Refractory High-Entropy Alloys (RHEAs) have emerged as one of the most promising material systems for next‑generation Nuclear Thermal Propulsion (NTP) reactors due to their exceptional combination of ultra‑high‑temperature stability, radiation damage tolerance, hydrogen compatibility, and resistance to thermomechanical degradation. NTP systems expose materials to conditions far beyond the capability of conventional alloys temperatures of 2500–3000 K, neutron fluence exceeding 10²²–10²⁴ n/cm², hydrogen embrittlement, thermal gradients above 1000 K/mm, and multi‑cycle start–shutdown loading. Historic NTP programmes including NERVA and Rover relied heavily on W‑Re alloys, carbides, and graphite‑based fuel elements, but these materials suffered from swelling, grain growth, hydrogen attack, radiation‑induced microstructural damage, and cracking under thermal shock. In contrast, RHEAs particularly Mo‑Nb‑Ta‑W and Ti‑Zr‑Nb‑Ta‑Mo configurations demonstrate high melting points (> 2500°C), sluggish diffusion, severe lattice distortion, tunable valence electron concentration (VEC), resistance to hydrogen embrittlement, and strong anisotropic radiation‑damage suppression. Their multi‑principal element architecture stabilises single‑phase BCC structures, enhances defect annihilation, reduces vacancy mobility, and mitigates radiation‑induced segregation (RIS) and radiation‑enhanced diffusion (RED). This work presents a comprehensive evaluation of RHEA behaviour within NTP reactor conditions, integrating thermodynamics, microstructural evolution, radiation physics, hydrogen transport modelling, creep and fatigue behaviour, neutron‑irradiation effects, and multi‑cycle thermo‑mechanical loading. The analysis demonstrates that RHEAs significantly outperform conventional refractory alloys, exhibiting reduced dpa‑induced swelling, higher resistance to He/H bubbleformation, lower hydrogen diffusivity, enhanced grain‑boundary cohesion, and superior fatigue and creep properties at temperatures above 2000 K. These results position RHEAs as leading candidates for structural elements, flow‑channel components, fuel‑element claddings, and control‑drum materials in future NASA, ESA, and DARPA NTP reactorarchitectures.