Nuclear Weapons and Materials

Transcription

1 CHAPTER 3 Nuclear Weapons and Materials Nuclear weapons have existed for more than fifty years, and the technology required to produce them is well understood and widely available. Nine countries (Britain, China, France, India, Israel, Pakistan, Russia, South Africa, and the United States) have produced nuclear weapons, and more than 40 countries have the technical knowledge and capability of doing so with the application of adequate resources. Even well-organized subnational organizations and terrorist groups, given adequate time and resources, could possibly produce a basic nuclear device. Despite these realities, nuclear weapons remain highly complex devices and are difficult to produce. Their production requires a significant level of technical capability and a major investment of time and money. By far the most costly, complicated, and observable part of producing a nuclear weapon is acquiring sufficient amounts of weapons-usable nuclear materials from which the explosive power of nuclear weapons derives. If this material can be purchased or stolen, it dramatically reduces but does not eliminate the challenges associated with producing a nuclear weapon. Producing specialized nuclear materials and designing and building a well-engineered explosive device requires the construction of large and thus highly visible facilities (those required for nuclear materials production), making the clandestine acquisition of nuclear weapons extremely difficult. The challenge of preventing the spread of nuclear weapons is complicated, however, by the fact that nuclear materials, including weaponusable materials, have legitimate peaceful uses. These uses include fuel for nuclear power and research reactors and even industrial uses (tritium and plutonium are used in small amounts in smoke detectors and emergency lighting systems). The physical protection of nuclear materials in the civilian sector, therefore, is a critical component of preventing the spread of nuclear weapons. In addition, the international system of safeguards, administered by the International Atomic Energy Agency, can help to deter theft and alert governments if peaceful nuclear materials are diverted for nonlegitimate uses. Basic Nuclear Concepts Conventional explosives release energy through the manipulation of molecules, forming and breaking apart clusters of atoms. Nuclear explosions are much more powerful, and harness their energy by splitting or fusing together individual atoms. 35

2 36 Assessments and Weapons Atoms consist of a nucleus of neutrons and protons surrounded by a number of orbiting electrons. Elements are defined by their atomic number, which is equal to the number of protons they possess. Elements can exist in nature in different forms called isotopes, meaning that they have the same atomic number but a different number of neutrons in their nuclei. For example, three forms of hydrogen atoms are found in nature: simple hydrogen, deuterium, and tritium. All three have one proton in their nucleus, but have zero, one, and two neutrons, respectively. Various isotopes interact with other particles (such as other neutrons) and molecules differently. These differences allow some isotopes to be fused together or split apart more easily, creating new (larger or smaller) atoms and releasing various forms of energy and radiation in the form of alpha, beta, gamma or x-rays, and neutrons. It is this splitting or fusing of atoms that produces the energy released in a nuclear reactor or in a nuclear weapon. The different forms of atoms are designated by their element name (i.e., hydrogen, iron, uranium, and plutonium) and an isotopic number equal to the combined number of neutrons and protons in the nucleus. All atoms, small or large, are referred to in this way. For example, all uranium atoms have 92 protons in their nuclei, but can exist (among other forms) with 143, 145, or 147 neutrons in their nuclei. The different isotopes are referred to as Uranium 233, or U 233 (92 protons plus 143 neutrons), U 235 (92 protons plus 145 neutrons), and U 238 (92 protons plus 147 neutrons). Plutonium (Pu) atoms can exist (among other forms) with 238, 239, and 240 neutrons and protons in their nuclei and are referred to as Pu 238, Pu 239, Pu 240, etc. Uranium and plutonium are the two main elements that are of relevance when discussing the production of nuclear weapons. (At least one other human-made element, neptunium, can also be used in nuclear weapons, but it exists in very limited quantities globally and is only now being addressed as a proliferation concern.) Some isotopes can be readily split or fissioned into smaller atoms and are referred to as fissile materials. U 233, U 235, and Pu 239 are all fissile materials that can be used in a nuclear weapon. These atoms split apart after absorbing a neutron into their nucleus. The absorption of the neutron makes the atomic nucleus unstable, and the isotope will then split, seeking a more stable form. Each atom that splits releases energy and additional neutrons that can, in turn, be absorbed by other atoms. These atoms, too, then split and release energy and additional neutrons that go on to split even more atoms. This chain reaction is what enables nuclear materials to be harnessed for various purposes, including the production of heat in a nuclear reactor (for creating steam and then electricity) and the explosive power of a nuclear weapon. In a nuclear reactor, the chain reaction is controlled and limited, while in nuclear weapons the released energy is uncontrolled, enormous, and takes place in a very short time (in tenths of a microsecond). Basic Nuclear Weapon Concepts All nuclear weapons rely on a central core of fissile material. The uncontrolled chain reaction releases vast amounts of energy in a small fraction of a second

3 Nuclear Weapons and Materials 37 before the central core of nuclear material is blown apart. To create a chain reaction, the core of nuclear material must be formed into a critical mass, meaning that enough fissionable material is in a sufficiently small area to enable a selfsustaining number of fissions to take place (that is, more neutrons are released than are absorbed). The amount of material required to produce this mass differs according to the isotopic composition of the material being used (for example, the mass is different for different purity levels of U 235 and Pu 239). A pure form of U 235 (say 90 percent U 235) will have a much smaller critical mass than that of a less pure form (say 45 percent). The IAEA publishes figures on the quantities of material required to produce a nuclear weapon. The significant quantities that IAEA specifies are 25 kilograms of highly enriched uranium and 8 kilograms of plutonium. The U.S. Department of Energy and the U.S. National Academy of Sciences have published reports saying that only 4 kilograms of plutonium are required for a basic nuclear weapon. The minimum or exact amount of nuclear material needed to produce nuclear weapons is classified information in all nuclear-weapon states. Basic Nuclear Weapon Designs All nuclear weapons use a basic fission chain reaction that is caused by the creation of a critical mass of fissile material. Some weapons rely entirely on this primary, while others use the primary fission explosion to fuel a second fusion reaction, which can produce much larger explosions. This initial critical mass can be created in one of two ways: by shooting one subcritical mass into another subcritical mass known as a gun design or by taking a subcritical sphere of fissile material and compressing it uniformly into a critical mass known as an implosion weapon. The gun design is the least complex of the known nuclear weapon designs, having been around for more than a half-century. The nuclear weapon dropped on Hiroshima, Japan, on August 6, 1945, was a gun-type weapon and was so well understood, even at that time, that it was used without being tested beforehand. A gun design shoots a uranium plug into a shaped uranium vessel. The rapid combining of the two parts creates a critical mass, leading to a chain reaction and a nuclear explosion. The implosion weapon design compresses a subcritical sphere of nuclear material uniformly into a sphere sufficiently small to create a critical mass. The first nuclear explosion (the Trinity test) at Alamogordo, New Mexico, on July 16, 1945, and the nuclear weapon dropped on Nagasaki, Japan, on August 9, 1945, were implosion designs. The implosion design is more complicated but allows for the construction of a more compact weapon, such as those used in missile warheads. The concept is similar to trying to turn a basketball into a baseball with explosives. Gun-design weapons can only use uranium as a fissile material, since in a gun assembly plutonium will begin producing too many neutrons as the two halves approach each other. These fissions will prevent the formation of a critical mass and result in what is called a fizzle, where only a small amount of energy is released. Because only uranium can be used in the simpler gun design, particu-

4 38 Assessments and Weapons lar concern surrounds the diversion of produced highly enriched uranium. Both uranium and plutonium can be used in the more complex implosion design. Design elements in a nuclear weapon can also change the amount of material needed to achieve a critical mass. Adding a basic neutron reflector around the outside of a weapon so that escaping neutrons are reflected back into the mass of material is a common feature used to reduce the amount of material needed to create a nuclear chain reaction. Advanced Nuclear Designs Other nuclear weapons boost the explosive yield produced by the same amount of fissile materials in a weapon by adding other design elements. This is done mainly by introducing other elements, such as tritium, into the heart of a nuclear explosion. The intense heat and pressures generated by the continuing nuclear explosion result in the fusion of the boosting materials, which in turn release large amounts of energy and additional neutrons that can increase the number of fissile atoms that are split in a reaction. The use of heat to fuse these materials is why these designs are referred to as thermonuclear. The effect is similar to adding gasoline to a campfire adding fuel to an already occurring series of reactions. More advanced, or full-up, thermonuclear weapons use the initial nuclear release from a primary to ignite a fusion reaction. In a fusion reaction, lighter atoms are fused by the heat and radiation released by the primary fission explosion. The first multi-stage, thermonuclear device also referred to as the hydrogen bomb (because it used liquid deuterium, which is an isotope of hydrogen) was exploded on November 1, The explosive used a basic fission primary to produce the heat and radiation necessary to ignite the secondary explosive of liquid deuterium. Whereas the first fission nuclear explosions had a force of 20,000 metric tons of TNT (20 kilotons), the first hydrogen explosion had a force of 10,400,000 metric tons of TNT (10.4 megatons). The Production of Nuclear Materials Fortunately, from a non-proliferation perspective, fissile materials are not readily found in nature. Uranium 235 is the only naturally occurring fissile material but makes up only.07 percent of the uranium found in nature. This isotope can be increased through a process known as enrichment. To be used in a weapon, uranium must be enriched to at least 20 percent. Weapon-grade uranium is much higher, reaching to and going above 93 percent U 235. All the other fissile materials, including U 233 and plutonium, must be created artificially in a nuclear reactor and subsequently separated in a process referred to as reprocessing, or chemical separation. Uranium Enrichment Several methods have been developed for enriching uranium. All of them ultimately rely on differentiating between the isotopes of uranium and isolating material with increased concentrations of U 235. Two principal techniques are

5 Nuclear Weapons and Materials 39 in use today: the gaseous diffusion method, in which uranium hexafluoride gas is forced through a selectively porous barrier, and the ultracentrifuge, or gas centrifuge, method, in which uranium hexafluoride gas is swirled in a cylinder that is rotating at extremely high speeds. Electromagnetic isotope separation (EMIS), a process that was used in refining uranium for the first nuclear weapon in the United States and a technology pursued by Iraq in its nuclear weapon program (see chapter 16) separates uranium tetrachloride into different isotopes. EMIS is a highly inefficient but less complex method that was abandoned in the 1950s. Considerable research and development have been conducted on two additional enrichment techniques, the chemical method and laser isotope separation, but neither is used in the commercial production of enriched uranium or for weapon manufacturing. Other methods have been used in weapon programs, including the South African jet nozzle design. Producing highly enriched uranium entails many steps, apart from the enrichment process itself, and many other installations and capabilities are necessary. Nations wishing to obtain highly enriched uranium, without international restrictions prohibiting its use for nuclear explosives, would have to develop an enrichment technology independently or obtain it illegally, since virtually all nuclear exporter states are unwilling to sell nuclear equipment and materials unless the recipients pledge not to use them for nuclear explosives and agree to place them under the inspection system of the International Atomic Energy Agency. (See the appendixes for IAEA safeguards.) For illustrative purposes, the basic nuclear resources and facilities that would be needed to produce weapons-grade uranium indigenously include: uranium deposits a uranium mine a uranium mill (for processing uranium ore that usually contains less than 1 percent uranium into uranium oxide concentrate, or yellowcake) a conversion plant (for purifying yellowcake and converting it into uranium hexafluoride (UF 6 ) or uranium tetrachloride (UCl 4 ), the material processed in the enrichment plant) an enrichment plant (for enriching the uranium hexafluoride gas or uranium tetrachloride in the isotope U 235), and a capability for converting the enriched uranium hexafluoride gas or uranium tetrachloride into solid uranium oxide or metal Nuclear Reactors Other fissile materials can be created artificially in nuclear reactors. The United States and other nuclear-weapon states have tended to use dedicated military reactors for the production of these weapon-usable materials, but others (such as India and North Korea) have ostensibly used civilian reactors for the production of plutonium.

6 40 Assessments and Weapons In a reactor, uranium fuel (either natural uranium or slightly enriched uranium, depending on the reactor design) is used to create a controlled chain reaction. This reaction releases neutrons that in turn are captured by fertile nuclear materials, such as U 232 or U 238. These materials, with the addition of a new neutron, are converted to U 233 or Pu 239, which are fissile materials. However, the fuel rods containing these materials also contain other reaction by-products through the accumulation of neutrons and the fissioning of elements. Many of these are highly radioactive and require processing to recover the weapon-usable materials. To do this, spent fuel rods are taken to a reprocessing plant, where they are dissolved in nitric acid, and the plutonium is separated from the solution in a series of chemical processing steps. Since the spent fuel rods are highly radioactive, heavy lead casks must be used to transport them. In addition, the rooms at the reprocessing plant where the chemical extraction of the plutonium occurs must have thick walls, lead shielding, and special ventilation to contain radiation hazards. Although detailed information about reprocessing was declassified by the United States and France in the 1950s and is generally available, it is still a complex procedure from an engineering point of view. Indeed, almost every nation that has tried to develop nuclear weapons by the plutonium route India, Iraq, Israel, and Pakistan has sought outside help from the advanced nuclear-supplier countries. North Korea, however, has apparently succeeded in constructing a reprocessing facility at Yongbyon without such foreign assistance. Like enrichment facilities, however, reprocessing plants can also be used for legitimate civilian purposes because plutonium can be used as fuel in nuclear power reactors. Indeed, through the 1970s it was generally assumed that as the use of nuclear power grew and worldwide uranium resources were depleted, plutonium extracted from spent fuel would have to be recycled as a substitute fuel in conventional power reactors. In addition, research and development are under way in several nations on a new generation of reactors known as breeder reactors, most notably in France, Japan, and Russia. Breeder reactors use mixed plutonium-uranium fuel surrounded with a blanket of natural uranium. As the reactor operates, slightly more plutonium is created in the core and the blanket together than is consumed in the core, thereby breeding new fuel. These programs have encountered complex technical and political challenges, not the least of which relate to the overabundance of plutonium and questions about safety and waste produced from these types of reactors and their spent fuel handling. A growing area of research relates to proliferation-resistant reactors that could be used to consume those large amounts of excess plutonium and whose spent fuel would be less well suited for use in the production of plutonium and nuclear weapons. Like plutonium recycling, the economic advantages of breeders depend on natural uranium s becoming scarce and expensive. Over the past three decades, however, new uranium reserves have been discovered; nuclear power has reached only a fraction of its expected growth levels; and reprocessing spent fuel to extract plutonium (a critical step in the manufacture of plutonium-based

7 Nuclear Weapons and Materials 41 fuels) has proved to be far more expensive and complex than anticipated. Moreover, concern has grown over the proliferation risks of the wide-scale use of plutonium as a fuel. These factors led the United States in the late 1970s to abandon its plans to recycle plutonium in light-water reactors and, in the early 1980s, to abandon its breeder reactor development program. Germany has abandoned its breeder reactor program and is phasing out its recycling of plutonium. The United Kingdom, too, has frozen its program to develop breeder reactors, although it is continuing to reprocess spent fuel on a commercial basis for itself and several advanced nations. The principal proponents of the use of plutonium for civilian purposes are France, Japan, and Russia, which are all continuing to develop the breeder reactor option and are moving forward with sizable plutonium recycling programs. Belgium and Switzerland, although they do not have breeder reactor programs, are using increasing amounts of recycled plutonium in light-water reactors. Broadly speaking, the proponents of nuclear energy in these countries have maintained support for the civil use of plutonium by arguing that, although it may not be economical, it represents an advanced technology that will pay off in the future and reduce a dependence on foreign sources of energy. Like the production of enriched uranium, the production of plutonium entails many steps, and many installations and capabilities besides the reactor and reprocessing plant are needed. For illustrative purposes, the following facilities and resources would be required for an independent plutonium production capability, assuming that a research or power reactor, moderated by either heavy water or graphite and employing natural uranium fuel, were used: uranium deposits a uranium mine a uranium mill (for processing uranium ore containing less than 1 percent uranium into uranium oxide concentrate, or yellowcake) a uranium purification plant (to further improve the yellowcake into reactorgrade uranium dioxide) a fuel fabrication plant (to manufacture the fuel elements placed in the reactor), including a capability to fabricate zircaloy or aluminum tubing a research or power reactor moderated by heavy water or graphite a heavy-water production plant or a reactor-grade graphite production plant, and a reprocessing plant In contrast to heavy-water and graphite-moderated reactors, which use natural uranium as fuel, a light-water moderated reactor would necessitate use of low-enriched uranium, implying that an enrichment capability may be available. If so, highly enriched uranium could, in theory, be produced, obviating the need for plutonium as a weapon material. (It is also possible that a state

8 42 Assessments and Weapons might import fuel for a light-water reactor under IAEA inspection and, after using the material to produce electricity, reprocess it to extract plutonium. Although IAEA rules would require the country involved to place any such plutonium under IAEA monitoring, the state might one day abrogate its IAEA obligations and seize that material for use in nuclear arms.)

9 NPT nuclearweapon states Non-NPT nuclearweapon states Suspected clandestine programs U.S. NUCLEAR WEAPON STATUS 2002 UNITED KINGDOM FRANCE UNITED STATES ISRAEL IRAQ IRAN Abstaining Countries The following industrialized countries have the technological base, but thus far not the desire, to develop nuclear weapons. Some have installations that can produce weapons-grade material under international inspection: Australia, Austria, Belgium, Canada, Czech Republic, Denmark, Finland, Germany, Hungary, Italy, Japan, Netherlands, Norway, Slovak Republic, South Korea, Spain, Sweden, and Switzerland. Renunciations Argentina, Brazil, and South Africa had active nuclear weapon programs during the 1980s, but renounced such activities by opening all of their nuclear facilities to international inspection and by joining the non-proliferation regime. Belarus, Kazakhstan, and Ukraine acceded to the NPT as non-nuclear-weapon states and cooperated in the removal of all remaining nuclear weapons to Russia after the breakup of the Soviet Union. Worldwide Nuclear Stockpiles Country Total Nuclear Warheads China 410 France 348 India Israel Pakistan Russia ~20,000 United Kingdom 185 United States ~10,700 Maximum Total 31,055 Carnegie Endowment for International Peace, Deadly Arsenals (2002), RUSSIA NORTH KOREA PAKISTAN CHINA INDIA 1. India is thought to have produced enough weapons-grade plutonium to produce between 50 and 90 nuclear weapons. The number of actual weapons assembled or capable of being assembled is unknown. No weapons are known to be deployed among active military units or on missiles. 2. Israel is thought to possess enough nuclear material for between 98 and 172 nuclear weapons. The number of weapons assembled or capable of being assembled is unknown, but likely to be on the lower end of this range. 3. Pakistan may have produced enough weapons-grade uranium to produce up to 50 nuclear weapons. The number of actual weapons assembled or capable of being assembled is unknown. Pakistan s nuclear weapons are reportedly stored in component form, with the fissile core separated from the non-nuclear explosives.

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