With the further improvement of engine thrust, specific impulse, and inherent load requirements in modern aerospace systems, nuclear thermal propulsion (NTP) 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.

This study aims to explore the response of air-breathing NTP 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.

First of all, 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. Then, multiple verification and validation (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%. Finally, 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 was investigated.

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 1.53 MPa, 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 1.53 MPa 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.

Simulation results of this study provide experience for early warning and intervention of flow-blockage accidents in air-breathing nuclear thermal propulsion engine working.