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IOP Concise Physics

Atomic Bomb: The Story of the Manhattan ProjectHow nuclear physics became a global geopolitical game-changer

Bruce Cameron Reed

Chapter 1

Introduction and overview

This chapter offers a brief tour of why and for whom this book was written, a surveyof some essential background scientific concepts and an overview of how this bookis organized.

1.1 PrologueIn August 1945, two United States Army Air Force B-29 bombers each droppedsingle ‘atomic’ bombs on the Japanese cities of Hiroshima and Nagasaki. Thesedevices, known colloquially as Little Boy and Fat Man, each exploded with energiesequivalent to over ten thousand tons of conventional explosive, the normal payloadof 1000 such bombers deployed simultaneously. Hiroshima and Nagasaki weredestroyed and within days Japan surrendered, bringing an end to World War II.A 1946 analysis by the United States Strategic Bombing Survey estimated thetotal number of people killed in the bombings to be about 125 000, with a further130 000–160 000 injured.

These bombings represented the culmination of the work of the United StatesArmy’s ‘Manhattan Engineer District’, which in mid-1942 had been assignedresponsibility for developing the bombs. After the war this effort became popularlyknown as ‘The Manhattan Project’. At the time, Manhattan was the most complexand costly research and development project ever undertaken. It has beenestimated that nearly a half-million people were employed in the Project’s factoriesand laboratories, with a peak steady-state employment of about 150 000. ByAugust 1945, the cost of the Project had reached $1.9 billion, but secrecysurrounding the work was so intense that perhaps only a few dozen individualsheld synoptic knowledge of the overall enterprise. As vast and successful as it was,however, the Project was by no means a sure thing: the entire undertaking waspredicated on turning a recently discovered and incompletely understood physicalphenomenon into a weapon that could be configured for operation in combatconditions. Manhattan represented an immense gamble of manpower, industrial

doi:10.1088/978-1-6270-5991-6ch1 1-1 ª Morgan & Claypool Publishers 2015

capacity, and scientific and engineering ingenuity at a time when virtually everyenterprise in the United States was turned to war production. Had history evolvedslightly differently, Manhattan might equally well have made no contribution toending the war. But it did succeed and its legacy includes America’s postwarmilitary and political power, the Cold War, the thousands of nuclear weaponsstill held in the arsenals of various countries today, the threat of nuclear terrorism,and public apprehension with radiation and nuclear energy. No other twentieth-century scientific/technological development has had such a profound impact onhuman affairs.

This volume is probably in your hands or on your computer screen because youhave heard about the Manhattan Project and want to know more about it. Why wasthe organization called the Manhattan Engineer District? How do nuclear weaponsfunction? Why is there such a thing as a critical mass? What makes uranium andplutonium so special in this regard, and how were they obtained? How were thetarget cities chosen? What were the effects of the bombs? But because the Project wasso vast, you may have found the available material on it to be utterly overwhelming;an online search will yield literally millions of hits and it is practically impossible toknow where to start and what information is credible. This volume has beenprepared for readers who have some background in physics and chemistry at about asenior high-school/early college level who seek a survey-level treatment on thescience, history and legacy of the Manhattan Project.

The scale of the Manhattan Project was so great that no single-volume history ofit can ever hope to be comprehensive; bear in mind that this book is a survey volume.After the Project came under Army auspices in mid-1942 it split into a number ofcomponents which operated in parallel, with the results of these componentscombining to produce Hiroshima and Nagasaki. This parallelism makes tellingthe story of the Manhattan Project a challenge. A strict chronological narrativewould require constant jumping around from place to place to keep up with rapidlyevolving events at each location. To avoid this confusion I have opted to deal withvarious Project components in individual sections. While this allows for topicalcoherence it does comes at the price of sacrificing a linear story line, a limitation Iask my readers to keep in mind.

This book comprises six chapters including this one (see figure 1.1 for conceptmap of this book). Chapter 2 surveys the science behind the project: the nature ofatomic nuclei, radioactivity, nuclear reactions, the discovery of nuclear fission andhow, by mid-1941, it had come to be appreciated that particular forms of theelements uranium and plutonium could be turned into powerful fission-basedexplosives. Chapter 3, the heart of this book, describes how the ManhattanEngineer District was organized, how the fissile forms of uranium and plutoniumwere obtained, bomb design considerations and the first test of a nuclear weapon.Chapter 4 covers how the target cities came to be chosen, the preparation ofaircraft to carry the bombs, political considerations as to actual use of the bombsversus a demonstration shot, the context of the war in mid-1945, plans for theproposed invasion of Japan and the bombing missions themselves. Chapter 5 offersa brief survey of some of the legacies of the Manhattan Project, the currentnumbers of nuclear weapons in the world and efforts to control their proliferation.

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Chapter 6 lists a number of books, articles and electronic links to sources whichoffer more detailed information on various aspects of the Project.

The remainder of this chapter comprises two sections which serve as brieforientation tours for the rest of this book. Section 1.2 surveys some of the underlying

Figure 1.1. Concept map of the layout of this book.

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scientific material that is examined in more detail in chapter 2: isotopes, nuclearfission, the role of neutrons in maintaining fission-based chain reactions, howneutron speed is critical in determining whether a chain reaction will be controlled(as in reactors) or uncontrolled (as in bombs), and how only certain particularisotopes of uranium and plutonium can be used to make nuclear weapons. Section 1.3overviews the Manhattan Project itself in a few pages: how the work was organized,the large-scale factories that were constructed to produce uranium and plutonium,the notion of a critical mass for a nuclear weapon, and the nature of the designs of thetwo types of bombs that were developed: the Hiroshima ‘gun-type’ device andthe much more complex Nagasaki ‘implosion-type’ unit.

1.2 Some scientific preliminariesBy the early 1930s, physicists had come to the understanding of the structure of atomsthat is still taught in elementary and high-school physics and chemistry classes today:atomic nuclei consisting of positively charged protons bound together with electricallyneutral neutrons, with the nucleus orbited by a cloud of fast-moving negativelycharged electrons. This picture represented the culmination of over 30 years ofresearch that began with the discovery of natural radioactivity in 1896.

Along the way to the understanding of atomic structure also came appreciation ofthe fact that different elements occur in a variety of isotopic forms. Isotopes ofuranium and plutonium play critical roles in the functioning of nuclear weapons; it isessential to understand this concept very clearly. In any atom, the number ofnegatively charged orbiting electrons normally equals the number of positivelycharged protons in the nucleus of the atom; atoms are thus usually electricallyneutral overall. This number is the same for all atoms of the same element, dictatesthe chemical properties of the element and is called the atomic number of theelement. Physicists and chemists commonly designate atomic number by the upper-case letter Z. For oxygen atoms, Z= 8, that is, all oxygen atoms contain eightprotons in their nuclei and eight orbiting electrons. Different isotopes of the sameelement contain differing numbers of neutrons in their nuclei, but have the samechemical properties because they have the same number Z of protons in their nuclei.For example, there are three naturally occurring isotopes of oxygen: one containseight neutrons, one contains nine neutrons and one contains ten neutrons. In thesethree types you consequently have a total of 16, 17 or 18 protons plus neutrons in thenuclei. To discriminate these cases, the three types are referred to as oxygen-16,oxygen-17 and oxygen-18, or simply O-16, O-17 and O-18. O-16 is by far the mostcommon type, comprising over 99.7% of naturally occurring oxygen, but you canand regularly do quite happily breathe the other two.

The total number of neutrons plus protons in the nucleus of an atom is known asthe atom’s atomic weight or mass number. This is commonly designated with theupper-case letter A. If the letter N is similarly used to designate the number ofneutrons in the nucleus, then it follows that N=A − Z.

There is a very convenient shorthand method for specifying isotopes. Thisnotation will be used extensively throughout this book, and has the general form

X .AZ

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In this expression, X is the symbol for the element involved, the upper-leftsuperscript A is the atomic weight and the lower-left subscript is the atomic numberZ corresponding to element X. In this notation, the oxygen isotopes described abovecan be written as O,16

8 O178 and O18

8 .An essential element in the functioning of nuclear weapons is uranium. All

uranium atoms contain 92 protons in their nuclei, so for this element Z= 92. Thereare two important isotopes of uranium in the story of the Manhattan Project, U-235and U-238 or, in the above notation, U235

92 and U.23892 All U-235 nuclei contain 143

neutrons, while all U-238 nuclei contain 146 neutrons. The three-neutron differencemight seem miniscule, but it played a huge role in the Manhattan Project and iscentral to how nuclear weapons function. An equally important difference is thatthese two isotopes occur in nature in very unequal proportions: only about 0.7% ofnaturally occurring uranium is of the U-235 variety, while the other 99.3% is U-238.Another important isotope involved in the Project is one of the synthetic elementplutonium (symbol: Pu), all nuclei of which contain 94 protons; the isotope that willbe of interest to us is Pu-239, Pu239

94 .There is a very intimate relationship between radioactivity and isotopes. Early in

the twentieth century, Ernest Rutherford and Frederick Soddy deduced that when anucleus undergoes ‘radioactive decay’, it spontaneously transforms itself from anisotope of one element into one of another. Different types of decay mechanisms arepossible and result in different transformation patterns and different resultingisotopes, but in naturally occurring decays the product-element nucleus only everdiffers from that of the starting-element nucleus by one or two protons. Two decaymechanisms that we will be concerned with are so-called alpha-decay and beta-decay; these are described in chapter 2. During the mid-1930s, it was discovered thatradioactive decay could be induced to occur ‘artificially’ in many elements throughexperimental conditions set up by human beings as opposed to waiting for thephenomenon to happen through random natural processes.

The discovery of artificially induced radioactivity led, indirectly, to the discoveryof uranium fission in late 1938. The word ‘fission’ is synonymous with ‘splitting’, andthe discovery involved the realization that nuclei of uranium atoms could be causedto break apart when struck by incoming neutrons. The fact that the bombardingparticles are neutrons is important. Fission cannot be induced by trying to strike auranium nucleus with either another uranium nucleus or one of another element; therepulsive forces between the protons of the nuclei are normally too great to permitthe nuclei to come into contact with each other. But because neutrons are electricallyneutral, they experience no such repulsion and so can easily be made to come intocontact with a target nucleus. In the splitting process, the struck nucleus loses a smallamount of mass, but this mass corresponds to a great amount of energy thanksto Albert Einstein’s famous equation E=mc2. The amount of energy releasedper reaction in these splittings proved to be millions of times that released in anyknown chemical reaction. This process, formally termed nuclear fission, lies at theheart of how nuclear reactors and weapons function, and it was the factor of millionsthat pointed to the possibility of developing explosives of almost unimaginablepower: if such reactions could be created on a large scale, one could potentiallyreplace millions of pounds of a conventional explosive with a few pounds of

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nuclear explosive. A few weeks after the discovery of fission, it was found that aby-product of each fission was the simultaneous liberation of two or three neutronsfrom the struck nucleus. These ‘secondary’ neutrons, if they do not escape the massof uranium involved, can go on to fission other nuclei and hence initiate a so-calledchain reaction. In theory, this process can continue until all of the uranium isfissioned.

The discovery of fission raised a number of questions. Could any other elementsundergo fission? Was there a minimum amount of uranium that would have to bearranged in one place to have any hope of realizing a chain reaction? Did one orboth of the isotopes of uranium undergo fission? Could the process be controlled byhuman intervention to give the possibility of a power source, or would the result bean uncontrollable explosion? Why had all of the uranium ores of the Earth notspontaneously fissioned themselves into other elements millennia ago?

By the time of the outbreak of World War II in September 1939, it had beentheoretically predicted that only the rare U-235 isotope would fission under neutronbombardment, while U-238 nuclei would tend to capture incoming neutrons withoutfissioning. These predictions were confirmed experimentally in early 1940. Given theoverwhelming preponderance of U-238 in natural uranium, this capture effectpromised to literally poison the prospect for a chain reaction using natural-abundance uranium. To obtain a chain reaction, it appeared that it would benecessary to isolate a sample of U-235 from its sister isotope, or at least process asample of uranium in some way so as to isolate a sub-sample with a dramaticallyincreased percentage of U-235. Such processing is now known as isotope enrichmentand is always a very difficult task. Since isotopes of any element behave identicallyso far as their chemical properties are concerned, no chemical technique canbe employed to achieve enrichment; only a technique that depends on the slightmass difference (~1%) between the two isotopes could be a possibility. But theprospects were limited. Only three enrichment techniques, namely centrifugation,mass spectrometry and a process known as diffusion, looked to be plausible, andthese had been applied only in cases involving light elements where the percentagedifferences between isotopic masses is much greater than in the case of an elementsuch as uranium.

By the middle of 1940, understanding of the differing responses of the twouranium isotopes to bombarding neutrons had led to the development of a new ideafor obtaining a controlled (not explosive) chain reaction using natural uraniumwithout enrichment. The key lay in how nuclei react to bombarding neutrons. Whena nucleus is struck by a neutron, various reactions are possible: the nucleus mightfission, it might capture the neutron without fissioning, or it might simply deflect theneutron as a billiard ball would an incoming marble. Each process has someprobability of occurring and these probabilities depend on the speed of the incomingneutrons. Neutrons released in fission reactions are extremely energetic, emergingwith average speeds of about 20 million meters per second, or about 45 million milesper hour. For obvious reasons, such neutrons are termed ‘fast’. As remarked above,U-238 tends to capture fast neutrons emitted in fissions of U-235 nuclei. However,when a nucleus of U-238 is struck by a very slow neutron—one traveling at a mere

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couple thousand meters per second—it behaves as a much more benign target, withscattering of the neutron being about three times as likely as capture. But—and thisis a crucial point—U-235 nuclei turn out to have an enormous probability forundergoing fission when struck by slow neutrons: over 200 times greater than thecapture probability for U-238. This factor is large enough to compensate for the smallnatural abundance of U-235 to the extent that an individual neutron is about as likelyto fission a nucleus of U-235 as it is to be captured by one of U-238 (figure 1.2); this iswhat renders possible a ‘slow-neutron’ chain reaction. Slowing fission-liberatedneutrons is effectively equivalent to enriching the abundance percentage of U-235;if neutrons emitted in fissions can be slowed, then they have a good chance of goingon to fission other U-235 nuclei before being lost by being captured by U-238 nuclei.In actuality, both processes will proceed simultaneously. Surprisingly, neutroncapture by U-238 nuclei turns out to be indirectly advantageous for bomb-makers,as is explained in the third paragraph following this one.

How can one slow a neutron during the very brief interval between when it isemitted in a fission and when it strikes another nucleus? The trick is to work not witha single large lump of uranium, but rather to disperse it as small chunks throughouta surrounding medium which slows neutrons without capturing them. Such amedium is known as a moderator and the entire assemblage is a reactor. Duringthe war the synonymous term ‘pile’ was used in the literal sense of metallic uraniumslugs embedded within a heap of moderating material. Ordinary water can serve as amoderator, but at the time graphite (like that used in pencils) proved easier toemploy. By introducing moveable rods of neutron-capturing material into the pileand adjusting their positions as necessary, the reaction can be controlled. It is in thisway that natural-abundance uranium proved capable of sustaining a controllednuclear reaction, but not an explosive one. All modern power-producing reactorsoperate via chain reactions mediated by neutrons. Along this line, do not be alarmedby fictional stories wherein reactors behave like bombs. The reaction is far too slowand even if the control rods are rendered inoperative, the reactor will melt asopposed to blowing up: Fukushima.

Returning to 1940, it still appeared that to make a chain reaction mediated by fastneutrons—a bomb—it would be necessary to isolate pure U-235. However, it was

Figure 1.2. Schematic illustration of a chain reaction utilizing moderated neutrons. Each fission of a U-235liberates two secondary neutrons, one of which goes on to fission another U-235 nucleus while the other iscaptured by a nucleus of U-238.

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soon appreciated that the moderated-neutron concept could be used here as well, atleast in an indirect way.

It was mentioned above that neutron capture by U-238 nuclei will still proceed ina reactor alongside fission of U-235 nuclei. On capturing a neutron, a U-238 nucleusbecomes a U-239 nucleus. Based on some experimentally established patternsregarding the stability of nuclei, it was predicted that U-239 nuclei might decaywithin a short time to nuclei of atomic number 94, the element now known asplutonium (Pu). It was further predicted that element 94 might be very similar in itsfissionability properties to U-235. If this proved to be the case, then a reactor couldbe used to ‘breed’ plutonium from neutron capture by U-238 nuclei while main-taining a self-sustaining controlled chain reaction via U-235 fissions. The advantageof this would be that the plutonium could be separated from the mass of parenturanium fuel by chemical processes and then used to construct a bomb; this is whatobviates the need to develop enrichment technologies. These predictions were soonconfirmed on a laboratory scale by creating a tiny sample of plutonium via neutronbombardment of uranium.

By the time of the Japanese attack on Pearl Harbor in December 1941, it appearedthat there were two possible routes to developing a nuclear explosive: (i) isolate tens ofkilograms of U-235, or (ii) develop reactors with which to breed plutonium. U-235was considered to be almost certain to make an excellent nuclear explosive, but thetens of kilograms would have to be separated atom by atom from a parent mass ofuranium ore. As for plutonium, the probable chemical separation techniques wereunderstood by chemical engineers, but nobody had ever isolated any significantquantity of plutonium or constructed an operating reactor. Could a large-scalereactor be safely controlled? Might plutonium prove to have some property thatobviated its value as an explosive?

Motivated by the possibility that German scientists could be thinking along thesame lines, the scientific and military leaders of the Manhattan Project made the onlydecision that they could in such circumstances: both methods would be pursued.

1.3 The Manhattan Project—a surveyA project involving hundreds of thousands of people, hundreds of contractors and abudget of nearly $2 billion (over $20 billion in 2015 dollars) could be an organiza-tional nightmare in the best of circumstances. How was such a monumental effortorganized and carried out with secrecy and minimal political interference?

The possibility of military applications of nuclear fission was first brought to theattention of the President of the United States in the fall of 1939 and governmentsupport for research was soon organized. Until mid-1942 this support was under theauthority of various civilian branches of the government, although it was beingconducted in secrecy. By mid-1942, various lines of research in Britain and Americahad converged on the conclusion that both reactors and weapons could be feasible,but that they would require very large-scale engineering and construction efforts.The only organization capable of mounting such an effort with the requisite secrecywas the United States Army. In the fall of 1942 the work was assigned to the Army’sCorps of Engineers.

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Two major production facilities and a highly secret bomb-design laboratory wereestablished to advance the work of the Manhattan Project. These facilities are thesubjects of sections 3.3–3.5. The bomb design laboratory was located at Los Alamos,New Mexico (section 3.3) and the production facilities were located in the states ofTennessee and Washington (sections 3.4 and 3.5); these were respectively devoted toisolating nearly pure U-235 and breeding Pu-239. One bomb, a plutonium device,was tested in an explosion code-named Trinity in mid-July 1945 (section 3.6).

The Los Alamos Laboratory was established in the spring of 1943 in northernNewMexico, and was directed by physicist J Robert Oppenheimer of the Universityof California. In theory, the work facing Los Alamos scientists seemed straight-forward. Fissile isotopes such as U-235 or Pu-239 possess a so-called critical mass, aminimum mass necessary to achieve a chain reaction. The precise value of thecritical mass depends on factors such as the density of the material, its probabilityfor undergoing fission and the number of neutrons liberated per fission reaction.Much of the experimental work at Los Alamos involved obtaining accuratemeasurements of these quantities. With these data, critical masses can be calculatedvia mathematical relationships from an area of physics known as diffusion theory,which was well-established long before 1943.

For the sake of argument, suppose that the critical mass for some material is 50 kg,which is in fact not far off the mark for U-235. It turns out that you will obtain amore efficient explosion if you have more material available than just one criticalmass, so let’s say that you have 70 kg. To make your bomb, form your 70 kg into twopieces, say each of mass 35 kg, and simply arrange to bring them together when youare ready to detonate your device. In effect, this is exactly what was done in theuranium-based Hiroshima bomb. Inside a long cylindrical bomb casing was mountedthe barrel of a naval artillery gun (figure 1.3). One piece of uranium, the ‘target’ piece,was mounted at the nose end of the barrel, while a second piece, the ‘projectile’,was loaded into the tail end. When sensors indicated that the bomb had fallento a pre-programmed detonation height, a conventional powder charge was ignited

Figure 1.3. Schematic illustration of a gun-type fission weapon. The uranium projectile is fired toward amating target piece in the nose. Reproduced from Reed B C 2014 The History and Science of the ManhattanProject (Berlin: Springer), figure 1.4.

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to propel the projectile piece into the mating target piece. There are ancillaryconsiderations such as providing for a source of neutrons to initiate the chainreaction, but this is the basic idea of how a so-called fission ‘gun bomb’ operates.

The Hiroshima bomb contained about 60 kg of U-235, but overall weighed nearlyfive tons. Much of this was the weight of the cannon, but there was anothersignificant contributor: the target end of the cannon was surrounded by a steeltamper several hundred kilograms in mass. The tamper served several functions.First, it stopped the projectile piece from bashing its way through the front end ofthe bomb; unlike in an artillery cannon, the projectile piece has to remain seatedaround the target piece! Second, by briefly retarding the expansion of the bomb coreas it detonates, one buys a bit more time (microseconds) over which the chainreaction can operate. Third, if the tamper is made of a material which reflectsescaping neutrons back into the assembled pieces, they have another chance to causefissions. The latter two effects both enhance weapon efficiency; an efficiency increaseof a factor of ten over an untamped device is quite possible, so it is certainly worthgoing to the effort of providing a tamper.

The plutonium bomb proved to be a very different matter. Reactor-producedplutonium proved to exhibit a high level of spontaneous fission—a natural, completelyuncontrollable process. Because of this, it was predicted that if one tried to make agun-type bomb using plutonium, the nuclear explosion would start itself sponta-neously before the target and projectile pieces were fully mated. The result would bean expensive but very low-efficiency explosion, a so-called ‘fizzle’. Two approaches toavoiding this problem were evident: either find a way to use less fissile material(a lower spontaneous-fission rate) and/or assemble the sub-critical pieces more rapidlythan could be achieved with the gun mechanism. Both approaches were utilized. Thecritical mass of a fissile material depends on its density; greater density means a lowercritical mass. Hence, if you have a mass of material that would be subcritical at normaldensity, it can be made critical by crushing it to a higher density; the result is that youcan get away with using less material than would ‘normally’ be required. This led tothe idea of an implosion weapon wherein a small subcritical core with a naturally lowrate of spontaneous fission is surrounded with a fast-burning explosive configured todetonate inwards to crush the core to high density in a very short time. The difficultpart is that the implosion has to be essentially perfectly symmetric, with all of thepieces of surrounding explosive detonating within about a microsecond of each other.Questions as to the feasibility of implosion were so serious that it was decided to usesome plutonium in a full-scale test of the method; this was the Trinity test of July 16,1945, the world’s first nuclear explosion (figure 1.4). The test was a complete successand just three weeks later the method was put to use in the Nagasaki bomb.

With the above descriptions, you now have an idea why the Hiroshima U-235Little Boy uranium bomb was a long, cylindrically shaped mechanism, while theNagasaki Pu-239 Fat Man plutonium bomb was a bulbous, nearly sphericalarrangement (figure 1.5).

At its peak, the Los Alamos Laboratory employed only about 2500 people, butwithout their efforts the work of hundreds of thousands of others in Tennessee andWashington would have been for nothing.

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The uranium-enrichment complex located in Tennessee was designated by thename Clinton Engineer Works (CEW) after the small town near Knoxville where itwas located. Spread over a 90 square mile military reservation were three separateenrichment facilities, plus a pilot-scale nuclear reactor, supporting shops, and a cityto house workers and their families. Overall, the CEW consumed nearly $1.2 billionin construction and operating costs. The three uranium-enrichment facilities werecode named Y-12, K-25 and S-50. Each of these is described briefly in the followingparagraphs.

Y-12. This facility enriched uranium by the process of electromagnetic massspectroscopy. Mass spectroscopy is an experimental technique used to make veryprecise measurements of the identifications and amounts of various chemicalelements contained within a sample of material to be analyzed. The fundamentalprinciple is that when an ionized atom or molecule is directed into a region of space

Figure 1.5. Left: Little Boy in its loading pit. Source: http://commons.wikimedia.org/wiki/File:Atombombe_Little_Boy_2.jpg. Right: The Fat Man bomb. Note the signatures on the tail. Source: http://commons.wikimedia.org/wiki/File:Fat_Man_on_Trailer.jpg.

Figure 1.4. Left: The Trinity fireball 25 ms after detonation. Source: http://commons.wikimedia.org/wiki/File:Trinity_Test_Fireball_25ms.jpg. Right: The fireball a few seconds later. Courtesy of the Los Alamos NationalLaboratory Archives.

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where a magnetic field is present, it will follow a trajectory whose path depends,among other things, on the mass of the atom or molecule. (An atom is said to beionized when one or more of its orbital electrons have been removed, leaving theatom with a net positive charge; electrical charges experience a force in the presenceof a magnetic field.) To put this into practice, a uranium compound was heated untilit became vaporized. The vapor was then ionized and directed as a narrow streaminto a vacuum tank sandwiched between the coils of a huge electromagnet. Atoms ofthe U-235 and U-238 isotopes then follow slightly different trajectories and can becollected separately (see figure 2.11). To obtain a sensible rate of production ofbomb-grade uranium (which is defined as 90% U-235), over 900 magnet coils andnearly 1200 vacuum tanks were put into operation; some 5000 operating andmaintenance personnel kept Y-12 running. This process can be inefficient, but everyatom of U-235 in the Hiroshima Little Boy bomb eventually passed at least oncethrough the Y-12 complex.

K-25. At over $500 million in construction and operating costs, this was the singlemost expensive building of the entire Manhattan Project: a U-shaped factory fourstories high (one underground), half a mile long and about 1000 ft wide (figure 3.13).This enormous structure housed the gaseous diffusion plant of the Project; thisprocess was also known as barrier diffusion. The premise of this technique is that if agas of atoms of mixed isotopic composition is pumped against a thin, porous metalbarrier containing millions of microscopic holes, atoms of lower mass will passthrough the barrier slightly more frequently than those of higher mass. The result is avery minute level of enrichment of the gas in the lighter isotope on the ‘back’ side ofthe barrier. However, only a very small level of enrichment can be achieved bypassing the gas through the barrier on any one occasion; to obtain bomb-gradematerial the process has to be repeated sequentially thousands of times. K-25incorporated nearly 3000 enrichment stages and, at the time, was the largestconstruction project in the history of the world. Some 12 000 people were requiredto operate the plant.

S-50. The S-50 plant enriched uranium by a second diffusion-based processknown as liquid thermal diffusion, which is often simply termed thermal diffusion todifferentiate it from the gaseous process employed in the K-25 plant. While liquiddiffusion is rather inefficient, it is conceptually simple. If a liquid or gas comprisingtwo different isotopes of an element is fed into a narrow space between a heated walland a cooled wall, material containing atoms of the lighter isotope will preferentiallymove toward the hotter surface while heavier-isotope material collects toward thecooler one. The hotter material then rises by convection while the cooler descends,leading to an accumulation of material very slightly enriched in the lighter isotope atthe top of the narrow space. The lighter-isotope-enriched material can then beharvested and sent on to another such structure for further enrichment. In the S-50plant, this process was realized by pumping a uranium compound into a narrowannulus between nested vertical pipes; the plant utilized 2142 such arrangements,each 48 ft high (figure 3.14).

These enrichment methods were initially thought of as individual horses com-peting in a race to see which one could start with natural uranium and most

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efficiently produce bomb-grade material in one process. But as they were put intooperation it became clear that they worked better as a team; some proved moreefficient at various stages of enrichment than at others. Ultimately, uranium beganits journey by being processed through the S-50 plant to be enriched to 0.86% U-235,after which it went on to the K-25 plant (to 7%) and thence through one or twostages of the Y-12 complex to obtain 90% U-235.

Under Project auspices, the world’s first nuclear reactor, an experimental graph-ite-moderated pile, achieved a self-sustaining chain reaction in early December 1942.This pile, code-named CP-1, operated at an estimated power output of one-half of awatt—less than a flashlight bulb. CP-1 was strictly experimental; its purpose was todemonstrate that a chain reaction could be created and controlled. The rate offormation of plutonium in a reactor depends on the reactor’s power output and thepower of CP-1 was a far cry from the millions of watts (megawatts, MW) estimatedto be required to breed plutonium rapidly enough to produce a bomb in a sensiblelength of time. Following the successful operation of a 1 MW ‘pilot-scale’ reactor atthe CEW site, the Manhattan Project’s production-scale reactors were designed tooperate at 250 MW. Three such piles were built at a remote site in south-centralWashington State, where they could be cooled with water drawn from the ColumbiaRiver. The Hanford Engineer Works (or simply ‘Hanford’) occupied an area of over600 square miles; the reactors were placed 6 miles apart to provide safety zones incase of a disaster at any one of them. The first pile achieved criticality in September1944, but unanticipated problems caused a three-month shut-down while modifica-tions were effected. Ultimately, the Hanford reactors produced the kilograms ofplutonium necessary for the Trinity test and the Nagasaki bomb.

This brings our whirlwind tour of the Manhattan Project to a close. Now it is timeto go back to the point from which we began—the underlying physics—and begin amore detailed tour with many more stops.

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