Nuclear Weapon Hydrodynamic Testing
Since the late 1940s, weapons engineers have used hydrodynamic tests and dynamic experiments in conjunction with nuclear tests to study and assess the performance and reliability of nuclear weapons primaries. In these types of experiments, test assemblies that mock the conditions of an actual nuclear weapon are detonated using high explosives. In hydrodynamic testing, non-fissile isotopes, such as uranium-238 and plutonium-242, are subjected to enough pressure and shock that they start to behave like liquids (hence the 'hydro' in hydrodynamic). Radiographs (x-ray photographs) can be used to obtain information on the resulting implosion; computer calculations based on these test results are used to predict how a nuclear weapon would perform.
Multiple view hydrodynamic testing (experiments to look at the flow of adjacent materials as they are driven by high explosives) and dynamic testing (experiments to study other effects of high explosives), combined with computer modeling, provide the only means of obtaining design data in the absence of nuclear testing.
The primary contains HE which surrounds a metal pit. When a weapon is detonated a series of steps occur very rapidly in a controlled sequence. First the HE is detonated. After the detonators are triggered, a wave of detonation passes through the main HE charge. The HE burn and the detonation wave can be affected by the type of explosive and its chemistry, the grain size, impurities, manufacturing method, and gaps in the HE assembly, among other things. If the HE does not detonate as designed, the pit may not implode properly but may still blow apart, scattering plutonium metal or other materials.
The pressure caused by the detonating HE causes a shock wave to travel through the pit material. The pit responds in a complex set of interactions as it implodes radially to a compact shape. As the shock wave crosses the pit, small amounts of material may be ejected from each interface, which may or may not affect the implosion. The response of the pit _ how the metal moves, flows, or melts, for example _ is complex and depends on dynamic materials properties which can be affected by factors associated with component fabrication as well as by the intrinsic properties of specific materials (particularly plutonium). If the pit does not implode properly, the boosting process may be affected.
The tritium-deuterium boost gas is heated by the pit implosion and the onset of the fissioning process. The heated boost gas undergoes nuclear fusion and generates large numbers of high-energy neutrons. These enter the fissile pit material and cause subsequent fissioning. These boost-induced nuclear interactions generate additional fission yield, "boosting" the nuclear yield of the primary. If boosting does not occur properly or is inadequate, weapons performance may be dramatically decreased.
Hydrodynamic tests and dynamic experiments have been an historical requirement to assist in the understanding and evaluation of nuclear weapons performance. Dynamic experiments are used to gain information on the physical properties and dynamic behavior of materials used in nuclear weapons, including changes due to aging. Hydrodynamic tests are used to obtain diagnostic information on the behavior of a nuclear weapons primary (using simulant materials for the fissile materials in an actual weapon) and to evaluate the effects of aging on the nuclear weapons remaining in the greatly reduced stockpile. The information that comes from these types of tests and experiments cannot be obtained in any other way.
Non-nuclear hydrodynamic experiments reveal the behavior of a nuclear weapon from ignition to the beginning of the nuclear chain reaction. These experiments consist of wrapping inert (nonfissile) material in a high explosive that is then detonated. The resulting explosive compression deforms the material, makes it denser, and even melts it. This process replicates the effects in the core of a nuclear device.
In a hydrodynamic test, inert material (e.g., 238U or a simulant for plutonium) is imploded to determine how well the high-explosive system functions. The testing program for an unboosted implosion device primarily ensures that the hydrodynamic behavior of the implosion (particularly of a hollow pit) is correct. The simplest way to do hydrodynamic testing is to implode inert pits made of a simulant for fissile material (e.g., natural uranium instead of HEU) while using any of several "old fashioned" means to observe the behavior of the heavy metal. One such technique is to use a pin-dome, essentially nothing more than a precisely machined insulating "champagne cork" with a large number of protruding radial pins of different distances placed at the center of the implosion region.
Hydroshots tests are conducted to test the hydrodynamic performance of the shaped explosives used in the ordnance. The explosive device used in the hydroshot testing comprised an explosive charge shaped as a hemisphere, about half the size of a basketball and weighing from 1-3 kg (2.2 to 6.6 lb). The explosive charge was surrounded by a DU ring about 1-2 inches in height and weighing about 22 kg (48.5 lb). The purpose of the DU ring was to simulate the hydrodynamic conditions in a fully spherical weapon.
Pin dome experiments are probably the easiest hydrodynamic diagnostics available. However, backlighting the pit with a flash x-ray or neutron source to obtain an actual picture of the imploding material is also a possibility. Generally, the flash x-ray source needed has to have very high peak power available in a single pulse, and the timing and firing of the source in concert with the implosion of the device requires very sophisticated system design. Backlighting the imploding system with a neutron source is a bit more straightforward, but requires very sophisticated neutron optics and imaging capability, which could be difficult to obtain. Iraq used flash x-ray diagnostics.
Pin hydrodynamic tests monitors changes in the implosion behavior of HE. The test assembly comprises three main subassemblies: a pin-dome assembly, a mock pit, and the HE. The test measures elapsed time from initiation until the explosive drives the mock pit into an array of timing pins, a "pin dome," of known length and location. The HE implodes the mock pit onto the timing pins, which provide data about the temporal and spatial uniformity of implosion. A nonuniform implosion could indicate an HE problem. Excessive density variations, voids, or cracks in the HE, for example, can disrupt the shock-wave propagation from the detonation.
High-speed radiographic images of the implosion process are taken with a powerful x-ray machine. Data from the FXR's x-ray images are used to verify and normalize computer models of device implosions. In the absence of nuclear testing, scientists rely on these computer calculations to develop the judgment necessary to certify the safety and reliability of nuclear weapons. The x-rays penetrate and are scattered or absorbed by the materials in the device, depending upon the density and absorption cross section of the various interior parts. The x rays that are neither absorbed or scattered by the device form the image on photographic emulsions or on the recording surface in a gamma-ray camera.
The Radio Lanthanum (RaLa) method, which does permit time-dependent measurements of the symmetry of an implosion, should be mentioned because of its conceptual simplicity. RaLa was used extensively during the Manhattan Project, but has probably not been employed very often since then. An intensely radioactive sample of the element lanthanum was prepared in an accelerator or reactor and then quickly inserted into the center of the implosion test device. Highly collimated Geiger-Mueller counters observed the behavior of the material as it imploded. The RaLa technique is inherently fairly crude in its ability to detect asymmetries and environmentally unappealing because the radioactive material is scattered about the test stand. However, the isotopes have half lives of only a few hours to a few days, so the residual radioactivity decreases significantly in a week or so.
A Snowball test checks reliability of the initiation chain by confirming that the booster initiates the HE. A machined shell of explosive is assembled with a booster and detonator to form a "snowball." When this assembly is fired, a streak camera captures spatial and temporal information of the initial, or "breakout," detonation wave on the outer surface of the explosive snowball. The relatively flat curves at the bottom of the image data indicate a good, uniform explosion. Changes in the breakout profile would be used to track the performance of the booster and the condition of the interface with the HE.
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