"Pentagon eyes age of 'mini-nukes' ; Small weapons for small conflicts"

1993-06-16, syndicated to editors of monopolist newspapers in the USA owned by the Knight-Ridder company, and published in an edited form [http://articles.baltimoresun.com/1993-06-16/news/1993167188_1_nuclear-weapons-warhead-small-nuclear] [https://archive.today/gBtEh].
An extended version was posted 2014-09-11 at [http://www.4thmedia.org/2014/09/pentagon-eyes-age-of-mini-nukes-small-weapons-for-small-conflicts/] [https://archive.today/CFXHq].
What follows is the complete version, including all 3 parts, with additional educational materials, as syndicated 1993 to 1997.
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"Pentagon eyes age of 'mini-nukes' ; Small weapons for small conflicts"
(1993-06-16, by Knight-Ridder Newspapers):
WASHINGTON — Some Pentagon planners hope the end of the Cold War will signal the start of a new era — the age of “mini-nukes,” small nuclear weapons that might be used in future Third World conflicts.
Among the gleams in the nuclear planners’ eyes is a “micro-nuke” with the explosive power of just 10 tons of TNT, an item that might be suitable for jobs like blasting Iraqi dictator Saddam Hussein out of his Baghdad bunker.
A request to fund research into the new generation of small atomic weapons is included in President Clinton’s 1994 budget proposal for the Department of Energy.
Not surprisingly, the idea has sparked a blast from nuclear foes.
“Nuclear zealots couldn’t care less that the Cold War is over,” Bill Arkin, a nuclear researcher with the environmental group Greenpeace, complained yesterday. “What is shocking, though, is that the Clinton administration tolerates, and even supports, these new programs.”
Mr. Arkin described the fledgling program in a report in the July-August issue of the Bulletin of Atomic Scientists. White House and Pentagon officials acknowledged yesterday that such work is under way, but they would not comment further.
In April, Gen. Lee Butler, commander of the Pentagon’s nuclear forces, told Congress that he “is working with selected regional commands to explore the transfer of planning responsibilities for employment of nuclear weapons in theater conflicts.”
And two of the nation’s pre-eminent nuclear research labs — the Lawrence Livermore Lab in California and the Los Alamos Lab in New Mexico — want to press ahead with development of what the Clinton budget proposal calls a “precision, low-yield warhead.”
The proposed 10-ton “micro-nuke,” would pack a punch 10 times the size of the largest non-nuclear bombs dropped by U.S. forces during the Persian Gulf War. It would be 1/500th the size of the B-61, currently the smallest nuclear warhead in the Pentagon inventory.
The labs also are weighing development of a “mini-nuke,” with the explosive power of 100 tons of TNT, to destroy nuclear, biological and chemical warheads in flight, according to Los Alamos documents.
A third warhead — known as the “tiny-nuke” — would have the power of 1,000 tons of TNT and might be used against enemy ground troops. The Army, which in recent years gave up all of its battlefield nuclear weapons, had nuclear artillery shells about this size.
The Los Alamos documents declare that “any long-term nuclear stockpile should include several hundred low-yield nuclear weapons systems.”
Mr. Arkin’s report traces the growing support for these weapons among the military, which is a key element in gaining Pentagon support for their production.
The small nukes would “protect U.S. deployed forces” while denying “sanctuary to nuclear-armed leadership” of Third World nations, it said.
The weapons, according to the Los Alomos documents, also would “discourage proliferation” by deterring “future Third World nuclear states.”

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The first approach to lowering the yield of a nuclear explosion is based on how high you compress the fissile material.
It is well known that the critical mass of a fissile material is inversely proportional to the square of its density. Thus the more you compress it the less you need or the more that you compress it the bigger the bang. At very high compression levels of 1.6 and above. There is no limit on the minimum amount of fissile material required to construct a nuclear weapon. 1 gram of fissile material will be equal to 1kilogram or more of TNT. There fore mini nukes with explosive yields in the several ton range will only require fissile material in the 100’s of grams range or less. This is only limited by current implosion technology. In the future Electro-Magnetic, Microwave and L.E.D. Laser compression will eventually replace simple chemical implosion systems.

The second approach is called the fizzle effect.
In the fizzle mode you simply pre-initiate the nuclear chain reaction in the fissile material while it is in a super critical state. As a result the yield of the explosion is reduced compared to it’s normal detonation value. All nuclear weapons will have a non zero fizzle value no mater what size. There are two types of nuclear weapons composed of fast and slow implosion systems. Depending on the waiting time between the start of criticality and the moment of optimal condition. The fizzle effect is more probable in a slower gun type compression system with a fissile material with a higher level of neutron self emission (spontaneous fission). Therefore a very small nuclear device using a single gun type compression system can be made out of reactor grade or weapons grade Plutonium. The fizzle effect is of a statical nature where the main concern would be at the moment of neutron occurrence during the waiting period just before criticality occurs.
Natural fizzle yields of up to 10 to 20 tons have been estimated for a high speed compression system that combines two sub-critical masses of plutonium at about 300 meters per second. A yield of about 5 times lower would be expected at a speed of 100m/s. 2 to 5 tons. With a 5KG mass of plutonium accelerated at 100 meters per second, its kinetic energy would be equal to 25 KJ of energy. The energy if provided by TNT would be about 4 MJ/kg. So the amount of explosives needed to properly compress the Plutonium into a critical mass would be far less than 1KG of TNT. Simply by using a greater amount of high explosives for the same amount of plutonium, (5KG) it will produce higher velocities and compression rates that would create an even bigger yield in the 100 ton and up to the low kiloton range. 5KT max.

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It was not long before the scientists realized that in creating the tiny “core weapon” for the hydrogen bomb, they had also created a relativelylightweight micro nuclear weapon that could be carried by a single soldier for various uses against high value targets, including hydroelectric power stations and bridges. Less than two years later, the first of the SADM [Special Atomic Demolition Munition] series shown at the top of this page was pressed into operational service. The “standard” SADM that evolved would eventually have a core of Plutonium 239 encased in a thin shell of non-fissile Uranium 238 known as a “neutron reflector”. Plutonium and Napalm flash burns – but can you tell which are which?
When the 10-ton TNT equivalence SADM went critical, it obviously created far less radiation than the huge and inappropriately named “Little Boy” at Hiroshima, but still produced dangerously high levels of residual radiation. Most of this came from SADM’s very “dirty” Uranium 238 reflector, which along with its Plutonium 239 core, exploded into millions of particles at the point of criticality. This same non-fissile Uranium 238 material still causes serious illnesses today, after being fired by American tanks and aircraft as sub-critical Depleted Uranium [DU] shells or missile warheads. Ask anyone in southern Iraq and Kosovo how sick this stuff can make you.
The years rolled by and top-secret projects were initiated in America and Israel to replace the old SADM with its heavy weight and excess radioactivity, culminating in the successful development and testing at Dimona during 1981 of the “new” micro nuclear device. Using advanced nuclear physics, the scientists found a way of detonating the new “suitcase” bomb without the use of a Uranium 238 reflector, and further refined the Plutonium 239 in its core to 99.78%. These measures resulted in a weapon considerably smaller and lighter than SADM, which also had another enormous advantage.
The new Dimona micro nuke was the very first critical weapon that could be used in “stealth” mode. Gone was the dirty Uranium 238 reflector, and up went the purity of the smaller Plutonium 239 core. Plutonium emits only alpha radiation, which is for all practical purposes “invisible” to a standard Geiger counter. In direct contrast with its more deadly cousins beta and gamma, alpha can travel only a few feet and is incapable of penetrating human skin.
Remember that this micro nuke is a tiny weapon in terms of critical mass, with its limited number of particles distributed over a very wide area. You will have to be within five feet to detect a single particle. Though the alpha particles cannot penetrate the skin, such radiation is extremely hazardous if inhaled because Plutonium is the most toxic substance known to man. If you breathed in a mouthful immediately after the blast you would be dead in less than an hour, perhaps within minutes.

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Smallest fissile material mass which under fixed conditions (type of fissile material, geometry, moderated/immoderate system, etc.) initiates a self-perpetuating chain reaction.
The table contains the minimum critical mass for some nuclides under certain conditions. Note in a non compressed form.

Smallest critical mass in spherical shape for aqueous solution at optimum moderation (1st set of numbers, on left): Unreflected (kg) / Water-reflected (kg)
Smallest critical mass in spherical shape for metal (fast systems) (2nd set of numbers, on right): Unreflected (kg) / Steel-reflected (kg)
Isotope U-233: (1.080 / 0.568) (15.8 / 6.1)
Isotope U-235: (1.420 / 0.784) (46.7 / 16.8)
Isotope Np-237: (– /  –) (63.6 / 38.6)
Isotope Pu-238: (– /  –) (9.5 / 4.7)
Isotope Pu-239: (0.877 / 0.494) (10.0 / 4.5)
Isotope Pu-240: (– /  –) (35.7 / 19.8)
Isotope Pu-241: (0.511 / 0.246) (12.3 / 5.1)
Isotope Am-241: (– /  –) (57.6 / 33.8)
Isotope Am-242: (0.042 / 0.020) (8.8 / 3.0)
Isotope Cm-243: (0.280 / 0.127) (8.4 / 3.1)
Isotope Cm-244: (– /  –) (26.6 / 13.2)
Isotope Cm-245: (0.116 / 0.054) (9.1 / 3.5)
Isotope Cm-247: (4.060 / 2.180) (6.9 / 2.8)
Isotope Cf-249: (0.129 / 0.060) (5.9 / 2.4)
Isotope Cf-251: (0.048 / 0.025) (5.5 / 2.3)
Smallest critical masses for some fissile material under certain boundary conditions

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Techniques for Warhead Design
There are seven broad categories of techniques that can assist in the design of new warheads without full-scale testing:

1. Nuclear explosions ranging from a few tens of pounds to a few hundred tons of TNT equivalent or less that are not quite full-scale explosions, but which yield most of the crucial information about the functioning of the weapon, other than its exact explosive yield.

2. Small-scale nuclear explosions with a nuclear yield of a few tens of pounds or less (hydronuclear testing).

3. Tests of many of the properties of nuclear charges using materials that cannot sustain chain reactions (hydrodynamic testing).

4. Experiments in nuclear fusion to develop understanding of the thermonuclear component of weapons as well of the deuterium-tritium boosters that make the fission components of warheads more efficient.

5. Computer modelling

6. Theoretical models and calculations (other than computer models).

7. Other related experiments, field tests, theoretical work, and modeling exercises, for instance using nuclear reactors, conventional explosives, etc. to determine the properties of various components and subassemblies of warheads. This includes work on basic science in various disciplines such as nuclear physics and radiochemistry.

Nuclear weapons have been successfully designed without full-scale tests. In fact, the bomb dropped over Hiroshima was not tested prior to its wartime use. That is because Manhattan Project scientists and engineers were very confident that the relative simplicity of the “gun-type” design combined with the various theoretical, laboratory and non-nuclear field tests they conducted were sufficient to guarantee success. In contrast they were far less sure of the implosion design that was needed for the plutonium weapon. One reason was that the timing of the firing of the conventional explosives was so critical that they could not predict the performance of the weapon based on theoretical considerations and laboratory experiments and non-nuclear field tests alone.

Hydronuclear and Hydrodynamic Testing -
Nuclear weapons designers have been using hydrodynamic testing as well as full-scale testing for designing new weapons, which includes ensuring their safety. Since full-scale testing would be ended by a comprehensive test ban, some scientists claim that testing at some level, such as hydronuclear testing, is essential for determining the safety of nuclear weapons. In particular, such testing can be important for helping to determine what is called “one-point safety” or “single-point-safety” of warheads in the absence of full-scale testing. One-point safety means ensuring a nuclear explosion will not result if any point on the conventional explosive that surrounds the fissile material were accidentally detonated. The purpose of determining one-point safety is to help prevent accidental detonations of nuclear weapons. The Unites States have used nuclear tests extensively to determine one-point safety since 1955. During the 1958-1961 moratorium, Los Alamos put together a program for hydronuclear testing for studying one-point safety. The risks of a failure to determine one-point safety prior to putting a warhead into production have been recognized for well over three decades.
A recent report of the Natural Resources Defence Council set forth what can be learned about nuclear weapons design at various levels of nuclear explosive yield (expressed at equivalent weights of the conventional explosive TNT)
· a yield of less than about one-tenth of a pound: information about one-point safety;
· a yield of less than about half-a-pound: information about the achievement of criticality (needed to initiate the nuclear explosion);
· a yield under four pounds: criticality tests, measurement of temperature and pressure conditions as criticality is achieved, and other essential design information on the characteristics of the weapon at the very start of the nuclear explosives;
· a yield of a few pounds to several hundred pounds: data enabling estimation of the yield of the weapon, and all the other data from lower yield tests;
· a yield of a few tens of tons: development of advanced weapons consisting of fissile materials only (that is no thermonuclear component);
· a yield of a few hundred tons: most of the essential data about boosted fission as well as thermonuclear weapons.

The Unites States seek a “comprehensive” test ban that would permit hydronuclear tests of up to four pounds; Russia would like to do tests of a few tens of tons; France would like several hundred tons. China would like to have a several-hundred-ton limit if any exceptions are allowed because it feels that lower limits would enable the superpowers to design new weapons more easily than states with less technologically sophisticated equipment.
The French government last May joined other nuclear weapon states in pressing the non-nuclear weapon states to agree to an indefinite extension of the Non-Proliferation Treaty. As part of the bargain, France endorsed a statement that pending completion of a comprehensive test treaty, the nuclear weapon powers “should exercise utmost restraint”.
Everyone assumed this to mean that France had agreed to extend its moratorium on tests until the treaty was completed. In the same document France endorsed a statement which approved the idea of nuclear-weapon-free zones and which urged the nuclear-weapon states to support them and sign the relevant protocols. There is a nuclear-weapon-free zone in the South Pacific. France refuses to recognise it. (Source: Letter to the Editor, Frank Barnaby. The Guardian (UK), 14 July 1995).
The United States would carry out its hydronuclear testing program at the Nevada Test Site. This would be in addition to its extensive hydrodynamic testing program at Los Alamos and Lawrence Livermore National Laboratories. The main stated public purpose of these facilities is to ensure “safety and reliability” of the U.S. nuclear arsenal. he devices could also aid in designing new weapons. The U.S. is also building an advanced hydrodynamic testing facility called Dual-Axis Radiographic Hydrotest (DARHT) facility at Los Alamos. The term “dual-axis” refers to two X-ray machines that would be built to photograph the interior of material being compressed to represent a nuclear warhead pit. The materials tested in DARHT could be a dense non-radioactive element used to simulate the pit, depleted uranium, or perhaps even plutonium-242 (photographs with X-rays are called radiographs). The X-rays are generated by creating powerful electron beams in an accelerator and stopping the beam in a tungsten target.
DARHT is to be built in two stages, with one X-ray machine being built by 1997 and a second to be added (if the first works well) by the year 2000. The two independent axes of observation will enable three-dimensional observation of the compression of materials simulating the pit of a warhead. This could possibly provide far more data for nuclear warhead design. Construction of DARHT began in 1994 without a full environmental impact statement. It was stopped in January 1995 by a court order pending completion of an environmental impact statement.
The utility of DARHT for its stated safety and reliability purposes is a matter of some dispute within the nuclear establishment. Los Alamos is, of course, convinced of the need for it. But a 1992 Sandia National Laboratory report stated that the aims of the first DARHT could be accomplished by an $8 million upgrade to the FXR machine at Lawrence Livermore National Laboratory (compared to the $85.6 million cost of the first part of DARHT) and that for reasons dealing with uncertainty of performance and other factors the second arm of DARHT should be postponed. The total estimated cost of DARHT is $123.8 million. The labs also want an even more advanced hydrotest facility (AHF) with four to six X-ray beams, currently estimated to cost $422 million.

Laser Fusion -
Another new facility, called the National Ignition Facility (NIF) is proposed to be built at Lawrence Livermore National Laboratory. This has not yet obtained final approval, pending the outcome of a technical study. NIF is a larger version of a laser fusion machine that already exists at Livermore.
Laser fusion is a process in which powerful lasers are simultaneously focused on a minute pellet of tritium and deuterium, raising temperatures to levels comparable to those in the interior of the sun. This initiates a tiny thermonuclear reaction, which is essentially a very small-scale version of a thermonuclear bomb. The process is also called inertial confinement fusion (ICF). The often stated purpose of such experiments for over two decades has been to develop a device for generating electricity from fusion. But, while the scientific and commercial feasibility of this or any other method of generating electricity from fusion is decades way at best, the more immediate weapons applications of inertial confinement fusion have been officially acknowledged.
According to a Livermore document about NIF, the inertial confinement fusion program has, besides its potential application to laser fusion power generation, “an essential role in accessing physics regimes of interest to nuclear weapons design and to provide nuclear weapon related physics data, particularly in the area of secondary design.” It would also “provide an aboveground simulation capacity for nuclear weapons effects on strategic, tactical, and space assets (including sensors and command and control)…”
In sum, the new hydrotest facility, DARHT, and the new laser fusion machine, NIF, as well as various other preparations will enable the United States to design new nuclear weapons and to maintain the capacity to do so for long term, despite its Non-Proliferation Treaty obligations to pursue nuclear disarmament in good faith and despite the end of the Cold War.
Source and Contact: Reprinted with permission for Science for Democratic Action, Vol. 4 No 2, Spring 1995. Published by Institute for Energy and Environmental Research (IEER), 6935 Takoma Park, MD 20912 USA. Tel: +1-301-270.5500

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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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Nuclear Weapon Hydronuclear Testing -
In a hydronuclear test, fissile material is imploded, but a supercritical mass is not maintained for a long enough time to permit the device to deliver “full” nuclear yield. Depending upon the conditions of the test, nuclear energy releases may range from the unmeasurably small (milligrams or less) to kilograms or even metric tons of TNT equivalent yield.
Hydronuclear experiments, as distinguished from hydrodynamic ones, use actual fissile material assembled to form a supercritical mass in which a chain reaction be-gins. Normally, hydronuclear experiments are designed to use nuclear devices modified in one of several ways, including substituting inert material or less-fissile material for some of the HEU or plutonium in the pit, so that very little nuclear energy release occurs. Yields in experiments described as “hydronuclear” by various countries have ranged from much less than 1 kg TNT equivalent to many tons.
Subcritical Experiments are scientific experiments to obtain technical information in support of the U.S. Department of Energy, National Nuclear Security Administration’s (NNSA) Stockpile Stewardship and Management Programs — the NNSA programs are to maintain the safety and reliability of the U.S. nuclear weapons stockpile without nuclear testing. They involve chemical high explosives to generate high pressures that are applied to nuclear weapon materials, such as plutonium. The configuration and quantities of explosives and nuclear materials will be such that no nuclear explosion will take place. Thus, the experiments are consistent with the Comprehensive Test Ban Treaty. They are called “subcritical” because there will be no critical mass formed, i.e., no self-sustaining nuclear fission chain reaction will occur. Scientific data is obtained on the behavior of nuclear weapon materials by the use of complex, high speed measurement instruments.
Los Alamos National Laboratory conducted the first subcritical experiment Rebound on July 2, 1997. The purpose of the experiment was to obtain information on the response of plutonium to shock wave compression at different pressures. This information was obtained by performing fundamental shock wave experiments on plutonium. This data is a hugoniot curve, that is, a curve showing density. Rebound involved three measurements of different pressures, which were done in a single experiment room, 10 feet by 15 feet by 30 feet, located 962 feet below ground in the U1a Complex. All three of the experiments utilized high explosives for driving stainless-steel flyer plates into target assemblies generate pressure in the plutonium targets.
The second subcritical experiment Holog was conducted by Lawrence Livermore National Laboratory scientists on September 18, 1997. It was designed to yield information on the nonnuclear properties of plutonium under extreme shock conditions. The name Holog was taken from the Laboratory-developed holography technology that allows scientists to capture three-dimensional images of the particles ejected from the surface of materials shocked by high explosives. The Holog experiment was to allow scientists to answer basic questions like how plutonium reacts when shocked — which cannot be determined today with the required precision by experimenting with substitute materials. It is anticipated that this data will be used in complex computer simulations which will help assure the safety and reliability of U.S. nuclear weapons without nuclear testing. The explosion was comparable to that of a large fire cracker or shotgun blast.
The Comprehensive Test Ban Treaty is a zero-yield ban because we determined that this was in our interest and because no threshold above zero yield would have been negotiable, for reasons that remain true. The original U.S. and British scope position, which would have allowed “hydronuclear” tests with yields up to four pounds, might have been reluctantly accepted by the non-nuclear weapon states. But it was vigorously rejected by Russia, France, and China, which preferred yield limits of 10 tons, 300 tons, or an exemption for “peaceful” nuclear explosions, respectively. Such yield thresholds would have been politically unacceptable to many non-nuclear weapon states, and the PNE exemption was rejected by almost everyone.
Subcritical tests have become largely accepted internationally for ensuring the safety and reliability of a nation’s nuclear force without resorting to nuclear testing. Russia has been conducting subcritical tests involving both weapons-grade plutonium and uranium since 1995 at its Novaya Zemlya test site near the Arctic Circle.