Here, a model for the nonlinear Rayleigh-Taylor instability (RTI) of a steady ablation front based on a sharp boundary approximation is presented. The model includes the effect of mass ablation and represents a basic tool for investigating many aspects of the nonlinear ablative RTI relevant to inertial confinement fusion. The single mode analysis shows the development of a nonlinear exponential instability for wave numbers close to the linear cutoff. Such a nonlinear instability grows at a rate

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... faster than the linear growth rate and leads to saturation amplitudes significantly larger than the classical value 0.1λ. We also found that linearly stable perturbations with wave numbers larger than the linear cutoff become unstable when their initial amplitudes exceed a threshold value. The shedding of long wavelength modes via mode coupling is much greater than predicted by the classical RTI theory. The effects of ablation on the evolution of a front of bubbles is also investigated and the front acceleration is computed.