efficiency. The model also checks to ensure no second law of thermodynamics violation for the regenerators. While high effectiveness is desired for regeneration (to maximize thermal efficiency), the physical sizes of the regenerators must be reasonably sized, to keep the cost managable. The heat exchanger sizes (area) are determined using ε-NTU method.