The mechanism by which intense X-rays ablate a material surface and generate recoil momentum underpins critical engineering applications such as asteroid defense and X-ray-driven propulsion. Although this process is reasonably well understood in plasma physics under typical laboratory conditions, a systematic understanding of momentum generation and its evolution under the low-specific-energy, long-duration and long-distance conditions relevant to asteroid defense and ablation propulsion is still lacking. In this study, we employ the FLASH radiation hydrodynamics code to construct a one-dimensional irradiation-driven model for such regimes. The model incorporates radiation transport, a material equation of state and energy deposition physics, and reproduces the coupled evolution of the temperature, pressure and density fields in aluminum targets under intense X-ray irradiation.

Based on this model, the momentum generation process induced by intense X-ray irradiation is divided into four stages according to the dominant physical mechanism: (I) radiation ablation, (II) material ejection, (III) shock impedance matching, and (IV) spallation expansion.

For each stage, its contribution to the total momentum growth is analyzed quantitatively, and the underlying physical phenomena are clarified, including the “temperature–pressure peak misalignment” induced by radiative preheating and the “separation between the main shock wave and the phase-transition wave” caused by the solid-liquid phase transition. Stage II is found to dominate the momentum growth.

To overcome the systematic deviation caused by the “spontaneous expansion” that arises from the neglect of material constitutive relations in radiation hydrodynamic codes, we propose a correction method for the impulse coupling coefficient based on linearized Riemann invariants. This method provides a self-consistent way to extract the asymptotic expansion velocity and the effective ablation pressure, thereby improving the accuracy of the impulse coupling coefficient. Our numerical results show that, for intense X-ray irradiation with a spectrum concentrated within 1 keV and a fluence of 50–200 J/cm², the impulse coupling coefficient of aluminum lies between 0.3 and 0.7 Pa·s·cm²/J. These values are in good agreement with existing experimental measurements and with the theoretical predictions of the vaporization-impulse theory.

This paper establishes a clear stage-resolved physical picture of momentum generation under intense X-ray irradiation, clarifying the respective contributions of radiation ablation, material ejection, impedance matching and spallation. A correction method for the “spontaneous expansion” numerical artifact—previously overlooked and unresolved in radiation hydrodynamic simulations of recoil momentum—is proposed. This work achieves an accurate determination of the impulse coupling coefficient through high-fidelity simulation combined with a self-consistent numerical correction, and provides a unified and generalizable framework for understanding the dynamic response of materials under extreme radiation loading. The framework is applicable not only to laboratory Z-pinch experiments but also to real-world asteroid deflection missions, where accurate momentum prediction is essential for assessing deflection effectiveness and optimizing parameters.