Nuclear Weapon Testing Signatures
Every stage of nuclear weapon development, from material production to deployment, can generate signatures that provide some indication of a weapon program's existence or status, although only a few of them point fairly un-ambiguously to a nuclear weapon program.
Repeated high explosive (HE) tests are generally required before a workable implosion-type nuclear weapon can be designed. Explosive tests to study either the HE alone or its ability to propel metal objects would usually require electronic or optical instrumentation. Some indicators of high-explosive testing activity are the following:
- expansion of facilities or personnel at or near an existing ordnance plant;
- purchase or production of explosives more energetic than pure TNT, such as RDX, HMX, or PETN, any of which could be mixed with TNT;
- equipment for compacting or melting and casting HE, perhaps modified from what would be used at a standard ammunition loading plant;
- alternatively, for different types of explosives, isostatic or hydrostatic presses, weighing many tons and likely remotely controlled (some antitank shaped-charges are also made using such presses);
- precision, possibly template or computer-numerically controlled, two-axis machining facilities for HE, especially if suited for machining curved contours and surrounded by blast-protection shielding;
- waste and scrap from the above operations, possibly including effluent waste-water systems involving filters or catch basins;
- pronounced red coloration in waste water caused by dissolved TNT;
- solid scrap periodically destroyed by burning or detonation; and
- instrumented firing stations and control bunkers for HE or HE-metal tests using charges weighing up to hundreds of pounds.
Test-firing of HE-metal systems containing uranium would be indicated by the following:
- bright streamers radiating from the test (caused by burning fragments of uranium) visible to the eye;
- local debris or dust that contained uranium; and
- nearby fire-extinguishing equipment, portable radiation monitoring equipment, or permanent air-sampling radiation-monitors.
Since highly dense (but nonfissile) uranium-238 is widely used in certain types of antitank weapon, these indicators could also stem from advanced nonnuclear munition programs. Therefore, most or all of these could be associated with conventional munitions production and do not give unambiguous evidence of nuclear weapon development. However, spherically symmetric implosions would be more likely connected with a nuclear program.
Gun-type weapons generally require highly enriched uranium surrounded by neutron-reflecting material such as natural uranium, tungsten alloy, or beryllium metal or oxide (ceramic). A development program might use hundreds of pounds of beryllium or thousands of pounds of uranium or tungsten for the neutron reflector alone. Unusually high importation of some of these items by certain countries might suggest weapon-development activity. In addition, ground cover at the detonation test-site may be cleared in only one direction, since the debris from tests-and especially burning uranium streamers if natural uranium were used as a mockup for HEU-would be concentrated in a cone coaxial with the direction of projectile firing (however, a test program for nonfissioning shaped charges or kinetic-energy rounds could also have such a configuration); special fast-acting very-high-pressure gauges might be used to record the pressures in the gun breech; and distinct acoustic features might be observable.
Observers who had access to suspicious laboratories might detect the following signatures:
- Criticality tests -- weapon designers using near-critical fissile assemblies may wish to measure criticality with closed-circuit television and neutron counters in remotely operated (possibly underground) experiments. However, experiments can also be performed at the bench-top level, not needing elaborate equipment, and much of the relevant data is already available in the open literature. Moreover, similar facilities are also used for agricultural and biological neutron-irradiation research. Any kind of criticality accident at a suspect site, however, would be a strong indicator of weapon-design activity, since other applications would be ununlikely to work with near-critical assemblies.
- Neutron background measurements -- for gun-type devices, neutron-flux measurements would be required to assure that background neutron counts were sufficiently low. Such measurements might be indicated by a room containing neutron detectors that was shielded from external sources of neutrons, for example with water- or polyethylene-filled walls. Such facilities might also be used to test neutron initiators.
- Development of neutron initiators -- neutron initiators produce a pulse of neutrons to initiate the nuclear chain reaction at the optimum moment. They use either alpha-particle-emitting radioactive substances or small particle accelerators con-taining radioactive tritium gas. Therefore, import or production of alpha-emitting materials, tritium, or the special facilities to handle them (similar to those used for spent-fuel reprocessing) could indicate weapon development. However, small accelerator-based neutron sources are produced commercially for oil-well logging and laboratory use, so that they do not necessarily indicate a weapon program.
- Special tests -- Since neutron initiation is so important to the proper detonation of a nuclear device, tests involving actual HE with very small (sub-critical) amounts of nuclear material would likely be carried out as well. These might be conducted in shallow underground chambers designed for neutron shielding. (A series of such hydronuclear experiments was conducted by the United States during the testing moratorium of 1958-61.) Some of the surface equipment associated with these tests might also be telling.
Regardless of whether a country chooses to test a nuclear device at full yield or at reduced yields, a suitable underground site would be highly desirable. Since underground tests can be contained quite effectively when carried out properly, test-preparation activities would often be more observable than would atmospheric releases from tests themselves. Drilling rigs, sections of one-meter-diameter or larger pipe, mining operations, or road construction in new remote locations could all indicate such preparations and could probably be observed by reconnaissance satellites. (Determining that such activities actually do pertain to nuclear testing, however, may prove more difficult.) Large drilling equipment, cranes, heavy electrical cables, and roads could all provide visual indicators of such a test site. Electronic data-acquisition systems, which are widely available around the world, would require extensive cabling systems suitable for transmitting diagnostic signals might also be visible.
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