This study presents two new robust nonlinear control algorithms based on the theory of time-varying sliding mode control (TVSMC) strategy to stabilize the attitude of an unmanned aerial vehicle (UAV) for nuclear decommissioning applications. Constant radiation affects the UAV performance. For instance, its parameters are time-varying and subject to uncertainty all the time. This is especially important in designing sliding mode control as the motion of the control system in the reaching phase is highly sensitive against environmental disturbances and parameter uncertainties. In this study, two types of time-varying sliding manifolds are proposed to eliminate the reaching phase and to enhance the robust performance in the aforementioned phase. Therefore, two novel types of time-varying sliding surfaces are introduced based on the initial condition as intercept-varying sliding mode control (IVSMC) approaches. In the first proposed method, the reaching time from initial manifold to the desired one is similar to that of the conventional SMC method. While in the second proposed IVSMC scheme, one can accelerate or decelerate the motion of the time-varying sliding manifolds at any selected time. 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 is simulated.