Schedules of rates and charges for electric energy produced at the Bonneville project and sold to purchasers as in this chapter provided shall be prepared by the administrator and become effective upon confirmation and approval thereof by the Secretary of Energy; and such rates and charges shall also be applicable to dispositions of electric energy to Federal agencies. Subject to confirmation and approval by the Secretary of Energy, such rate schedules may be modified from time to time by the administrator, and shall be fixed and established with a view to encouraging the widest possible diversified use of electric energy. The said rate schedules may provide for uniform rates or rates uniform throughout prescribed transmission areas in order to extend the benefits of an integrated transmission system and encourage the equitable distribution of the electric energy developed at the Bonneville project.