Manhattan Project - Plutonium - Weapon Design

Weapon Design

In 1943, development efforts were directed to a gun-type fission weapon with plutonium called Thin Man. Initial research on the properties of plutonium was done using cyclotron-generated plutonium-239, which was extremely pure, but could only be created in very small amounts. Los Alamos received the first sample of plutonium from the Clinton X-10 reactor in April 1944 and within days Emilio Segrè discovered a problem: the reactor-bred plutonium had a higher concentration of plutonium-240, resulting in up to five times the spontaneous fission rate of cyclotron plutonium. Seaborg had correctly predicted in March 1943 that some of the plutonium-239 would absorb a neutron and become plutonium-240.

This made reactor plutonium unsuitable for use in a gun-type weapon. The plutonium-240 would start the chain reaction too quickly, causing a predetonation that would release enough energy to disperse the critical mass with a minimal amount of plutonium reacted (a fizzle). A faster gun was suggested but found to be impractical. The possibility of separating the isotopes was considered and rejected, as plutonium-240 is even harder to separate from plutonium-239 than uranium-235 from uranium-238.

Work on an alternative method of bomb design, known as implosion, had begun earlier at the instigation of the physicist Seth Neddermeyer. Implosion used explosives to crush a subcritical sphere of fissile material into a smaller and denser form. When the fissile atoms are packed closer together, the rate of neutron capture increases, and the mass becomes a critical mass. The metal needs to travel only a very short distance, so the critical mass is be assembled in much less time than it would take with the gun method. Neddermeyer's 1943 and early 1944 investigations into implosion showed promise, but also made it clear that the problem would be much more difficult from a theoretical and engineering perspective than the gun design. In September 1943, John von Neumann, who had experience with shaped charges used in armor-piercing shells, argued that not only would implosion reduce the danger of predetonation and fizzle, but would make more efficient use of the fissionable material. He proposed using a spherical configuration instead of the cylindrical one that Neddermeyer was working on.

By July 1944, Oppenheimer had concluded plutonium could not be used in a gun design, and opted for implosion. The accelerated effort on an implosion design, codenamed Fat Man, began in August 1944 when Oppenheimer implemented a sweeping reorganization of the Los Alamos laboratory to focus on implosion. Two new groups were created at Los Alamos to develop the implosion weapon, X (for explosives) Division headed by George Kistiakowsky and G (for gadget) Division under Robert Bacher. The new design that von Neumann and T (for theoretical) Division, most notably Rudolf Peierls, had devised used explosive lenses to focus the explosion onto a spherical shape using a combination of both slow and fast high explosives.

The design of lenses that detonated with the proper shape and velocity turned out to be slow, difficult and frustrating. Various explosives were tested before settling on composition B as the fast explosive and baratol as the slow explosive. The final design resembled a soccer ball, with 20 hexagonal and 12 pentagonal lenses, each weighing about 80 pounds (36 kg). Getting the detonation just right required fast, reliable and safe electrical detonators, of which there were two for each lens for reliability. It was therefore decided to use exploding-bridgewire detonators. A contract for their manufacture was given to Raytheon. To study the behavior of converging shock waves, Serber devised the RaLa Experiment, which used the short-lived radioisotope lanthanum-140, a potent source of gamma radiation, in an ionization chamber.

Within the explosives was the 4.5-inch (110 mm) thick aluminum pusher, which provided a smooth transition from the relatively low density explosive to the next layer, the 3-inch (76 mm) thick tamper of natural uranium. Its main job was to hold the critical mass together as long as possible. It would also reflect neutrons back into the core. Some part of it might fission as well. To prevent predetonation by an external neutron, the tamper was coated in a thin layer of boron. A polonium-beryllium modulated neutron initiator, known as an "urchin" because its shape resembled a sea urchin, was developed by the Monsanto Company to start the chain reaction at precisely the right moment. This work with the chemistry and metallurgy of radioactive polonium was directed by Charles Allen Thomas and became known as the Dayton Project. Testing required up to 500 curies per month of polonium, which Monsanto was able to deliver. The whole assembly was encased in a duralumin bomb casing to protect it from bullets and flak.

The ultimate task of the metallurgists was to determine how to cast plutonium into a sphere. The difficulties became apparent when attempts to measure the density of plutonium gave inconsistent results. At first contamination was believed to be the cause, but it was soon determined that there were multiple allotropes of plutonium. The brittle α phase that exists at room temperature changes to the plastic β phase at higher temperatures. Attention then shifted to the even more malleable δ phase that normally exists in the 300 °C to 450 °C range. It was found that this was stable at room temperature when alloyed with aluminum, but aluminum emits neutrons when bombarded with alpha particles, which would exacerbate the pre-ignition problem. The metallurgists then hit upon a plutonium-gallium alloy, which stabilized the δ phase and could be hot pressed into the desired spherical shape. As plutonium was found to corrode readily, the sphere was coated with nickel.

The work proved dangerous. By the end of the war, half the experienced chemists and metallurgists had to be removed from work with plutonium when unacceptably high levels of the element appeared in their urine. A minor fire at Los Alamos in January 1945 led to a fear that a fire in the plutonium laboratory might contaminate the whole town, and Groves authorized the construction of a new facility for plutonium chemistry and metallurgy, which became known as the DP-site. The hemispheres for the first plutonium pit (or core) were produced and delivered on 2 July 1945. Three more hemispheres followed on 23 July and were delivered three days later.