Thermodynamic and system analysis of a hydrogen–helium blended closed expander nuclear thermal propulsion system under dual operating modes

In advanced high temperature energy systems, tailoring the working fluid can provide an additional way to relieve thermal load constraints while reshaping overall system performance. This study develops a system level model for a nuclear thermal propulsion (NTP) cycle using a blended hydrogen–helium propellant, in order to examine how fluid blending affects system thermodynamics, reactor power demand, and nozzle thermal management. The model couples the thermophysical properties of the hydrogen–helium blended working fluid, cryogenic storage and downstream mixing, nozzle regenerative cooling, turbopump behavior, and reactor thermal neutronic response, and it examines both a constant power mode and a constant thrust mode under temperature limits. The pure hydrogen reference cycle yields a maximum vacuum specific impulse of 932.78 s, while helium blending introduces only minor neutronic perturbations within the present model. Under the constant power mode, increasing the helium mass fraction from 0 % to 60 % raises thrust from 91.94 to 118.80 kN, while reducing the maximum nozzle wall temperature from 1024.02 to 911.66 K and the peak heat flux from 69.74 to 56.58 MW/m². Under the constant thrust mode at 100 kN, the same helium addition lowers the required reactor thermal power from 399.46 to 308.25 MW. Overall, within the present modeling boundaries, hydrogen and helium blending serves as an additional operating variable that can redistribute specific impulse, thrust, reactor power demand, and nozzle thermal load in a high temperature energy system.