Nuclear Weapon Testing
Nuclear weapons, to quote Sidney D. Drell, are "sophisticated but not complicated." That is, the working principles are straightforward, although the equipment needed to make a device function, and function reliably, is quite sophisticated and requires high-quality engineering to design and build. A nuclear weapon may contain over 6,000 parts; the nuclear package, which contains the weapons primary and secondary assemblies, has about 300 parts. Although it is generally believed that a proliferator need not test a conservatively designed device at full yield to have confidence in it, some experimentation and testing along the way is necessary to demonstrate the behavior of the non-nuclear components including the firing set, detonators, and neutron generators. If there is not to be a full-yield nuclear test, then the non-nuclear experiments must be carried out with greater care and competence.
One reason for believing that a full-yield nuclear test is unnecessary is that each of the six states known to have tested nuclear devices has achieved a nuclear detonation on the first try. The first nuclear weapon used in combat used an untested gun-assembled design, but a very simple and inefficient one. The first implosion device was tested on July 16, 1945, near Alamogordo, New Mexico, and an identical "physics package" (the portion of the weapon including fissile and fusion fuels plus high explosives) was swiftly incorporated into the bomb dropped on Nagasaki.
The term "nuclear testing" encompasses all experiments in which special nuclear material (or a simulant) is placed in contact with high explosives, which are then detonated, or with a propellant, which is ignited. This limitation deliberately excludes activities which are more scientific in nature and not intimately connected with the progression from fissile material and/or fusion fuel to a nuclear explosive device. This definition is far broader than that of the Comprehensive Test Ban Treaty (CTBT) of 1996, which prohibits only nuclear weapon test explosions and other nuclear explosions. Many states of concern for nuclear proliferation have subscribed to the CTBT, and may, therefore, find it difficult to conduct full-yield tests either underground or in the atmosphere. At the lowest end of the nuclear yield distribution from hydronuclear tests, some states might reckon that the knowledge gained from a small explosive release of nuclear energy would be worth the risk of getting caught. Generally, within the U.S. Government, the condition of prompt nuclear criticality distinguishes, under the CTBT, a prohibited test of an explosively assembled device from one which is allowed.
Nuclear Yield Testing
Fundamentally, test programs can be divided into two major categories: those for an HEU-fueled, gun-assembled device and those for an implosion device using either plutonium or HEU. The first Chinese test was of an HEU implosion device, Iraq intended to develop just such a weapon, and the South Africans conducted no nuclear tests of their gun-assembled devices. The general design of a gun-assembled device is straightforward and based on well-understood principles of artillery weapons; however, the technology for obtaining enriched uranium is complex. On the other hand, implosion-assembled devices using plutonium-which could be extracted simply using chemical techniques from reactor rods-are more difficult to manufacture. If a nation had an indigenous reactor industry, such extraction would be straightforward.
The testing program for a gun-assembled device is moderately complex, but it is essential to realize that nothing nuclear need be tested to verify the probable operation of such a device-only its conventional components. The design of Little Boy, the bomb dropped on Hiroshima, had not been proof tested before the war shot.
The testing program for a simple fission device using plutonium must be more extensive than that for a gun-assembled device using enriched uranium. For example, the constructor must know that his fissile "pit" will be uniformly compressed and that the compression will be rapid enough to minimize the chances for a pre-initiation "fizzle," that any neutron generator present will fire at the correct moment, and that compression is likely to be maintained long enough to result in significant nuclear yield.
A proliferator hoping to demonstrate its technical prowess may elect to pursue an implosion device despite the availability of enriched uranium. Alternatively, it may choose implosion to achieve greater efficiency in the use of special material. It can be presumed that this type of proliferator will forego the development of thermonuclear weapons.
From 1945 through much of 1991, the United States detonated more than 1,200 nuclear devices with yields from a few pounds to about 15 megatons. Until the middle of 1963, most U.S. (and Soviet) tests took place in the atmosphere; some were con-ducted underground, a few were below the surface of the ocean, and roughly a dozen American shots took place at altitudes above 10 km. The largest test ever conducted, that of a 60-megaton device, was carried out in the Arctic by the USSR. Since the Limited Test Ban Treaty (LTBT) was signed in 1963, all U.S., UK, and Soviet nuclear detonations have been underground. The French and Chinese, while not parties to the LTBT, gradually moved their testing from the open atmosphere to subterranean sites-in boreholes, mine shafts, and in drill holes beneath the ocean floor.
Atmospheric tests are easier to carry out -- although impossible to conceal -- and for technically less-sophisticated powers provide more information in a more direct manner than do underground explosions. A weapon detonated from a several hundred foot high tower or suspended from a tethered balloon permits photography of the evolution of the nuclear fireball and the cloud. The shock wave in air can be observed, and one can determine the effects of the weapon on real targets such as structures and vehicles.
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