Because of the short time interval between spontaneous neutron emissions (and, therefore, the large number of background neutrons) found in plutonium because of the decay by spontaneous fission of the isotope Pu-240, Manhattan Project scientists devised the implosion method of assembly in which high explosives are arranged to form an imploding shock wave which compresses the fissile material to supercriticality.
The core of fissile material that is formed into a super-critical mass by chemical high explosives (HE) or propellants. When the high explosive is detonated, an inwardly directed implosion wave is produced. This wave compresses the sphere of fissionable material. The decrease in surface to volume ratio of this compressed mass plus its increased density is then such as to make the mass supercritical. The HE is exploded by detonators timed electronically by a fuzing system, which may use altitude sensors or other means of control.
The nuclear chain-reaction is normally started by an initiator that injects a burst of neutrons into the fissile core at an appropriate moment. The timing of the initiation of the chain reaction is important and must be carefully designed for the weapon to have a predictable yield. A neutron generator emits a burst of neutrons to initiate the chain reaction at the proper moment -- near the point of maximum compression in an implosion design or of full assembly in the gun-barrel design.
A surrounding tamper may help keep the nuclear material assembled for a longer time before it blows itself apart, thus increasing the yield. The tamper often doubles as a neutron reflector.
A thin beryllium reflector (thickness no more than the core radius) can reduce the total mass of the system, although it increases its overall diameter. For beryllium thicknesses of a few centimeters, the radius of a plutonium core is reduced by 40-60% of the reflector thickness. The critical mass for alpha-phase plutonium is 10.5 kg, and an additional 20-30% of mass is needed to make a significant explosion. A thin beryllium reflector can reduce this by a couple of kilograms.
Implosion systems can be built using either Pu-239 or U-235 but the gun assembly only works for uranium. Implosion weapons are more difficult to build than gun weapons, but they are also more efficient, requiring less SNM and producing larger yields. Iraq attempted to build an implosion bomb using U-235. In contrast, North Korea chose to use 239 Pu produced in a nuclear reactor.
In weapons with severe size (especially radius) and mass constraints (like artillery shells) some technique other than gun assembly may be desired. For example, plutonium cannot be used in guns at all [because the gun assembly is so slow it only produces a pre-detonation fizzle] so a plutonium fueled artillery shell [desirable because plutonium has a much smaller critical mass than uranium] requires some other approach. The two ends of a cylinder, could be driven toward each other to create a high-density sphere. This two-point detonation greatly reduced the diameter and the weight of the primary. The planar implosion process is some two orders of magnitude faster than gun assembly, and can be used with materials with high neutron background (i.e. plutonium).
A linear implosion allows for a low density, elongated non-spherical (football shaped) mass to be compressed into a supercritical configuration without using symmetric implosion designs. This assembly is accomplished by embedding an elliptical shaped mass in a cylinder of explosive. The explosive is detonated on both ends, and an inert wave shaping device is required in front of the detonation points. Extensive experimentation was needed to create a workable form, but this design enables the use of Plutonium as well as Uranium.
Two-Point Detonation Implosion
The Fat Man bomb had two concentric, spherical shells of high explosives, each about 10 inches (25 cm) thick. The outer shell consisted of a soccer-ball pattern of 32 high explosive lenses, each of which converted the convex wave from its detonator into a concave wave matching the contour of the outer surface of the inner shell. The inner shell drove the implosion. If the 32 [or more] lenses could be replaced with only two, the high explosive sphere could become an ellipsoid (prolate spheroid) of much smaller diameter. A carefully shaped ovoid of explosive, shaped like a rugby football, can create a spherical wave at the center by using only two-point detonation, one at each end.
Two-point detonation greatly reduced the diameter and the weight of the primary. This makes the weapon more reliable and of smaller cross section. The two-point detonation system for primaries made miniaturization of missile warheads possible. Modern US hydrogen bombs use a non-spherical core. Two-point detonation is used on warheads like the W88.
‘Joint Services Publication 538: Regulation of the Nuclear Weapon Programme’ (JSP 538) is the definitive UK Ministry of Defence guidance document on nuclear and radiological safety requirements within the UK nuclear weapons programme. the configuration of high explosive and nuclear components in the Trident-2 warhead ensure that, if the explosive were initiated at any single point, a nuclear explosion could not occur.
Under the guidance in Annex F of JSP 538 nuclear warhead designs should always take into account the possibility that the inadvertent detonation of the supercharge explosive at a single point could, in principle, lead to inadvertent nuclear yield rather than just a release of radioactive material. A nuclear warhead must be shown to be single point safe – an accident leading to detonation of the high explosive trigger at one point among many will not cause the warhead to go critical. This Safety Design Principle also applies to all warhead configurations that occur during assembly and disassembly processes. Nuclear warheads are designed to be single point safe such that in the event of inadvertent detonation of the supercharge initiated at a single point anywhere in its volume, the probability of any nuclear yield exceeding more than 2kg of TNT equivalent shall be less than 10-6.
In the first months of operations at Los Alamos in the spring of 1943, Oppenheimer and others believed that the first atomic bomb would be a gun that would shoot one piece of uranium or plutonium at a second piece of identical material. When the two pieces came together, a nuclear explosion would take place. From April 1943 until mid-summer 1944, almost all work at Los Alamos centered on designing and building such a gun. Experiments directed by future Nobel Prize winner Emilio Segre, however, demonstrated that plutonium could not be used in a gun. Impurities in the metal, which could not be removed, would cause a fizzle. It seemed, for a short time, that plutonium could not be used to make an atomic bomb. Because of serious problems in producing uranium, the plutonium problem put the entire atomic bomb program at risk.
The technical solution to this problem lay in the use of high explosives. Seth Neddermeyer proposed using the supersonic shock waves produced by high explosives to crush, or implode, a ball of plutonium to a supercritical state. If a ball of plutonium could be imploded symmetrically to a supercritical state, a nuclear explosion would follow. Seeing the technical merit of this approach, Oppenheimer reorganized the Los Alamos Laboratory in the summer of 1944 to concentrate work in this area. However, since this possible solution was new and untried, a test of such a gadget would be necessary.
In the late summer of 1943, experimental work at Los Alamos was focused on the designs for two gun-type atomic weapons. One would fire a uranium "bullet" into a uranium "target," while the other would use plutonium bullets and targets and, to overcome problems that might be caused by impurities in plutonium, would fire the bullet at a higher velocity.
The use of a gun to assemble nuclear explosives had been proposed as early as June 1942, and although it required considerable study of the nuclear properties of uranium-235 and plutonium in the shapes they would be cast in for use as bullets and targets, it seemed the most straightforward way to proceed.
It had also occurred to Richard Tolman, a professor of physics at the California Institute of Technology and vice-chairman of the National Defense Research Committee, that fissionable material might be assembled by detonating a high explosive around a hollow sphere and crushing it into a critical mass. Because of the difficulty of implementing this idea, however, few paid much attention to it. Robert Serber, a University of California physics professor, mentioned it in his indoctrination lectures at Los Alamos in April 1943 as one of "various other shooting arrangements" that had been suggested "but as yet not carefully analyzed."
Upon hearing Serber's lectures, Seth Neddermeyer, another professor of physics from Cal Tech, seized upon the idea enthusiastically. He recognized that blowing a sphere of uranium-235 or plutonium together in this matter would assemble these materials more rapidly than a gun could and proposed that it be explored. Oppenheimer agreed to a small program, which was set up on South Mesa.
The Ordnance Engineering Group (E-5) under Neddermeyer's direction, pursued experiments there and in Pennsylvania, at the Bruceton Explosives Research Laboratory of the NDRC. At Bruceton, "implosion charges" were fabricated for them by George Kistiakowsky of Harvard University, who was head of the project. Neddermeyer and Edwin M. McMillan, a University of California physicist who traveled there with him, were impressed that when a shell of explosives surrounding an iron pipe was set off, it closed the pipe. They returned to Los Alamos to repeat the experiment, varying the explosives, the pipe size and the arrangements, and studying the remains. A small plant was built at Anchor Ranch to cast the high explosives used in these experiments.
In September 1943, Oppenheimer asked John von Neumann, a Princeton mathematician who had been working on shaped charges, fluid dynamics and the computation of ballistic trajectories as a consultant to the Army's Aberdeen Proving Ground in Maryland, to look into the theoretical problems faced at Los Alamos.
Von Neumann agreed to spend some time as a consultant, working primarily in his office at the National Academy of Sciences in Washington, D.C., but with an occasional visit to Los Alamos. His first, in September, acquainted him with the implosion program. He suggested that shaped charges would produce an appropriate spherical detonation wave and pointed out that the method was not only likely to be faster than the gun, but that it would produce higher pressures and reduce the amount of active material required, making the bomb more efficient.
The Laboratory was galvanized by von Neumann's insight. Theorist Edward Teller scolded Charles Critchfield, who had been working on the project, for overlooking the greater efficiency to be expected from implosion, and Manhattan Engineer District Commander Leslie Groves chided Navy Capt. William Parsons for focusing on the "safer" gun method.
Teller advocated that the Laboratory should devote major effort to its development. In 1944 he was given the responsibility for all theoretical work on this problem. Teller made two important contributions. He was the first to suggest that the implosion would compress the fissile material to higher than normal density inside the bomb. Furthermore he calculated, with others, the equation of state of highly compressed materials, which might be expected to result from a successful implosion. However, he declined to take charge of the group which would perform the detailed calculations of the implosion.
Kistiakowsky was persuaded to come to Los Alamos to head a new program to develop the high explosives. A diagnostic program, involving X-ray and photographic techniques as well as the "terminal observations" Neddermeyer had employed, was begun.
New ideas for diagnosing an imploding system, including the use of a betatron electron accelerator, magnetic fields, electric pins and natural sources of radioactivity to produce signals that would indicate the rate of collapse inside the sphere, were subsequently introduced.
Calculations showed that an inward-moving spherical shock wave would be disrupted by the interference of detonation waves from the high-explosive segments and by instabilities arising as the tamper material was pushed into the heavier nuclear core by the implosion. This led to a fuller understanding of the behavior of a symmetric implosion and greater doubt that it could be achieved. What was needed was an explosive lens to convert the detonation wave to a spherically convergent form.
Under Kistiakowsky's direction, a new site, Sawmill, off S-Site, was constructed between December 1943 and May 1944. James Tuck, a member of the British Mission at Los Alamos, had worked in England on the use of combinations of different explosives to "focus" detonation waves and headed a group to develop an explosive lens for the implosion gadget. After von Neumann suggested a workable design for the lens, Lt. Cmdr. Norris Bradbury, a Stanford physics professor assigned to the Dahlgren Proving Ground of the U.S. Navy Ordnance Bureau, was recruited in June 1944 to solve the problem of casting the high explosives for the design.
Even if the appropriate explosive lenses could be produced, they would have to be set off simultaneously to create a symmetrical implosion. After experiments with a variety of Primacord and electric detonators, Luis Alvarez, a University of California Radiation Laboratory physicist who had come to the Laboratory from radar work at the Massachusetts Institute of Technology, and his student, Lawrence Johnson, devised such a system in May 1944.
Although progress had been made, Kistiakowsky was skeptical about the success of the program in the spring of 1944. He predicted that by October they might be able to "recommend a design of the gadget that will have a finite chance of properly functioning," but added that in "November or December the test of the gadget failed. Project staff resumes frantic work, Kistiakowsky goes nuts and is locked up." The consequences of such a failure, however, would be devastating to the program.
In the summer of 1944, Emilio Segre's group at Pajarito Site found that plutonium from nuclear reactors had an isotopic impurity, plutonium-240, that prohibited its use in a gun-type assembly. Since all of the plutonium that would be used in the atomic bomb would be produced in reactors, this meant that the vast investment in the Hanford production reactors built by DuPont would go down the drain unless implosion could be perfected.
The Laboratory was reorganized to accomplish this. New division, G for gadget and X for explosives, were set up to develop the nuclear and high-explosive components of the implosion device. The Laboratory's Governing Board was divided into administrative and technical boards to manage the growing effort. Even then, the Technical Board's tasks were increasingly assumed by lower-level interdivisional committees and conferences that coordinated the effort required.
The reorganization of the Laboratory was accompanied by a vast expansion in personnel, as no stone was left unturned in the search for a suitable design and the development of suitable components for the gadget. From roughly 1,100 personnel, Laboratory employment grew within a year to more than 2,500. Implosion meant an explosion of the Laboratory population.
It was not clear, however, that the much more complicated implosion device would work. Before it could be used in combat, a test would be required.
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