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Do our nuclear weapons work?
The answer, it turns out is “probably” or “we’re pretty sure they do.” or “Almost for sure, most of them should.”
That’s right. We’re not entirely sure, and as time goes on we’re becoming less sure. That’s because we don’t test them and haven’t done so for two decades.
How it was and how we got here:
A nuclear weapon is a very complex piece of engineering and physics. There are many parts that have to work properly for the weapon to actually detonate. The core must implode in a manner that results in the correct final geometry. It must undergo fission before it is blown apart, sometimes requiring additional neutrons be provided by a pulsed neutron generator or by boosting with a small amount of fusion. In hydrogen bombs, energy from the primary must be channeled into the secondary and produce fusion. The time tolerances involved are less than nanoseconds.
For this reason, nuclear weapon designs were initially always tested at full scale, in prototype devices that would then become production weapons. The first tests were conducted in the atmosphere. Hundreds of such tests, some of multiple megatons were conducted by the United States and Soviet Union in the 1940’s, 1950’s and 1960’s. These tests had multiple purposes. In addition to validating the viability of the weapons designs, they were used to better understand the physics involved, with data collected to help guide future weapons design. Tests were also used to determine the effects of weapons on structures, aiding in the design of nuclear-resistant structures, communications systems and weapons platforms.
In 1963 the United States and Soviet Union signed the Partial Test Ban Treaty. The treaty ended the testing of nuclear devices in the atmosphere, underwater or outer space by the signing parties. After 1963, all US and Soviet tests would take place underground, in shafts designed to completely contain the explosions and prevent any fallout from entering the atmosphere. For the most part, this was successful, although there were occasional minor leaks and at least one major breach of containment due to an unmapped fissure in 1970. France and China continued to conduct atmospheric testing, having not been party to the 1963 treaty. The last atmospheric nuclear test was conducted by China in 1980. Since that time, all tests have been underground.
By the late 1960’s, the superpowers had generally ended the practice of testing nuclear weapons at their full yield. Having acquired a much better understanding of the physics and engineering behind nuclear weapons, it was no longer considered necessary to test the secondary stages of nuclear weapons at their full yield. Testing the fission primaries, with either no secondary component, or a greatly reduced secondary yield provided ample data on the reliability of the weapon design.
The only exception to this was the rare circumstance where a new type of weapon was developed, with a vastly different design than previous weapons. The 1971 Cannikin test was one example of a high yield weapon tested underground. At five megatons, the exceptional yield of the test device required extreme measures be taken to contain the blast. The test was conducted at the bottom of a 1.8 kilometer deep shaft, drilled through solid rock on a remote island off the coast of Alaska. The weapon tested was the W71, a highly unique warhead designed for the Spartan anti-ballistic missile system. The new warhead was designed to produce an extremely high x-ray and neutron flux and to operate in the extreme environment of outer space, possibly being subjected to radiation from other nuclear explosions. Given these special design criteria, it was determined that a full scale test of the system was necessary.
In 1974, the US and Soviet Union signed the Threshold Test Ban Treaty, limiting nuclear tests to a maximum of 150 kilotons. By the time the treaty was signed, it was no longer necessary to test weapons at their full design yield, so the treaty was largely symbolic. Since larger tests require more complex and extensive containment measures, and because they were no longer necessary, both countries had generally abandoned large tests by that time. Although other nuclear powers were not party to the treaty, by the 1970’s, full yield weapons testing was no longer necessary for established nuclear powers.
The United States and Soviet Union continued to conduct nuclear tests, mostly with yields of a few kilotons, throughout the 1980’s. France, China and the UK also conducted nuclear tests through the 1980s and into the early 1990’s.
The End of Nuclear Tests (for established nuclear powers):
The Soviet Union conducted its last underground nuclear test in 1990. After the fall of the USSR, the Russian Federation did not conduct any further tests, although it did make preparations for tests which were ultimately suspended of canceled. The United States conducted its last nuclear test in 1992, before suspending the testing program. China and France continued until 1996. In the 1990’s, political pressure began to increase dramatically on countries to end nuclear testing completely. At present, Russia, the United States, China, France and the UK have completely shelved their nuclear test programs since the mid 1990’s, although the assets for nuclear testing may remain on standby.
In 1996 the Comprehensive Test Ban Treaty was introduced to the United Nations. It would end all nuclear testing of any kind by the parties of the treaty. Although the United States signed the treaty, it was never ratified by the US Congress,it is therefore officially unrecognized by the US until ratified. It also has not been signed by all nuclear powers in the world, and as such, it has not come into force.
None the less, given the amount of time that has passed since nuclear tests have been conducted by the major nuclear powers and the political pressure associated with testing, there are no plans to resume nuclear testing by the US, Russia, France or the UK. India and Pakistan conducted tests as recently as 1998 and have indicated that they may conduct more. North Korea conducted the most recent nuclear test in 2009.
The dangers of not testing weapons:
All of the nuclear weapons in the inventory of the United States have been extensively tested. That’s also true for Russia, the UK, France and the other major nuclear powers.That is definitely a good thing, because despite the extensive knowledge and experience the US has with nuclear weapons design, building a new weapon without extensive testing can result in some very unwelcome surprises. The extremely high tolerances and complex physics and engineering involved in weapons design makes it difficult to ever be absolutely sure they work without testing.
In the early 1960’s, the United States was at the top of its game when it came to nuclear weapons design, and was highly confident in the ability of scientists and engineers to design effective and reliable nuclear weapons. However, in the early 1960’s, both the Soviet Union and the United States suspended most nuclear testing for a period of about two and a half years, as they engaged in diplomacy over testing requirements. While this was not an officially sanctioned ban, it did result in a brief de facto nuclear testing moratorium.
When the United States began testing again, in the mid 1960’s, it was discovered that the two weapons primaries, which had since become the most important component of the US strategic nuclear force had a major reliability. Both the Python and Tsetse primaries reportedly did not function as expected during testing. The extent and nature of the reliability problem remains classified, but it is known that hundreds or thousands of weapons had to be pulled from service and modified.
In the case of weapons that have been fully tested, it’s not entirely uncommon for surprises during the testing phase. During testing of the W47 warhead, it was discovered that the design had the potential to accidentally detonate, even if the core was only partially imploded, as might happen in an accident. As a result, additional safing mechanisms were added to the production version of the warhead. Later tests, however, revealed that the safing mechanism employed could reduce the reliability of the warhead, eventually leading to its retirement from the stockpile. Tests also revealed unexpected reliability issues with the W52 and W45 nuclear warheads.
It should be noted that the extent to which unexpected results have occurred during nuclear tests, even in relatively mature designs, is not publicly known. In most circumstances, the precise results from nuclear tests remains top secret. What we do know is that they have occurred in at least a few noteworthy cases.
Reliability problems are not confined to older weapons designs. The W76, which is a mainstay of the US submarine-launched nuclear missile force, has been criticized for containing a possible design flaw that could, at least in some circumstances, result in a failure to detonate or a dramatically reduced yield. Whether these concerns are valid is impossible to know for certain without further testing of the warhead.
There are, however, plans to design and construct a nuclear weapon with absolutely no testing whatsoever. The Reliable Replacement Warhead was proposed as the first new American nuclear weapon since the late 1980’s. It was intended to be a post-Cold War design, replacing older warheads with one that would have an extended reliable shelf-life, minimal maintenance and enhanced safety. Such a warhead would fulfill the need for a long term stockpile with minimal upkeep and greater reliability. Some designs were to be based on existing, tested warheads, but concerns were raised about the use of any warhead whose final design would never be the subject of any testing. In 2009 funding was cut and the program shelved, but the possible need for a modernized warhead continues to be debated.
The Aging Stockpile:
At present, the United States relies on existing weapons as the backbone of the nuclear deterrent. Some, like the B61, are more than forty years old. During this time period, they have received periodic upgrades of electronics and components like chemical explosives, batteries and tritium supplies have been replaced due to age. However, the weapons cores are original.
This presents a problem. Nuclear weapons have extremely high design tolerances, and this is especially true for the advanced designs of most active US nuclear weapons. Modern nuclear weapons use a minimal amount of explosive combined with air lenses and specially shaped cavities to rapidly and precisely implode a plutonium core. Some also use neutron generators to speed the initiation of the fission reaction as the core implodes. The tolerances of these systems are in the nanoseconds, and if the core does not implode with perfect symmetry or if the material does not come together fast enough, it will not function properly.
Making a core that will preform properly is made more difficult by the nature of plutonium. Plutonium, as it turns out, has some of the most undesirable characteristics of any metal. It’s extremely hard, brittle and difficult to machine. Plutonium is also pyrophoric, auto-igniting in some circumstances. Plutonium is known to form a complex microcrystalline structure that can be prone to cleavage. In fact, the properties of plutonium are so poor that pure plutonium is not used for weapons cores. Rather, an alloy, composed mostly of plutonium, but also containing small amounts of gallium and possibly other materials is used to provide acceptable physical properties.
But plutonium has another property that is especially worrisome when testing is not being conducted. Because plutonium is radioactive and produces strong alpha emissions, it will, over time, self-irradiate and produce changes to the structure of a plutonium core. Over time, some of the alpha particles produced by the decay of plutonium become entrapped in the crystalline structure of the material. These form tiny pockets of helium gas and may have the effect of cleaving the material, reducing strength or changing the physical properties. Some swelling can occur, potentially changing the cores precise geometry. The plutonium-gallium alloy also tends to separate, with gallium grains migrating to the center of the core, leaving a gallium-depleted region with potentially problematic physical properties.
The problem is by no means unique to the United States. Although less public information is available on the condition of Russia’s nuclear warheads, the average of the core material is believed to be much older than those of American weapons. The material has also been subject to considerably less testing and may not be as durable as American plutonium alloys. The average age of the weapons held by China, India and Pakistan is less, but the weapons designs have not been subject to as extensive testing as US or Russian weapons.
So how do we know this is not reducing the reliability of the weapons?
The most direct and reliable way of knowing is to periodically test the weapons. It’s otherwise quite difficult to know for sure, given that we have only a half-century of experience with plutonium. Also, as different weapons have different core designs, different isotopic ratios of plutonium and different ages, it’s hard to be sure that all design types work, just because one did. As the cores of weapons age, the certainty that they will preform as expected is reduced.
There is, however, one way of assuring weapons are effective, even if the aging plutonium is not tested. The plutonium pits could simply be re-smelted into new pits, remoulded to the original design criteria. Doing so would eliminate any microscopic flaws in the structure and redistribute the material throughout the cores. Unfortunately, the US does not have the capacity to do this. The Rocky Flats Plant, where plutonium cores were fabricated ended production of weapons in 1989. Today, the hot cells, specialized furnaces and other equipment needed for smelting plutonium exist in only small numbers at a few national laboratories. While it’s possible that a few cores a year could be re-fabricated, the US lacks the ability to restart large scale core smelting and fabrication without major upgrades to equipment.
As such, the US has perused a program of “Stockpile Stewardship” that involves attempting to assess and maintain the reliability of nuclear weapons without any actual tests.
Some of these include:
Computer simulations - Computer simulations have become a mainstay of the current no-test nuclear weapon stewardship program. Computer calculations are used to predict how the physical properties of plutonium may change over time, how these properties may effect the operation of weapons and also to evaluate weapons designs and basic weapons physics. The calculations required to preform simulations of these processes are extremely complex. As a result, some of the largest and most powerful supercomputers ever built are used primarily for this purpose.As of this article’s writing, the world’s two fastest supercomputers are located at the Lawrence Livermore National Laboratory and the Oak Ridge National Laboratory and are used primarily for nuclear weapons-related calculations and simulations.
Gun-based Materials Shock Tests – The properties of nuclear materials, most especially plutonium and how they respond to rapid physical shocks, similar in effect to the types of forces during a weapon pit implosion are simulated at a series of facilities using specialized high-velocity gun systems. These are regarded as “subcritical tests” as they use actual weapon material but in quantities too small to produce a fission chain reaction. Facilities include the Lawrence Livermore National Laboratory’s Contained Firing Facility and Joint Actinide Shock Physics Experimental Research located at the Nevada National Security Site (formerly known as the Nevada Test Site.)
Hydrodynamic Testing - Another form of subcritical materials testing, hydrodynamic testing involves subjecting nuclear materials and core mockups to extreme hydrodynamic shock, simulating the implosion that occurs when a weapon detonates. The implosion that occurs during the tests are recorded at extremely high speed through the use of powerful pulsed x-ray generators, which allow for the interior of the cores to be imaged as the test takes place. 
Different facilities are utilized to provide different types and scales of hydrodynamic stress tests. Some of these tests use large quantities of conventional explosive to simulate the kind of compression that occurs during a nuclear weapon’s detonation, while other tests are of a much smaller scale.
The facilities used for hydrodynamic testing exist at the Los Alamos National Laboratory, the Lawrence Livermore National Laboratory and the Nevada National Security Site. The tests conducted are classified as subcritical nuclear weapons tests.
Materials Radiography Experiments – In order to assess the properties of nuclear materials and measure their reactivity with extreme precision, a wide variety of radiographic experiments are conducted. Facilities to support this include the Los Alamos Neutron Science Center and Los Alamos Proton Radiography Facilities. One aim of continued research into radiography is to provide enhanced imaging capabilities for use in hydrodynamic and other types of shock tests.
The Z-Machine – The Z-machine is the world’s largest electrical discharge pulsed power generator. It is located at the Sandia National Laboratory, the Z-machine was first demonstrated to the public in 1998. It is basically an enormous bank of capacitors which can be discharged in a period of nanoseconds, producing power levels in the petawatt range.
The Z-machine has numerous uses, including generating super high power x-rays, testing how materials react to extreme electromagnetic shocks and producing small amounts of nuclear fusion by creating powerful magnetic pulsed fields around tiny capsules of easily fusible isotopes such as deuterium and tritium. It has also been used to produce small amounts of fusion in heavier elements.
Although the Z-machine has a myriad of uses in areas as widely varied as materials engineering, radiographic analysis and basic plasma physics, it was built primarily for the purpose of nuclear weapons research and this remains one of its main functions. The tiny amounts of fusion plasma produced by the Z-machine can be used to research the conditions that occur during a nuclear detonation as a way of validating computer simulations. The extremely energetic pulses produced by the machine are also used to test the properties of plutonium samples.
An interesting aside is that earlier this year, a congressional committed report on nuclear weapons research and stewardship stated:
The Committee understands that these experiments yielded fundamentally new and surprising data about the behavior of plutonium at high pressure and this new data has been one of the most valuable contributions to the stockpile stewardship program
Exactly what this “surprising” data is was not disclosed, but it goes to show that we still do not have a complete understanding of the unique properties of plutonium.
The National Ignition Facility – The National Ignition Facility represents one of the largest (and most expensive) scientific research programs currently underway by the United States Government. Construction began in 1997, but only recently has it begun to approach its full planned capacity. It is also possible that the facility will be upgraded further, as current systems have, thus far, been unable to produce full “ignition” of a nuclear fusion reaction – the point at which the reaction produces more energy than was used to initiate it. 
NIF is the culmination of decades of research into inertial confinement fusion. In inertial confinement fusion, nuclear fusion is produced by subjecting a tiny target of material to a pulse of energy produced by high power lasers. With 192 of the world’s most powerful lasers, NIF dwarfs all facilities of this type that have come before it and is capable of achieving momentary temperatures as high as 3.3 million Kelvin. The energy directed at a small cylinder called a hohlraum. The laser energy vaporizes the cylinder and releases a burst of high intensity radiation which heats and implodes the fusion pellet.
If this sounds familiar, its because it’s the same way a hydrogen bomb works, except it is much smaller and the high intensity radiation which does the heating and imploding comes from the lasers striking the hohlraum and not from a fission primary explosive. Thus, the type of fusion reactions produced by the National Ignition Facility are really just tiny h-bombs.
When reported in the press, NIF is often described as a project to create clean, sustainable fusion-based energy. That, however, is not the reason NIF was built and it is not the primary type of research conducted at NIF. Although the facility may provide opportunities to better understand the physics of nuclear fusion in general, the actual purpose of NIF is to provide the ability to study the conditions that occur during nuclear explosions as a way of refining and validating computer models. Although it has received little attention, plans call for the us of weapons grade plutonium in NIF experiments, subjecting the plutonium directly to the kind of environment that occurs when a bomb detonates.
Why it matters:
- Weapons tests not only assure that the weapon will work properly and reliably if fired, but also that it will not detonate inadvertently. The compact two-point implosion systems used in some modern nuclear weapons can, if not designed properly, produce a significant nuclear yield, even if only part of the core is imploded. The only way to fully assess the core characteristics under implosion and to determine if such hazards may exist is by full scale core testing. Since partial implosions could produce significant fission yields, they are not currently conducted.
- Questions about the reliability of US nuclear weapons could result in increased proliferation by causing other nations to feel the need to develop their own nuclear arsenals. The United States is not the only country which is protected by the American nuclear umbrella. Mutual defense and “nuclear sharing” agreements exist, assuring allies that they are protected against aggression by US nuclear arms. After nuclear tests were conducted in North Korea, a number of officials in South Korea began to openly call for development of a South Korean nuclear program. Similar calls to action have been made in Taiwan and Japan. The United States countered this by reaffirming that American nuclear-equipped forces in the region were committed to responding to any aggression by North Korea, including a nuclear strike. However, if doubts exist about the reliability of American weapons, countries like South Korea would have a very powerful motive to develop their own weapons.
- Questions about the ability of nuclear weapons to continue to operate reliably into the future could result in pressure to use such weapons systems in the near term. For example an argument could be made that “If we are going to use our nuclear bunker busters to destroy the Iranian nuclear program, we should do it now, when they will probably work. If we want ten years, they might not be operational at all.”
- Questionable reliability of weapons reduces their deterrent value and increases the likelihood of destabilization.
- If the reliability of nuclear weapons is not well established, a larger arsenal will be required, as not all the weapons used will be expected to function properly. For example, if up to 50% might fail, then twice as large an arsenal is required.
- It is possible that an adversary, which has come to believe that the US arsenal is highly unreliable, could attempt to provoke a nuclear response from the US simply to determine if the weapons are, in fact, still fully functional.
- If major reliability problems were discovered after they had become severe enough to impair a large portion of weapons from functioning properly, much of the current stockpile would be forced to be withdrawn. Since the US has retired many weapons and consolidated its forces into only a handful of designs, it is conceivable that discovering that the aging pits of one design had become extremely unreliable could result in much of the arsenal needing to be pulled from service.
- In the future it may become desirable to modify existing weapons designs or to create new designs in order to improve safety and security. For example, new methods of pit fabrication might reduce the likelihood of area contamination of a weapon were destroyed in a fire. However, without testing, it would be difficult to incorporate such new designs into weapons systems while maintaining confidence that the weapon would function as expected.
It should be noted that these concerns are not exclusive to the United States. Other countries with nuclear arsenals, which do not engage in testing, could also face such pressures, as the result of uncertainty about their stockpiles. In fact, it could be much worse, as not all nuclear powers have access to the data from decades of testing which the United States does.
In terms of the ability to simulate full scale tests or acquire useful data by other means, most other nuclear powers do not possess the same level of computing power or the kinds of pulsed-power, hydrodynamic shock or high intensity pulsed fusion facilities the US has. Like the United States, France has been perusing Inertial Confinement Fusion as a means of gaining data related to nuclear weapons function, constructing the Laser Mégajoule, a facility similar to the National Ignition Facility.
Unfortunately, Russia, China, India and Pakistan do not have such facilities, which could bode poorly for their continued confidence in their nuclear stockpiles, possibly even to the point of some developing an “itchy trigger finger.”
Conclusion:
For those who have not already realized it, I personally favor the resumption of a limited number of periodic nuclear tests, conducted underground with rigorous measures taken to insure containment. Such tests provide data that is very difficult to obtain otherwise and are the only way of absolute assurance of reliability in a way no other method can. They also have the benefit of providing additional scientific data, offering an excellent opertunity for seismic sounding and providing data on nuclear reactions, geological forces and materials properties.
From the standpoint of peace and security, it should be noted that nuclear weapons exist and they are not going anywhere. Nobody wants to see the day when such powerful weapons are fired in anger, but simply conducting tests does not increase the likelihood of this and may even reduce it.
2012-11-24 08:20:07
Source: http://depletedcranium.com/do-our-nuclear-weapons-work/
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