d e s i g n i n g t h e b o m b

The world’s first man-made atomic explosiontook place only 28 months after the arrival of thefirst scientific contingent at Los Alamos. Few greatertributes to human ingenuity have ever been written.

The theoretical basis for nuclear weapons wasalready understood, in its outlines, when the Lab-oratory was established. Many of the engineeringproblems were foreseen in a general way, but muchremained to be done. The following summary ofweapon theory (all of it known in early 1943) willserve to suggest the enormous difficulty of the taskthat lay ahead.

The nucleus of an atom of uranium-235 contains92 protons and 143 neutrons. When this nucleusabsorbs an additional neutron, it becomes unstableand usually divides approximately in half. The twofragments become nuclei of two lighter elements,having a total mass somewhat less than the mass ofthe original uranium nucleus plus the additionalneutron. Most of the mass difference between theoriginal material and the products is converted intokinetic energy–rapid flight of the fragments.

The product nuclei emit several neutrons (in1943 nobody knew exactly how many, on the aver-age) for every U-235 nucleus split. Other U-235nuclei may absorb these neutrons and undergofission in turn, producing still more neutrons. Sucha chain reaction will proceed very rapidly, as longas one neutron (at least) from each fission causesanother fission.

Theoretically, the energy release from one kilo-gram (about 2.2 pounds) of U-235 would equal theenergy release from the detonation of 17,000 tons ofT N T .

Given these facts, the problem was to devise abomb deriving its explosive energy from the fissionof uranium-235 (or of plutonium-239, the onlyother fissionable material under consideration in1943).

Neither U-235 nor Pu-239 was available in suffi-cient quantity to make a bomb. It was thought thatthese materials would become available within twoyears. The task of the Los Alamos staff was todesign the bomb, and to devise methods of manu-facturing it, in advance of the scheduled deliveriesof the needed material.

A separate and remarkable story lies behind thefact that the scheduled deliveries were made ontime. The story being told here will limit itselfprimarily to what happened at Los Alamos.

The Los Alamos task was to discover means ofmaking the desired explosive liberation of energy

take place efficiently and at the right time. A verit-able mountain of difficulties stood in the way.

No one knew how much fissionable material hadto be put together to support an explosive chainreaction, but it was known that the reaction couldnot occur if the amount were insuff ic ient . Theburning of conventional explosives is a chain reac-tion of a different kind; a tiny quantity of TNTburns as readily as a larger amount. Fission chainscannot occur in the saline way, because the neutronson which they depend must remain within the fueluntil they encounter fissionable nuclei. If the sur-face area of the fuel mass is large compared to thevolume (i.e., if the fuel mass is too small or toomuch flattened out), then the neutron escape areais too large compared to the neutron source volume,and too many neutrons will find their way out with-out causing fission.

By now it is possible to determine mathematicallyhow much fuel is enough, but only when the shape,density, and purity of the fuel material are known.In 1943 additional unknowns stood in the way. Theexact average number of neutrons emitted in fis-sion had not yet been measured. Neither had thepertinent “cross sections.”

The term “cross section” is an extremely usefulone in the study of nuclear reactions. It is a measureof the 1ikelihood that a certain reaction will occur,stated in terms of effective target area. Perhaps thefollowing analogy will help to make this clear:

A popular carnival game is one in which the cus-tomer throws baseballs at ranks of woolly dolls. Ifwe suppose that the solid portion of each doll pre-sents a front surface of one square foot, and if wcneglect the diameter of the baseball, then the effec-tive area, (“cross section”) of each doll, for the reac-tion we might call “direct hit,” is one square foot.But if we investigate other possible reactions, wefind the effective area of the same doll changing. As-sume, for instance, that only one direct hit out oftwo causes the doll to fall down. For the “knock-down” reaction, then, the cross section of each dollis .5 square foot. If we throw ping pong balls, the“direct hit” cross section may remain one squarefoot, while the “knockdown” cross section becomeszero. Or, if the fringe on the doll is unusually stiff,so that a baseball passing through the fringe some-times causes the doll to fall, then the doll’s crosssection for baseball knockdown may rise toward twosquare feet. And the knockdown cross section willbe different for baseballs of different speeds. Cross

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In Los Alamos by day great minds pondered the bombproblems, by night, talk turned to less secret discus-sions. From left to right across both pages: Enrico Fermi

section is the effective target area for a specific reac-tion or event.

If we consider a free neutron, traveling through asphere of pure U-235 metal, we need to know howlikely it is to cause fission, instead of escaping fromthe sphere. We need to know, in other words, nothow large the U-235 nucleus actually is, but howlarge a target it presents statistically, for the fissionreaction, to a neutron of given velocity. Fissioncross sections of U-235 for neutrons of a wide rangeof velocities (energies) needed to be known beforea bomb could be designed. Also, since the uraniumwould not actually be pure U-235, it was necessaryto know the cross sections of various impurities, es-pecially U-238, for neutron absorption withoutfission.

Only on the basis of careful experiments andmeasurements could the needed cross sections belearned. Particle accelerators (atom smashers) wereused in these experiments, because they could pro-duce, indirectly, beams of neutrons with which tobombard samples of bomb material.

The mass of a sphere of fissionable material justsufficient to sustain a chain reaction is called the“critical mass.” By surrounding this material withan envelope of other material, to bounce escapingneutrons back into the active volume, it was possibleto improve neutron economy and thus reduce thecritical mass. Such a reflecting- envelope is some-times called a tamper.

The tamper in a weapon serves a second purpose.As the fissionable mass expands during the explo-sion, it quickly becomes less dense, at the sametime, its surface area increases. These two effectsact together to “quench” the chain reaction, sincethey facilitate neutron escape and reduce the likeli-

with L. D. P. King; Eric Jette, Charles Critchfield, J.Robert Oppenheimer; Edward Teller with Norris Brad-bury; E. O. Lawrence, Fermi and 1. 1. Rabi.

hood that a given neutron will encounter a fission-able nucleus. A massive tamper slows the expan-sion and allows more energy to be liberated beforethe reaction is quenched.

Tamper materials, especially their cross sectionsfor capturing neutrons and for scattering neutronsback into the fissionable material, needed intensiveStudy.

It happens that the fission cross section of U-235is greater for slow neutrons than for fast (presum-ably because the slow, neutron spends more timenear the nucleus.) In spite of this fact, the reactionin a bomb must depend almost solely on fast neu-trons. This is partly because the neutrons producedin fission arc naturally fast, partly because an air-delivered bomb must be as light as possible (there-fore not permitting the inclusion of moderatingmaterial to slow the neutrons), and partly because aslow-neutron reaction system would not have timeto liberate a large enough fraction of the potentialenergy before the bomb blew itself apart.

Therefore, is was necessary to establish facts re-lating to the efficiency 01 a tamped atomic explo-sion produced by fast neutrons. And these facts hadto be established in advance of the delivery of fis-sionable material for the first bomb-in advance,of course, of any atomic explosion at all.

For the sake of explosion efficiency, it was inad-visable to depend on “background” neutrons (freeneutrons unavoidably present in the bomb at alltimes) to start the reaction. The only way to be surethe reaction would start fast, and a t exactly theright moment, was to arrange an internal neutronsource that would deliver millions of neutrons in asingle burst at the instant of complete assembly.Devices called “initiators” had to be developed to

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supply these neutrons.As if such difficulties were not enough, a whole

new supply of problems was introduced by the needto make the bomb go off at the right time and onlyat the right time.

A stick of dynamite is capable of exploding.When its cap or igniter sets it off, it explodes. Acritical mass of fissionable material is not onlycapable of sustaining a chain reaction; it is in-capable of not doing so. No percussion cap is neces-sary. Nobody lights a fuse. Once the critical condi-tions exist, the reaction begins. (This is because theone free neutron needed to trigger the reaction willalways be supplied within a fraction of a second byneutrons from cosmic rays, spontaneous fission, orother sources.) Therefore, the detonation of a nu-clear bomb occurs whenever its core is actually andfully assembled for the first time. The final assem-bly must occur only at the target. To say that thisintroduces a problem is putting it mildly.

Furthermore, the final assembly must be accom-plished rapidly. As the core passes from its sub-critical, or safe, configuration to its supercritical, orexplosive, configuration, it must inevitably passthrough configurations that are barely critical. Fastassembly is necessary because there must be no timefor the reaction to occur and destroy the bomb be-fore the optimum configuration is reached.

Since no assembly method would be fast enoughunless i t made use of high explosives, an intensivestudy of the potentialities of chemical explosivesfor this purpose had to be made.

In principle, two general methods of assemblyappeared possible. One was the so-called “gun”method, in which one subcritical mass of fissionablematerial would be fired as a projectile at a targetconsisting of another subcritical mass of fissionablematerial. When projectile met target, the two to-gether would constitute a supercritical mass. (Thegun, with its explosive charge and its fissionable

projectile, would have to be enclosed in the bombcasing, along with the target.) The other assemblymethod was “implosion,” in which a slightly sub-critical mass of fissionable material would be sur-rounded by high explosives. When these explosiveswere detonated, they would compress the fissionablematerial, thereby increasing its density (decreasingthe distances between target nuclei), thus renderingit supercritical.

The gun method appeared to be the easier todevelop. It involved principles already well under-stood by ordnance experts, while the implosionmethod introduced entirely new principles of guid-ing explosive energy It was hoped that the gunmethod might work for- both uranium and plu-tonium bombs. It was a somewhat slower detona-tion system than implosion was, but its develop-ment would require fewer technological innovations.

In 1944 came the verification of a piece of badnews rumored a little earlier: The gun methodwas unsuitable for plutonium bombs. The reasonwas that plutonium produced in nuclear reactors(such as those at Oak Ridge and Hanford) con-tained a significant percentage of an isotopeidentified as Pu-240. Plutonium of this mass num-ber had a strong tendency toward spontaneous fis-sion, releasing neutrons. This produced an un-usually high neutron background in plutoniumcontaining the 240 isotope. Therefore, assembly of aplutonium bomb would have to be lightning-fast toprevent premature initiation of the chain reaction.Assembly by the gun method would be too slow; ina plutonium bomb, it would have to be implosionor nothing.

The simplest way to proceed might have beento build a few experimental bombs in the earlynineteen-forties and try them out. Not the least ofthe Laboratory’s problems arose from the impos-sibility of doing this. By the time the preciousshipments of fissionable material arrived at Los

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Alamos, a workable bomb design had to be ready.Various components and sub-assemblies could betested by themselves, but no integral test of theweapon would be possible until long after the timewhen such testing might have served its purposebest.

As soon as the Laboratory had its first skeletonstaff and a minimum of equipment (a cyclotronloaned by Harvard University, two electrostaticaccelerators from the University of Wisconsin, aCockcrow t-Walton accelerator from the Universityof Illinois, and much other borrowed equipment)the work began. In many ways it was a continuationof research already begun in a dozen laboratories,all over the country. But it had a focus for the firsttime. Los Alamos, and no other laboratory, wouldmake the first bomb.

Research got under way on several fronts duringthe first half of 1943. Measurements of the “neu-tron number” (average number of neutrons emittedper fission) of plutonium-239 and uranium-235 wereundertaken immediately, though the plutoniummeasurements had to be made on a sample scarcelyvisible to the naked eye.

Other research projects begun in the first monthswere these:

***Measurements of the fission spectrum (energyrange) of neutrons from U-235.

***Measurements of fission cross sections Of U-235 and Pu-239 for neutrons of high, low, and allintermediate energies.

***Measurements of the time (a fraction Of a mil-lionth of a second) between fission and the emissionof virtually all the fission neutrons.

***Measurements of cross sections of neutroncapture and neutron scattering in various possibletamper materials.

***Development of experimental techniques (in-cluding ways of producing. and counting neutronsof specific energies, measuring fission in variousmaterials, and measuring certain non-fission reac-tions induced by neutrons.)

***Radiochemical studies aimed toward the de-velopment of an initiator (the neutron source men-tioned earlier) for the bomb.

***Research on uranium hydride, an early possi-bility for bomb fuel, later abandoned.

***Research on the chemistry and metallurgy ofuranium and plutonium, and of possible tampermaterials (including development of purificationprocesses and analytical methods for measuringsmall amounts of impurities).

***Research on projectile and target materialsfor the gun program.

***Planning for construction of a deuteriumliquefaction plant at Los Alamos, to supply liquiddeuterium for experiments useful in the develop-ment of a thermonuclear bomb.

The Water Boiler reactor under con-struction. This reactor was constructedfor use as a research tool during thedevelopment of the bomb. The world’sfirst enriched uranium reactor, theWater Boiler is still in operation.

***An intensive ordnance program, studyingthe uses of high explosives for bomb assembly.

***A tremendous theoretical effort devoted tocalculations of all kinds related to the physics andthermodynamics of the bomb.

At this early stage in the work of the Laboratory,it was believed that the development of the bombwould have two more-or-less distinct phases: (a) re-search in physics, chemistry, and metallurgy, then

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Wartime photo showing one of thepractical, if primitive, ways in whichradioactive materials w e r e s a f e l yhandled during the early years of thebomb development.

(b) technology in engineering ordnance design. Theoriginal directive set forth a plan for putting theLaboratory on a military basis in the second phase,commissioning the scientists in military rank, andso on. As it turned out, the commissioning plan wasnever put into effect.

In the fall of 1943, as a result of a conference be-tween President Roosevelt and Prime MinisterChurchill, it was decided to assign about two dozenBritish citizens to work at Los Alamos among themwere some of the world’s most distinguished scien-tists.

Laboratory personnel were all civilians until thefall of 1943, when a detachment of WACs and sev-eral technicians and scientists drafted into theArmy’s Special Engineer Detachment (SED) joinedthe staff. By July, 1945, 50% of Laboratory person-nel were military, mostly men of the SED. TotalLaboratory personnal increased steadily from 250in July, 1943, to 2,500 in JuIy, 1945.

Thc most important single characteristic of LosAlamos Scientific Laboratory became apparent inthe very first days. It has remained LASL’S most im-portant single characteristic: T h e L a b o r a t o r y i spredominantly a scientific, not an engineering, in-stitution. The fact that its first mission was thecreation of a practical piece of hardware wouldseem to contradict such a statement, but the con-tradiction is only an apparent one. The nature ofthis specific piece of hardware was such that itscreation called for a massive program of scientificresearch.

That it also called for a massive engineering pro-gram is equally true, but not central to the charac-ter of the institution as it was in 1943 or as it hasbeen ever since. Every development program under-taken by the Laboratory has been of such an ad-vanced kind that the technological effort was small-er, in terms of man hours, than the scientific one. Itis impossible, of course, to separate the two kindsof effort in a clear-cut way, but anyone who hasworked in the intellectual climate 01 Los Alamosknows that “Los Alamos Engineering Laboratory”or even “Los Alamos Research and DevelopmentLaboratory” would have been a misnomer-. “Sci-entific” is right, and the word was inserted in theearlier name, “Los Alamos Laboratory” in 1947.

The ice house, a stone building longused, before the war, for storage ofice from Ashley Pond. It later assumedhistoric significance as the building inwhich nuclear components of the firstatomic bomb were assembled,

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. .

Atomic bomb research was conducted in these hurriedlyconstructed laboratory buildings which made up the

technical area. Gamma building, below and at rightabove, wrapped around and obscured red Ashley Pond.

p r e p a r i n g t h e m a t e r i a l s

Preparation of the two fissionable core materials,meanwhile was being accomplished elsewhereagainst almost overwhelming odds.

In the case of uranium, the difficulty arose fromthe fact that uranium 238 and uranium 235 arealmost identical substances. Each of the two kindsof atoms has 92 nuclear protons and 92 orbitalelectrons. Since the chemical behavior of anyatom is almost entirely governed by its orbitalelectrons, the two kinds of uranium could not beseparated by chemical processes. Some other pro-cess—a purely physical one—was required.

U-235 has 143 neutrons in each nucleus. U-238has 146. Somehow, those three extra neutrons hadto be used to make the U-238 atoms go one wayand the U-235 atoms go another. A great manymethods were suggested. Half a dozen or so, includ-ing a centrifuge process like the one used to separatecream from milk, were given extensive trials, Almostevery idea worked, but no idea worked very well.The difference between the two isotopes was toosmall.

Furthermore, all of the separation methods triedwere expensive. If the isotope separation programhad been an industrial enterprise, aimed at mak-ing a profit, the only sensible course would havebeen to close up shop.

But there was a war on. Nobody knew how mucheffort, if any, Germany might be devoting to nuc-lear weapon development (actually it was verylittle), but one thing was almost certain: If theGermans were first to develop a nuclear weapon,Hitler would win the war. This was no time topinch pennies.

America’s decision, based partly on extremelygood work by scientists in Britain, was to con-tinue at any cost. Investigation of many ways ofseparating isotopes would go on, and a really vasteffort would be made on the two most promisingprocesses. One of these was gaseous diffusion sep-aration and the other was electromagnetic sep-aration.

Both processes are based on the difference inweight (more properly, in mass) between the twokinds of uranium nuclei,

The molecules in a gas are in constant motion.The warmer the gas, the faster its molecules move;but some move faster than others. On the average,heavier molecules are more sluggish than lightones. They move more slowly. Therefore, when agas diffuses through a porous barrier, the lightermolecules get through a little more often (at first)

than the heavier ones.Perhaps unfortunately, uranium is not a gas. For

the gaseous diffusion process, the uranium has to becombined with fluorine to produce an easily-vapor-ized compound called uranium hexafluoride.Uranium hexafluoride gas is extremely corrosive,tending to attack pumps, piping, barriers, andalmost anything else it happens to touch.

But the gaseous diffusion method works. Passagethrough each barrier in a multi-stage separationplant increases (very slightly) the concentration ofU-235 in some of the gas. By using thousands ofstages, thousands of miles of piping, and hundredsof acres of barriers, it is possible to produce veryhighly enriched uranium hexafluoride. Uraniummetal made from the enriched gas has a very lowconcentration of U-238.

A large part of the wartime project consisted ofplanning and building a separation plant to employthe principle just described. The plant was builtin Oak Ridge, Tennessee, in the years from 1943 to1945.

The electromagnetic separation process is quitedifferent, but it also exploits the very slight massdifferences created by the presence of those threeextra neutrons in each U-238 nucleus.

Everything possessing mass has inertia. The moremass, the more inertia. It is perhaps usual to thinkof inertia as a reluctance to move, but inertia is abroader phenomenon than that. It is less a resist-ance to movement than a kind of resistance tochange. If an object is stationary, inertia makes itreluctant to move; if it is moving, inertia makes itreluctant to stop or to change direction.

Since inertia is proportional to mass, the U-238nucleus has a little more inertia than the U-235nucleus. If both are traveling at the same speed,the heavier nucleus will have a slightly strongerresistance to any change in direction. Therefore, agiven force tending to change the direction ofmotion will have a slightly greater effect on thelighter nuclei than on the heavier.

This principle is exploited in the electromagneticseparation of isotopes in the following way: First,the uranium atoms are “ionized,” usually by beingdeprived of one orbital electron each. This leavesthe atoms positively charged, so that they can beaccelerated electrically and acted on magnetically.When they have been accelerated–many millions ofthem at a time—they are formed into a beam, alltraveling in the same direction. The beam of ura-

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nium ions is then passed through a magnetic fieldwhich has been arranged in such a way as to bendtheir trajectories. Under the influence of the mag-netic field, the U-235 ions change direction morethan the ‘U-238 ions. The beam becomes two beams,each of which can be caught in a separate receptacle.

Though the development of the electromagneticseparation process encountered many difficulties,the method ultimately succeeded in producing im-portant quantities of U-235. Electromagnetic sep-arating machines called “calutrons,” developed bythe University of California, were installed at OakRidge. They were used mainly to increase the en-richment of already-enriched products of the im-mense gaseous diffusion plant and of a smallerthermal diffusion plant, which used uranium hexa-fluoride in liquid form.

By late 1944, highly enriched uranium com-pounds were being produced at Oak Ridge in kilo-gram quantities.

Meanwhile, the program to produce the pre-viously unknown element of atomic number 94 hadmade great strides.

Berkeley scientists produced minute quantitiesof plutonium in the winter of 1940-41, by bombard-ment of uranium with particles from an accelerator.The new element proved to be readily fissionable, ashad been predicted.

However, production in quantities of militarysignificance could not be carried out with particle

accelerators. What was needed was a rea lly plentifulsource of’ free neutrons. The only sufficient sourcewould be a nuclear fission reactor.

Fission reactors are devices in which a chain re-action is maintained under controlled conditions.No such device had been built when the Berkeleyscientists produced their first plutonium. It wouldtake two more years to achieve the first man-madefission chain reaction.

The leader in that achievement was the sameEnrico Fermi who had first split the uraniumnucleus. He had come to the United States and wasworking at the University of Chicago.

Fermi and his associates sought to demonstratethe possibility of a fission chain reaction in naturaluranium—uranium containing less than one percent U-235. Though a natural-uranium reactionwould release energy at a rate unsuitable for anefficient nuclear explosion, the demonstration thatsuch a reaction could be maintained would havegreat significance. Among other things, it couldlead to the construction of reactors capable of pro-ducing large quantities of plutonium.

In Fermi’s experiment, lumps of natural uraniummetal and of natural uranium oxide were placedin a “lattice” (a system of regular spacing) withina pile of graphite blocks.

The graphite was necessary to enable the pile tosustain a chain reaction. Here is why:

When a uranium nucleus undergoes fission, neu-

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trons come out at high velocity. In a natural ura-nium system, these high-speed neutrons collide withuranium nuclei of both kinds. Some of the collisionscause fission, but many others do not. Most of theneutrons become involved in a series of “elastic”(glancing or bouncing) collisions with nuclei. Suchcollisions do not cause fission, and each such col-lision robs a neutron of some of its speed.

It happens that the velocity of a neutron has alarge effect on what the neutron can do to a ura-nium nucleus. As the velocity goes down, the neu-tron loses its ability to cause fission in U-238, whileacquiring even greater ability to cause fission inU-235. At what is called “thermal” velocity (whenthe neutron has lost all of the initial impulse itreceived from the fissioning nucleus) its ability tocause fission in U-235 is very high.

Unfortunately, there is a certain intermediatevelocity at which a neutron is most likely to becaptured by a U-238 nucleus, without causing fis-sion. In a chain-reacting pile using natural ura-nium, it is therefore desirable to prevent collisionsbetween medium-speed neutrons and U-238 nuclei.Otherwise, so many neutrons will be captured thatthe chain reaction will die out. (It is exactly suchcaptures that result in the formation of plutonium,but Fermi was not yet trying for that; his pile wouldrequire a maxium number of free neutrons, justto keep the chain reaction alive.)

By using lumps of uranium separated by blocksof graphite, it is possible to avoid many of theneutron captures that would occur in a structure ofpure uranium. Neutrons produced by fissions in one

lump of fuel fly out of that 1ump and into thegraphite before they have lost enough speed to becaptured readily by U-238 nuclei. In the graphite,they lose much of their velocity, because of elasticcollisions with carbon nuclei. By the time the neu-trons reach the next lump of fuel, they are “ther-mal” (slow), and are not so likely to be captured bythe U-238.

Fermi’s pile produced its first sustained chain re-action in December, 1942, exactly one week afterthe Under Secretary of War had directed that asite at Los Alamos, New Mexico, be acquired fora nuclear weapon laboratory. Fermi’s success de-monstrated the possibility of the sustained chainreaction and gave great encouragement to thosewho planned to use larger piles as neutron sourcesfor the production of plutonium.

Construction of one such pile began in Tennesseein 1943. By November of the same year, it was inoperation. Within a few months after that, it hadpi-educed several grams of plutonium.

However, much larger plutonium productionreactors would be necessary for the production ofenough plutonium to be used in bomb cores. InJune, 1943, construction of such reactors began atHanford, Washington, where water from theColumbia River could be used as a reactor coolant.By September, 1944, the first Hanford pile was inoperation. Plutonium nitrate from Hanford wouldsoon join the flow of fissionable material that wasalready moving from the uranium and plutoniumproduction facilities in Tennessee toward the LosAlamos Laboratory.

A sketch of Fermi’s chain-reacting pile at the University of Chicago.

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By September, 1944, the first kilogram of highlyenriched uranium (63% U-235) had been receivedfrom the separation plant at Oak Ridge. By July,1945, 50 kilograms had been received, and the en-richment had increased to 89%.

The first small quantities of plutonium (as nitrate,not as metal) arrived at the Laboratory in October,1943. Gram amounts were delivered early in 1944,and soon after that still larger amounts begancoming in, first from Oak Ridge and later fromHanford.

Both the uranium and the plutonium neededpurification (an unprecedented job) before becom-ing suitable for weapons use. Means of purifyingthese elements were developed at Los Alamos andhave been constantly improved.

The problem 01 preparing plutonium metal ofhigh purity was started in the Laboratory in August,1943 at a time when no plutonium was available forresearch. It gradually became available in amountsvarying from micrograms to grams, but in themeantime, extensive preliminary investigations ofpossible methods of preparation had been made us-ing other elements as stand-ins.

By the spring of 1944, the world’s first piece ofplutonium metal prepared in any scale larger thana few micrograms was produced by the graphitecentrifuge method which used centrifugal force tothrow down molten metal into the tip of a cone dur-ing reduction. This was accomplished by placingthe reaction mixture in a cone-shaped refractoryliner sealed inside a steel “bomb.” The bomb wasthen placed in a graphite centrifuge which washeated rapidly to a high temperature while rota-ting. As the reduction took place the metal wasthrown together in the tip of the liner, producing agood yield of coherent metal.

Meantime, however, research on the stationarybomb method indicated that gravitational forcealone was adequate to separate the metal fromplutonium fluoride, using calcium and iodine withappropriate heating conditions. The stationarybomb method, more suited to large scale produc-tion and much less complicated than the centrifugemethod, was then adopted and is still used for rou-tine production of pure plutonium metal. Thecentrifuge method, however , served its purpose at atime when it was most desperately needed.

A section of the electromagnetic process equipment, Units ofused for enriching uranium at Oak Ridge, Tennessee. produce

the electromagneticstable isotopes for

system are used today topeaceful purposes.

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