The original physically-grounded formulization of the operative readiness coefficient has been proposed as a quantitative index of hydro-turbine reliability. Computations of dynamics demonstrate that one can detect three typical periods of turbine exploitation: initial period with low reliability indices, period of normal exploitation with high indices and final period characterized by a gradual decrease in reliability. The proposed index is used in the special method which allows estimating the necessity of turbine reconstruction for reliability reasons. It is based on comparing the estimated value of the operative readiness coefficient for a hydro-power station (HPS) planned for reconstruction with the respective coefficient for the similar HPS. If such a value is less than the same one for the considered analogue HPS, it proves to be necessary to perform the reconstruction of the checked hydro turbine for reliability reasons.