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8.0 - The First Nuclear Weapons

Version 2.17: 1 August 2002


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8.0 - The First Nuclear Weapons
This section describes the first fission and fusion bombs that were developed and tested. The purpose is 3-fold.
First, these devices are of considerable historical and public interest (i.e., 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.
8.1 The First Atomic Bombs

8.1.1 The Design of 'Gadget', 'Fat Man', and 'Joe 1' (RDS-1)

8.1.2 TRINITY - The 'Gadget' Test

8.1.3 'Little Boy'

8.1.4 'Fat Man'

8.1.5 Availability of Additional Bombs

8.2 The First Hydrogen Bombs

8.2.1 Early Research on Fusion Weapons

8.2.2 Design and Testing of the First Fusion Weapons

8.1 - The First Atomic Bombs
This subsection describes the three atomic bombs, which were constructed and detonated in 1945.
8.1.1 The Design of 'Gadget', 'Fat Man', and 'Joe 1' (RDS-1)
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 U.S. made Gadget/Fat Man, it also applies to the first Soviet atomic bomb codenamed "RDS-1" (Reaktivnyi Dvigatel Stalina; 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. 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 information the Soviets received were not complete specifications of the final design and -- in particular -- lacked precise measurements. So even an attempt to make an exact replica would inevitably vary in exact dimensions and other respects.
The 2 key spies at Los Alamos were Theodore ("Ted") Hall and Klaus Fuchs. Hall was involved in certain aspects of the experimental work involved with the implosion experiments and was the first source to provide the Soviets with information about the implosion concept (while it was in preliminary development) in the fall of 1944. The detailed design information about the bomb was provided by Fuchs who actually had a key role in its development. Significant information about lens manufacture was also passed on by David Greenglass. But this was mostly or entirely redundant with the Hall and Fuchs information. In the early 1990s, KGB/RFIS officials disseminated an account of an unidentified scientist-spy code named "Perseus". But this has since been determined to be a fictional composite.
A fairly complete description of the Gadget's design emerged into the public domain over a span of 45 years (from 1950 to 1995), mostly as an indirect result of Soviet espionage. The Rosenberg trial resulted in a first description of the implosion process and explosive lens design due to the release of testimony provided by David Greenglass and Klaus Fuchs. The very detailed information provided by Klaus Fuchs emerged after the collapse of the Soviet Union. Many additional FBI records relating to the Fuchs and Rosenberg investigations have recently been released in recent years also.
The intelligence-based descriptions of the Gadget and Fat Man suffered somewhat from the fact that they actually recounted different stages of design evolution and not the specifications of the actual devices exploded at Trinity and Nagasaki. Since 1995, more specific information about the actual Gadget and Fat Man designs has been uncovered and published through the efforts of John Coster-Mullen who has assembled a great deal of data through interviews, field investigations, archive searches, and FOIA requests. Coster-Mullen's work is available in the form of his self-published book Atom Bombs: The Top Secret Inside Story of Little Boy and Fat Man.

Implosion Bomb Cross Section (drawn to scale)

Explosive Sphere Component


(Outside Diameters)

Pit Assembly

Neutron initiator

2.0 cm (0.79 inches)

Initiator cavity

2.5 cm (0.98 inches)

Plutonium core

9.20 cm (3.62 inches)

Uranium tamper shell

22.225 cm (8.75 inches)

Boron-plastic shell

22.86 cm (9 inches)

Implosion Assembly

Aluminum pusher shell

46.99 cm (18.5 inches)

Inner HE booster shell

92.075 cm (36.25 inches)

Explosive lens

137.8 cm (54.25 inches)

Explosive Sphere Casing

Cork liner

140.3 cm (55.25 inches)

Duraluminum case

145.4 cm (57.25 inches)

Fat Man Bomb Ballistic Case Dimensions

Ballistic case (ID)

152.1 cm (59.875 inches)

Ballistic case (OD)

153.0 cm (60.25 inches)

Scale drawing of 'Fat Man' (click for large image).

Cut-away view showing the implosion bomb lens block arrangement. Lens and booster blocks are combined in this diagram. Click for large image. The Pit Assembly
The heart of the pit assembly was the Plutonium core whose fission in its compressed state would release the explosive energy of the bomb. The core contained 6.2 kg of a delta-phase Plutonium-Gallium alloy. The core was a 9.20 cm sphere -- solid except for a 2.5 cm cavity in the center for the 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 in the first core. Wartime production meant that the Plutonium had to be separated as quickly as feasible after being bred in the reactor. Due to the very short irradiation periods used (about 150 days), this was super-grade weapon Plutonium containing only about 1.2% Pu240.
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 weapon design at the start of 1945 called for the use of pure Plutonium in the alpha phase (its densest phase and the phase that is stable at room temperature). At that time, knowledge of Plutonium phase behavior and alloys were slight. And using Plutonium with no diluents and in its densest phase favored the most efficient explosion possible.
But as increasing quantities of Plutonium came available for metallurgists to work with problems with making using castings of alpha Plutonium became apparent. Hot pressing 0.9 inch (2.29 cm) hemispheres weighing 60 g had been successful in April. But attempts to repeat this in May with 2-inch (5.08 cm) hemispheres weighing 650 g failed. The pressings warped and split due to the phase change that occurred, much of it after the metal had cooled. Fortunately for the program, a suitable alloy that stabilized the low density delta phase had already been developed.
The previous summer after the Plutonium crisis had struck Los Alamos, the theoretical division had calculated the tolerance for diluent atoms in Plutonium (that is, the concentration of non-Plutonium atoms that would not significantly affect the explosion efficiency). This level was set at 5% (molar). In February 1945, the metallurgists identified 2 alloying elements that stabilized the metal’s delta phase -- Silicon and Aluminum. Further work found that only Aluminum produced alloys with satisfactory stability. Suitable ones with an Aluminum content ranging from 1-to-4% were tested.
In early-March, new tolerances for various light element impurities were calculated based on the production of neutrons from the "alpha,n" reaction. The levels were quite conservative although the Pu240 had imposed neutron backgrounds some 620 times higher than originally expected. The light element neutron background was allowed to rise only 62 times so that it would not be a significant contributor to the neutron emission. It was soon noticed that these limits set the maximum content of Aluminum at 0.5% (molar), making the only candidate delta-stable alloy unacceptable. Tests were soon made with Aluminum’s next heaviest chemically similar neighbor -- Gallium. In April, the 3% Gallium alloy was developed and tested and thus offered a ready solution to the Plutonium fabrication problem.
Switching to the low density delta phase alloy had other benefits. The higher critical mass of the lower density metal required the use of more Plutonium. But it also allowed the use of more Plutonium. The more Plutonium used in the bomb, the bigger the explosion due both to the direct increase in fuel mass and an increase in efficiency. Serendipitously, the expected drawback of using lower density metal -- a less efficient reaction -- did not materialize. Even stabilized, the delta phase collapsed quite easily so that during implosion the Plutonium would compress to the same density as would a pure Plutonium alpha phase core.
Another advantage of the alloy was that the stabilization eliminated any possibility of phase transition expansion due to inadvertent overheating of the core after manufacture. A real risk for the self-heating Plutonium which would distort and ruin it for weapon use. A final advantage was that the alloy proved relatively corrosion resistant, undergoing corrosion at only 4% the rate of pure Plutonium. This alloy has remained in use in subsequent nuclear weapons down to the present day.
The bomb cores were formed in 2 hemispheres by hot pressing an ingot of the proper weight in steel dies. The alloy proved easy to press-form at 400 ºC and 30,000 psi (200 MPa).
Since Plutonium is a very chemically-reactive metal as well as a significant health hazard, each half-sphere was plated with an inert metal to protect it. The initial technique used was electroplating Silver to form a layer 0.005 inches (0.13 mm) thick. This protection method was used for the 'Gadget core', but created a problem. Hasty preparation had left plating solution trapped under the Silver resulting in blistering that ruined the corrosion protection and the fit. Careful grinding and layering with Gold leaf restored the necessary smooth surface. 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.
After Trinity, 2 changes were made in bomb core fabrication. Silver plating was replaced by a new process that coated the Plutonium with a much tougher and more effective protective layer of Nickel. The Plutonium was exposed to an atmosphere of a Nickel Carbonyl which reacted with the Plutonium to deposit a film of Nickel. This coating technique would remain in use at least through the 1950s at Rocky Flats. Later cores also had a design change that eliminated the need for the Gold gasket. This possibly involved changing the mating surfaces of the hemispheres from being flat to having some sort of step or bevel to block shock wave jets.
In the center of core was a 2.5 cm cavity that held the neutron initiator. The 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.
The Urchin had 15 concentric latitudinal grooves cut into the inner surface of the shell. Each groove was wedge-shaped and 2.09 mm deep. Like the Plutonium core, the shell was formed in 2 halves by hot pressing in a Nickel Carbonyl atmosphere. The surfaces of the shell and central sphere were coated by a layer of Nickel and then plated with 0.1 mm of Gold. 50 curies of Polonium210 (11 mg) were 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 and surrounding Plutonium. The Urchin was attached to a mounting bracket inside the core’s central cavity.
The Urchin was activated by the arrival of the implosion shock wave at the center of the core. 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 Po210 then struck Beryllium atoms, periodically knocking loose neutrons (perhaps one every 5-10 nanoseconds).
Surrounding the core was a natural-Uranium tamper weighing 108 kg with a diameter of 9 inches (22.225 cm). The tamper formed a 6.56 cm thick layer around the core. Together, the core and tamper formed the "pit assembly." The thickness of the tamper layer was determined by neutron conservation considerations since a few centimeters are sufficient to provide effective 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.
With the neutron reflection provided by the tamper the Plutonium core was about 78% of a critical mass before implosion. An additional margin of safety was provided by the use of a Cadmium wire in the pit before initiator insertion. When compressed by the implosion to over twice its original density, the pit became an assembly of some 3-to-4 critical masses.
On the outside surface of the tamper was another measure to improve weapon reliability -- a brown 1/8- inch thick (0.32 cm) layer of neutron-absorbing enriched Boron10 bonded with acrylic thermoplastic. This feature reduced the neutron background in the core. The very fast spontaneous fission neutrons originating in the pit assembly had a very short residence time. They would quickly either be captured in the pit assembly or escape from it. But upon doing so, they would encounter the thick hydrogen-rich explosive layer which would act as an effective moderator and neutron reflector. The strongly absorbing Boron captured these slow neutrons before they could be scattered back into the pit assembly where they would then persist for a relatively long time. The Implosion Assembly
The implosion assembly comprised 3 layers: the outermost explosive lens layer which created the converging implosion shock wave; an inner booster explosive layer that strengthened the converging wave, and an Aluminum sphere called the "pusher" which further enhanced the implosion wave. Every aspect of the implosion assembly had to conform to high standards of precision to ensure a highly symmetric implosion wave. No more than a 5% variation was tolerable.
The entire high-explosive implosion system (made up of the lens and booster layers) was 17.875 inches (45.4 cm) thick. These layers each consisted of 32 explosive blocks (20 hexagonal and 12 pentagonal blocks) which fitted together in the same pattern as a soccer ball (see Figures 3-7 and 3-8 in Section - Implosion Assembly). The complete spherical explosive assembly was 54.25 inches (137.8 cm) wide and weighed 5,300 lb (2,400 kg).
Each lens block had 2 components. The body made of high velocity explosive and a parabolic low velocity explosive focusing element on the inner surface. These pieces formed the lens that shaped a convex, expanding shock wave into a convex converging one (see Figure 3-6). Each lens block was 9 inches (22.9 cm) thick. The hexagonal lens blocks each weighed about 145 lb (66 kg), each pentagonal block about 95 lb (43 kg). The complete lens layer weighed about 4,000 lb (1,800 kg).
The lenses were made by precision casting which required explosive mixtures that could be safely melted. The high-velocity explosive was Composition B ("Comp B") -- a mixture of 60% RDX (a very high velocity but unmeltable explosive); 39% TNT (a good explosive that melts easily --m.p. 80.35 °C); and 1% wax as a binder. The slower 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. The high density of Barium Nitrate gives Baratol a density of at least 2.5.
The inner layer of explosive blocks had a thickness of 8 7/8 inches (22.5 cm) thick. The hexagonal blocks each weighed 47 lb (19 kg). Each pentagonal one was 31 lb (14 kg) for a total mass of about 1,300 lb (590 kg).
The entire explosive assembly 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 1/32 of an inch (0.8 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 and blotting paper and adhesive tape was also used to make everything fit snugly together with no air gaps.

To achieve the most precise detonation synchronization possible, conventional detonators (which consisted 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 were needed to explode all 32 detonators simultaneously. A cascade of spark gap switches was used to trigger the discharge of the capacitor bank. Known as the "X-Unit", this system weighed 400 lb (180 kg) and was one of the most difficult components to qualify in time for the second atomic attack.
Surrounding the tamper and Boron-plastic layer was a 4¾ inch (12.0 cm) thick Aluminum sphere weighing 130 kg, called the "pusher" replacing a similar thickness of high explosive. This layer of inert higher density material (a density ratio of 1.64) improved the implosion wave in a number of ways. Upon encountering the higher density layer, the shock wave slows (which tends to reduce the size of irregularities that have developed) and creates a partial shock reflection of increased pressure that propagates outward. This strengthens the implosion wave and reduces the drop in pressure than tends occurs behind the shock front -- both of which contribute to enhancing the compression of the core. At this smaller radius, the effect of implosion convergence had concentrated the shock wave energy to the point that the explosive that was replaced no longer contributed significant amounts of energy to it in any case. The Explosive Sphere Casing
The development of the casing that held the explosive sphere together progressed independently from the work on the design of the implosion system itself. (It was taken for granted that whatever the final design it would be high explosive sphere sized to fit the 59 inch case.) A complex early design designated the "1222" model involved 12 pentagonal sections of the aluminum alloy Duraluminum (Dural) inside an icosahedral steel shell, the assembly of which required 1,500 bolts.
This was abandoned in the summer of 1944 in favor of a much simpler system -- the "1561" model. This consisted of a spherical shell made up of 2 polar caps and 5 equatorial segments of machined Dural castings which required only 90 bolts for assembly. The overall thickness of the Aluminum was 1 inch. This sphere was enveloped by an ellipsoidal shell of steel attached at the equator with the tail bolted to the ellipsoid. The electrical detonating and fusing equipment was mounted on the sphere in the space between the sphere and the outer ellipsoid.
A layer of ½-inch thick cork lined the inside of the Aluminum sphere and cushioned and compressed the explosive sphere, holding the whole implosion system together. The outermost steel shell had a thickness of 3/8 of an inch. It was intended to be heat-treated steel armor plate but warping during the heat treatment process caused replacement with a case of ordinary steel.
Both the implosion bomb prototype Gadget and the combat Fat Man bomb were assembled on the site from separate pieces. The explosive sphere was built up from separate explosive blocks; the tamper and pusher spheres were lowered in by a small crane; and the pieces of the Dural shell was bolted together. Due to the complexity of the weapon, this was a process that took at least 2 days (including checkout procedures).
The final bomb design after Fat Man allowed "trap door" assembly. The entire bomb was assembled ahead of time except for the pit assembly. To complete the bomb, one of the domed caps was removed along with one of the explosive lenses and inner explosive blocks. 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 block and lens were 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 range from on the order of tens of tons (roughly a factor of 10 higher than the energy of the high explosive itself) to perhaps as high as hundreds of tons. The main hazard would be from gamma radiation however, which would be deadly well outside the main area of blast effects. A 2- ton explosion would produce a lethal 640 cSv prompt gamma radiation exposure 250 m from the bomb!

8.1.2 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 codenamed 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 4,624 ft above sea level.
The assembly of 'Gadget' took 5 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 10 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 at 0300 hours after zero

14.0 R/hr at 0330 hours

6.0 R/hr at 0830 hours

0.6 R/hr at 3600 hours
The total exposure as this site was 212-to-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 2 adults and a child. Levels at this location were "0.4 R/hr at 3600 hours after zero and 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.
8.1.3 Little Boy
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. Sufficient U235 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. Thus despite being straightforward technically, the program still required extraordinary attention to detail. The design arrived at was a very conservative one. The principle risk was whether the fuzing system with trigger the bomb at the appropriate time.
Three 6.5-inch (165 mm) smooth-bore gun tubes with 2-inch (5.08 cm) thick walls and designed for a maximum pressure of 40,000 psi (2,700 bar) were ordered in March 1944 and received in October. Proof firings -- consisting of firing a 200 lb (90 kg) projectile at 1,000 ft/sec (300 m/sec) 2-or-3 times from each tube -- were conducted in December. To hold weight down, the tubes were not designed for many repeated firings, unlike conventional artillery.
The gun tubes were 6-feet (1.8 m) long and weighed about 1,000 pounds (450 kg) -- little more than 10 percent of the final bomb weight. Far more massive -- weighing in at over 5,000 pounds (2,300 kg) -- was the high alloy steel case forging that held the target assembly. The target case measuring 28 inches (71 cm) wide and 36 inches (91 cm) long was screwed on to the end of barrel (lest recoil from the gun’s firing pull them apart) and had to absorb the full momentum of the projectile and bring it to rest without cracking or deforming. The target case that was used in Little Boy was the first one ever made. Which surprisingly had proved also to be the best ever made. It had been used in 4 proof-test firings without damage before its use in combat.
Inside the target case was a cavity that held the cylindrical Tungsten Carbide tamper -- 13 inches (33 cm) wide and long and weighing 680 pounds (310 kg). The tamper acted as a neutron reflector, increasing the effective the number of critical masses in the Uranium core. And with a density of 14.8, it also served as an inertial tamper to hold the core together as long as possible to increase the explosive yield. When fully assembled, the Uranium core resided in a 6.5-inch wide (16.5 cm) cylindrical cavity surrounded by a 3.25-inch (8.255 cm) thick layer of Tungsten Carbide.
The choice of Tungsten Carbide as a tamper instead of the natural-Uranium used in Fat Man was dictated by the need to keep neutrons out of the assembly. U238 undergoes spontaneous fission 100 times more frequently than U235. And a piece large enough to be useful as a tamper (200 kg) would generate 3,400 neutrons a second. Too many for gun assembly to be feasible.
The enriched-Uranium core (weighing 64.15 kg fully assembled) was divided 40/60 between the target insert (25.6 kg) and the hollow core in the projectile (38.53 kg). The hollow core was slightly less than 6.5 inches wide (16.5 cm) and 7 inches long (17.8 cm). It was fabricated as a stack of 9 rings of varying thickness with a 4-inch (10.2 cm) inside diameter. The target insert was a cylinder 4 inches (10.2 cm) wide and 7 inches long (17.8 cm) also made up of a stack of rings. The 6 target insert rings had a 1-inch inside diameter to accommodate a rod that held the stack together.
With an enrichment of around 80%, the spontaneous fission rate in the 64 kg of Uranium amounted to 79 fissions/second -- only 1/6th of the limit previously established for Thin Man. The assembly velocity was reduced to 1/3 of requirement for the Plutonium gun, and even with the larger Uranium core (which gave a greater assembly distance) the chance of predetonation was still significantly reduced. Since the mass of a gun varies with the square of its velocity to the 1/3 reduction in velocity allowed a potential decrease in gun mass to only 1/9th of the original design. In fact, the mass of the final gun was greater than this, providing an increased margin of reliability.
Both the target insert and uranium projectile were backed by disks of Tungsten Carbide reflector 6.5 inches wide and 3.25 inches thick The projectile consisted of the Uranium core and Carbide reflector disk packed in a steel sheet can with a wall thickness of 0.0625 inches (1.59 mm) thick along with the 6-inch (15.2 cm) long steel projectile body. The projectile body was equipped with brass bands to provide the gas-tight seal with the smooth-bore barrel. The complete weight of the projectile was 190 pounds (86 kg).
When the projectile united with insert, the impact drove the assembled core and the Tungsten Carbide disk backing the target against a malleable steel disk (the “anvil”) mounted inside the target case. It was the anvil’s job to deform, spreading out like a pancake, thus cushioning the impact and absorbing the bulk of the projectile’s kinetic energy. Its momentum was absorbed by the massive target case. The engineering design of the complete bomb (designated "Model 1850") 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 U235 core which waited on deliveries of enriched-Uranium.
The complete weapon was 28 inches in diameter, 126 inches long, and weighed 8900 lb. The target case and tamper and the gun barrel accounted for nearly all of 'Little Boy’s length and 3/4 of its weight. Most of the remaining weight was in the steel plates that stretched back from where they joined the target case to form the hollow bomb body. A 1-inch (2.54 cm) thick, 27.25 inch wide (69.2 cm) steel bulkhead disk slid over the rear of the gun barrel to create the aft end of the bomb body. 5 steel plates were bolted between the bulkhead and target case to form a compartment around the barrel. Inside this compartment was the fuzing system (the electrical and mechanical components required to detonate the bomb).
Little Boy used the same air burst detonator system as Fat Man (see below). Essentially all the enriched-Uranium on hand was used to fabricate the weapon core. Since deliveries had taken place over about a 7-month period with varying degrees of enrichment, the material varied in quality. It was important to place the most highly-enriched material in the center of the core where the neutron flux would be highest. So the target insert was fabricated last using the latest and most highly enriched shipments. The insert was completed on July 24, the projectile having been cast in pieces from June 15 to July 3. The insert had an enrichment of 86%. The projectile was enriched to 82% for an average enrichment for the core of 83.5% -- equal to slightly more than 3 critical masses.
At the bottom of the core cavity, one-or-more Beryllium/Polonium "Abner" initiators (different from the implosion initiators -- simpler in design and with less Polonium) could be mounted. 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 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. These were fastened radially around the target assembly.
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 3 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, 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 5 other Little Boy units were built. But no others entered the U.S. arsenal. It appears that not even one additional complete set of components required to assemble a combat-ready weapon were ever procured.
The first U235 projectile component was completed at Los Alamos on June 15, 1945. Casting of the U235 projectile for Little Boy was completed on July 3. On July 14, Little Boy bomb units accompanied by the U235 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 U235 target insert -- was completed and was tested the next day. The Indianapolis delivered Little Boy bomb units and the U235 projectile to Tinian on July 26. On the same day, the target assembly -- divided into 3 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. On July 31, the U235 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.

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