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Jun 17, 2008 | 21:45 GMT

Nuclear Weapons: Devices and Deliverable Warheads

Summary
On July 16, 1945, at a remote testing range in southern New Mexico, the United States detonated the world's first atomic bomb. Developing the device was probably the most complex and expensive exercise in applied physics in human history. Even today, weaponizing the atom remains one of the most challenging endeavors a country can engage in — and one few ultimately choose.
Editor’s Note: This is the first in a series of analyses on the feasibility and relevance of nuclear weapons in the 21st century. The Washington Post reported June 15 that the A.Q. Khan network could have circulated plans for building an effective nuclear warhead, i.e., a nuclear device small enough to be effectively delivered atop a ballistic missile. The possibility that Iran or North Korea could have received the plans makes an understanding of nuclear weaponry — its historical development and current feasibility — all the more relevant in a discussion of contemporary geopolitics. First, two key definitions and an important distinction. Developing a nuclear weapon is far more complex than simply obtaining a device capable of initiating a nuclear explosion, and there is a vast difference between a nuclear device and a nuclear weapon:
  • A nuclear device is simply an apparatus that can initiate an uncontrolled nuclear chain reaction with sufficient fissile material to make a very large hole in the ground. Indeed, both “Little Boy” and “Fat Man” — the atomic bombs dropped on Hiroshima and Nagasaki, respectively — were little more than crude nuclear devices, despite the immense complexity of their groundbreaking design and construction. A nuclear device can be as large as a room. In 1952, the world's first detonation of a “thermonuclear” or fusion device (a hydrogen bomb) was achieved with a device the size of a small building (the so-called “Ivy Mike” apparatus reportedly was referred to as a “thermonuclear installation”). In 2006, the most that North Korea could have tested was a nuclear device, and it may have been something even less. A nuclear device may be “deliverable” in some scenarios, but it is not necessarily of the appropriate scale or robustness to offer a reliable military-strike capability.
  • A nuclear weapon, on the other hand, is a robust, reliable and miniaturized nuclear device (a warhead) that has been combined with a similarly robust and reliable delivery system. The importance of this synthesis should not be underestimated. Deliverability is a key feature of a nuclear weapon — and it must be a practical, militarily efficient means of delivery with a high probability of success. The challenges of achieving this synthesis are extensive. For a nuclear device to be deployed as a ballistic missile warhead, as a cruise missile warhead or as a gravity bomb, a series of very significant technical hurdles must be surmounted, including nuclear physics, materials science, rocketry, missile guidance and the like.
The delivery of Little Boy and Fat Man, crude devices, was made possible only by the parallel development of the B-29 “Superfortress,” at the time the world's largest and longest-range heavy bomber. It was the B-29 that weaponized Little Boy and Fat Man. Today, modern strategic warheads sit in clusters atop intercontinental ballistic missiles with guidance systems that ensure accuracy within a few hundred yards and fuses that ensure detonation after the extreme stress of launch, the cold vacuum of space and the heat and speed of re-entry. A nuclear device does not come easy. A nuclear weapon is one of the most advanced syntheses of complex technologies ever achieved by man.

The Beginning

By the end of World War II, the United States' Manhattan Engineering District involved the abundance of people and resources of a major U.S. industry. Begun in 1942, the so-called Manhattan Project was a privileged beneficiary of the country's massive wartime industrial base and drew upon the scientific expertise of the country's — and the world’s — most esteemed and talented physicists. The entire undertaking was driven by the urgency that can only be applied by a fully mobilized nation engaged in a global two-front war. And the product of all that urgent effort almost came too late. By the summer of 1945, the war in Europe had been won and the Japanese had been beaten back, unbowed, in the Pacific. Most major cities on the home island of Honshu had been devastated by incendiary bombing, and the Japanese were talking about surrendering. By this point, the highly enriched uranium sufficiently refined for use in a nuclear weapon was still in such short supply that the relatively simple gun-type design of Little Boy — used against Hiroshima on August 6 — was not tested before the bomb was released on the city. (The Trinity device tested on July 16 was of the same more complex implosion design as Fat Man, which was used against Nagasaki three days later.)

Getting There

Ultimately, what made the Manhattan Project unique (aside from its almost limitless resources) was the fact that it succeeding in developing a nuclear device before anyone else did. The basic principle of an uncontrolled nuclear chain reaction was theoretically sound (there was even the short-lived concern that the uncontrolled nuclear chain reaction would spread to the nitrogen in the Earth's atmosphere, with apocalyptic consequences). But because of the complexity of its development, the success of the Trinity device was far from certain. When it proved successful, every subsequent nuclear program in the world could work toward a known goal with increasingly known parameters — and with increasing help from early adopters. The French, for example, had limited ties to the original Manhattan Project, while Soviet efforts were propelled by the ruthlessness of Joseph Stalin and a successful espionage program (which accelerated the nuclear program by several years). The Soviets then helped the Chinese (very nearly giving them a fully assembled nuclear device) and the North Koreans before eventually cutting off their support. Both China and North Korea were left with substantial foundations on which to build nuclear weapons programs. The inherent dual-use of civilian nuclear technology for power generation has also proven pivotal at times. Even well-established nuclear powers occasionally shop around for assistance with reactor construction for power-generation purposes. Both Pakistan’s and North Korea's nuclear efforts have ties to A.Q. Khan, who learned much about civilian nuclear technology while working as a scientist in the Netherlands. Nevertheless, to this day, no country other than the United States has ever completely and independently developed its own nuclear infrastructure and its own nuclear weapon. The fabrication of fissile material alone — the one true limiting factor in the development of a nuclear device — presents significant challenges. The concept of separating a heavier isotope of uranium from a lighter isotope of uranium in order to enrich the stock to higher than 80 percent U235 — sufficient for use in weapons — is well understood. Separating something heavier from something lighter in a gaseous state is not all that hard. But doing it on a sufficiently refined level to separate two isotopes differentiated by only a few subatomic particles is extremely difficult. The alternative, reprocessing plutonium, is a chemical process not nearly as challenging as enrichment but it is extremely nasty, producing deadly levels of radioactivity, and it can only be done after plutonium has been created inside a nuclear reactor. Suffice it to say that, in practice, neither way of fabricating fissile material is simple. While Iran is currently enriching uranium in centrifuges, it is not clear that the centrifuges are anywhere near sufficient quality to achieve high levels of enrichment. And despite a concerted national effort, the Iranians seem to be struggling to bring a meaningful number of centrifuges online. Compared to the challenges of enrichment, the fabrication of a simple gun-type device like Little Boy is comparatively simple, though precise and extensive calculations are still required. But only uranium can be used in a gun-type device; plutonium requires the far more complex method of implosion, which presents numerous challenges, including the precise “lensing” of high-grade explosives. The purity of the lenses, their arrangement and the timing of the detonation must all be carefully crafted and coordinated to create a perfectly symmetrical explosion that compresses the plutonium core to a supercritical mass. Again, theoretically, it is a fairly understandable concept. In practice, however, it requires a great deal of knowledge and expertise. The creation of even the most primitive implosion device during the Manhattan Project challenged the best scientific minds and technology available at the time. The fabrication of fissile material and the development of either a gun-type device or an implosion device is a process that only nine or 10 countries in the world have accomplished. Of those countries, South Africa has since renounced and dismantled its nuclear weapons program while North Korea may or may not have a working device.

Weaponization

To move beyond the device stage toward weaponization, numerous other technological barriers come into play. First, delivery systems must be devised and both the bomb design and the payload capacity for the delivery system appropriately tailored. The delivery system itself — whether air-drop, cruise missile or ballistic missile — involves significant technological challenges, including aircraft design, subsystems integration and the development of complex guidance and propulsion systems. Indeed, these remain developmental challenges for many established nuclear powers. Ballistic missile design is an especially complex undertaking — to say nothing of mating such missiles with a submarine for undersea launch. In each case, the physics package (the components of the bomb that actually initiate a nuclear explosion) must be significantly miniaturized to one degree or another. A modern re-entry vehicle is a steep conical shape shorter than a human being that contains an even smaller physics package weighing only a few hundred pounds. Getting a warhead down to this size is no easy task. It requires, among other things, precision manufacturing, exceptional quality control and a keen understanding of nuclear physics. Then there are the decades of testing and practice necessary to ensure detonation upon delivery, national command authority controls and the like. Indeed, U.S. national laboratories still use some of the world's most powerful supercomputers to model the effects of age on the current U.S. nuclear arsenal. Developing a nuclear weapon is not simply a matter of money, resources and brains. It also is the product of decades of testing (now frowned upon by the world community), design experience, numerous fielded weapons and a sustained annual investment of billions of dollars. An aspiring nuclear power today does not have such options. The frantic pace of the Cold War arms race is over, nuclear testing is almost universally banned and the costs imposed by the international community (economic sanctions, geopolitical ostracism) can be higher than the costs of developing and maintaining a program. Thus, the calculus to proceed with such an endeavor has proved to be a discouraging one for most countries. Only Pakistan and possibly North Korea have joined the club since the fall of the Berlin Wall in 1989. Next: Are nuclear weapons relevant?
Stratfor
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