The accident released more radioactivity than the Three Mile Island accident . The spill has been called " the largest radioactive accident in U.S. history , " but the Nuclear Regulatory Commission has said that this is " an overstatement , " and that " there have been a number of other events that have been more significant in terms of radiological impact . The event was more significant from an environmental perspective than from a human one . " The dam formed the southern wall of one of the mill 's three holding ponds , which were used to evaporate tailings solution until the remaining solid waste could be buried . From 1967 to 1982 , the mill produced an average of of tailings every day , for a total of . The high embankment was constructed on a deposit of collapsible clayey , silty sand a deep . United Nuclear used a new design , recommended by the Nuclear Regulatory Commission , that used earth rather than tailings themselves as building material . The holding pond was not lined , a violation of the Uranium Mill Tailings Radiation Control Act of 1978 . This allowed tailings solution to seep into the ground , weakening the foundation of the dam and contaminating the groundwater
Three Mile Island accident
Three Mile Island accident
Church Rock uranium mill spill