[Background] With the further improvement of engine thrust

specific impulse

and inherent load requirements in modern aerospace systems

nuclear thermal propulsion technology has considerable development potential. Air-breathing nuclear thermal propulsion engines do not need to carry oxidants and chemical fuels

directly drawing air from the atmosphere

heating

and generating thrust

further reducing the inherent load of the engine. [Purpose] This study aims to study the response of air-breathing nuclear thermal propulsion reactors to flow-blockage accidents

and study the influence of blockage factors on the stability of reactor operation under different blockage ratios and positions to obtain the pre-judgment parameters. [Methods] A cross dimensional Neutronics-Thermal hydraulics-Mechanicals multi-physics field coupling model was established with zero-dimensional point reactor kinetics equations

one-dimensional fluid

and three-dimensional solid thermo-mechanical forms. Multiple V&V tests were conducted to verify the correctness of the code for efficiently calculating the transient response of the system

including multiple feedback mechanisms

with a deviation of less than 5%. A study was conducted on the response of reactor flow-blockage accidents at different degrees and positions at a blockage area rate of 0 to 100% under rated operating conditions. [Results] The calculation results indicate that as the proportion of blockage areas increases

the limiting factor for core safety and stability shifts from temperature to blockage thrust loss

and the maximum blockage factor allowed for the engine to achieve self-stabilization generally decreases to 0.74. In large-scale blockage accidents

temperature and stress are maintained within safe levels. When the blockage factor exceeds the limit

the low total pressure behind the reactor rapidly decreases the thrust and forms positive feedback

causing engine instability. As the blockage position moves downstream

the reactor is more likely to enter an unstable state under the same blockage factor conditions. When the initial total pressure behind the reactor after blockage is less than 1.53MPa

it can be considered that the current blockage factor of the reactor is only 15.3% at most different from the critical instability value. After organizing a large amount of data

the total pressure of 1.53MPa behind the reactor can be considered as a pre-judgment condition for approaching instability. The value may vary under different reactor conditions

but the parameter has system conservation and consistency. Taking the allowable temperature limit and the initial total pressure behind the reactor into account

it can provide early warning for any uncontrolled flow conditions. [Conclusions] The article can provide experience for early warning and intervention of flow-blockage accidents in air-breathing nuclear thermal propulsion engine working.