Themes > Science > Chemistry > Nuclear Chemistry > Nuclear Weapons > The First Nuclear Chain Reaction > The First Nuclear Weapons > The Design of Gadget, Fat Man, and "Joe 1" (RDS-1)

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 tamper/reflector shell 11.5 cm
Plutonium pit                   4.5 cm
Beryllium neutron initiator     1.0 cm

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.

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.

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.

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.

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, to perhaps as high as the low 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 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.

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