This site was selected on the basis of several criteria. The plutonium pilot facilities needed to be from the site boundary and any other installation, in case radioactive fission products escaped. While security and safety concerns suggested a remote site, it still needed to be near sources of labor, and accessible by road and rail transportation. A mild climate that allowed construction to proceed throughout the year was desirable. Terrain separated by ridges would reduce the impact of accidental explosions, but they could not be so steep as to complicate construction. The substratum needed to be firm enough to provide good foundations, but not so rocky that it would hinder excavation work. It needed large amounts of electrical power (available from the Tennessee Valley Authority) and cooling water. Finally, a War Department policy held that, as a rule, munitions facilities should not be located west of the Sierra or Cascade Ranges, east of the Appalachian Mountains, or within of the Canadian or Mexican borders.