We present hereafter a new approach to estimate the reliability of complex circuits used in harmful environments like radiation. This goal can be attained in an early stage of the design process. Usually, this step is performed in laboratory, by means of radiation facilities (particle accelerators). In our case, we estimate the expected tolerance of the complex circuit with respect to SEU during the VHDL specification step. By doing so, the early-estimated reliability level is used to balance the design process into a trade-off between maximum area overhead due to the insertion of redundancy and the minimum reliability required for a given application. This approach is being automated through the development of a CAD tool.