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
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
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
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
* 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
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
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