Schedules of rates and charges for electric energy produced at the Fort Peck project and sold to purchasers as in this chapter provided shall be prepared by the Bureau and become effective upon confirmation and approval thereof by the Secretary of Energy. Subject to confirmation and approval by the Secretary of Energy, such rate schedules may be modified from time to time by the Bureau 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 rate 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 Fort Peck project.