During World War II, the United States built the Hanford Nuclear Weapons Reservation (Hanford) in Washington for the production of plutonium. The government contracted with E.I. Dupont de Nemours Co. (Dupont) (defendant) to operate the facility. Plutonium production emitted a fission byproduct called 1-131, or radioiodine. 1-131 was known to be dangerous to health, and the government issued recommendations for exposure. These recommendations were not legally binding. Many years later, research showed that 1-131 could cause thyroid-related diseases, such as thyroid cancer and hypothyroidism. In 1988, congress enacted the Price-Anderson Act (the act), placing all claims relating to nuclear incidents under federal jurisdiction, and allowing plaintiffs to sue private parties, such as government contractors. Plaintiffs who believed they had been injured by 1-131 emitted from Hanford filed multiple class-action suits against Dupont and other defendants. They alleged that Dupont was strictly liable for their injuries because Dupont was engaged in an abnormally dangerous activity. These cases were consolidated, and ultimately 12 plaintiffs were chosen to proceed with a “bellwether” case to determine the strengths and weaknesses of the litigation. Half of these plaintiffs’ suits were dismissed. The remaining six went to trial. Before the trial, Dupont argued that the common-law affirmative defense of contractor immunity was a complete defense to liability. The trial court held that the act preempted this defense. The trial court instructed the jury that the plaintiffs must prove that Hanford’s 1-131 emissions were a cause-in-fact of their injuries. A jury found in favor of two plaintiffs, awarding them damages of nearly $550,000 collectively. The non-prevailing plaintiffs and the defendants appealed. On appeal, the plaintiffs argued that the correct standard of proof was substantial causation, rather than but-for causation. Dupont argued that the district court erred when it held that contractor-immunity did not give Dupont a complete defense against the plaintiffs’ claims. Additionally, Dupont argued that it was not engaged in an abnormally dangerous activity because it did not fully know the dangers of plutonium byproducts at the time it operated Hanford.