The National Environmental Protection Act (NEPA) directed federal agencies to consider environmental impacts before taking any major action. The Nuclear Regulatory Commission (defendant) determined as part of a rulemaking that nuclear licensing boards should assume as part of their NEPA review that nuclear-waste storage facilities did not leak and thus had no environmental impact. The commission inserted this assumption into a numerical table estimating the environmental impact of fuel-cycle activities of a light-water nuclear reactor over a period of one year. Licensing boards then used the table when acting on license applications. Although noting that there were uncertainties accompanying the assumption, the commission determined that the probability of leakage was so low that the issue need not be reconsidered in each and every license application. Further, the commission counterbalanced the assumption with higher estimates of environmental impacts elsewhere in the table. The commission concluded that despite the no-leakage assumption, the table’s overall environmental impact estimate erred on the side of caution. The rule implementing the no-leakage assumption also provided that the table should be supplemented in each license proceeding with narrative evidence about fuel-cycle activities’ health, socioeconomic, and cumulative impacts. The Natural Resources Defense Council (NRDC) (plaintiff) challenged the rule in the United States Court of Appeals for the D.C. Circuit. The court of appeals stated that an agency can make generic findings under NEPA and insert them into individual licensing determinations. However, the court of appeals found the commission’s no-leakage assumption arbitrary and capricious and vacated the rule. The United States Supreme Court granted certiorari.