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atomic bomb

Source:
The Oxford Companion to World War II
Author(s):
R. V. JonesR. V. Jones, Martin J. SherwinMartin J. Sherwin

atomic bomb. 

1. Development

Following the discovery of radioactivity by the French scientist Henri Becquerel in 1896, measurements of the kinetic energies of alpha-particles emitted by radioactive atoms led the British scientists Ernest Rutherford and Frederick Soddy to conclude in 1903 that atoms contained immense stores of energy; and in 1904 Soddy was speculating to the British Corps of Royal Engineers that this energy might ultimately form the basis of a devastating weapon based on atomic fission. In 1911 Rutherford found that the mass of an atom, and therefore its energy, was almost entirely concentrated in its nucleus; and in 1920 Arthur Eddington concluded, from Einstein's equivalence of mass and energy and from Francis Aston's measurements of the masses of hydrogen and helium nuclei, that enormous amounts of energy would be released if hydrogen nuclei could be fused together to make helium nuclei, and that this process was a main source of energy in stars. Thus by 1920 it was realized that weapons might be based on the fission of heavy nuclei or the fusion of light ones.

In 1932, at Cambridge, James Chadwick discovered the neutron, and this proved easily capable of penetrating atomic nuclei because, unlike protons and electrons, it was neither repelled nor attracted by the inherent nuclear charge. Some nuclei, when they absorbed a neutron, became radioactive and then might emit charged particles or even further neutrons. If these in turn could penetrate the nuclei of nearby atoms, a chain reaction might be started, as Leo Szilard patented in 1934. Two German scientists working in Berlin, Otto Hahn and Fritz Strassmann, investigating the effects of bombarding uranium with neutrons, found in 1938 that this created new elements, and their careful work forced them to the surprising conclusion that one of these elements was barium and not, as they had expected, an element heavier than uranium. Over Christmas 1938 the Austrian scientists Otto Frisch and Lise Meitner, then working in Denmark and Sweden respectively, successfully explained the results as indicating that neutrons were penetrating uranium nuclei, each of which then split into two smaller nuclei, sometimes with some smaller fragments. Since the total mass of the fission products was less than that of a uranium nucleus, much energy must have been emitted in the process. The global physics community then realized that if there were neutrons among the minor fragments in the fission process, a chain reaction might be possible; and so thoughts were now seriously directed towards atomic weapons.

To appreciate the subsequent turn of events, a survey of the relevant knowledge in early 1940 may help. In 1939, Niels Bohr and John Wheeler, building on George Gamow's model of a nucleus as a minute drop of liquid, developed a model of the fission process; and Bohr concluded from this model that it was the lighter uranium isotope U235 that was being split. The drop was held together by attractive forces between the protons and neutrons of which it was composed, against the electric forces of repulsion between its protons. In the common isotope of uranium, U238, there were 92 protons and 146 neutrons. This nucleus was hard to split by bombarding it with a further neutron; but the rarer isotope, U235, with 92 protons and 143 neutrons was less stable, particularly to ‘slow’ neutrons. One possible route to a bomb might therefore depend on the fission of U235 by slow neutrons, but these would take so long (a millisecond or so) to diffuse through a block of uranium that the bomb would blow itself apart long before its full explosive power could be realized. This difficulty, and others, inevitably appealed to many physicists who tended to hope that nature had subtly set insuperable obstacles in the way of humanity's achieving command of such devastating power; and, anyway, most British physicists were already completely absorbed in the effort to develop radar and electronic warfare, both of which were much more urgent in 1940.

There was, though, a group of physicists so far excluded from the war effort: the Jewish physicists from Germany and Austria to whom the British had given shelter. Two of these, Otto Frisch and Rudolph Peierls, working in the University of Birmingham, concluded that a bomb might be made if the U235 isotope could be isolated from its less reactive and preponderating counterpart, U238, and Peierls's calculation suggested that, against the opinion of most other workers in Europe and the US, the amount required for a bomb might be no more than a few kilograms (there was a critical size, which had to be great enough for neutrons from one fission to have sufficient chance of striking a second nucleus before escaping outside). It might be small enough to be within the potential of existing methods for separating isotopes, if vigorously developed. For a bomb with 5 kg. (11 lbs.) of U235, Peierls and Frisch estimated an explosive power equivalent to ‘several thousand tons of dynamite’: these figures proved impressively close to those which were realized in the atomic bombs of 1945.

The consequent memorandum by Peierls and Frisch led to the formation in the UK of the M.A.U.D. committee under George Thomson in April 1940 to investigate the bomb's potential. A second route towards a bomb had been suggested by Hans von Halban and Lew Kowarski, who had escaped from France and joined the British effort. This route, which had also been identified in the USA, started with the 140:1 mixture of U238 and U235 in natural uranium, where bombardment of the U235 nuclei could result in surplus neutrons which might then penetrate U238 nuclei, converting them into a new element, neptunium, of atomic weight 239, which by radioactive decay could produce nuclei of a new element of atomic weight 239 and atomic number 94 (uranium's atomic number was 92). The attractive possibility of this hitherto unknown element, for which the name ‘plutonium’ was suggested, was that from the Bohr–Wheeler model it was expected to resemble U235 as a bomb material, while being capable of separation by simple chemistry from its uranium parent, whereas U235 separation involved very difficult physical techniques. Moreover, the ‘useless’ and much commoner U238 isotope could now itself be transformed into a bomb material.

One snag in this route was that the neutrons emitted by U235 on fission had too high a speed for them to be captured most efficiently by other U235 nuclei, and so the chances were that any such neutrons would be captured by the much more numerous U238 nuclei. While this capture would ultimately result in plutonium nuclei, if it occurred too frequently there would not be enough neutrons left over to maintain the fission chain in the U235 nuclei. The solution was therefore to form the natural uranium into small blocks from which the fission neutrons could temporarily escape and encounter a surrounding ‘moderator’ made up of light nuclei such as deuterium (heavy water) or carbon (graphite) from which they would rebound back into a uranium block with a speed reduced sufficiently to favour their preferential capture by U235 nuclei, and so maintain the fission chain in these nuclei while the surplus neutrons were used to convert U238 into plutonium. The best source of deuterium was heavy water produced at the Norske Hydro Works at Rjukan in Norway (see Vemork); and this, or its less effective counterpart, pure carbon (graphite), was therefore vital to the production of plutonium.

Such was the state of knowledge in the UK in 1940 where work started the following year on the large-scale separation of uranium isotopes; and in October 1941 a new body, a division of the Department of Scientific and Industrial Research, codenamed TUBE ALLOYS, was formed to supervise all British nuclear energy research. At that time scientists in other countries had failed to spur their respective governments into urgent action. In Japan K. Hagiwara foresaw the possibility of a bomb with U235, but no serious effort ensued. In Germany, the possibility of a fast-neutron bomb based on U235 was largely overlooked; it was thought that some tons of separated uranium would be necessary for a bomb, and this would not be feasible before the end of the war, although a ‘boiler’—that is, a controlled reaction releasing a new kind of energy—might be achieved. In the USA scientists who had fled the Nazi threat became deeply concerned that a nuclear bomb might be under development in Germany. Three of them, Szilard, Eugene Wigner, and Edward Teller, went in August 1939 to see the doyen of the scientific community, Albert Einstein (1879–1955), and persuaded him to write to President Roosevelt to warn him of this danger. However, the American research that resulted was based on fission by slow neutrons and, with the USA still at peace, lacked the necessary urgency, though by April 1941 enough plutonium (one four-millionth of a gram) had been made to suggest that it was indeed fissionable by slow neutrons.

In the USSR Igor Kurchatov had also alerted his government to the possibility of a weapon based on nuclear fission. The idea was not pursued, but after the Soviet physicist Georgi Flerov stated in May 1942 that his country ‘must build a uranium bomb without delay’ an intense espionage effort against the US programme began, and the knowledge thereby gained was of much help to Kurchatov's post-war programme.

During 1941 American scientists visited the UK to see the progress being made there, and were so impressed by the British conviction, based on the analysis of Frisch and Peierls, that in December 1941 they recommended a full-scale American effort. Up to that time, according to the American physicist Arthur Compton, not a single member of the American committee ‘really believed that uranium fission would become of critical importance in the Second World War’. A further agent in convincing the Americans was Mark Oliphant, an Australian pupil of Rutherford, who pressed on Ernest Lawrence (the inventor of the cyclotron) and Vannevar Bush, who headed the Office of Scientific Research and Development, the British conviction that a bomb could be made. The result was a huge effort in the USA, codenamed MANHATTAN PROJECT, to which workers from the existing British effort were transferred (see Map 7). An establishment for basic bomb development was formed at Los Alamos, New Mexico, under Oppenheimer; another, at Oak Ridge in Tennessee, worked on the separation of U235 from U238 by gaseous diffusion and electromagnetic techniques; and a third, at Hanford on the Columbia river, produced plutonium in graphite piles. As head of the entire project in Washington a military engineer, Colonel (later Lt-General) Leslie R. Groves was appointed in September 1942; previously he had built the Pentagon. In addition, in 1944 J. D. Cockcroft headed an Anglo-Canadian-French establishment at Chalk River, west of Ottawa, where ample hydroelectric power was available to produce heavy water for plutonium production.

MANHATTAN PROJECT for development of the atomic bombClick to view larger

7. MANHATTAN PROJECT for development of the atomic bomb

In 1942 Enrico Fermi, working in Chicago with Compton, built a ‘pile’ of suitably disposed ‘bricks’ of graphite interspersed with sealed ‘cans’ each containing about 27 kg. (60 lb.) of uranium oxide powder and also some blocks of natural uranium metal. As with a bomb, there was a critical size for a pile: if it was too small too many neutrons would escape outside before they were absorbed by uranium nuclei; and Fermi gradually built up his pile until on 2 December 1942 it ‘went critical’ and showed that it could produce both energy and plutonium.

By early 1945 bomb-grade U235 was being produced at Oak Ridge and plutonium at Hanford. A key question then was how to keep the explosive in a bomb inert until the instant of detonation. The obvious solution, once the critical mass had been determined from previous measurements, was to keep such a mass in two separate halves, each of which would be safe by itself; if these were then brought rapidly together to form a mass of critical size an explosion would result. With uranium, this solution proved practicable: one half was made into a ‘slug’ for a ‘gun’ which fired the slug at the other half as a target. This was the device used in the trial bomb ‘Little Boy’ dropped on Hiroshima on 6 August 1945. With Plutonium, however, even a velocity of approach of the masses at 915 m./sec. (3,000 ft./sec.) was too slow—the spontaneous radioactivity in the plutonium halves would cause them to heat up too quickly as they approached, and the ‘bomb’ would fizzle rather than explode.

An alternative approach was to use the fact that plutonium will suddenly collapse to a more compact form at high pressure which forces its atoms together, almost doubling its density; neutrons generated in one nuclear fission will then have a greater chance of encountering other nuclei before they can escape outside. A suitably chosen mass of plutonium of normal density can then be placed between suitably disposed and shaped masses of conventional explosives, which on detonation produce shock waves which will compress the plutonium to below its critical volume; it will then explode. This is a much faster process than impact from a gun, and it was tried out at the first nuclear explosion, ‘Trinity’, at Alamogordo on 16 July 1945; it was then used in the bomb ‘Fat Man’ dropped on Nagasaki on 9 August 1945.

By then the war with Germany had already ended, and it was clear that German atomic work had been far behind. Although a bomb had been contemplated, the project had been given up in 1942, partly because the amount of U235 required for a bomb had not been nearly so carefully estimated as in Britain, resulting in overestimates running up to tons, and these would have required an effort far beyond German resources. And although the plutonium route had been conceived by individuals, it was not seriously pursued. The German effort, led by Werner Heisenberg, from 1942 onwards therefore concentrated on developing a ‘boiler’ with natural uranium and heavy water; but this work, which might incidentally have opened up the plutonium route, was seriously handicapped because insufficient heavy water was available for a pile, thanks to the sabotage by Norwegian patriots at the plant at Vemork.

The making of the atomic bomb had been a prodigious enterprise, by far the most sophisticated large-scale effort ever made by man. According to Groves (see below) the cost was $2,000,000,000, and the workforce was more than 600,000. For comparison, the Great Pyramid, Herodotus relates, required a continuous force of 100,000 men working for twenty years; and the Great Wall of China may have involved 1,000,000 men.

The atomic bomb effort, particularly in engineering and construction was, of course, predominantly American, while much of the earlier stimulus came from émigré scientists, where the memorandum by Frisch and Peierls was crucial. It was that memorandum, and the subsequent report of the M.A.U.D. committee under G. P. Thomson, which was responsible for ‘the encouragement and support at the highest level’ acknowledged by General Groves, who also wrote that: ‘Prime Minister Churchill was probably the best friend that the Manhattan Project ever had.’

It was natural that thoughts among the outstanding body of scientific talent at Los Alamos should turn to the possible release of energy by the fusion of hydrogen into helium. For this it was necessary that the nuclei of hydrogen (or better, deuterium, because of its greater mass) should be forced together at speeds sufficient to overcome the repulsion between the nuclear charges. This entailed heating to very high temperatures, some hundreds of millions of degrees centigrade, which might be generated in a priming explosion by a uranium or plutonium bomb, giving an explosion equivalent to millions rather than thousands of tons of TNT. The development of such bombs in the USA, the USSR, and the UK belongs to post-war history.

R. V. Jones

2. Politics

The two most revolutionary events of the 20th century, the discovery of nuclear fission and the Second World War, marked the year 1939. In February, the news of fission was published in the British science journal Nature, and on the first day of September German forces invaded Poland. In the six years that followed, as the international order was destroyed on the battlefields of Europe and Asia, the nuclear age was created, in fear and in secrecy, by American, British, and Canadian scientists in laboratories located throughout North America. This marriage of Mars and Minerva, of cataclysmic war and revolutionary science, altered the course of history.

The birth of the nuclear age marked both an end and a beginning. The war that launched the atomic bomb project destroyed the rising German and Japanese empires, crippled the global influence of the UK and France, and set the ambitions of the most powerful members of the Grand Alliance, the USA and the USSR, on a collision course. As these expanding great powers amassed their strength during the last year of the war, scientists at the secret and isolated Los Alamos laboratory, on a mesa high above the desert in New Mexico, were completing the design, assembly, and testing of the weapon that would radically alter the military and diplomatic power of the USA.

The idea that the atomic bomb would play an important role in US–Soviet relations after the war took hold early. ‘There was never from about two weeks from the time I took charge of this project,’ General Leslie R. Groves reported, ‘any illusion on my part but that Russia was our enemy and the project was conducted on that basis’. Groves's boss, Secretary of War Henry L. Stimson, less harsh in his judgement of America's wartime ally, nevertheless informed Roosevelt in late 1944 that, ‘troubles with Russia…[were connected] to the future of S-1 the [atomic bomb].’

The more awesome implications of the revolution that science was creating were also recognized and brought to the president's attention. ‘Modern civilization might be completely destroyed’ by a future nuclear war, if some form of post-war international control of atomic energy were not implemented, Stimson told President Truman in April 1945, less than two weeks after Roosevelt's death. It is one of the many ironies associated with the early history of the atomic bomb that the concern to avoid a post-war nuclear arms race—and a war that was expected to be the inevitable result—contributed to the atomic bombings of Hiroshima and Nagasaki on 6 and 9 August 1945. ‘If the bomb were not used in the present war,’ Arthur Compton, the director of the Metallurgical (Nuclear Research) Laboratory at the University of Chicago, wrote to Stimson, criticizing opposition to the military use of the atomic bomb, ‘the world would have no adequate warning as to what was to be expected if war should break out again.’

The organization of the MANHATTAN PROJECT, the codename for the massive two-billion-dollar industrial, technical, and scientific enterprise responsible for the construction of nuclear weapons, was not an inevitable response to the discovery of fission. On the contrary, it was a tardy (and ultimately miscal culated) response to a terrifying prospect: that scientists in Germany, where fission had been discovered, were racing to develop this new weapon. Why German scientists failed to take this initiative has become an increasingly controversial issue. While most historians have argued that it resulted from scientific miscalculations by Germany's leading physicist, Werner Heisenberg, historian Thomas Powers has asserted (Heisenberg's War: The Secret History of the German Atomic Bomb, London, 1993) that Heisenberg's pessimistic estimates were based on his moral opposition to the development of such a weapon. Whichever interpretation one accepts, the scientific and technological uncertainties surrounding the question of whether a German atomic bomb could have been constructed are staggeringly complex.

Scientists in the USA also had the highest regard for German science, but their esteem inspired the opposite reaction, and they managed to bring their fears to Roosevelt's attention. Introduced to the implications of the discovery of fission in 1939, through a letter from Albert Einstein, which concluded, ominously, that ‘Germany has actually stopped the sale of uranium from the Czechoslovakian mines’, the president's initial response was disappointingly cautious. He authorized the formation of a scientific committee to study whether a nuclear weapon was feasible. The question remained unanswered for two years, when Otto Frisch and Rudolph Peierls, refugee physicists from Germany working in the UK, suggested a method of uranium separation that held out the possibility that a nuclear weapon could be produced within a few years.

In the summer of 1941 the British government forwarded the report of the M.A.U.D. committee, ‘we have now reached the conclusion that it will be possible to make an effective uranium bomb…’ to the American ‘uranium committee’. Following the strong recommendation of scientists, who were now convinced that their German rivals had a two-year head start, Roosevelt approved a crash programme to build atomic bombs. Vannevar Bush, head of the Office of Scientific Research and Development (OSRD), organized the project and, in the autumn of 1942, the War Department took over, assigning it the code name MANHATTAN ENGINEER DISTRICT, soon abbreviated to the MANHATTAN PROJECT. Even before the project had been launched, émigré physicists such as the Hungarians Leo Szilard, Edward Teller, and Eugen Wigner, the Italian Enrico Fermi, the German Swiss Albert Einstein, and numerous native American physicists such as Ernest Lawrence, J. Robert Oppenheimer, and Arthur and Karl Compton, among others, warned high officials that the first nation to attain nuclear weapons could not be defeated. The atomic bomb was not merely a better bomb, it was an unprecedented force, ‘a winning weapon’.

Although viewed at first as a defensive response to a perceived German threat, the bomb quickly evolved into an instrument for defeating the Japanese and controlling the Soviets. The point of use of the first bomb was discussed, the minutes of the Military Policy Committee meeting of 5 May 1943 report, and the general view appeared to be that its best point of use would be on a Japanese fleet concentration in the Harbor of Truk…The Japanese were selected, ‘the minutes go on to say without any touch of irony, as they would not be so apt to secure knowledge from it as would the Germans’.

Having launched the project believing that atomic bombs were powerful enough to win the war, planners did not require exceptional perspicacity to conclude that a monopoly of such a weapon had significant potential post-war military and diplomatic advantages. Roosevelt shared this conclusion with Churchill; Stimson shared it with Bush and with Lord Cherwell (see Lindemann), the prime minister's scientific adviser; and Stalin, informed of the MANHATTAN PROJECT by Soviet agents, shared it with the great Soviet physicist Igor Kurchatov. Ideas that threatened to neutralize the Anglo-American post-war nuclear advantage, such as the great Danish physicist Niels Bohr's proposal to Roosevelt and Churchill that they approach Stalin with a plan for the international control of atomic energy, were rejected, and Bohr became suspect. Enquiries should be made regarding the activities of Professor Bohr, Roosevelt and Churchill agreed in September 1944, and steps taken to ensure that he is responsible for no leakage of information, particularly to the Soviets. Thus, by this date, the Churchill–Roosevelt strategy was set. As the Hyde Park aide-mémoire noted: ‘Full collaboration between the United States and the British government in developing TUBE ALLOYS atomic weapons for military and commercial purposes should continue after the defeat of Japan unless and until terminated by joint agreement.’

When Roosevelt died on 12 April 1945 he left both a wartime and a post-war legacy. The wartime legacy was that the bomb would be used militarily against Japan as soon as it was ready. The post-war legacy was that in the aftermath of the war it would be used diplomatically, as a bargaining chip against the USSR. During the early months of Truman's administration these legacies merged. The expectation that the military use of the bomb during the war would reinforce its post-war diplomatic value emerged as a central consideration for Stimson, for the new Secretary of State, James F. Byrnes, and for Truman.

As problems with the Soviets mounted, the hopes invested in the value of the bomb increased. It was viewed as a virtual panacea for confronting impending post-war diplomatic difficulties. Stimson referred to it in his diary as a royal straight flush and a ‘mastercard’, a potential ‘Frankenstein’ or the saviour of ‘the peace of the world’. But in almost every instance, he noted, through one remark or another, that ‘over any such tangled wave of problems between the USA and the USSR the S-1 secret would be dominant.’

At the Potsdam conference in July–August 1945 (see TERMINAL), the news of the successful test of the first atomic bomb, on 16 July at Alamogordo, New Mexico, prompted joy and relief among the Americans. Churchill reported that Truman suddenly ‘stood up to the Russians in a most emphatic and decisive manner’. Virtually cooing with relief, Stimson wrote in his diary, ‘Now with our new weapon we would not need the assistance of the Russians to conquer Japan.’

Whether the USA needed the ‘Russians’ or even the atomic bomb to ‘conquer Japan’ (by which Stimson meant inducing the Japanese to surrender) will for ever remain a source of controversy. But the argument that the decision to use those weapons was taken strictly to save the lives of American soldiers, as Truman and Stimson reported in books, articles, and public statements, has become increasingly unsupportable as documents are prised out of their archival shelters.

In the end, the issue of Japanese surrender turned on clarifying the doctrine of ‘unconditional surrender’. Introduced by Roosevelt in 1943, and accepted by the American public as the appropriate basis upon which to end the war, the doctrine had become something of a political shibboleth by the time Truman entered office. But it had also become a barrier to surrender. Having broken the Japanese diplomatic code (see MAGIC) early in the war, the Department of State's Far Eastern specialists were united in their belief that Japan would surrender if assured that it could maintain Emperor Hirohito and the imperial dynasty. In the end, after the two existing atomic bombs had destroyed their targets, and the Soviets had entered the war, the Japanese continued to insist on such guarantees. They would surrender, the Japanese announced on 10 August, on the condition that the Potsdam Declaration ‘does not comprise any demand which prejudices the prerogatives of His Majesty as Sovereign Ruler’. In response to subtle assurances from the USA that the emperor would continue to occupy the throne, Japan surrendered on 14 August. ‘History might find that the United States,’ Stimson wrote in his autobiography, ‘in its delay in stating its position on unconditional surrender terms, had prolonged the war.’

What Stimson did not say was that the availability of nuclear weapons was the source of that delay. Until they were used, until the power of the atomic bomb had been demonstrated, the nuclear option precluded all other options— clarifying unconditional surrender among them—because it promised dividends. The shock of the bombs dropped on Hiroshima and Nagasaki would not only be felt in Tokyo, American leaders calculated, they also would be noted in Moscow. The military use of atomic weapons was expected not only to end the war; it was assumed that it would help to organize an American peace. While these expectations and decisions may be understandable in the context of four years of scientific secrecy and brutal war, they were not inevitable. They were avoidable. In the end, that is the most important lesson of Hiroshima for the nuclear age.

Martin J. Sherwin

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