Section 102(2)(C) of the National Environmental Policy Act (NEPA) requires federal agencies to consider the environmental impact of any major federal action. The Nuclear Regulatory Commission (NRC) decided that licensing boards should assume, for purposes of NEPA, that the permanent storage of certain nuclear wastes would have no significant environmental impact and thus should not affect the decision whether to license a particular nuclear power plant. The environmental impact of operating a light-water nuclear power plant includes the effects of offsite activities necessary to provide fuel for the plant ('front end' activities), and of offsite activities necessary to dispose of the highly toxic and long-lived nuclear wastes generated by the plant ('back end' activities). NRC first adopted Table S-3 in 1974, after extensive informal rulemaking proceedings. The Natural Resources Defense Council (NRDC), challenged the original rule and a license issued under the rule to the Vermont Yankee Nuclear Power Corp. The Court of Appeals for the District of Columbia Circuit affirmed Table S-3's treatment of the 'front end' of the fuel cycle, but vacated and remanded the portion of the rule relating to the 'back end' because of perceived inadequacies in the rulemaking procedures. While Vermont Yankee, 435 U.S. 519, was pending in this Court, the NRC proposed a new 'interim' rulemaking proceeding to determine whether to adopt a revised Table S-3. NRC acknowledged that the risks from long-term repository failure were uncertain, but suggested that research should resolve most of those uncertainties in the near future. NRC promulgated the interim rule in March 1977. Table S-3 now explicitly stated that solidified high-level and transuranic wastes would remain buried in a federal repository and therefore would have no effect on the environment. In 1979, the NRC adopted the 'final' Table S-3 rule. It expressly stated that Table S-3 should be supplemented in individual proceedings by evidence about the health, socioeconomic, and cumulative aspects of fuel cycle activities. The NRC continued to adhere to the zero-release assumption that the solidified waste would not escape and harm the environment once the repository was sealed. The NRC predicted that a bedded-salt repository would maintain its integrity, and found the evidence 'tentative but favorable' that an appropriate site would be found. The NRDC petitioned for review of the final rule. The Court of Appeals concluded that the Table S-3 rules were arbitrary and capricious and inconsistent with NEPA because the NRC had not factored the consideration of uncertainties surrounding the zero-release assumption into the licensing process in such a manner that the uncertainties could potentially affect the outcome of any decision to license a particular plant. The NRC appealed.