This section describes the first fission and fusion bombs that were
developed and tested. The purpose is three-fold. First, these devices are of
considerable historical and public interest, the "first" of anything
garners special attention. Second, these devices serve as archetypal examples
of basic designs, and more information is available about these devices than
later ones. Third, the effort and technology that was required to develop these
devices provide indications of how easily primitive nuclear weapons can be
developed by others.
This subsection describes the three atomic
bombs, which were constructed and detonated in 1945.
The design of the Gadget and Fat Man devices are discussed together
since they are basically the same. Gadget was an experimental test version of
the implosion system used in Fat Man. 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 US made Gadget/Fat Man,
it also applies to the first Soviet atomic bomb, code named RDS-1 (Reaktivnyi
Dvigatel Stalina; Stalin's Rocket Engine) by the Soviet Union and designated
Joe-1 by US 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 US, and many FBI
records relating to the Fuchs and Rosenberg investigations have recently been
released that contain design data given to FBI interrogators by Fuchs and
Greenglass.
The basic structure of this design was based on a series of concentric
nested spheres (each discussed in detail in the paragraphs below) Starting from
the outside (listing by outside radius) these were:
Explosive lens system |
65 cm |
Pusher/neutron absorber shell |
23 cm |
Uranium temper/reflector shell |
11.5 cm |
Plutonium pit |
4.5 cm |
Beryllium neutron initiator |
1.0 cm |
1.1.1.1 The Pit
The pit of these devices contained 6.2 kg of a delta-phase plutonium
alloy. The mass was provided in 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 a 9.0 cm sphere, solid except for an approximately 2.5 cm
cavity in the center for the modulated neutron initiator. 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
15.9) 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 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 stabilization 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 multi-megabar pressures generated by the implosive shock wave cause this
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 first 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 (or silver, as has been
reported for the Gadget core). This created a problem with the Gadget pit since
hasty electroplating had left plating solution trapped under the nickel (or
silver), 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.
1.1.1.2 The Neutron Initiator
The beryllium initiator used was called the "Urchin" or
"screwball" design. It was a sphere consisting of a hollow beryllium
shell, with a solid beryllium pellet inside, the whole initiator weighing about
7 grams. The outer shell was 2 cm wide and 0.6 cm thick, the solid inner sphere
was 0.8 cm wide. 15 parallel wedge-shaped grooves, each 2.09 mm deep, were cut
into the inner surface of the shell. Like the pit, the shell was formed in two
halves by hot pressing in a nickel carbonyl atmosphere. The surfaces of the
shell and central sphere were coated with 0.1 mm of gold, and also a nickel layer
deposited by the nickel carbonyl atmosphere. 50 curies polonium-210 (11 mg) was
deposited on the grooves inside the shell and on the central sphere. The gold
and nickel layers protected the beryllium from alpha particles emitted by the
polonium or surrounding plutonium. The Urchin was attached to a mounting
bracket inside the central cavity of the pit, which was probably 2.5 cm wide.
The Urchin was activated by the arrival of the implosion shock wave at
the center of the pit. When the shock wave reached the walls of the cavity,
they vaporized and the plutonium gas shock wave then struck the initiator,
collapsing the grooves and creating Munroe-effect jets that rapidly mixed the
polonium and beryllium of the inner and outer spheres together. The alpha particles
emitted by the Po-210 then struck beryllium atoms, periodically knocking loose
neutrons, perhaps one every 5-10 nanoseconds.
1.1.1.3 The Reflector/Tamper
The pit was surrounded by a natural uranium tamper weighing 120 kg,
with a diameter of 23 cm. The tamper formed a 7 cm thick layer around the pit.
The thickness of this layer was determined by neutron conservation
considerations, since a few cm is sufficient to provide inertial confinement.
Thicker natural uranium reflectors (exceeding 10 cm) provide significant
additional savings to ordinary critical assemblies. But the "time
absorption" effect inherent to fast exponential chain reactions reduced
the benefits of a thicker reflector. About 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 up to 2.5 times its original density (possibly
somewhat less), the pit became an assembly of some 4-5 critical masses. Before
use, the bomb was safed by use of a cadmium wire in the pit.
1.1.1.4 The Pusher/Neutron Absorber Shell
Surrounding the tamper was an 11.5 cm thick aluminum sphere also
weighing 120 kg. The primary purpose of this sphere, called the
"pusher", seems to have been to reduce the effect of the Taylor wave,
the rapid drop in pressure the occurs behind a detonation front. The Taylor
wave tends to steepen in an implosion, causing pressure to drop more and more
rapidly as the wave converges. A shock reflection occurs at the Composition B/aluminum
interface (due to the 1.65/2.71 density ratio) sending a higher pressure second
shock back into the explosive and suppressing the Taylor wave. This also
increases the pressure of the transmitted wave, enhancing the pressure reached
at the center of core.
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 with a relatively low boron content. 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.
1.1.1.5 The High Explosive Lens System
The entire high explosive implosion system made a layer some 47 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 aluminum 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 snugly 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 400 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
sphere.
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.
Safety was a serious problem for Fat Man, though in a comparison of
worst case accidents, not as serious a problem as it was for Little Boy. The
critical mass of the uranium reflected core in the delta phase was 7.5 kg, but
only 5.5 kg in the alpha phase. Any accidental detonation of the high explosive
(in a fire or plane crash for example) would be certain to collapse the 6.2 kg
delta phase core to the supercritical alpha phase state. The expected yield
from the explosion would be on the order of tens of tons, roughly a factor of
ten higher than the energy of the high explosive itself. The main hazard would
be from gamma radiation however, which would be deadly well outside the main
area of blast effects. A 20 ton explosion would produce a lethal 640 rem prompt
gamma radiation exposure 250 m from the bomb!
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.
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.
The heaviest fallout was detected about 20 miles northeast of ground
zero. In this area radiation levels recorded along U.S. Highway 380 for a
distance of ten miles reached "approximately 50 R total." Also in
this area was a site dubbed "Hot Canyon". The canyon was 5 miles east
of the town of Bingham, 1.1 miles east of a road junction. This is a summary of
radiation levels:
15.0 R/hr @ 0300 hours after zero |
14.0 R/hr @ 0330 hours |
6.0 R/hr @ 0830 hours |
0.6 R/hr @ 3600 hours |
The total exposure as this site was 212-230 R.
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.
0.9 miles east of "Hot Canyon", was a house containing the
Raitliff family, consisting of two adults and a child. Levels at this location
were "0.4 R/hr at 3600 hours after zero & after a rain. Accumulated
total dose 57-60 R." Also nearby was another house with a couple named
Wilson. None of these people were evacuated.
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.
The design of Little Boy was completely different from Gadget/Fat Man.
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
(the final version was designated the Model 1850), 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.
All of the uranium used in Little Boy 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 pit contained 64.1 kg of highly enriched uranium. By the time
Little Boy was assembled, 50 kg of uranium enriched to 89% had been produced by
Oak Ridge, and an additional 14 kg of 50% enrichment uranium was on hand. All
of it was used in the bomb, giving an average enrichment of 80%, or
approximately 2.4 critical masses. This is less than the 5 or so critical
masses achieved by Gadget/Fat Man, 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 >90% enrichment) and an inferior reflector, but a superior
tamper (tungsten carbide gives a 15% lower critical mass, compared to tungsten
metal, but is 25% less dense).
The U-235 mass of Little boy was divided into two pieces: the bullet
and the target. The "bullet": a cylindrical stack of U-235 rings
about 10 cm wide and 16 cm long, containing 40% of the mass (25.6 kg). It was
constructed from six rings, the stack backed by a tungsten carbide disk and a
steel backplate, all within a 1/16 inch thick steel can to make the complete
projectile. The "target": a hollow cylinder 16 cm long and wide, weighing
38.4 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 supercritical. Almost
certainly the bullet was made entirely of 89% enrichment uranium since placing
the most fissile material at the center of the core is a basic principle of
efficient bomb design.
The bullet was sheathed in a boron "safety sabot" that
absorbed neutrons and reduced the chance of a criticality accident. The target
also contained a boron safety plug. When the projectile reached the target, the
boron sabot would be stripped off, and then the plug would be ejected into a
recess in the nose.
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.
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.
Although it only took some 0.5 milliseconds for the fissile material in
the bullet to traverse the length of the target, the reactivity insertion time
for Little Boy was 1.35 milliseconds, indicating that a critical configuration
was achieved well before the bullet reached the target. The uranium/steel
assembly was designed as a "blind target", one that would stop and
hold the bullet upon impact due to expansion of the bullet rings. 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 ABNER initiators out of a batch of 16 shipped to Tinian were used in
Little Boy. These were fastened radially to the front end of the target
assembly.
The gun was a 3" (inside diameter) anti-aircraft barrel, 6.5"
wide, and 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.
To reduce the possibility of the bullet being driven into the target by
a crash, the fit was intentionally made very tight. The bullet had to be rammed
into the breech to assemble the weapon, and about 300,000 newtons of force
(70,000 lb) were required to drive it forward. The weapon striking a hard
surface in a crash could conceivably produce the 500 Gs of acceleration required
however.
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 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 126 inches long, was 28 inches in diameter and
weighed 8900 lb. Little Boy used the same air burst detonator system as Fat Man
(see below).
No other weapon of this design was ever detonated. Only five other
Little Boy units were built, but no others entered the US arsenal. It appears
that not even one additional complete set of components required to assemble a
combat-ready weapon were ever procured.
The first U-235 projectile component was completed at Los Alamos on
June 15, 1945. Casting of the U-235 projectile for Little Boy was completed on
July 3. On July 14 Little Boy bomb units, accompanied by the U-235 projectile,
were shipped out of San Francisco. They were picked up by the USS Indianapolis
(CA-35) at the U.S. Navy's Hunter's Point shipyard at San Francisco on July 16,
bound for Tinian Island in the Mariana Islands. On July 24 the last component
for Little Boy, the U-235 target insert, was completed and was tested the next
day. The Indianapolis delivered Little Boy bomb units, and the U-235 projectile
to Tinian on July 26. On the same day the target assembly, divided into three
parts flew out of Kirtland Air Force Base, Albuquerque on three C-54 transport
planes, which arrived July 28 at Tinian.
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.
0000 |
Final briefing, the target of choice is Hiroshima |
0245 |
Enola Gay begins takeoof roll |
0730 |
The bomb is armed |
0850 |
Flying at 31,000 Enola Gay crosses Shikoku due east of Hiroshima |
|
Bombing conditions are good, the aim point is easily visible, no
opposition is encountered |
0916:02 (8:16:02 Hiroshima Time |
Little Boy explodes at an altitude of 1900 +/- 50 ft (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 instrumental test with this
weapon design. |
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
significant fallout over Japan.
The combat configuration for the implosion bomb (the Model 1561)
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 Fat Man
bomb required assembly almost from scratch - a demanding and time consuming
job. Assembly of a Fat Man bomb was (and may still be) the most complex field
preparation operation for any weapon ever made.
Like Little Boy, Fat Man 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'.
Fat Man 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. It arrived
on Tinian on July 28. Also on July 28, three specially-modified B-29s flew from
Kirtland Field carrying three Fat Man bomb assemblies, including units F-31 and
F-32, each encased in an outer ballistic shell. These arrived at Tinian on
August 2, the first Fat Man units to do so. 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".
0347 |
Bock’s Car takes off from Tinian, the target choice is Kokorua
Arsenal. The pilot 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. Pilot turns toward Nagasak, the only
secondary target in range. |
|
Upon arriving at Nagasaki, Bock’c 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 aimpoint. |
11:02 (Nagasaki Time) |
Fat Man explodes at 1,650 =/-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.
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 scientific research, although scientific
work continued - primarily toward improving the bomb designs.
The three reactors (B and D which went started up for production in
December 1944, and F which started up February 1945) at Hanford had a combined
design thermal output of 750 megawatts and were theoretically capable of
producing 19.4 kg of plutonium a month (6.5 kg/reactor), enough for over 3 Fat
Man bombs. Monthly or annual production figures are unavailable for 1945 and
1946, but by the end of FY 1947 (30 June 1947) 493 kg of plutonium had been
produced. Neglecting the startup month of each reactor, this indicates an
average plutonium production 5.6 kg/reactor even though they were operated at
reduced power or even shut down intermittently beginning in 1946.
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 0.89% enriched uranium,
followed by 1.1% enriched feed from the K-25 gaseous diffusion plant. The
established production process was then: thermal diffusion (to 0.89%) ->
gaseous diffusion (to 1.1%) -> alpha calutron (to 20%) -> beta calutron
(up to 89%). 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. When enough stages had been added to
K-25 to allow 20% enrichment, the alpha calutrons were slated to be shut down
even if the war continued.
After Japan's surrender in August 1945, S-50 was shut down; the alpha
calutrons followed in September. But K-25 was complete on August 15, and these
shutdowns would have occurred in any case. At this point gaseous diffusion was
incapable of producing weapon grade uranium, a planned "top plant"
had been cancelled in favor of more beta calutrons. An expansion of K-25,
called K-27, to produce a larger flow of 20% enriched feed was under
construction and due to go in full operation by 1 February 1946. In October
production had increased to 32 kg of U-235 per month.
In November and December additional beta tracks went on line, and the
percentage of downtime for all beta tracks fell, boosting production further.
Between October 1945 and June 1946, these improvements led to a 117% increase
in output at Oak Ridge, to about 69 kg of U-235 per month.
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.
The improved composite core weapon was in full development at Los
Alamos when the war ended. It combined 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,
creating shock waves that propagated inward and outward simultaneously for more
rapid and even compression.
The composite pit had several advantages over using the materials
separately:
·
A
single design could be used employing both of the available weapon materials.
·
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 25% less than fissile material for the same yield, or a doubled
yield with the same amount of material.
Production estimates given to Sec. Stimson in July 1945 projected 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.
Improvements in bomb design being prepared at the end of the war would have
permitted one bomb to be produced for every 5 kg of plutonium or 12 kg of
uranium in output. These improvements were apparently taken into account in
this estimate. Assuming these bomb improvements were used, the October capacity
would have permitted up to 6 bombs a month. Note that with the peak monthly
plutonium and HEU production figures (19.4 kg and 69 kg respectively),
production of close to 10 bombs a month was possible.
When the war ended on August 15 1945 there was an abrupt change in
priorities, so a wartime development and production schedule did not continue.
Development of the levitated pit/composite core bomb ground to a halt
immediately. It did not enter the US arsenal until the late forties. Plans to
increase initiator production to ten times the July 1945 level were abandoned.
Fissile material production continued unabated after the S-50 and alpha
calutron shutdowns though the fall, but plutonium shipments from Hanford were
halted, and plutonium nitrate concentrates were stockpiled there.
In early 1946, K-25 and K-27 were reconfigured to produce weapon grade
uranium directly, but the extremely costly Y-12 beta tracks continued to
operate until the end of 1946. By that time Y-12 had separated about 1000 kg of
weapon grade uranium. From this point on gaseous diffusion enriched uranium was
the mainstay of weapon grade fissile material production in the US, dwarfing
plutonium production, until highly enriched uranium production for weapons use
was halted in 1964.
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.
The effects of these priority changes can be seen in the post war
stockpile. Although Los Alamos had 60 Fat Man units on hand in October 1945,
the US 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
bombs. There was probably sufficient fissile material on hand for over 100
bombs though.
The discovery of fusion reactions arose early in the twentieth century
out of the growing understanding of atomic physics. By the early 20s it was
realized that hydrogen fusion was the source of the sun's power output,
although the details were still obscure. This work culminated in the paper
published by Hans Bethe in _Physical Review_ in 1939 describing the role of
fusion reactions in the sun, for which he received the Nobel Prize in Physics
in 1967.
The possibility of creating weapons employing fusion reactions was not
seriously considered until the discovery of fission. Almost immediately
physicists around the world realized that fission explosions generating high
temperatures might be possible, but a few years passed before the idea of using
these temperatures to ignite fusion reactions was suggested. Tokutaro Hagiwara
at the University of Kyoto proposed this idea in a speech in May 1941,
apparently the first such mention.
While working on atomic bomb research a few months later, in September
1941, Enrico Fermi muses to Edward Teller ("out of the blue") whether
a fission explosion could ignite a fusion reaction in deuterium. After some
study Teller concluded that it is impossible and although no further work on
the subject followed for awhile, this conversation began Teller's eventual obsession
with fusion bombs.
[Historical footnote: During World War II, the
idea occurred in Germany that convergent shock waves and collapsing shells
might focus enough energy to allow conventional high explosives to ignite
limited fusion reactions. This idea was probably inspired by Gudderly's work in
converging shock waves, and certainly by the Allied attempts to destroy the
heavy water plant at Vemork, Norway. Since German physicists considered fission
weapons to be beyond reach during the current war, they concluded that the
Allied interest in heavy water must be due its application in high explosive
weapons. The Germans actually checked craters left by the British "Grand
Slam", the largest conventional bomb dropped during the war, to discover
whether its unusual power was due to fusion boosting. Polish researchers in the
60s and 70s reported actually generating fusion neutrons through convergent
shock waves. Although the theoretical possibility remains, no one has
apparently ever released significant amounts of energy this way.]
Research into the possibility of fusion weapons took an irregular and
halting journey from the time of Fermi and Teller's conversation until bombs
were actually built in the early 1950s. During WW II there was an initial surge
of interest once fission bomb physics was fairly well understood. After
preliminary theoretical investigation it was realized that much better
experimental data was needed, and a fusion research program was included in the
Manhattan Project at Los Alamos. Continuing theoretical investigations took
repeated turns towards optimism, then pessimism, and back again. As the
difficulty of the enterprise came clear, its priority was steadily downgraded.
Teller on the other hand grew so captivated by the problem that he became
unable to fulfill his duties at Los Alamos, was relieved of all technical
leadership responsibilities, and was eventually transferred to a separate study
group to prevent him from interfering in the work of others on the atomic bomb.
During July-September 1942 Oppenheimer's theoretical study group
(Oppenheimer, Bethe, Teller, John Van Vleck, Felix Bloch, Robert Serber, and
Emil Konopinski) in Berkeley examined the principles of atomic bomb design, and
also considered the feasibility of fusion bombs. Megaton range fusion bombs
were considered highly likely.
April, 1943 - During the initial organization effort at Los Alamos,
Bethe is selected over Teller to head the Theoretical Division. Teller is soon
placed in charge of lower priority research on fusion weapon design (designated
the Super), but remains responsible for much theoretical work on the fission
weapon as well.
February, 1944 - The Los Alamos Governing Board reevaluates deuterium
fusion research and determines that tritium would be necessary to make an
explosive reaction. Priority of fusion bomb work is further downgraded.
May, 1944 - Teller is removed entirely from the Theoretical Division to
prevent his interference with fission bomb work. He is placed in charge of a
small independent group for fusion research.
At the end of the war most of Los Alamos' scientific and technical
talent, and virtually all of its leadership, left for civilian careers. Teller
was among those who left. For a period of time very little progress on weapon
research of any kind occurred. A conference, chaired by Teller, was held in
April 1946 to review the wartime progress on the Super.
The design at that time was for a gun-type uranium fission bomb to be
surrounded by about a cubic meter of liquid deuterium, with the whole assembly
being encased in a heavy tamper. A large but undetermined amount of tritium
would be required to ignite the reaction. If the amount of tritium required was
too large, then the bomb would be impractical. Since the fusion of one T atom releases
8% the energy of the fission of a Pu-239 atom, with which it competes for
neutrons in production reactors, the energy boost from D+D fusion must be
considerably more than a factor of 10 greater than that released by the tritium
starter fuel before the Super could be worthwhile.
The assessment at the time of the conference was that the Super was
basically sound, but that more detailed calculations would be required verify
it. Also present at the conference was Klaus Fuchs, who was spying for the Soviet
Union. The Soviets thus were well informed about American interest and optimism
about fusion weapons.
In mid-1946 Teller developed an idea he called the Alarm Clock. This
involved the use of fusion fuel, specifically lithium-6 deuteride inside a
uranium tamper of an implosion fission bomb. The idea was that the fission
neutrons would breed tritium form the lithium, and fission energy would
compress and heat the fusion fuel and ignite a reaction. A fusion-fission chain
reaction would then proceed between the fusion fuel and tamper until the bomb
disassembled.
By the end of 1946 Teller thought the Alarm Clock idea unpromising. In
his September 1947 memorandum "On the Development of Thermonuclear
Bombs" he was pessimistic about Alarm Clock's potential, but felt that it,
like the Super were possible and required further study. Due to limitations in
computing devices then available, he proposed delaying further work on both
approaches for two years. If work had proceeded on the Alarm Clock design at
this time the U.S. could probably have tested a device similar to Joe 4 (see
below) before the end of 1949.
In the four years following the end of the war about 50% of the Los
Alamos Theoretical Division's effort went into studying the Super, although its
size and talent were much reduced from wartime levels. The absence of good
calculating machines hampered the massive numerical computations that were
required and greatly slowed progress.
By 1949 the Cold War was in full swing, with the Berlin blockade and
Communist governments seizing control throughout Eastern Europe. This included
Teller's homeland of Hungary, where much of his family still lived. Early in
the summer of 1949 he thus rejoined Los Alamos to pursue the Super. On August
29 the first Soviet atomic bomb, code named RDS-1 and called Joe 1 by US
intelligence, was exploded breaking the US nuclear weapon monopoly.
Up to this time the more detailed work on the classical Super design
had showed that it was marginal at best. The large amounts of tritium required
made it extremely expensive for the yield produced, and it was not even certain
that the design would work at all. Teller remained optimistic however. During
the next few months Robert Oppenheimer, as head of the Atomic Energy
Commission's General Advisory Council (GAC) consistently opposed accelerating
work on the Super due to its demonstrated shortcomings.
Despite this, on 31 January 1950, Pres. Truman announced that the US
would proceed to develop hydrogen bombs. A couple of weeks after Truman's announcement,
Teller issued a 72 page update of "On the Development of Thermonuclear
Bombs". In this paper he again regarded both the Super and Alarm Clock as
viable candidates for weapons development, but again proposed delaying decision
on full scale development of either for another two years.
At this time Soviet research on the subject was already well underway,
focusing on the Sakharov-Ginsberg version of the Alarm Clock concept which they
called the Layer Cake. A special department was set up in March 1950 to proceed
with actual Layer Cake weapon development.
By February 1950, immediately after Truman's decision, Stanislaw Ulam
had discovered by hand calculation that even more immense amounts of tritium
than previously believed would be necessary for the Super to have any chance of
success. When Ulam and Cornelius Everett completed more detailed computations
on June 16, the design even these huge amounts of tritium appeared to be
inadequate. Additional analysis by Ulam and Enrico Fermi nailed the coffin shut
on the classical Super. When John Von Neumann's newly invented ENIAC began
doing extensive calculations on the problem later in the year, the negative
results were simply piling more dirt on the grave. Until early 1951 real
progress on hydrogen bomb development was impossible because no one knew how to
proceed.
In January 1951, Ulam broke the barrier to progress by inventing the
idea of staging: using the energy released by an atomic bomb primary to
compress an external fuel capsule. He initially developed the idea as a means
to create improved fission bombs, the second stage being a mass of fissionable
material. By late in the month he realized that the powerful compression that
was possible would overcome the obstacles to efficient large scale fusion
reactions. By multiple staging, bombs of virtually unlimited size could be
created.
This key idea was not sufficient by itself. Before a workable design
could be developed a scheme was needed for generating efficient compression
using this energy flux, as was a means for igniting the fuel once it was
compressed. Ulam's idea was to use the neutron flux or the hydrodynamic shock
wave of the expanding bomb core to achieve compression. Working with Ulam,
Teller added additional refinements to this insight during the month of
February. Teller's principal contribution during this period was realizing that
the thermal radiation flux from the primary was a more promising means of
generating the necessary implosive forces. On 9 March 1951, Ulam and Teller
jointly wrote a report, _On Heterocatalytic Detonations I. Hydrodynamic Lenses
and Radiation Mirrors_, that summarized these ideas.
From this point on Teller increasingly began to claim exclusive credit
for the breakthrough, and eventually came to deny that Ulam had made any
original or significant contribution.
Later in March Teller added an important additional element to the
radiation implosion scheme. Adapting Ulam's idea to use staged implosion to
trigger a fission reaction, Teller suggested placing a fissile mass in the
center of the fusion fuel. The convergent shock wave would compress this to
supercriticality upon arriving at the center, making it act as a "spark
plug" to ignite the fusion reaction. This idea is perhaps not strictly
necessary, the convergent shock wave will generate very high temperatures in
the center any way and might suffice to initiate fusion as it does in modern
laboratory inertial confinement fusion experiments.
Since the continuing compression on the fusion fuel would act to
confine the fission spark plug, this final combined design concept was termed
the "equilibrium thermonuclear". Teller wrote this idea up in a
report on 4 April, 1951.
It was only in April 1951 that the necessary physical principles were
in hand to allow the development and testing of an actual hydrogen bomb to go
forward. More computations were required to design the device than for any
other project in human history up to this point (made possible by the recent
invention of the programmable computer). The elapsed time from this point until
the detonation of the Mike device was less than 19 months, an achievement as
remarkable in its own way as the Manhattan Project.
In April 1951 experiments with fusion reactions and atomic bombs were
already being prepared by the US as part of the Greenhouse test series,
including a test of the idea of fusion boosting. The Greenhouse George test in
particular provided a valuable opportunity to evaluate the Teller-Ulam ideas by
allowing the observation of radiation effects in heating and compressing
(although not imploding) an external mass of fusion fuel.
Since there are several known designs for incorporating fusion
reactions into weapons we come to a question that is largely a matter of
definition: Which design qualifies as a *true* hydrogen bomb? I will not try to
debate this issue here (see Section 11: Questions and Answers), instead I am
including descriptions of all of the significant tests that lead to the
development and deployment of early thermonuclear weapons.
The tests are listed in chronological order. Each is followed by a
brief discussion of its significance to weapons development.
Greenhouse George
Detonated 5/9/51 at 0930 (local time) on a 200 ft tower on Ebireru/Ruby
island at Eniwetok atoll.
Total yield: 225 Kt
George was a test of a pure fission bomb, and the highest yield bomb
tested up to that time. The bomb was a cylindrical implosion U-235 bomb,
perhaps based on a design by physicist George Gamow. An experiment called the
Cylinder device was piggy-backed on George to test the ignition of a
thermonuclear reaction. The cylindrical implosion design allowed the fusion
experiment to be heated directly by the pit without the shielding effects of a
high explosive layer, and avoided disruption by expanding detonation gases. A
deuterium-tritium mixture external to the large fission core was ignited by the
thermal flux, and produced detectable fusion neutrons. This was the first
ignition of a thermonuclear reaction by an atomic bomb. Rhodes, in _Dark Sun_,
estimates the fusion energy yield as 25 Kt, although the mass of fuel given
("less than an ounce") is at a factor of 12 too small for this. Other
sources simply give the fusion yield as "small". This approach provided
no prospect for development into a high yield thermonuclear weapon. This test
fortuitously provided useful data for evaluating the Teller-Ulam design which
had been devised two months prior.
Greenhouse Item
Detonated 5/25/51 at 0617 (local time) on a 300 ft tower on
Engebi/Janet island at Eniwetok atoll.
Total yield: 45.5 Kt
First test of a boosted fission device. A deuterium-tritium mixture in
the U-235 bomb core boosted fission yield by 100% over its expected unboosted
yield. This innovation was eventually incorporated into most or all strategic
weapons, but the fusion yield was negligible and overall yield was still
limited by the capabilities of fission designs.
Ivy Mike
Detonated 11/1/52 at
0714:59.4 +/- 0.2 sec (local time) at ground level on Elugelab/Flora island at
Enewetak atoll.
Total yield: 10.4 megatons.
This was the first test of the Teller-Ulam (or Ulam-Teller)
configuration. The Mike device used liquid deuterium as the fusion fuel. It was
a massive laboratory apparatus installed on Elugelab Island in the Enewetak
Atoll consisting of a cylinder about 20 feet high (more exactly 243.625 inches
or 6.19 m), 6 ft 8 in wide, and weighing 164,000 lb (including attached
diagnostic instruments); also said to weigh 140,000 lb without "the cryogenic
unit" (this may mean the casing by itself). It was housed in an open
hanger-like structure 88 ft x 46 ft, and 61 ft high, where assembly started in
September of 1952.
The Mike device consisted of a massive steel cylinder with rounded
ends, a TX-5 implosion bomb at one end acted as the primary, and a giant
stainless steel dewar (thermos) flask holding several hundred liters of liquid
deuterium surrounded by a massive natural uranium pusher/tamper constituted the
secondary fusion stage (know as the "Sausage").
The welded steel casing was lined with a layer of lead. A layer of
polyethylene several centimeters thick was attached to the lead with copper
nails. This layer of plastic generated plasma pressure during the implosion.
The Sausage consisted of a triple-walled stainless steel dewar. The
inner most wall contained the liquid deuterium. Between this wall and the
middle wall was a vacuum to prevent heat conduction. Between the middle wall
and the outer wall was another vacuum, and a liquid nitrogen-cooled thermal
radiation shield made of copper.
To reduce thermal radiation leakage even further, the uranium pusher
(which was oxidized to a purple-black color, making it an excellent thermal
radiator) was lined with gold leaf.
Down the axis of the dewar, suspended in the liquid deuterium was a
plutonium rod that acted as the "spark plug" to ignite the fusion
reaction once the compression shock wave arrived at the center. It did not run
the entire length of the dewar, but was supported at each end by axial columns.
The spark plug was a boosted fission device, it was hollow and was charged with
a few grams of tritium/deuterium gas (which of course liquified once the dewar
was charged with liquid deuterium).
The Mike device had a conservative design. The external casing was made
of steel and was extraordinarily thick (usually described as "a foot
thick", but more likely 10 inches to be consistent with the weight) to
maximize the confinement of the radiation induced pressure inside. The interior
diameter was thus about 60 inches. A very wide radiation channel was provided
around the secondary stage to minimize thermal gradients, and to make success
less dependent on sophisticated analysis. Due to the low density of liquid
deuterium, and the necessity of thermal insulation, the secondary itself was
quite voluminous which, when combined with the wide channel between the
secondary and the casing led to the 80 inch diameter. The massive casing
accounted for most of Mike's weight (about 85%).
The TX-5 device was an experimental version of the implosion system
that was also deployed as the Mk-5 fission bomb. It used a 92 point ignition
system, that is, 92 detonators and explosive lenses were used to make the
spherical imploding shock wave. This allows the formation of the implosion
shock wave with a thinner layer of explosive than earlier designs. The TX-5 was
designed to use different fission pits to allow variable yields. The highest
reported yield for a TX-5 test was Greenhouse Easy at 47 Kt on 20 April 1951,
with a 2700 lb device. The smaller mass compared with earlier designs kept the
temperature higher and allowed thermal radiation to escape more quickly from
the primary, thus enhancing the radiation implosion process. If the Easy
configuration was used in Mike, then the secondary fusion/primary yield ratio
was 50/1. The deployed Mk-5 had an external diameter of 43.75 inches, the TX-5
would have been substantially smaller since it lacked the Mk-5 bomb casing.
Three fuels were considered for Mike: liquid deuterium, deuterated
ammonia (ND3), and lithium deuteride. The reason for choosing liquid deuterium
for this test was primarily due to two factors: the physics was simpler to
study and analyze, and extensive studies had already been conducted over the
previous decade on pure deuterium fuel. The desirability of lithium-6 deuteride
as a fuel was known, but sufficient Li-6 could not be produced in time to make
the November 1952 target date (in fact construction of the first lithium
enrichment plant had just begun at the time of the test).
Liquid deuterium produces energy through four reactions:
For Mike to function successfully, densities and temperatures in the
secondary sufficient to ignite reactions 2 and 3 were required. This requires
densities hundreds of times normal, and temperatures in the tens of millions of
degrees K (say, 75 g/cm^3 and 3x10^7 K).
Since the reaction cross section of 1 is some 100 times higher than the
combined value of 2 and 3 the tritium is burned as fast as it is produced,
contributing most of the energy early in the reaction. Reaction 4, on the other
hand, requires temperatures exceeding 200 million K before its cross section
becomes large enough to contribute significantly. Whether sufficient
temperatures are reached and quantities of He-3 are produced to make 4 a major
contributor depends on the combustion efficiency (percentage of fuel burned).
If only reactions 1-3 contribute significantly, corresponding to the
combustion of 25% of the deuterium fuel or less, then the energy output is 57
Kt/kg. If reaction 4 contributes to the maximum extent, the output is 82.4
Kt/kg. The maximum temperature generated by an efficient burn reaches 350
million K.
The fission fraction for Mike was quite high - 77%. The total fusion
yield was thus 2.4 megatons, which corresponds to the efficient thermonuclear
combustion of 29.1 kg of deuterium (172 liters), or the inefficient combustion
of 41.6 kg (249 liters). The total fission yield was 7.9 megatons, the fission
of 465 kg of uranium. All but some 50 Kt of this was due to fast fission of the
uranium secondary stage tamper by fusion neutrons, a 3.3 fold boost.
The amount of deuterium actually present in Mike was no more than 1000
liters, which is the amount of liquid deuterium handled by Operation Ivy. In
fact, it was probably substantially less than this since excess LD2 was
undoubtedly brought along in case leakage or other losses occurred.
Prior to test, Mike's yield was estimated at 1-10 megatons, with a most
likely yield of 5 Mt, but with a remote possibility of yields in the range of
50-90 Mt. The principal uncertainties here would have been the efficiency of
the fusion burn, and the efficiency with which the tamper captured neutrons.
Both of these factors are strongly influenced by the success of the compression
process. The fusion efficiency involved novel and complex physics which could
not be calculated reliably even if the degree of compression were known. The
physics for determining the efficiency of neutron capture on the other hand
were well understood and could be calculated if the conditions could be
predicted.
The upper limit estimate provides some insight into the mass of the
uranium fusion tamper. Presumably the 90 Mt figure was calculated by assuming
complete fusion and fission of all materials in the secondary. If 1000 liters
of deuterium were burned with complete efficiency, the yield would be 13.9 Mt.
Fission must account for 76.1 Mt, corresponding to a uranium tamper mass of
4475 kg. Lower amounts of deuterium would lead to higher tamper estimates (a
ratio of 0.82 kg of U for each liter of LD2).
The detonation of Mike completely obliterated Elugelab, leaving an
underwater crater a 6240 feet wide and 164 ft deep in the atoll where an island
had once been. Mike created a fireball 3 miles wide; the "mushroom"
cloud rose to 57,000 ft in 90 seconds, and topped out in 5 minutes at 135,000
ft - the top of the stratosphere- with a stem eight miles across. The cloud
eventually spread to 1000 miles wide, with a stem 30 miles across. 80 million
tons of soil were lifted into the air by the blast.
TX-16/EC-16
The Mike design was actually
converted into a deliverable weapon, demonstrating that lithium deuteride is
not essential to making a usable weapon. The weaponized design, designated the
TX-16, went into engineering development in June 1952 (5 months before the Ivy
Mike test). The design eliminated the cryogenic refrigerator, reduced the
weight of the tamper, drastically reduced the dimensions and mass of the
casing, used a lighter and less powerful primary, and pared the weight in other
areas. The expected yield was reduced to 7 Mt. The device was about 60 inches
in diameter, 25 ft long, and weighed 30,000 lb. This weapon design would have
been filled with liquid deuterium at a cryogenic filling station before
take-off, a reservoir in the weapon held sufficient liquid hydrogen to replace
boil-off losses during flight. Components for about five of these bombs were
built in late 1953, and had reached deplyment by the time of the Castle tests.
A unit of the TX-16, code named Jughead, was slated for proof test
detonation on 22 March 1954 as part of the Castle series, prior to its expected
deployment as the EC-16 (Emergency Capability) gravity bomb in May 1954. The
excellent results with the solid-fueled Shrimp device in the Castle Bravo test
on 1 March(see below) resulted in the cancellation of this test, and then of
the entire EC-16 program on 2 April 1954.
Soviet Test: Joe 4/RDS-6s
Detonated: 12 August 1953,
on a tower at Semipalatinsk in Kazakhstan
Total yield: 400 kilotons
This was the fifth Soviet trest, and first Soviet test of a weapon with
substantial yield enhancement from fusion reactions. This bomb (designated
RDS-6s) did not employ the Teller-Ulam configuration, instead it used the
"Sloika" design invented by Andrei Sakharov and Vitalii Ginzburg. A
sloika is a layered Russian pastry, rather like a napoleon, and has thus been
translated as "Layer Cake". The design was first invented in the
United States by Edward Teller (who called it "Alarm Clock") but it
was not developed into a weapon there.
This design is based on a combination of what Sakharov has called the
"First and Second Ideas". The First Idea, developed by Sakharov,
calls for using a layer of fusion fuel (deuterium and tritium in his original
concept) around a fission primary, with an outermost layer of U-238 acting as a
fusion tamper. The U-238 tamper confines the fusion fuel so that the
radiation-driven shock wave from the fission core can efficiently compress and
heat the fusion fuel to the ignition point, while the low conductivity of the
fusion tamper prevents heat loss and at the same time yields addition energy
from fast fission by the fusion-generated neutrons. The Second Idea,
contributed by Ginzburg used lithium-6 deuteride (with some tritium) as the
fusion fuel. Being a solid, this is a convenient material for designing a bomb,
and it also produces additional tritium from fission neutrons through the Li-6
+ n reaction. This establishes a coupled fission -> fusion -> fission
chain reaction in the U-238 tamper, with the fusion fuel acting in effect as a
neutron accelerator. Larger bombs can be created by placing additional
successive layers of Li-6 D and U around the bomb. The device tested in 1953
probalby had two layers.
A small U-235 fission bomb acted as the trigger (about 40 Kt). The
total yield was 400 Kt, and 15-20% of the energy was released by fusion, and
90% due directly or indirectly to the fusion reaction.
A few weeks before the test it was belatedly realized that despite the
sparse population of the area around Semipalatinsk, a serious fallout hazard
nonetheless existed for tens of thousands of people. The options were to carry
out a mass evacuation or delay the test until an air-dropped system could be
arranged, which would take at least six months. Rather than delay the test, a
hasty evacuation was conducted. [Note: This implies that the Layer Cake was not
available as a usable weapon until after Feb. 1954, a time at which the US had
actually deployed the EC-14, a megaton-range lithium deuteride fueled
Teller-Ulam design. See the Castle Union test below.]
Castle Bravo
Detonated: 1 March 1954 (0645 local time) on reef 2950 ft off of
Nam/Charlie island, Bikini Atoll
Total yield: 15 megatons
The Shrimp device detonated in the Bravo test was the first test of a
Teller-Ulam configuration bomb fueled with lithium deuteride. This became the
standard design for all subsequent hydrogen bombs (including Soviet designs).
Shrimp was a cylinder 179.5 in long, and 53.9 in wide, weighing 23,500 lb. The
lithium in Shrimp was enriched to a level of 40% Li-6. The predicted yield of
this device was only 6 Mt (range 4-6 Mt), but the production of unexpectedly
large amounts of tritium through the fast neutron fission of Li-7 boosted the
yield to 250% of the predicted value, making it the largest bomb ever tested by
the US (and destroying much of the measuring equipment). The fission yield was
10 Mt, the fusion yield was 5 Mt for a fusion fraction of 33%.
The explosion created a 6000 ft crater, 240 ft deep in the atoll reef.
The cloud top rose to 114,000 ft.
The Bravo test created the worst radiological disaster in US history.
Due to failure to postpone the test following unfavorable changes in the
weather, combined with the unexpectedly high yield, the Marshallese Islanders
on Rongerik, Rongelap, Ailinginae, and Utirik atolls were blanketed with the
fallout plume. They were evacuated on March 3 but 64 Marshallese received doses
of 175 R. In addition, the Japanese fishing vessel Daigo Fukuryu Maru (Fifth
Lucky Dragon) was also heavily contaminated, with the 23 crewmen received
exposures of 300 R (one later died from complications). The entire Bikini Atoll
was contaminated to varying degrees, and many operation Castle personnel were
subsequently over-exposed as a result. After this test the exclusion zone
around the Castle tests was increased to 570,000 square miles, a circle 850
miles across (for comparison this is equal to about 1% of the entire Earth's
land area).
The two stage device Shrimp design was used as the basis for the Mk-21
bomb. The weaponization effort began on 26 March, only three weeks after Bravo.
By mid April the military characteristics were defined. On 1 July an expedited
schedule for deployment was approved. The use of the final fast fission stage
was apparently eliminated. After a number of efforts to reduce the weight, the
design seems to have stabilized in mid-July 1955 with a projected yield of 4
megatons (subsequently tested at 4.5 megatons in Redwing Navajo, 95% fusion, 11
July 1956). Quantity production began in December 1955 and ended in July 1956
with 275 units being produced. The Mk 21 weighed about 15,000 lb; it was 12.5
ft long, and 56 in. in diameter. During June-November 1957 it was converted to
the Mk 36 design.
Castle Romeo
Detonated 27 March 1954 on barge in Bikini atoll lagoon near Bravo test
site at 0630:00.4 (local time).
Total yield: 11 megatons
The Runt I device (the second in the Castle series) was another solid
fueled two stage design. This device was 224.9 in. long, 61.4 in. in diameter,
and weighed 39,600 lb. The fuel for Runt was natural lithium deuteride, a major
advantage considering the high cost of lithium-6 enrichment. It exceeded its
predicted yield by an even larger margin than Bravo, with a most probable yield
of 4 Mt out of a 1.5-7 Mt range. This is consistent with the higher proportion
of Li-7, compared to Bravo. The fission yield was 7 Mt, for a fusion fraction
of 36%.
The Runt I and Runt II devices (seen Castle Yankee below) were design
tests for the EC-17 and EC-24 bombs respectively. These two weapons were very
similar (externally identical, similar internal configurations, but with
different primaries). They were the most powerful weapons ever built by the US,
with predicted yields of 15-20 megatons, and were also the largest and heaviest
bombs ever deployed by the US The Mk 17/24 (as the deployed versions were eventually
designated) was 24 ft. 8 in. long, with a 61.4 in. diameter, and a weight of
41,400-42,000 lb (30,000 lb of this was the 3.5 in. steel casing).
Although the initial work on these weapons dates at least to Feb. 1953,
they went into development engineering in Oct. 1953. The EC-17 and EC-24 became
the second and third models of hydrogen bomb to enter the US arsenal. From
April to September in 1954 EC-17 and EC-24 bombs were stockpiled (5 EC-17, and
10 EC-24). These bombs were removed in October, modified for better safety
features and with drogue parachutes for slower fall, and returned to duty as
the Mk 17 Mod 0 and Mk 24 Mod 0 in November 1954. These weapons went through
two subsequent modifications, and stockpiles reached 200 Mk 17s and 105 Mk 24s during
the October 1954 - November 1955 production run. The Mk 24s were retired in
Sept-Oct 1956; the Mk 17s were retired between Nov. 1956 and Aug. 1957.
Castle Union
Detonated 26 March 1954 (0610:00.7 local time) on barge in Bikini atoll
lagoon off Yurochi Island.
Total yield: 6.9 megatons
This was actually the fourth test in the Bravo series (the third test -
Koon - failed when the fusion stage did not ignite). This was the test of the
EC-14 Alarm Clock bomb (unrelated to Teller's earlier Alarm Clock concept),
which was the first hydrogen bomb actually to enter the US arsenal and the
first Teller-Ulam bomb ever to be deployed anywhere. This was a solid fueled
two stage device using 95% enriched lithium-6. It also exceeded expectations,
the predicted yield was 3-4 Mt (range 1-6 Mt). The fission yield was 5 Mt for a
fusion fraction of 28%. The tested device had a length of 151 in., a diameter
of 61.4 in,. and weighed 27,700 lb.
The TX-14 Alarm Clock went into development engineering in August 1952,
and procurement was approved in mid-September (some 6 weeks before Mike had
even been tested). The first EC-14 weapons were produced in Feb. 1954, two
months prior to test of the design. The design was simple but had very poor
safety features. A total of 5 were deployed, this low figure can probably be
attributed to scarcity of Li-6 at the time. Safety could presumably have been
improved through retrofitting, but the high cost of these weapons probably led
to their rapid retirement. They were removed from the arsenal in October with
the deployment of the EC-17. The Mk 14 (its final deployed designation) had a
diameter of 61.4 in., a length of 18 ft. 6 in., and weighed 28,954 lbs. After
refitting with a drogue parachute its weight increased to 29,851 lbs.
Castle Yankee
Detonated 5 May 1954 (0610:00.1 local time) on barge in Bikini atoll
lagoon, above the Union crater.
Total yield: 13.5 megatons
The Runt II device was very similar to Runt I, mostly differing in the
design of the primaries. The fuel for Runt II was also natural lithium
deuteride. It also exceeded its predicted yield, with a most probable yield of
8 Mt out of a 6-10 Mt range. See Castle Union for a discussion of weapons
derived from this test. This device was 225 in. long, 61 in. in diameter, and
weighed 39,600 lb. The fission yield was 7 Mt for a fusion fraction of 48%.
Soviet Test No. 19 Test
11/22/55 (No common name) Detonated 11/22/55
Total yield: 1.6 megatons
The first Soviet test of a Teller-Ulam/Sakharov Third Idea bomb. It
used radiation implosion to detonate a lithium deuteride fueled capsule. This
was the world's first air-dropped fusion bomb test. After this test the Soviet
Union used radiation implosion bombs as the basis for their strategic arsenal.
Exploded underneath an inversion layer, the refracted shock did unexpected
collateral damage, killing three people.