This study introduces a new multi-channel robust nonlinear control algorithm based on the theory of the time-varying sliding mode control (TVSMC) strategy to stabilize the attitude of an unmanned aerial vehicle (UAV) for nuclear decommissioning applications. Since the UAV is imposed by constant radiations, its parameters are always time-varying and uncertain. This is particularly important in designing sliding mode control because the motion of the control system in the reaching phase can be influenced by environmental disturbances and parameter uncertainties. In this study, a time-varying sliding manifold is proposed to eliminate the reaching phase and to enhance the robust performance. Therefore, a novel type of time-varying sliding surface is introduced based on the initial condition as a slope-varying manifold. Then, a procedure for determining the control parameters is investigated. Furthermore, chattering phenomenon can be avoided using two techniques known as boundary layer and continuous SMC. Finally, to highlight the robust performance of the proposed methods, a quadrotor UAV subject to external disturbances and uncertainties is simulated.