A revelatory paper by UCal historian Sean Malloy shocked me. Check out the final sentence of this conclusion to his masterful analysis:
This survey of pre-Hiroshima knowledge of radiation effects in the United States makes it clear that most of the immediate and long-term biological effects of radiation on victims of the bomb were predictable at the time of the A-bomb decision, even if still imperfectly understood. Much of the research that made such predications possible was generated by men and women directly employed by the Manhattan Project during the course of the war. It is also clear that this knowledge played little or no role in the decision to use the atomic bomb. The policy of compartmentalization combined with the single-minded drive at Los Alamos to build a working bomb and the primacy of the blast damage model meant that few even inside the Manhattan Project were aware of, or interested in, the emerging body of knowledge on radiation effects generated during the war. The scientists and physicians tasked with researching those effects knew little or nothing about the efforts to build and deliver the atomic bomb and played no role in the decisions about its wartime use. The high-level American leaders who made the final decisions about the bomb, including President Truman, Secretary of State Byrnes, and Secretary of War Stimson, were never informed that the weapon would continue to sicken and kill its victims long after use.
Malloy, Sean L. 2012. “‘A very pleasant way to die’: Radiation effects and the decision to use the atomic bomb against Japan.” Diplomatic History, 36, Jun., pp. 515-545.