7.1 The First Atomic Bombs


This subsection describes the three atomic bombs, which were constructed and

detonated in 1945. Although diagrams would be extremely useful in describing

these devices, I have not included any ASCII graphics due to the

unsatisfactory resolution they offer. I may create Postscript files at some

future time.

7.1.1 The Design of Gadget, Fatman, and "Joe 1" (RDS-1)

The design of the Gadget and Fatman devices are discussed together since they

are basically the same. Gadget was an experimental test version of the

implosion system used in Fatman. A test of the implosion bomb was considered

essential due to the newness of the explosive wave shaping technology, and the

complexity of the system.

Although the data given below is based on the U.S. made Gadget/Fatman, it also

applies to the first Soviet atomic bomb, code named RDS-1 (Stalin's Rocket

Engine) by the Soviet Union and designated Joe-1 by U.S. intelligence. This is

because detailed descriptions of the design were given to Soviet intelligence

by spies who worked at Los Alamos; and Lavrenti Beria, who was the Communist

Party official heading the project, insisted that the first bomb copy the

proven American design as closely as possible. The principal spy was Klaus

Fuchs, who actually had a very important role in bomb development. Significant

information was also passed on by David Greenglass, and possibly also an

unidentified scientist code named Perseus. In fact some key information about

Gadget given below was made public as an indirect result of Soviet spying:

post-Soviet Russia has released records on espionage that reveal information

still classified in the U.S., and many FBI records relating to the Fuchs and

Rosenthal investigations have recently been released that contain design data

given to FBI interrogators by Fuchs and Greenglass.

The pit of these devices contained 6.1-6.2 kg of delta-phase plutonium. The

source of the 6.1-6.2 kg figure is a declassified memorandum written by Gen.

Groves to the Sec. of War two days after the Trinity test. He describes the

device and the results of the test and states that the explosion was created

by "13 and a half pounds of plutonium". The pit was essentially solid, except

for a small cavity (approximately 2 cm) in the center for the

beryllium/polonium-210 neutron initiator. A solid sphere with this mass in the

delta-phase would have a diameter of 9.0 cm. The solid design was a

conservative one suggested by Robert Christy to minimize asymmetry and

instability problems during implosion. The sphere had a 2.5 cm hole and

plutonium plug to allow initiator insertion after assembly of the sphere.

The plutonium was produced by the nuclear reactors at Hanford, Washington;

although it is possible that about 200 g of plutonium produced by the

experimental X-Reactor at Oak Ridge was also used. Due to the very short 100

day irradiation periods used during the war (wartime production meant that the

plutonium had to be separated as quickly as feasible after being bred), this

was super-grade weapon plutonium containing only about 0.9% Pu-240.

The plutonium was stabilized in the low density delta phase (density 16.9) by

alloying it with 3% gallium (by molar content, 0.8% by weight), but was

otherwise of high purity. The advantages of using delta phase plutonium over

using the high density alpha phase (density 19.2), which is stable in pure

plutonium below 115 degrees C, are that the delta phase is malleable while the

alpha phase is brittle, and that delta phase stabilization prevents the

dramatic shrinkage during cooling that distorts cast or hot-worked pure

plutonium. In addition stablization eliminates any possibility of phase

transition expansion due to inadvertent overheating of the pit after

manufacture, which would distort and ruin it for weapon's use.

It would seem that the lower density delta phase has offsetting disadvantages

in a bomb, where high density translates into improved efficiency and reduced

material requirements, but this turns out not to be so. Delta stabilized

plutonium undergoes a phase transition to the alpha state at relatively low

pressures (tens of kilobars, i.e. tens of thousands of atmospheres). The

megabar pressures generated by the implosive shock wave cause the transition

to occur, in addition to the normal effects of shock compression. Thus a

greater density increase and larger reactivity insertion occurs with delta

phase plutonium than would have been the case with the denser alpha phase.

The pit was formed in two hemispheres, probably by casting a blank followed by

hot pressing in a nickel carbonyl atmosphere. Since plutonium is a chemically

very reactive metal, as well as a significant health hazard, each half-sphere

was electroplated with nickel. This created a problem with the Gadget pit,

since hasty electroplating had left plating solution trapped under the nickel,

resulting in blistering that ruined the fit. Careful grinding and layering

with gold leaf restored the necessary smooth finish. However a thin gold

gasket (about 0.1 mm thick) between the hemispheres was a necessary feature of

the design in any case to prevent premature penetration of shock wave jets

between the hemispheres that could have prematurely activated the initiator.

The beryllium initiator used was called the "Urchin" or "screwball" design. It

consisted of a beryllium shell with parallel wedge-shaped grooves cut on the

inner surface. Like the pit, this shell was formed by hot pressing in a nickel

carbonyl atmosphere. The 50 curies or so of polonium-210 was inside the shell,

probably deposited on gold or platinum foil, and sealed between foil layers to

prevent evaporation. The beryllium was also plated with gold and nickel to

prevent it from contacting any stray alpha-emitting plutonium or polonium. The

Urchin was "levitated" inside the pit, that is, equipped with supports that

maintained a gap with the walls of the central cavity.

The Urchin was activated by the arrival of the implosion shockwave at the

center of the pit. When the shock wave reached the walls if the cavity, they

vaporized and the plutonium gas shock wave then struck the initiator,

collapsing the grooves and creating jets that rapidly mixed the polonium and

beryllium together. The alpha particles emitted by the Po-210 then generated

neutrons, perhaps one every 10 nanoseonds or so.

The pit was surrounded by a natural uranium tamper weighing about 260 kg, with

a diameter of about 30 cm. The tamper formed a layer about 10-11 cm around the

pit, which is about optimal for an efficient bomb since additional benefits

from inertial confinement and neutron conservation are not gained past this

point. At least 20% of the bomb yield was from fast fission of this tamper.

The pit and the tamper together made a marginally subcritical system. When

compressed by the implosion to approximately 2.5 times its original density,

the pit became an assembly of at least 6 critical masses. Before use, the bomb

was safed by use of a cadmium wire in the pit.

Surrounding the tamper was an 11 cm thick aluminum sphere weighing 160 kg. The

primarily purpose of this sphere was to reduce the effects of Taylor

instability which occurs when a low density fluid exerts force against one of

higher density. The Composition B explosive has a density of 1.65, which would

make a density ratio of 11.5 with the denser uranium (18.9). Aluminum has a

density of 2.7, making a ratio of only 7. The substantial thickness of this

layer also enhanced the shock wave focusing of implosion. A layer of aluminum

between the very reactive uranium metal and the high explosive may also have

been desirable for chemical stability reasons, although obviously a very thin

layer would have sufficed for this purpose.

Surrounding the tamper was a layer containing boron. Since boron itself is a

brittle non-metal that is difficult to fabricate, this was most likely in the

form of a malleable boron/aluminum alloy called boral (the composition is

typically 35-50% boron). It is possible that the entire aluminum sphere might

have been boral. The presence of boron was intended to prevent spontaneous

fission neutrons generated in the tamper from being scattered back into the

tamper/pit assembly by the explosive and aluminum layers as thermal neutrons.

The entire high explosive implosion system made a layer some 45 cm thick

weighing at least 2500 kg. This system consisted of 32 explosive lenses; 20 of

them hexagonal, and 12 pentagonal. The lenses fitted together in the same

pattern as a soccer ball, forming a complete spherical explosive assembly that

was 140 cm wide. Each lens had three pieces: two made of high velocity

explosive, and one of low velocity explosive. The outermost piece of high

velocity explosive had a conical cavity in its inner surface into which fitted

an appropriately shaped piece of slow explosive. These mated pieces formed the

actual lens that shaped a convex, expanding shock wave into a convex

converging one. An inner piece of high velocity explosive lay next to the

alumunium sphere to amplify the convergent shock. The lenses were made by

precision casting, so explosives that could be melted were used. The main high

explosive was Composition B, a mixture of 60% RDX - a very high velocity but

unmeltable explosive, 39% TNT - a good explosive that is easy to melt, and 1%

wax. The slower second explosive was Baratol, it is a mixture of TNT and

barium nitrate of variable composition (TNT is typically 25-33% of the

mixture) with 1% wax as a binder. The high density of barium nitrate gives

baratol a density of at least 2.5.

The lens system had to be made to very precise tolerances. The composition and

densities of the explosives had to be accurately controlled and extremely

uniform. The pieces had to fit together with an accuracy of less than 1 mm to

prevent irregularities in the shock wave. Accurate alignment of the lens

surfaces was even more important than a close fit. A great deal of tissue

paper and scotch tape was also used to make everything fit snuggly together.

Each of the components of the bomb, from the lenses to the pit itself, were

made as accurately as possible to insure accurate implosion, and the highest

densities possible.

To achieve the most precise detonation synchronization possible, conventional

detonators consisting of an electrically heated wire, and a sequence of

primary and secondary explosives were not used. Instead newly invented

exploding wire detonators were used. This detonator consists of a thin wire

that is explosively vaporized by a surge of current generated by a powerful

capacitor. The shock wave of the exploding wire initiates the secondary

explosive of the detonator (PETN). The discharge of the capacitor, and the

generation of initiating shock waves by the exploding wires can be

synchronized to +/- 10 nanoseconds. A disadvantage of this system is that

large batteries, a high voltage power supply, and a very powerful capacitor

bank (known as the X-Unit, the system weighed some 500 lb) was needed to

explode all 32 detonators simultaneously. A cascade of spark gap switches was

used to trigger the capacitor bank.

The whole explosive assembly was held together by a shell made of a strong

aluminum alloy called dural (or duraluminum). A number of other shell designs

had been tried and discarded. This shell design, designated model 1561, was

made of an equatorial band bolted together from 5 segments of machined dural

castings, with domed caps bolted to the top and bottom to make a complete


The final bomb design allowed "trap door" assembly. The entire bomb could be

assembled ahead of time, except for the pit/initiator. To complete the bomb,

one of the domed caps was removed, along with one of the explosive lenses. The

initiator was inserted between the plutonium hemispheres, and the assembled

pit was inserted in a 40 kg uranium cylinder that slid into the tamper to make

the complete core. The explosive lens was replaced, its detonator wires

attached, and the cap bolted back into place.

For transportation feasibility, as well as safety reasons, the implosion bombs

were not transported in assembled form but were put together shortly before

use. Due to the complexity of the weapon, this was a process that took at

least 2 days (including checkout procedures). Weapons of this design could

only be left in the assembled state for a few days due to deterioration of the

X-Unit batteries.

7.1.2 TRINITY - The Gadget Test

The test of the first atomic explosion in history was conducted at the Jornada

del Muerto trail (Journey of Death) at the Alamagordo Bombing Range in New

Mexico at 33 deg. 40' 31" North latitude, 106 deg. 28' 29" West longitude

(33.675 deg. N, 106.475 deg W). The device was called Gadget, the whole test

operation was code-named TRINITY.

Gadget was a 150 cm sphere consisting of the basic explosive assembly

described above with its dural shell, the firing electronics and equipment

were mounted externally on the test platform which was atop a 100 foot steel

tower, giving Gadget an elevation of 4624 ft above sea level.

The assembly of Gadget took five days and began on July 11, 1945. By July 13,

the assembly of Gadget's explosive lens, uranium reflector, and plutonium core

were completed at Ground Zero. On July 14, Gadget was hoisted to the top of

the 100 foot test tower, and the detonators were connected, after which final

test preparations began. On July 16, 1945, 5:29:45 a.m. (Mountain War Time)

Gadget was detonated. The explosive yield was 20-22 Kt (by latest estimates),

vaporizing the steel tower. Since the bomb was exploded above the ground it

produced only a very shallow crater (mainly created by compression of the

soil) - 2 meters deep with an 80 m radius. The crater was surrounded by fused

(melted) sand dubbed "trinitite" (or "atomsite"). The exact yield was

originally placed at 18.6 Kt on the basis of radiochemical tests. Since the

projected yield was only 5-10 Kt, many of the experiments were damaged or

destroyed by the test and failed to yield useful (or any) data.

Gadget was exploded close enough to the ground that considerable local fallout

was generated (along with significant induced radioactivity at ground zero

from the emitted neutrons). The most intense induced radiation was in an

irregular circle, about 10 m in radius around ground zero. The cloud rose to

11,000 m. The wind was blowing to the northeast, but significant fallout did

not descend for about 20 km downwind. Some evacuations were conducted the path

of the fallout plume out to 30 km. At Bingham, New Mexico gamma intensities of

1.5 R/hr were recorded between 2 and 4 hours after the test. South of Bingham

readings reached 15 R/hr, but declined to 3.8 R/hr 5 hours after the

detonation, and had decreased to less than 0.032 R/hr one month later.

Radiation (beta) burns were later observed on cattle in the general vicinity

of the test. The main fallout pattern extended about 160 km from ground zero,

and was about 50 km wide.

7.1.3 Little Boy

The design of Little Boy was completely different from Gadget/Fatman. It used

the gun assembly method that had originally been proposed for the plutonium

bomb. The development of the uranium gun weapon was somewhat erratic. Early

design and experimental work directed towards developing a gun system for

uranium assembly was conducted during the summer and fall of 1943, after Los

Alamos began operating. It was soon discontinued as attention shifted to the

technically more demanding plutonium gun. It was felt that once the plutonium

gun was successfully developed, the uranium gun would be almost an

afterthought since the necessary speed of assembly was much lower.

When the very high neutron emission rate of reactor-produced plutonium was

discovered in April-July 1944, the gun method was abandoned for plutonium and

serious attention returned to the uranium gun. The uranium gun program (the O-

1 group of the Ordnance Division) was lead by A. Francis Birch. He faced an

odd combination of considerations in directing the work. The system was

straightforward to develop, and sufficient U-235 to build the bomb obviously

wouldn't be available until mid 1945, if then. Birch was nonetheless under a

great deal of pressure to complete development as quickly as possible so that

all of the laboratory's assets could be directed to the risky implosion bomb.

Furthermore since the feasibility of the plutonium bomb was now in doubt, he

had to make absolutely sure that the uranium bomb would work. Thus although it

was a comparatively easy project technically, it still required extraordinary

attention to detail.

The design arrived at was a very conservative one, that was as certain to work

as any untested device can be. The design was complete by February 1945, only

preparations for field use were required after that. The actual bomb was ready

for combat use by early May, 1945 - except for the U-235 pit.

The pit contained 64 kg of highly enriched uranium (enrichment was 80-90% U-

235), or approximately 2.4 critical masses, all that was available at the

time. This is less than the 6 or more critical masses achieved by

Gadget/Fatman, and is the principal reason for Little Boy's lower efficiency.

It is interesting to compare this to the published data on the South African

gun-assembly bomb, which used 55 kg of enriched uranium (probably at a higher

degree of enrichment) and a superior tamper.

All of the uranium had gone through its final stages of enrichment in the

Calutron electromagnetic isotope separators at Oak Ridge, Tenn. Other isotope

enrichment systems, also at Oak Ridge, contributed as they became available.

Most of the uranium went through a three stage enrichment process: the thermal

diffusion enriched the feed uranium from the natural concentration (0.72%) to

the range of 1-1.5%; gaseous diffusion plant took this as feed and enriched it

to increasing concentrations as enrichment stages came on-line.

The U-235 mass of Little boy was divided into two pieces: the bullet and the

target. The "bullet": a solid cylinder of U-235 containing 42% of the mass (27

kg) and about 10 cm wide and 16 cm long. The "target": a hollow cylinder 16 cm

long and wide, weighing 37 kg, embedded in the tamper assembly. The target was

fabricated as two separate rings that were inserted in the bomb separately.

Note that even an unreflected sphere of U-235 weighing 64 kg would be


The tamper assembly for Little Boy consisted of a thick tungsten carbide

tamper/reflector, surrounded by a steel tamper forging about 60 cm wide. The

combined tungsten carbide/steel tamper weighed 2300 kg. U-238 is a superior

tamper and reflector, but tungsten carbide and steel were used instead due to

the spontaneous fission rate of U-238. U-238 undergoes spontaneous fission 100

times more frequently than U-235, and a piece large enough to be useful as a

tamper (200 kg) would generate 3400 neutrons a second - too many for gun

assembly to be feasible. Solid tungsten metal would have been better than the

carbide/steel tamper, but lack of industrial experience with fabricating large

pieces may have precluded it.

A hole was bored into the steel forging, and the carbide tamper was inserted.

The target was inserted in the form of several rings. The hole above the

target was threaded and the gun barrel was screwed in to attach it securely

(otherwise recoil from the bullet's acceleration would pull the target/tamper

and barrel apart). At the bottom of the hole one or more beryllium/polonium

initiator (different from the implosion initiators; simpler in design, with

less polonium) could be mounted.

The uranium/steel assembly was designed as a "blind target", one that would

stop and hold the bullet upon impact. Even if the neutron initiator failed to

work, the bomb would have exploded from spontaneous fission in a fraction of a

second. The decision to include initiators in the final weapon wasn't even

finalized by Oppenheimer until March 15, 1945. In the end, 4 initiators out of

a batch of 16 shipped to Tinian were used in Little Boy.

The gun was a 3" anti-aircraft barrel six feet long that had been bored out to

4" to accommodate the bullet. It weighed about 450 kg, and had a breech block

weighing 34 kg. Cordite, a conventional artillery smokeless powder, was used

as the propellant, and the velocity achieved by the bullet was 300 m/sec.

Little Boy was a terribly unsafe weapon design. Once the propellant was

loaded, anything that ignited it would cause a full yield explosion. For this

reason "Deke" Parsons, acting as weaponeer, decided (without authorization) to

place the cordite in the gun after take-off in case a crash and fire occurred.

It is possible that a violent crash (or accidental drop) could have driven the

bullet into the target even without the propellant causing anything from a

fizzle (a few tons yield) to a full yield explosion. Little Boy also presented

a hazard if it fell into water. Since it contained nearly three critical

masses with only air space separating them, water entering the weapon would

have acted as a moderator, possibly making the weapon critical. A high yield

explosion would not have occurred, but a rapid melt-down or explosive fizzle

and possible violent dispersal of radioactive material could have resulted.

The complete weapon was 10.5 feet long, was 28-29 inches in diameter and

weighed 8900 lb (also reported as 9700 lb). Little Boy used the same air burst

detonator system as Fatman (see below).

No other weapon of this design was ever tested. Several Little Boy units were

built, but no others entered the U.S. arsenal.

Casting of the U-235 projectile for Little Boy was completed at Los Alamos on

July 3, 1945. On July 14 Little Boy bomb units, accompanied by the U-235

projectile, were shipped out of San Francisco on the U.S.S. Indianapolis for

Tinian Island. On July 24 the last component for Little Boy, the U-235 target,

was made. The Indianapolis delivers Little Boy bomb units, and the U-235

projectile to Tinian on July 26. On the same day the target flew out of

Kirtland Air Force Base, Albuquerque on a C-54 transport plane, which arrived

July 28. Bomb unit L11 was selected for combat use and on July 31 the U-235

projectile and target were installed, along with 4 initiators - making Little

Boy ready for use the next day. An approaching typhoon required postponing the

planned attack of Hiroshima on Aug. 1. Several days are required for weather

to clear, and on Aug. 4 the date was set for 2 days later. On August 5 Tibbets

named B-29 No. 82 the "Enola Gay" after his mother, over the objections of its

pilot Robert Lewis. Little Boy was loaded on the plane the same day.

August 6, 1945 -

* 0000, final briefing, the target of choice is Hiroshima. Tibbets is pilot,

Lewis is co-pilot.

* 0245, Enola Gay begins takeoff roll.

* 0730, the bomb is armed.

* 0850, Flying at 31,000 ft Enola Gay crosses Shikoku due east of Hiroshima.

* Bombing conditions are good, the aimpoint is easily visible, no opposition

is encountered.

* 0916:02 (8:16:02 Hiroshima time) Little Boy explodes at an altitude of 1900

+/- 50 feet (580 m), 550 feet from the aim point, the Aioi Bridge, with a

yield of 12-18 Kt (the yield is uncertain due partly from the absence of any

instrumented test with this weapon design). A state-of-the-art, six year study

ending in 1987, which used all available evidence, set the yield at 15 Kt (+/-


The yield of Little Boy had been predicted before delivery at 13.4 Kt, and

the burst height was set at 1850 ft. Using the 15 Kt figure, the actual burst

height was optimum for a blast pressure of about 12 psi (i.e. it maximized the

area subjected to a 12 psi or greater overpressure). To inflict damage on a

city a blast pressure of 5 psi is sufficient, so greater damage would have

resulted from an optimum burst height of 2700'. Due to the uncertainty in

predicting yield, and the fact that bursting too high causes a rapid

deterioration in effects, the burst height had been set conservatively low in

case a low yield explosion occurred. The 1900 foot burst height is optimal for

a 5 Kt weapon. The burst height was sufficient to prevent any fallout over


7.1.4 Fatman

The combat configuration for the implosion bomb basically consisted of the

Gadget device encapsulated in a steel armor egg. The two steel half-ellipsoids

were bolted to the dural equatorial band of the explosive assembly, with the

necessary X-Unit, batteries, and fuzing and firing electronics located in the

front and aft shell. For use in combat, each Fatman bomb required assembly

almost from scratch - a demanding and time consuming job. Assembly of a Fatman

bomb was (and may still be) the most complex field preparation operation for

any weapon ever made.

Like Little Boy, Fatman was fuzed by four radar units called "Archies", the

antennas for which were mounted on the tail of the bomb. Developed originally

as fighter tail warning systems, these units measured the bomb's height above

the ground and were set to detonate at a pre-calculated altitude (set to 1850

ft, +/- 100 ft). A barometric switch acted as a "fail-safe", preventing

detonation until the bomb had fallen below 7000'.

Fatman was 60 inches in diameter, was 12 feet long, and weighed 10,300 lb.

The Fat Man plutonium core, and its initiator, left Kirtland Air Force Base,

for Tinian Island on July 26, 1945 in a C-54 transport plane. I arrived on

Tinian on July 28. No Fat Man bomb assemblies arrived until August 2. The

bombing date was set for August 11 at this time, with Kokura as the target.

Assembly of practice (non-nuclear) weapons began shortly afterward, with the

first completed bomb (Fat Man unit F33) ready on Aug. 5. On August 7 a

forecast of 5 days of bad weather around the 11th moved the bombing date up to

August 10, then to August 9. This compressed the bomb assembly schedule so

much that many check-out procedures had to be skipped during assembly. On

August 8 the assembly of Fat Man unit F31, with the plutonium core, was

completed in the early morning. At 2200, Fat Man was loaded on the B-29

"Bock's Car".

August 9, 1945 -

* 0347, Bock's Car takes off from Tinian, the target of choice is Kokura

Arsenal. Charles Sweeney is pilot. Soon after takeoff he discovers that the

fuel system will not pump from the 600 gallon reserve tank.

* 1044, Bock's Car arrives at Kokura but finds it covered by haze, the

aimpoint cannot be seen. Flak and fighters appear, forcing the plane to stop

searching. Sweeney turns toward Nagasaki, the only secondary target in range.

* Upon arriving at Nagasaki, Bock's Car has enough fuel for only one pass

over the city even with an emergency landing at Okinawa. Nagasaki is covered

with clouds, but one gap allows a drop several miles from the intended


* 11:02 (Nagasaki time) Fat Man explodes at 1650 +/- 33 feet (503 m) near the

perimeter of the city with a yield of 22+/-2 Kt. Due to the hilly terrain

around ground zero, five shock waves were felt in the aircraft (the initial

shock, and four reflections).

Although Fat Man fell on the border of an uninhabited area, the eventual

casualties still exceeded 70,000. Also ground zero turned out to be the

Mitsubishi Arms Manufacturing Plant, the major military target in Nagasaki. It

was utterly destroyed.

The 1987 reassessment of the Japanese bombings placed the yield at 21 Kt. At

the extreme estimate ranges for Little Boy and Fat Man (low for Little Boy,

high for Fat Man), a ratio of nearly 2-to-1 has been implied. The 1987 best

estimate figures make Fat Man only about 40% larger than Little Boy (and

possibly as little as 15% more).

Using the 21 Kt figure, the optimal burst height for Fat Man would have been

about 3100 feet. The actual burst height was optimal for 15 psi overpressure

The burst height was sufficient to prevent any fallout over Japan.

7.1.5 Availability of Additional Bombs

The date that a third weapon could have been used against Japan was no later

than August 20. The core was prepared by August 13, and Fat Man assemblies

were already on Tinian Island. It would have required less than a week to ship

the core and prepare a bomb for combat.

By mid 1945 the production of atomic weapons was a problem for industrial

engineering rather than science, although scientific work continued -

primarily toward improving the bomb designs.

The two reactors at Hanford had a combined thermal output of 500 megawatts and

were capable of producing 15 kg of plutonium a month, enough for 2.5 bombs.

Enriched uranium production is more difficult to summarize since there were

three different enrichment processes in use that had interconnected

production. The Y-12 plant calutrons also had reached maximum output early in

1945, but the amount of weapon-grade uranium this translates into varies

depending on the enrichment of the feedstock. Initially this was natural

uranium giving a production of weapon-grade uranium of some 6 kg/month. But

soon the S-50 thermal diffusion plant began feeding 1% enriched uranium,

followed by the K-25 gaseous diffusion plant. The established production

process was then: thermal diffusion -> gaseous diffusion -> calutron. Of these

three plants, the K-25 plant had by far the greatest separation capacity and

as it progressively came on line throughout 1945 the importance of the other

plants decreased. The calutrons probably would have continued to be used for a

final enrichment stage into the next year had the war continued, but the bulk

of the enrichment would have been carried out by K-25. By mid fall some 60 kg

of U-235 was being produced monthly, enough for four implosion bombs, with

production still increasing.

The relative importance of K-25 can be judged by production estimates given to

Sec. Stimson in July. Stimson was told that a second plutonium bomb would be

ready by Aug. 24, that 3 bombs should be available in September, and more each

month - reaching 7 or more in December. Since plutonium production can only

account for 2.5 bombs a month, and Y-12 by itself could only produce one bomb

every several months, most of this projected production must be due to K-25.

During its period of operation (ending in 1946), Y-12 produced about 1000 kg

of weapon grade uranium. By itself, it could have produced perhaps 100 kg from

natural uranium and S-50 feed. Indeed, gaseous diffusion enriched uranium

production dwarfed plutonium production from this point onward in the U.S.

nuclear weapon program until highly enriched uranium production halted in


It is very unlikely any more Little Boy-type bombs would have been used even

if the war continued. Little Boy was very inefficient, and it required a large

critical mass. If the U-235 were used in a Fat Man type bomb, the efficiency

would have been increased by more than an order of magnitude. The smaller

critical mass (15 kg) meant more bombs could be built. Oppenheimer suggested

to Gen. Groves on July 19, 1945 (immediately after the Trinity test) that the

U-235 from Little Boy be reworked into uranium/plutonium composite cores for

making more implosion bombs (4 implosion bombs could be made from Little Boy's

pit). Groves rejected the idea since it would delay combat use.

A modified implosion design was under development at Los Alamos when the war

ended using two innovations: a composite pit containing both U-235 and Pu-239,

and core levitation which allowed the imploding tamper to accelerate across an

air gap before striking the pit.

The composite pit had several advantages over using the materials separately:

* A single design could be used employing both of the available weapon


* Using U-235 with plutonium reduced the amount of plutonium and thus the

neutron background, while requiring a smaller critical mass than U-235 alone.

The levitated pit design achieved greater compression densities. This

permitted using less than fissile material for the same yield, or an enhanced

yield with the same amount of material.

These considerations were apparently taken into account in the weapon

production estimates given above.

When the war ended on August 15 1945 there was an abrupt change in priorities,

so a war-time development and production schedule did not continue.

Y-12 was extremely costly to operate and was shut down permanently early in

1946. The Hanford reactors accumulated unexpected neutron irradiation damage

(the Wigner effect) and in 1946 they were shut down or operated at reduced

power. If war had continued they both would have been pushed to continue full

production regardless of cost or risk. Plans had been in progress to increase

initiator production to ten times the July 1945 level. The levitated composite

pit design was also not rushed into production. It did not enter the U.S.

aresenal until the late forties.

Although Los Alamos had 60 Fat Man units on hand in October 1945, the U.S.

arsenal after had only 9 actual Fat Man type bombs in July 1946, with

initiators for only 7 of them. In July 1947 the arsenal had increased to 13