In the wake of a damaging fire at the Browns Ferry Nuclear Power Plant, a 1976 Commission report recommended improved fire protection standards for operating nuclear power plants. D developed technical guidelines for evaluating the fire safety of both new and operating nuclear plants. The guidelines for operating plants differed from those for plants not completed. D pursued the approach of evaluating the safety of operating plants by applying the guidelines on a plant by plant basis. Disagreements persisted, however, on some issues that were common to a number of plants. D decided to embark on the rule-making challenged here. D’s notice did not address plants that had been approved in the past. The final rules applied new requirements to all plants. The rules required D to grant an exemption if a plant could show that a required modification either would not improve or would be detrimental to, the plant’s overall safety. The rules also provided that D’s final-exemption decisions were subject to judicial review. P contends that the notice of proposed rule-making was inadequate because it gave no indication of the technical basis on which D had relied in formulating the proposed rules and because the rules as adopted differed in major respects from the rules proposed in the notice. P complains that d allowed only thirty days for comment, the statutory minimum for notice and comment rule-making, 5 U.S.C. § 553(d) (1976), a period P contends was inadequate given the complexity and relatively innovative character of the rules at issue here. P also argues that D failed to offer an adequate technical justification for the fire protection rules in the form in which they were ultimately adopted. P claims that the Commission failed to comply with its own regulations governing the imposition of new requirements for nuclear plants already in service.