Dynamic Simulation of a Space Reactor System with Closed Brayton Cycle Loops

Abstract

A dynamic simulation model for space reactor power systems with multiple closed Brayton cycle loops for energy conversion is developed and demonstrated for a startup transient. The simulated power system employs a submersion-subcritical safe space S ^ 4 reactor with a negative temperature reactivity feedback and has no single-point failures in reactor cooling and energy conversion. The S ^ 4 reactor core is divided into three hydraulically independent sectors, and each sector has a separate closed Brayton cycle loop that is thermal-hydraulically coupled to a circulating liquid NaK-78 secondary loop with two water heat pipe heat rejection radiator panels. Each closed Brayton cycle loop has a Brayton rotating unit designed and optimized for high thermal efficiency and low specific mass (1.16 kg/kW e ). The reactor coolant and the closed Brayton cycle working fluid is a He-Xe binary gas mixture with a molecular weight of 40 g/mol. Results are presented for a startup transient of the S ^ 4 closed Brayton cycle power system to full-power operation at a reactor thermal power 471 kW th , a Brayton rotating unit shaft speed of 45 krpm, and turbine and compressor inlet temperatures of 1149 and 400 K, respectively. At these conditions, the nominal electrical power and thermal efficiency of the power system are 130.8 kW e and 27.8%.