Implosion bomb

It was possible at that point to estimate roughly the size and shape of a bomb that worked by fast implosion. The big gun bomb would be just under 2 feet in diameter and 17 feet long. An implosion bomb—a thick shell of high explosives stirrounding a thick shell of tamper surrounding a plutonium core surrounding an initiator—would be just under 5 feet in diameter and a little over 9 feet long a man-sized egg with tail fins. 

The challenge of initiator development was to design a source of sufficient neutron intensity that released those neutrons only at the precise moment they were needed to initiate the chain reaction. In the case of the uranium gun that requirement would be relatively easy to meet, since the alpha source and the beryllium could be separated with the bullet and the target core. But the implosion bomb offered no such convenient arrangement for separation and for mixing. Polonium and beryllium had to be intimately conjoined in Fat Man at the center of the plutonium core but inert as far as neutrons were concerned until the fraction of a microsecond when the imploding shock wave squeezed the plutonium to maximum density. Then the two materials needed instantaneously to mix. 

Early model Fat Man implosion bomb, upper segments removed to show interior. Overall diameter is about 5 feet. 

The complexity of the project for all the effort and support meant that atomic bombs (actually a misnomer, a more accurate name would be nuclear bomb) would not be available until 1945 and made the proposed target shift from Germany to Japan. But by July 1945 an implosion bomb was ready to be tested several days ahead of schedule. 

Oppenheimer acted quickly to maximize the laboratory s efforts to master implosion. Only if the implosion method could be perfected would the plutoniiun produced at Hanford come into play. Without either a plutonium gun bomb or implosion weapon, the burden would fall entirely on uranium and the less efficient gun method. Oppenheimer directed a major reorganization of Los Alamos in July 1944 that prepared the way for the final development of an implosion bomb. Robert Bacher took over G Division (for gadget) to experiment with implosion and design a bomb George Kistiakowsky led X Division (for explosives) in work on the explosive components Hans Bethe continued to head up theoretic studies and Deke Parsons now focused on overall bomb construction and... 

Los Alamos scientists successfully test a plutonium implosion bomb in the Trinity shot at Alamogordo, New Mexico. 

Inertial confinement fusion has long succeeded in the context of militai y explosions—the hydrogen bomb. In the militai y application a fission bomb produces x-rays that drive an implosion of D-T fuel to enormous temperatures and densities such that fusion reactions occur during the short time that inertia keeps the fusing nuclei densely packed and hot. 

In a nuclear weapon, the fissile material is initially subcritical. The challenge is to produce a supercritical mass so rapidly that the chain reaction takes place uniformly throughout the metal. Supercriticality can be achieved by shooting two subcritical blocks toward each other (as was done in the bomb that fell on Hiroshima) or by implosion of a single subcritical mass (the technique used in the bomb that destroyed Nagasaki). A strong neutron emitter, typically polonium, helps to initiate the chain reaction. 

Precisely controlled detonations are used to create an implosion of fissionable material to achieve critical mass in the explosion of an atomic bomb (See Vol 1, pp A499-504)... 

With uranium-235 separation started at Oak Ridge and plutonium-239 production under way at Hanford, a third laboratory was set up at Los Alamos, New Mexico, to work on bomb design. In order to create an explosion, many nuclei would have to fission almost simultaneously. The key concept was to bring together several pieces of fissionable material into a so-called critical mass, hi one design, two pieces of uranium-235 were shot toward each other from opposite ends of a cylindrical tube. A second design used a spherical shell of plutonium-239, to be detonated by an implosion toward the center of the sphere. 

A critical mass of material can also be arranged as a subcritical spherical shell which can be compressed into the supercritical sphere. This process is called implosion and is probably used in most weapons. The bomb over Hiroshima ( 15 kt) used uranium and was of the gun type, while the Nagasaki bomb ( 22 kt) was of the more efficient implosion type using plutonium. 

A three-dimensional squeeze inward was implosion. Neddermeyer had just defined a possible new way to fire an atomic bomb. The idea had been suggested previously, but no one had carried it beyond conversation. At a meeting on ordnance problems late in April, records the Los Alamos technical history, Neddermeyer presented the first serious theoretical analysis of the implosion. His arguments showed that the compression of a. .. sphere by detonation of a surrounding high-explosive layer was feasible, and that it would be superior to the gun method both in its high velocity and shorter path of assembly. ... 

It had been clear from the beginning that implosion, by squeezing a hollow shell of plutonium to a solid ball, could effectively assemble it as a critical mass much faster than the fastest gun could fire. What von Neumann and Teller now realized, and communicated to Oppenheimer in October 1943, was that implosion at more violent compressions than Neddermeyer had yet attempted should squeeze plutonium to such unearthly densities that a solid subcritical mass could serve as a bomb core, avoiding the complex problem of compressing hollow shells. Nor would predetonation threaten from light-element impurities. Develop implosion, in other words, and they could deliver a more reliable bomb more quickly. 

That necessity was painful, as the Los Alamos technical history makes clear The implosion was the only real hope, and from current evidence not a very good one. Oppenheimer agonized over the problem to the point that he considered resigning his directorship. Robert Bacher, the sturdy leader of the Experimental Physics Division, took long walks with him in those days to share his pain and eventually dissuaded him. There was no one else who could do the job, Bacher argued without Oppenheimer there would be no bomb in time to shorten the war and save lives. 

The implosion lens system von Neumann designed was made up of truncated pyramidal blocks about the size of car batteries. The assembled lenses formed a sphere with their smaller ends pointing inward. Each lens consisted of two different explosive materials fitted together—a thick, fast-buming outer layer and a shaped slow-burning solid inclusion that extended to the surface of the face of the block that pointed toward the bomb core ... 

Since Parsons had not succeeded in person in convincing Groves of the importance of bomb-assembly and bombing practice he wrote the general a forceful memorandum on the day after Christmas. There were major differences, he pointed out, between the gun gadget and the implosion gadget, particularly in terms of final assembly ... 

The demands of the implosion crisis in the autumn of 1944 reduced Trinity s priority, says Bainbridge, almost to zero. .. until the end of February 1945. With bomb physics well in hand by then Oppenheimer set the test shot s target date at July 4 and Bainbridge got busy. His staff of twenty-five increased across the next five months to more than 250. Herbert Anderson, P. B. Moon, Emilio Segr6 and Robert Wilson carried major re-... 

Disaster loomed again that day. The Creutz group at Los Alamos had fired the Chinese copy, measured the simultaneity of its implosion by the magnetic method and called Oppenheimer to report the dismaying news that the Trinity bomb was likely to fail So of course, says Kistiakowsky, I immediately became the chief villain and everybody lectured me. Groves flew in to Albuquerque in his official plane with Bush and Conant at noon they were appalled at the news and added their complaints to Kis-tiakowsky s full burden ... 

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