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National Museum of Nuclear Science & History

Conant & the Bomb

As one of the nation’s foremost scientists, James B. Conant served as the chairman of the National Defense Research Committee (NDRC) and played a critical role as the key scientific advisor overseeing the Manhattan Project. Conant felt great urgency to move quickly because of the threat of a Nazi atomic bomb, but was frustrated by indecision and delays. The high-level S-1 committee to investigate development of an atomic bomb and the newly created civilian Office of Scientific Research and Development (OSRD) were slow to sort out responsibilities for the project. Conant endorsed pursuing four different methods of producing the atomic bomb’s core ingredients. His work to hasten the process of developing a bomb is described in the excerpt below from James G. Hershberg’s book, “Harvard to Hiroshima and the Making of the Nuclear Age.”

Document Type:
James B. Conant

From Harvard to Hiroshima and the Making of the Nuclear Age by James G. Hershberg

By 1942, Conant’s incentive for clamping down on any potential security lapse, whether by garrulous relatives or scientists lacking a “need to know,” had steeply risen due to his belief that only a Nazi A-bomb could alter the war’s outcome. Roosevelt, influenced by reports from Bush and Conant, shared that calculation. “I think the whole thing should be pushed not only in regard to development, but also with due regard to time,” FDR wrote Bush in March, authorizing the OSRD to hand over development work to the army “on condition that you yourself are certain that the War Department has made all adequate provision for absolute secrecy.” But the atomic project could not go to the War Department for construction until the OSRD determined which method should be used in the immense plants that would fabricate the few kilograms of highly radioactive mass making up the core of each new weapon. Conant still lacked a firm conception of the fastest route to success. Four contestants in the fissionable material “horse race”—the metaphor that quickly gained vogue—appeared worthy of serious consideration. Three (gaseous diffusion, electromagnetic separation, centrifuge) aimed to isolate quantities of U-235, and one to produce a new element, plutonium, known as element 94 after its predicted atomic number, that would be even more fissionable than the uranium isotope.

It was, effectively, up to Conant to place the government’s bet. Yet, no obvious favorite had emerged by the time he met with S-1 Section program chiefs on May 23, 1942. Intensive conferences yielded informal predictions—optimistic, it turned out—that six bombs might be ready by July 1944, and possibly as early as January 1, 1944. But since there was no consensus on the best method to meet that timetable, Conant faced two choices, neither especially palatable. He could call for an intensive “Napoleonic” program on all fronts, costing hundreds of millions of dollars. Or he could place the project’s hopes on one method, which might or might not prove preferable to the others, and might result in no bomb at all if he chose wrong.

One factor complicating his decision, he admitted to Bush, was his awareness that unless he gave “a green light on everybody’s hopes and ambitions,” some “disheartened and discontented people” would “take the case to the court of public opinion, or at least the ‘top physicists’ of the country.” Such a public stink, of course, would be disastrous. But the main argument in favor of a crash, “all out” program remained fear of German competition. Since it now appeared to him that several of the alternate methods for devising a bomb were likely to work, “the probabilities of the Germans eventually getting such a weapon become very high.” As evidence to back this proposition Conant noted British intelligence information that the Germans had seized a ton of heavy water, needed for experiments leading to a self-sustaining chain reaction; reports as early as 1940 that German scientists were working on the problem; and, especially, “recent intercepted instructions to their agents in this country” showing interest in atomic weapons. Conant thus reasoned:

“If they are hard at work, they cannot be far behind since they started in 1939 with the same initial facts as the British and ourselves. There are still plenty of competent scientists left in Germany. They may be ahead of us by as much as a year, but hardly more. If the possession of the new weapon in sufficient quantities would be a determining factor in the war, then the question of who has it first is critical. Three months’ delay might be fatal. For example, the employment of a dozen bombs on England might be sufficient to enable an invasion to take place.”

If, instead, the military judged that possession of “a dozen or two atomic bombs” would be “not in reality determining but only supplemental,” the need for haste and for “betting heavily” would be much less. But Conant didn’t expect to get off the hook that easily and, not out of enthusiasm, but despair at the lack of consensus, he recommended continued work on all four processes. In his report to FDR, Bush “lifted verbatim” Conant’s views, having first carefully obtained the approval of [Vice President Henry] Wallace, [Henry] Stimson, and [General George C.] Marshall. On June 17, Roosevelt approved (“VB—OK—FDR”) this multiple approach, asking only one key question: “Do you have the money?” (Bush assured him that he did.) The project then entered one of its most frustrating periods, as a somewhat sluggish transfer of authority took place between the civilian OSRD and the army’s Manhattan Engineering District. Conant yearned during the summer of 1942 for signs of a clear winning method so the army could begin the job of constructing the factories that would produce the core material.


Despite the shift to army control, he remained—due to S-1’s rigorously enforced policy of “compartmentalizing” information—the only scientist in a position to assess the atomic data flowing from various research centers and thus the project’s overall progress. Despite desperate pleas from his S-1 Executive Committee, and army incredulity over the resources that would be needed for across-the-board development, Conant stuck to his view that no production method should be abandoned unless it clearly became inferior to others.

As the date for making a final decision on construction neared, and the problems in coordination between OSRD and the army became more and more apparent, a new joint group was formed to oversee development of the entire project. On September 23, 1942, the Military Policy Committee—with Bush as chairman and a representative each from the army and navy (Conant was named Bush’s alternate and attended all meetings)—began to act as “a sort of board of directors” for the Manhattan District’s new commander: the gruff, husky, ambitious, bumptious Gen. Leslie R. Groves.


Traveling to different sites to consult with top scientists working on various methods, Conant and his S-1 Executive Committee in the fall of 1942 took stock of the situation. In late October, Conant told Bush that the centrifuge method had proved the “weakest horse” and could be dropped, although strangling the project proved long and bitter, since its supporters repeatedly attempted to raise it from the crypt. And Conant remained particularly dubious of Compton’s plutonium project at the Metallurgical Laboratory at the University of Chicago. He still suspected the backers of this plan to produce plutonium—via a slow self-sustaining chain reaction in a uranium pile, or “boiler”—of being as interested in ascertaining the potential for nuclear power as they were in producing a bomb. He was also “boggled” by the complexities of planning a program around an element that had never been produced in visible quantities.

In November, Compton’s casual revelation that the prototype pile was being built under the stands of Stagg Field, the university’s football stadium, allegedly caused Conant’s face to turn white, but he thought it too late to stop the experiment for safety reasons. Then Conant received conflicting estimates of the amount of impurities that could be tolerated in element 94 (plutonium) without spoiling its usability as a weapon; this disturbing and “extremely embarrassing” report prompted him to order a review of the entire Chicago program. “Now is the time for faith,” implored the project’s leader, Arthur Compton, a devout Christian, in a special-delivery letter to Conant. But Conant did not think highly of such an appeal.  “It isn’t faith we need now, Arthur,” he replied.  “It’s works.” After seeing a pessimistic calculation from a British scientist, a “rather highly disturbed” Conant complained about the “present rather fuzzy state of our thinking” and forcefully reminded Compton of his duty to make honest estimates, even if they were discouraging. “I am sure you will agree with me,” he added, “that the record, which some day will be gone over with a fine tooth comb, is of importance, not because of its effect on any one of us, but because it will stand as to what American scientists can do under pressure. I should very much hate to have the record show that under the enthusiasm of the chase American scientists lost their critical acumen and failed to be realistic and hardboiled about the chance of success.”