Making of the Atomic Bomb

by Richard Rhodes

  The pile at Haigerloch had served for the KWI’s final round of neutron-multiplication studies. One and a half tons of carefully husbanded Norsk-Hydro heavy water moderated it; its fuel consisted of 664 cubes of metallic uranium attached to 78 chains that hung down into the water from the metal “shield” Pash describes. With this elegant arrangement and a central neutron source the KWI team in March had achieved nearly sevenfold neutron multiplication; Heisenberg had calculated at the time that a 50 percent increase in the size of the reactor would produce a sustained chain reaction.

  “The fact that the German atom bomb was not an immediate threat,” Boris Pash writes with justifiable pride, “was probably the most significant single piece of military intelligence developed throughout the war. Alone, that information was enough to justify Alsos.”2260 But Alsos managed more: it prevented the Soviet Union from capturing the leading German atomic scientists and acquiring a significant volume of high-quality uranium ore. The Belgian ore confiscated at Toulouse was already being processed through the Oak Ridge calutrons for Little Boy.

  * * *

  At Los Alamos in late 1944 Otto Frisch, always resourceful at invention, proposed a daring program of experiments. Enriched uranium had begun arriving on the Hill from Oak Ridge. By compounding the metal with hydrogen-rich material to make uranium hydride it had become possible to approach an assembly of critical mass responsive to fast as well as slow neutrons. Frisch was leader of the Critical Assemblies group in G Division. Making a critical assembly involved stacking several dozen 1½-inch bars of hydride one at a time and measuring the increased neutron activity as the cubical stack approached critical mass. Usually the small bars were stacked within a boxlike framework of larger machined bricks of beryllium tamper to reflect back neutrons and reduce the amount of uranium required. Dozens of these critical-assembly experiments had gone forward during 1944. “By successively lowering the hydrogen content of the material as more U235 became available,” the Los Alamos technical history points out, “experience was gained with faster and faster reactions.”2261

  But it was impossible to assemble a complete critical mass by stacking bars; such an assembly would run away, kill its sponsors with radiation and melt down. Frisch nearly caused a runaway reaction one day by leaning too close to a naked assembly—he called it a Lady Godiva—that was just subcritical, allowing the hydrogen in his body to reflect back neutrons. “At that moment,” he remembers, “out of the corner of my eye I saw that the little red [monitoring] lamps had stopped flickering.2262 They appeared to be glowing continuously. The flicker had speeded up so much that it could no longer be perceived.” Instantly Frisch swept his hand across the top of the assembly and knocked away some of the hydride bars. “The lamps slowed down again to a visible flicker.” In two seconds he had received by the generous standards of the wartime era a full day’s permissible dose of radiation.

  Despite that frightening experience, Frisch wanted to work with full critical masses to determine by experiment what Los Alamos had so far been able to determine only theoretically: how much uranium Little Boy would need. Hence his daring proposal:

  The idea was that the compound of uranium-235, which by then had arrived on the site, enough to make an explosive device, should indeed be assembled to make one, but leaving a big hole so that the central portion was missing; that would allow enough neutrons to escape so that no chain reaction could develop.2263 But the missing portion was to be made, ready to be dropped through the hole so that for a split second there was the condition for an atomic explosion, although only barely so.

  Brilliant young Richard Feynman laughed when he heard Frisch’s plan and named it: he said it would be like tickling the tail of a sleeping dragon.2264 The Dragon experiment it became.

  At a remote laboratory site in Omega Canyon that Fermi also used, Frisch’s group built a ten-foot iron frame, the “guillotine,” that supported upright aluminum guides. The experimenters surrounded the guides at table level with blocks of uranium hydride. To the top of the guillotine they raised a hydride core slug about two by six inches in size. It would fall under the influence of gravity, accelerating at 32 feet per second/per second. When it passed between the blocks it would momentarily form a critical mass. Mixed with hydride, the U235 would react much more slowly than pure metal would react later in Little Boy. But the Dragon would stir, and its dangerous stirring would give Frisch a measure of the fit between theory and experiment:

  It was as near as we could possibly go towards starting an atomic explosion without actually being blown up, and the results were most satisfactory. Everything happened exactly as it should. When the core was dropped through the hole we got a large burst of neutrons and a temperature rise of several degrees in that very short split second during which the chain reaction proceeded as a sort of stifled explosion. We worked under great pressure because the material had to be returned by a certain date to be made into metal. . . .2265 During those hectic weeks I worked about seventeen hours a day and slept from dawn till mid-morning.

  The official Los Alamos history measures the significance of Frisch’s Dragon-tickling:

  These experiments gave direct evidence of an explosive chain reaction. They gave an energy production of up to twenty million watts, with a temperature rise in the hydride up to 2°C per millisecond. The strongest burst obtained produced 1015 neutrons. The dragon is of historical importance. It was the first controlled nuclear reaction which was supercritical with prompt neutrons alone.2266

  By April 1945 Oak Ridge had produced enough U235 to allow a nearcritical assembly of pure metal without hydride dilution. The little bars arrived at the Omega site packed in small, heavy boxes everyone took pains to set well apart; unpacked and unwrapped, the metal shone silver in Frisch’s workbench light. Gradually it oxidized, to blue and then to rich plum. Frisch had walked in the snow at Kungälv puzzling out the meaning of Otto Hahn’s letters to his aunt; in the basement at Bohr’s institute in Copenhagen he had borrowed a name from biology for the process that made these small exotic bars deadly beyond measure; at Birmingham with Rudolf Peierls he had toyed with a formula and had first seen clearly that no more plum-colored metal than now lay scattered on his workbench would make a bomb that would change the world. At Los Alamos in Southwestern spring, dénouement: he would assemble as near a critical mass of U235 as anyone might ever assemble by hand and not be destroyed.

  April 12, Thursday, was the day Frisch completed his critical assembly experiments with metallic U235. The previous day Robert Oppenheimer had written Groves the cheering news that Kistiakowsky had managed to produce implosive compressions so smoothly symmetrical that their numbers agreed with theoretical prediction. April 12 in America was Friday, April 13, in Japan, and on the night of that unlucky day B-29’s bombing Tokyo bombed the Riken. The wooden building housing Yoshio Nishina’s unsuccessful gaseous thermal diffusion experiment did not immediately burn; firemen and staff managed to extinguish the fires that threatened it. But after the other fires were out the building suddenly burst into flame. It burned to the ground and took the Japanese atomic bomb project with it. In Europe John Lansdale was preparing to rush to Stassfurt to confiscate what remained of the Belgian uranium ore; when Groves heard of the success of that adventure later in April he wrote a memorandum to George Marshall that closed the German book:

  In 1940 the German Army in Belgium confiscated and removed to Germany about 1200 tons of uranium ore. So long as this material remained hidden under the control of the enemy we could not be sure but that he might be preparing to use atomic weapons.2267

  Yesterday I was notified by cable that personnel of my office had located this material near Stassfurt, Germany and that it was now being removed to a safe place outside of Germany where it will be under the complete control of American and British authorities.

  The capture of this material, which was the bulk of uranium supplies available in Europe, would seem to remove definitely any possibility of the Germans making
use of an atomic bomb in this war.

  The day these events cluster around, April 12, saw another book closed: at midday, in Warm Springs, Georgia, while sitting for a portrait, Franklin Delano Roosevelt in the sixty-third year of his life was shattered by a massive cerebral hemorrhage. He lingered comatose through the afternoon and died at 3:35 P.M. He had served his nation as President for thirteen years.

  When the news of Roosevelt’s death reached Los Alamos, Oppenheimer came out from his office onto the steps of the administration building and spoke to the men and women who had spontaneously gathered there. They grieved as Americans everywhere grieved for the loss of a national leader. Some also worried about whether the Manhattan Project would continue. Oppenheimer scheduled a Sunday morning memorial service that everyone in and out of the Tech Area might attend.

  “Sunday morning found the mesa deep in snow,” Philip Morrison recalls of that day, April 15. “A night’s fall had covered the rude textures of the town, silenced its business, and unified the view in a soft whiteness, over which the bright sun shone, casting deep blue shadows behind every wall. It was no costume for mourning, but it seemed recognition of something we needed, a gesture of consolation. Everybody came to the theater, where Oppie spoke very quietly for two or three minutes out of his heart and ours.”2268 It was Robert Oppenheimer at his best:

  When, three days ago, the world had word of the death of President Roosevelt, many wept who are unaccustomed to tears, many men and women, little enough accustomed to prayer, prayed to God.2269 Many of us looked with deep trouble to the future; many of us felt less certain that our works would be to a good end; all of us were reminded of how precious a thing human greatness is.

  We have been living through years of great evil, and of great terror. Roosevelt has been our President, our Commander-in-Chief and, in an old and unperverted sense, our leader. All over the world men have looked to him for guidance, and have seen symbolized in him their hope that the evils of this time would not be repeated; that the terrible sacrifices which have been made, and those that are still to be made, would lead to a world more fit for human habitation. . . .

  In the Hindu scripture, in the Bhagavad-Gita, it says, “Man is a creature whose substance is faith. What his faith is, he is.” The faith of Roosevelt is one that is shared by millions of men and women in every country of the world. For this reason it is possible to maintain the hope, for this reason it is right that we should dedicate ourselves to the hope, that his good works will not have ended with his death.

  Vice President Harry S. Truman of Independence, Missouri, who knew only the bare fact of the Manhattan Project’s existence, said later that when he heard from Eleanor Roosevelt that he must assume the Presidency in Franklin Roosevelt’s place, “I kept thinking, ‘The lightning has struck. The lightning has struck!’ ” Between the Thursday of Roosevelt’s death and the Sunday of the memorial service on the Hill, Otto Frisch delivered to Robert Oppenheimer his report on the first experimental determination of the critical mass of pure U235.2270 Little Boy needed more than one critical mass, but the fulfillment of that requirement was now only a matter of time. The lightning had struck at Los Alamos as well.

  PART THREE

  LIFE

  AND

  DEATH

  What will people of the future think of us? Will they say, as Roger Williams said of some of the Massachusetts Indians, that we were wolves with the minds of men? Will they think that we resigned our humanity? They will have the right.

  C. P. Snow

  I see that as human beings we have two great ecstatic impulses in us. One is to participate in life, which ends in the giving of life. The other is to avoid death, which ends tragically in the giving of death. Life and death are in our gift, we can activate life and activate death.

  Gil Elliot

  18

  Trinity

  Within twenty-four hours of Franklin Roosevelt’s death two men told Harry Truman about the atomic bomb. The first was Henry Lewis Stimson, the upright, white-haired, distinguished Secretary of War. He spoke to the newly sworn President following the brief cabinet meeting Truman called after taking the oath of office on the evening of the day Roosevelt died. “Stimson told me,” Truman reports in his memoirs, “that he wanted me to know about an immense project that was under way—a project looking to the development of a new explosive of almost unbelievable destructive power. That was all he felt free to say at the time, and his statement left me puzzled. It was the first bit of information that had come to me about the atomic bomb, but he gave me no details.”2271

  Truman had known of the Manhattan Project’s existence since his wartime Senate work as chairman of the Committee to Investigate the National Defense Program, when he had attempted to explore the expensive secret project’s purpose and had been rebuffed by the Secretary of War himself. That a senator of watchdog responsibility and bulldog tenacity would call off an investigation into unaccounted millions of dollars in defense-plant construction on Stimson’s word alone gives some measure of the quality of the Secretary’s reputation.

  Stimson was seventy-seven years old when Truman assumed the Presidency. He could remember stories his great-grandmother told him of her childhood talks with George Washington. He had attended Phillips Andover when the tuition at that distinguished New England preparatory school was sixty dollars a year and students cut their own firewood. He had graduated from Yale College and Harvard Law School, had served as Secretary of War under William Howard Taft, as Governor General of the Philippines under Calvin Coolidge, as Secretary of State under Herbert Hoover. Roosevelt had called him back to active service in 1940 and with able assistance especially from George Marshall and despite insomnia and migraines that frequently laid him low he had built and administered the most powerful military organization in the history of the world. He was a man of duty and of rectitude. “The chief lesson I have learned in a long life,” he wrote at the end of his career, “is that the only way you can make a man trustworthy is to trust him; and the surest way to make him untrustworthy is to distrust him and show your distrust.”2272 Stimson sought to apply the lesson impartially to men and to nations. In the spring of 1945 he was greatly worried about the use and consequences of the atomic bomb.

  The other man who spoke to Truman, on the following day, April 13, was James Francis Byrnes, known as Jimmy, sixty-six years old, a private citizen of South Carolina since the beginning of April but before then for three years what Franklin Roosevelt had styled “assistant President”: Director of Economic Stabilization and then Director of War Mobilization, with offices in the White House.2273 While FDR ran the war and foreign affairs, that is, Byrnes had run the country. “Jimmy Byrnes . . . came to see me,” writes Truman of his second briefing on the atomic bomb, “and even he told me few details, though with great solemnity he said that we were perfecting an explosive great enough to destroy the whole world.”2274 Then or soon afterward, before Truman met with Stimson again, Byrnes added a significant twist to his tale: “that in his belief the bomb might well put us in a position to dictate our own terms at the end of the war.”2275

  At that first Friday meeting Truman asked Byrnes to transcribe his shorthand notes on the Yalta Conference, three months past, which Byrnes had attended as one of Roosevelt’s advisers and about which Truman, merely the Vice President then, knew little. Yalta represented nearly all Byrnes’ direct experience of foreign affairs. It was more than Truman had. Under the circumstances the new President found it sufficient and informed his colleague that he meant to make him Secretary of State. Byrnes did not object. He insisted that he be given a free hand, however, as Roosevelt had given him in domestic affairs, and Truman agreed.

  “A small, wiry, neatly made man,” a team of contemporary observers describes Jimmy Byrnes, “with an odd, sharply angular face from which his sharp eyes peer out with an expression of quizzical geniality.”2276 Dean Acheson, then an Assistant Secretary of State, thought Byrnes overconfident and insensit
ive, “a vigorous extrovert, accustomed to the lusty exchange of South Carolina politics.”2277 Truman assayed the South Carolinian most shrewdly a few months after their April discussion in a private diary he intermittently kept:

  Had a long talk with my able and conniving Secretary of State. My but he has a keen mind! And he is an honest man. But all country politicians are alike. They are sure all other politicians are circuitous in their dealings. When they are told the straight truth, unvarnished, it is never believed—an asset sometimes.2278

  A politician’s politician, Byrnes had managed in his thirty-two years of public life to serve with distinction in all three branches of the federal government. He was self-made from the ground up. His father died before he was born. His mother learned dressmaking to survive. Young Jimmy found work at fourteen, his last year of formal education, in a law office, but in lieu of classroom study one of the law partners kindly guided him through a comprehensive reading list. His mother in the meantime taught him shorthand and in 1900, at twenty-one, he earned appointment as a court reporter. He read for the law under the judge whose circuit he reported and passed the bar in 1904. He ran first, in 1908, for solicitor, the South Carolina equivalent of district attorney, and made himself known prosecuting murderers. More than forty-six stump debates won him election to Congress in 1910; in 1930, after fourteen years in the House and five years out of office, he was elected to the Senate. By then he was already actively promoting Franklin Roosevelt’s approaching presidential bid. Byrnes served as one of the candidate’s speechwriters during the 1932 campaign and afterward worked hard as Roosevelt’s man in the Senate to push through the New Deal. His reward, in 1941, was a seat on the United States Supreme Court, which he resigned in 1942 to move to the White House to take over operating the complicated wartime emergency program of wage and price controls, the assistant Presidency of which Roosevelt spoke.