Appendix A
Physics and Technology of Nuclear-Explosive Materials

NEM and Fissile Materials

Nuclear weapons exploit the explosive release of nuclear energy from an exponentially growing chain reaction sustained by fissions triggered by “fast” neutrons (i.e., neutrons of energy in the thousands of electron-volts). Nuclides that are capable of supporting a chain reaction of this kind when present in suitable quantity, purity, and geometry are called “nuclear-explosive nuclides.” Any mixture of nuclear-explosive and other nuclides that can be made to support such a chain reaction when present in suitable quantity, purity, and geometry is called “nuclear-explosive material” (NEM). The most important nuclear-explosive nuclides are listed in Table A-1.

The term “fissile” refers to nuclides that can sustain a chain reaction under circumstances in which emitted neutrons are thermalized (i.e., slowed down to velocities corresponding to the temperature of the surroundings) before inducing further fissions. (This property is essential to the operation of the thermal-neutron reactors that have accounted for most nuclear electricity generation, nuclear naval propulsion, and weapon plutonium production around the world.) The underlying physics is such that all fissile nuclides are also nuclear explosives, but not all nuclear-explosive nuclides are fissile; for example, the even-numbered isotopes of plutonium—most importantly Pu-238, Pu-240, and Pu-242—are not fissile, but they are nuclear explosives.



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Monitoring Nuclear Weapons and Nuclear-Explosive Materials Appendix A Physics and Technology of Nuclear-Explosive Materials NEM and Fissile Materials Nuclear weapons exploit the explosive release of nuclear energy from an exponentially growing chain reaction sustained by fissions triggered by “fast” neutrons (i.e., neutrons of energy in the thousands of electron-volts). Nuclides that are capable of supporting a chain reaction of this kind when present in suitable quantity, purity, and geometry are called “nuclear-explosive nuclides.” Any mixture of nuclear-explosive and other nuclides that can be made to support such a chain reaction when present in suitable quantity, purity, and geometry is called “nuclear-explosive material” (NEM). The most important nuclear-explosive nuclides are listed in Table A-1. The term “fissile” refers to nuclides that can sustain a chain reaction under circumstances in which emitted neutrons are thermalized (i.e., slowed down to velocities corresponding to the temperature of the surroundings) before inducing further fissions. (This property is essential to the operation of the thermal-neutron reactors that have accounted for most nuclear electricity generation, nuclear naval propulsion, and weapon plutonium production around the world.) The underlying physics is such that all fissile nuclides are also nuclear explosives, but not all nuclear-explosive nuclides are fissile; for example, the even-numbered isotopes of plutonium—most importantly Pu-238, Pu-240, and Pu-242—are not fissile, but they are nuclear explosives.

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Monitoring Nuclear Weapons and Nuclear-Explosive Materials Reactivity, Critical Mass, and Explosive Yield TABLE A-1 Properties of Nuclear-Explosive Nuclides Isotope or Mixture Critical Mass (kg) Half Life (years) Decay Heat (watts/kg) Neutron Production From Spontaneous Fission (per kg-sec) Main Gamma Energies (MeV) U-233 16 160,000 0.28 1.2 2.6 from Tl-208 U-235 48 700,000,000 0.00006 0.36 0.19 Np-237 59 2,100,000 0.021 0.14 0.087 Pu-238 10 88 560 2,700,000 0.100 Pu-239 10 24,000 2.0 22 0.41 Pu-240 37 6,600 7.0 1,000,000 0.10 Pu-241 13 14 6.4 49 0.66 from Am-241 Pu-242 89 380,000 0.12 1,700,000 0.045 Am-241 57 430 110 1,500 0.66 The critical masses given are for a bare sphere of metal at normal density. Plutonium metal can exist in six different forms corresponding to different crystalline configurations, with different densities. The two of these that are most germane for nuclear weapons are alpha phase (density 19.6 grams per cubic centimeter) and delta phase (density 15.7 grams per cubic centimeter). The indicated critical masses are for alpha-phase plutonium. For delta-phase plutonium, the critical masses would be about 60 percent larger. In the case of Pu-239, neutron production is 22/kg-sec from spontaneous fission but 630/kg-sec from alpha-n reactions. In Pu-238, alpha-n reactions add 200,000/kg-sec to the 2,700,000/g-sec produced by spontaneous fission. In the other cases, augmentation by alpha-n reactions is not significant. Adapted from: Nuclear Energy Research Advisory Committee, Attributes of Proliferation Resistance for Civilian Nuclear Power Systems, U.S. Department of Energy, October 2000; General Electric, Nuclides and Isotopes, 14th ed., 1989. The nuclear reactivity of any nuclear-explosive nuclide or mixture of such nuclides depends on the cross sections (reaction probabilities) of the relevant nuclides for induced fission by incident neutrons of various energies and, alternatively, for absorbing such neutrons without fissioning. The reactivity also depends on the geometries, densities, and chemical forms in which the nuclear-explosive nuclides are present, and whether and to what extent the elements or compounds containing the nuclear-explosive isotopes are diluted or contaminated with other nuclides and compounds that can slow or absorb neutrons. A nuclear explosion is achieved by the rapid assembly, in a suitable geometry, of NEM embodying sufficient nuclear reactivity

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Monitoring Nuclear Weapons and Nuclear-Explosive Materials to initiate and sustain a chain reaction driven by fast neutrons. This means that, on the average, at least one of the several energetic neutrons released per fission will be “productively” captured by another nuclear-explosive nuclide—before the neutron escapes, is unproductively captured, or slows down—resulting in another fission. If that condition is met in a way such that each fission causes exactly one additional one, the configuration is said to be “critical;” if each fission causes more than one additional fission, the configuration is “supercritical.” The mass of NEM required to reach criticality if the material is in the form of a solid sphere at normal density in free space (i.e., not surrounded by material that can reflect neutrons) is called the “bare-sphere critical mass.” Table A-1 gives the bare-sphere critical masses for the most significant nuclear-explosive nuclides, along with some other properties that bear on the attractiveness of the nuclides as weapon material, namely, the radioactive half-life (longer is better for weapons use, inasmuch as shorter half-lives imply more rapid transformation of the nuclear-explosive nuclide into something else, the buildup of which may ultimately change the explosive properties of the material);1 the rate of heat generation by radioactive decay; high rates of heat generation can accelerate deterioration and/or internal distortion of weapon components if the heat is not removed by appropriate design. the rate of neutron production by spontaneous fission and reactions with alpha particles emitted in radioactive decay; the emission of neutrons by these processes may preinitiate a chain reaction earlier in the process of assembling a nuclear weapon than is optimal. the energies of the gamma rays emitted by radioactive decay of the nuclide or its progeny; energetic gamma rays are difficult to shield and therefore tend to lead both to detectable signals and to radiation doses to people handling NEM or weapons. The nuclides whose properties are tabulated in Table A-1 form the basis of the diverse grades of nuclear materials listed with their properties in Table A-2. 1   All nuclear-explosive nuclides are also radioactive. Most radioactive nuclides, however, are not nuclear explosives.

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Monitoring Nuclear Weapons and Nuclear-Explosive Materials TABLE A-2 Heat, Radioactivity and Radiation from Various Nuclear Materials Material Radioactivity (Ci/g) Neutron Generation (n/g-sec) Heat Release (W/kg) Gamma Dose (rem/hr) Natural U 0.0000007 0.013 0.000019 0.000012 LEU 0.0000019 0.012 0.000054 0.000057 Weapon-grade HEU 0.0000095 0.0014 0.00026 0.0015 Weapon-GradePu 0.22 52 2.5 0.94 Reactor-GradePu 6.2 340 14 15 Compositions (percentage by weight) of the indicated materials are assumed to be as follows: Natural U = 99.275 percent U-238, 0.7193 percent U-235, 0.0057 percent U-234 LEU (Low Enriched Uranium) = 96.475 percent U-238, 3.5 percent U-235, 0.025 percent U-234 HEU (Highly Enriched Uranium) = 5.88 percent U-238, 94.0 percent U-235, 0.12 percent U-234 Weapon Pu = 0.01 percent Pu-238, 93.8 percent Pu-239, 5.8 percent Pu-240, 0.13 percent Pu-241, 0.02 percent Pu-242, 0.22 percent Am-241 Reactor Pu = 1.3 percent Pu-238, 60.3 percent Pu-239, 24.3 percent Pu-240, 5.6 percent Pu-241, 5.0 percent Pu-242, 3.5 percent Am-241 The gamma-ray dose is calculated at the surface of a sphere of the metal with a mass of a few kilograms. Abbreviations: Ci = curie, g = gram, kg = kilogram, m3 = cubic meter, n = neutrons, W = watt. A nuclear weapon contains NEM stored in a subcritical configuration. Detonation of the weapon then requires that the NEM be rapidly assembled into a supercritical configuration, wherein the chain reaction grows almost instantaneously to explosive proportions. The explosion ceases once the unreacted part of the NEM has been sufficiently dispersed by the pressures resulting from the energy release (or the thermal expansion in case of a very modest supercriticality) to make the configuration again subcritical. Assembly can be effected either by rapidly joining two subcritical NEM components into a supercritical state (this is the principle of a gun-type weapon) or by rapidly compressing a subcritical NEM component to supercriticality (the implosion type weapon). In a gun-type weapon, a subcritical piece of NEM is propelled by chemical explosives into another subcritical piece of NEM; this process takes several milliseconds. In an implosion device, the NEM component—called a “pit”—is surrounded by chemical explosive lenses, the convergent implosion from which can compress the pit in a fraction of a millisecond. The reason highly enriched uranium (HEU) can be used in gun-type weapons, while plutonium cannot, is that the high rate of

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Monitoring Nuclear Weapons and Nuclear-Explosive Materials spontaneous neutron emission by all plutonium isotopes invariably pre-initiates the chain reaction, given the relatively slow rate of assembly of a critical mass achievable in a gun-type device. The more rapid implosion alternative to a gun-type design overcomes this pre-initiation liability of plutonium. The implosion approach can be effective enough in overcoming the problem of a high spontaneous rate of neutron generation to cope with even the extremely high neutron production rates associated with Pu-238, Pu-240, and Pu-242. Therefore plutonium of virtually any isotopic composition can be used in implosion weapons. Indeed, with sufficient sophistication in design and manufacturing, the less desirable mixtures of isotopes (such as the mixture in reactor-grade plutonium) can be used to make nuclear weapons with performance very similar to what is achievable with weapon-grade plutonium.2 For potential weapon makers with limited relevant knowledge and technical skills, however, the gun-type approach using HEU is a great advantage, since design and implementation are much simpler for gun-type than for implosion-type weapons. HEU has the further advantages of only weak radioactivity and negligible heat generation. Indeed, the gamma dose rates and radiological hazards from uranium at all levels of enrichment in U-235 are so low that radiation exposure is a negligible consideration for anyone stealing it or trying to make a weapon from it. Plutonium of any isotopic composition has a higher rate of heat generation than does HEU, and plutonium metal itself is more hazardous radiologically—and in other ways more difficult to work with—than HEU is. The difficulties of heat generation and radiological hazard are larger for reactor-grade plutonium than for weapon-grade plutonium, although these problems are by no means insurmountable. The critical mass can be made smaller than the bare-sphere value by surrounding the nuclear-explosive material with a “tamper” composed of materials that reflect neutrons. Note also that the implosion approach compresses the NEM to higher than normal density, thereby also reducing the critical mass. The reduction available from use of a reflector is in the range of factor of two or 2   See, e.g., U.S. DOE, Office of Arms Control and Nonproliferation, Final Nonproliferation and Arms Control Assessment of Weapons-Usable Fissile Material Storage and Excess Plutonium Disposition Alternatives (Washington, DC: Department of Energy, January 1997), which states at pp. 38-39: “[A]dvanced nuclear-weapon states such as the United States and Russia, using modern designs, could produce weapons from reactor-grade plutonium having reliable explosive yields, weight, and other characteristics generally comparable to those of weapons made from weapon-grade plutonium.”

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Monitoring Nuclear Weapons and Nuclear-Explosive Materials so. As for compression, the critical mass decreases with the square of the material density. Therefore, if it were possible to compress the nuclear-explosive material to twice its normal density the critical mass would decrease by a factor of four. Thus, a nuclear fission weapon might use a considerably smaller amount of nuclear-explosive material than the bare-sphere critical mass. The metallic forms of the relevant elements give the smallest critical-mass values and produce the most efficient weapons in terms of the fraction of the heavy nuclei present that actually fission. Other chemical forms may also be usable in a nuclear weapon: plutonium oxide, for example, with a bare-sphere critical mass about five times larger than that of plutonium metal, can be made to produce a nuclear explosion. The explosive yield (i.e., the release of energy) from a nuclear weapon is measured by convention by the corresponding quantity of the chemical high explosive, TNT. The explosion of one metric ton (1,000 kilograms) of TNT releases approximately 1 billion calories3 of energy, and the corresponding unit of measure (“one ton of TNT equivalent”) is defined as exactly 1 billion calories, or 4.2 billion joules. The first three nuclear weapons (the one tested at Alamogordo, New Mexico, in July 1945 and those dropped on Hiroshima and Nagasaki the following month) had yields in the range of 10 to 20 kilotons (10,000 to 20,000 tons) of TNT. Early efforts by proliferating states are likely to aim for the same range, as would the sorts of designs likely to be tried by terrorists. Fission weapons of more advanced design have involved a range of yields from a fraction of a kiloton to about 500 kilotons; thermonuclear weapons, combining fission and fusion processes may have yields extending into the multimegaton range. Production Technologies for NEM Here we review briefly what is entailed in producing the two most important classes of NEM, namely, highly enriched uranium and separated plutonium. 3   This is the “small” calorie (i.e., the heat required to increase the temperature of 1 gram of water by 1 degree C), not the “large,” or kilocalorie, used in specifying food consumption, which is 1,000 times larger.

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Monitoring Nuclear Weapons and Nuclear-Explosive Materials Highly Enriched Uranium The most commonly mined uranium bearing ores today are sandstones in which uranium occurs at concentrations of 0.03 percent to 0.2 percent by weight. Uranium is also found at concentrations on the order of 10 times lower in rather widely distributed shales, and at concentrations a few times lower still in even more widely occurring granites. Depending on the characteristics of the particular geologic formation, such uranium-bearing rocks may be extracted from underground mines or from open pits.4 Next the ore is crushed and leached with acid, which dissolves the uranium. It is then extracted as an oxide, U3O8. The extraction of uranium from ore is called ”uranium milling,” and the mildly radioactive, sand-like residues of the process are referred to as “uranium mill tailings.” Uranium mining and milling at a scale adequate to support a nuclear weapon program entail sizable and distinctive operations. The actual process of enriching the U-235 concentration above its value of 0.7 percent in natural uranium is even more demanding. Because different isotopes of uranium behave chemically almost identically, almost all separation methods have relied on physical means of separation, based on the 1.3 percent difference in mass between U-235 and U-238 atoms. In most approaches to this task the natural uranium is first converted to uranium hexafluoride gas (UF6), followed by physical separation of the lighter U-235F6 molecules from the slightly heavier U-238F6 molecules. The best-known technologies for accomplishing this separation are: gaseous diffusion plants, which exploit the difference in the diffusion rates of the lighter and heavier molecules through a “cascade” of thousands of porous barriers; or centrifuge plants, which use sets of hundreds or thousands of sophisticated, ultra-high-speed, gas-centrifuge machines to separate the molecules based on their differing inertial masses. Gaseous diffusion requires large factories containing complex piping arrangements and highly specialized membranes (the characteristics of which remain classified), and utilizes immense 4   Uranium also exists in seawater, at the concentration of about 3 parts per billion by weight. This means that a technology capable of extracting half the uranium from a given volume of seawater would need to process about 650,000 cubic meters of water to extract one kilogram of uranium. The means of doing this by selective absorption have been demonstrated on a small scale.

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Monitoring Nuclear Weapons and Nuclear-Explosive Materials amounts of electric power to run the compressors that force the uranium hexafluoride gas through the membranes. Centrifuge plants have electric power requirements 20-30 times smaller than those of gaseous diffusion plants, but the technology for the centrifuges is extremely demanding. Gas centrifuge plants can be considerably smaller than gaseous diffusion plants, but they still need room for many hundreds or thousands of centrifuges, so concealment poses some challenges. The gaseous diffusion and centrifuge plants operated for civilian nuclear power enrich uranium only to the 3 to 5 percent U-235 level, unsuitable for nuclear weapons. In terms of the “enrichment work” needed to separate isotopes, however, this concentration is half way or more toward the enrichment levels of 90 percent or greater that are desirable for nuclear weapons. In principle, any of the commercial enrichment plants could be operated in a manner to do the remaining work needed to bring this low enriched reactor fuel up to weapon-usable levels (see Box A-1). Other approaches to uranium enrichment besides gaseous diffusion and centrifuges have been utilized from time to time. Some have even larger power requirements than those of gaseous diffusion; the Helikon technology used by South Africa and the electromagnetic separation technology, which was developed by the United States during World War II and subsequently pursued by Iraq in its nuclear weapon program, both fall into this category. Others have very low separation factors and thus need a huge number of stages to reach high enrichment; chemically based processes that have been explored by France, Japan, and also by Iraq fall into this category. Technologies exploiting the capability of precisely tuned lasers to selectively excite U-235 atoms or molecules containing U-235, allowing their separation from the U-238 atoms or molecules containing U-238 by electromagnetic or other means, appear to have the potential for low energy requirements and high separation factors. These technologies have not yet been developed as practical options, however, and it remains unclear whether they might eventually make possible the production of HEU with modest resources and easier concealment. Separated Plutonium Any nuclear reactor that contains U-238 in its fuel produces Pu-239 as a result of the absorption of a fission neutron by a U-238 atom and its subsequent radioactive decay to Pu-239. Some of the Pu-239 that is produced is invariably fissioned itself in the course

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Monitoring Nuclear Weapons and Nuclear-Explosive Materials of the continuing reactor operation, and some undergoes successive absorption of further neutrons to become the heavier plutonium isotopes: Pu-240, Pu-241, and Pu-242. These isotopes also fission in the course of continuing reactor operation. The rate at which plutonium accumulates in a reactor’s fuel depends on many factors, including the type and thermal power output of the reactor and the characteristics of its fuel, its coolant, and its moderator (see Box A-2). The quantity and isotopic composition of the accumulated plutonium depend also on how the reactor is operated, particularly on how much fission occurs in fuel until the time it is removed from the reactor, a parameter called the “irradiation” or “burnup” of the fuel (see Box A-3). High burnup is desirable for electricity production because it means more saleable energy from the fuel, as well as less down-time for refueling (in the case of “batch refuelable” reactor types that need to be shut down in order to remove their spent fuel). But high burnup is undesirable for production of weapon plutonium, both because it leads to greater accumulation of the less desirable even-numbered plutonium isotopes, since much of the desirable Pu-239 product is lost through fission, and because higher burnup means the spent fuel contains larger amounts of radioactive fission products in relation to the quantity of fuel handled, making it more dangerous and difficult to separate out the plutonium. Plutonium Production Reactors and their Performance Reactors that countries built specifically to produce plutonium for weapons have nearly all been fueled with natural (unenriched) uranium and moderated by graphite or by heavy water. They have ranged in rated thermal power output from 20 to more than 4,000 megawatts. Many of these reactors were designed to be continuously refuelable, meaning that irradiated fuel can be removed from the reactor core and fresh fuel can be inserted while the chain reaction continues. This feature enables such reactors to operate at the low burnups needed to make weapon-grade plutonium without needing to be shut down frequently to remove and replace the slightly irradiated fuel. When operated at the very low burnup levels associated with production of weapon-grade plutonium, graphite-moderated and heavy-water-moderated reactors deliver a net rate of plutonium production in the range of 0.9-1.0 gram per megawatt-day of reactor operation. Thus, a very small production reactor with rated thermal capacity of 25 megawatts (the size range of the North Korean graphite-moderated production reactor at Yongbyon) can pro-

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Monitoring Nuclear Weapons and Nuclear-Explosive Materials duce in a year, if it achieves the equivalent of 250 full-power days of operation, about 5.5 kilograms of weapon-grade plutonium. A production reactor l00 times larger, typical of those the United States operated at Hanford and Savannah River, can produce 250 kilograms of plutonium per year. Reactors designed for electricity production generate plutonium as long as their fuel contains U-238. A typical light-water reactor—a batch-refuelable reactor type—operated at the high burnup optimum for the electric-generating role produces 0.22-0.27 gram of reactor-grade plutonium per megawatt-day. Hence a 3,000 thermal-megawatt light-water reactor operating at full power for 330 days per year will discharge 220-270 kilograms of plutonium per year in its spent fuel.5 If such a reactor were operated instead for purposes of optimum production of weapon plutonium at much lower burnup, the net amount of weapon-grade plutonium per year produced in a reactor of given thermal power might be comparable to or somewhat larger than the reactor-grade yield in normal commercial operations but electric power production would be reduced. Reprocessing Spent Fuel to Extract Plutonium In order to recover the plutonium produced in a nuclear reactor from the spent fuel it must be chemically separated or reprocessed from the fission products produced, and from the residual U-238. Reprocessing, like uranium enrichment, is a technically demanding and costly operation; because of the intense gamma radioactivity of the fission products, and the health risks posed by the alpha activity of plutonium if inhaled or otherwise taken into the body, reprocessing is much more hazardous than enrichment from the standpoint of health and safety. Standard practice is to allow the spent fuel to cool for a period of months to years before subjecting it to reprocessing, so that most of the shorter-half-life radionuclides decay away. Even after such cooling, the radiation hazards from spent fuel remain high. The dose rate at the surface of a spent fuel assembly from a modern light-water reactor, at typical commercial burnup and after 10 years of cooling, is around 20,000 rem6 per hour, and at distance of 5   This would correspond to a capacity factor of 330/365 = 0.904 or 90.4 percent (see Box A-3), a level of performance now quite commonly achieved in commercial light-water reactors. 6   The rem, or “roentgen equivalent man” represents the energy deposited per unit mass of tissue, weighted by the relative effectiveness in inducing health-affecting changes, for each type of radiation. A whole body dose of 500 rem leads to the death of about one half of the individuals exposed.

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Monitoring Nuclear Weapons and Nuclear-Explosive Materials a meter it is around 2,500 rem per hour. At the far lower burnups associated with production reactors operated to make weapon-grade plutonium, the dose rate is correspondingly lower, but impatience to get the plutonium out may reduce the length of time the fuel is allowed to cool. A fuel assembly from a light-water reactor that had experienced a burnup level appropriate to weapon plutonium production but then cooled for just two years would deliver a dose rate at its surface of nearly 40,000 rem per hour. The approach to reprocessing that has been used virtually universally for military and civilian purposes alike—called the “PUREX” process—consists of chopping up the radioactive spent fuel into pieces, dissolving these in nitric acid, and then performing a set of solvent extractions on the resulting solution to separate the plutonium, the uranium, and the fission products into three output streams. The uranium or plutonium may emerge finally as nitrates or as oxides. Ultimately, for weapon use, the plutonium would be transformed into metallic form by a simple process. In all cases, extensive shielding and equipment for remote handling of the materials are required in all stages of reprocessing up to the point where the fission products have been separated from the uranium and plutonium. The equipment must be designed to avoid the possibility that a critical mass of plutonium in a liquid form or as a precipitate could form at any point in the system. And pipes, valves, and vessels must be repairable by remote control, because they will be too radioactive to approach even with protective suits. The technology for this is so demanding and difficult that even major industrial nations have ended up building some reprocessing plants that failed almost immediately and were deemed so expensive to repair that they were abandoned.7 Denaturing Plutonium The best available way to render separated plutonium unusable for nuclear weapons is to unseparate it by remixing it with fission products. Remixing with fission products, to a suitable degree of difficulty for reversing the process, can be achieved by embodying the plutonium in mixed oxide (MOX) reactor fuel and then using that fuel in a power-generating reactor, or by mixing the plutonium with fission products that already exist in storage from prior mili- 7   For example, a reprocessing plant built by the General Electric Company in Morris, Illinois, met this fate.

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Monitoring Nuclear Weapons and Nuclear-Explosive Materials and gas centrifuge enrichment plants operated by United States, the United Kingdom, France, Russia, China, Japan, Germany, and the Netherlands is about 10 million SWU per year larger than current needs. If the excess capacity were to be used for producing weapon-grade HEU, it would be enough to turn 11,000 tons of natural uranium into 50 tons of 93 percent enriched HEU.13 These numbers underline the importance and the challenge associated with monitoring to ensure that civilian enrichment capacity is not diverted to production of NEM. The operation of the world’s civilian power reactors leads to the discharge of about 80 tons per year of plutonium embedded in 8,000 tons of spent nuclear fuel. This spent fuel, in which the plutonium is intimately mixed with intensely radioactive fission products and unfissioned U-235 and U-238, goes directly to spent fuel storage pools at the reactor sites. From these, some is later removed for transfer to dry cask storage; and in recent years in about 20 percent of the world’s reactors, the spent fuel is removed for transfer to a reprocessing plant where the plutonium is separated for eventual recycling in fresh reactor fuel. Large reprocessing plants for extracting plutonium from commercial power reactor fuel are in operation at La Hague in France, at Sellafield in England, and at Chelyabinsk in Russia. A much smaller commercial reprocessing plant is operating at Tokai-Mura in Japan. Japan has a larger reprocessing plant near completion at Rokkasho-Mura, but whether it will ever operate is unclear. France has a very small plant for reprocessing breeder reactor fuel at Marcoule. Belgium, the United States, and Germany operated pilot-scale reprocessing plants in the past motivated by commercial possibilities. In recent years, the rate of production of separated plutonium from reprocessing of spent civilian fuel has been about 20 tons per year, and the rate of fabrication of separated plutonium into mixed oxide fuel for actual loading into power reactors has been about one half that amount. Since the plutonium inventory in spent nuclear fuel has been growing at about 60 tons per year, the total plutonium inventory in spent plus active nuclear fuel has been growing at about 70 tons per year. Moreover, if currently operating commercial reprocessing plants were being utilized to their full 13   Producing 1 kilogram of 93 percent enriched HEU requires 226 kilograms of natural uranium input and 200 SWU if the depleted uranium “tails” contain 0.3 percent U-235. Thus 10 million SWU is enough to make 50 tons of 93 percent enriched HEU, starting from 226 × 50 = 11,300 tons of natural uranium.

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Monitoring Nuclear Weapons and Nuclear-Explosive Materials capacity, the civilian stocks of separated plutonium would be growing at about 30 tons per year. The greater part of the plutonium embedded in the spent fuel discharged by civil power reactors currently remains in unreprocessed spent fuel in wet or dry storage. Plutonium stored in this form amounted to about 1,250 tons at the end of 2002. Although this material cannot be used in nuclear weapons unless it is first separated in a reprocessing plant, the stocks that exist in this form nonetheless need to be monitored in addition to monitoring reprocessing plants, to ensure that reprocessing of civilian spent fuel for weapon purposes is not occurring in undeclared facilities. NEM in the civil sector in less developed countries is largely confined to HEU connected with research reactors. The principal exceptions are India, which has reprocessed plutonium from civil power reactors as well as from reactors dedicated to producing military plutonium, and China, which is just beginning the reprocessing of civil plutonium. There are approximately 135 operating research reactors fueled with HEU, in more than 40 countries around the world, ranging from the United States to Ghana.14 Most of these research reactors have only small amounts of HEU; but some, including a significant number outside the nuclear weapon states, have enough fresh HEU for a bomb. Even more have enough HEU for a bomb if spent HEU that is not radioactive enough to deter suicidal terrorists from taking it and using it in a bomb is taken into account.15 It should be noted, however, that the fresh fuel itself, although it is categorized as NEM, cannot be used directly to make a nuclear explosive until the uranium is separated from the aluminum or other inert matrix since the small density of the uranium greatly increases the critical mass. 14   Matthew Bunn, Anthony Wier, and John P. Holdren, Controlling Nuclear Warheads and Materials: A Report Card and Action Plan (Washington, DC: Nuclear Threat Initiative and Project on Managing the Atom, Harvard University, March 2003), and references therein, including especially International Atomic Energy Agency, Nuclear Research Reactors of the World (Vienna, Austria: IAEA, September, 2000), available as of January 2005, at: http://www.iaea.org/worldatom/rrdb/ supplemented with personal communications with James Matos, Argonne National Laboratory, and Iain Ritchie, International Atomic Energy Agency, 2002. 15   See Edwin Lyman and Alan Kuperman, “A Re-Evaluation of Physical Protection Standards for Irradiated HEU Fuel” paper presented at the 24th International Meeting on Reduced Enrichment for Research and Test Reactors (Bariloche, Argentina, November 5, 2002).

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Monitoring Nuclear Weapons and Nuclear-Explosive Materials BOX A-1 Uranium Enrichment Inputs and Outputs The magnitude of the task of uranium enrichment can be characterized in three particularly informative ways: the amount of unenriched or low enriched uranium (LEU) input required to obtain the desired, more highly enriched output; the amount of separative work required for the actual sorting of the heavy and light nuclei that enrichment entails; and the amount of electrical energy that a particular separation technology needs in order to perform this work. The amount of uranium feed required can be calculated from simple balance equations that track the unchanging total quantities of the U-235 and U-238 isotopes. The answer depends on the U-235 concentration in the feed, the U-235 concentration desired in the enriched product, and the concentration specified for U-235 in the depleted uranium waste stream (called “tails”). A materials balance calculation does not depend on which technological process one chooses for doing the enrichment, except to the degree that the final result needs to be adjusted for losses (such as, material ending up coating the insides of pipes), which can vary from one technology to the other. Natural uranium contains 0.72 percent U-235 and 99.27 percent U-238. (The remainder is 0.006 percent U-234, which can be neglected for our purposes here.) Enrichment levels for typical LEU power reactor fuels are 3-5 percent U-235 fuels, and the weapon-grade HEU preferred by bomb makers is 93 percent U-235. The amount of U-235 left in the tails is a matter of choice, but is usually between 0.2 and 0.4 percent. If natural uranium is cheap and enrichment work is expensive, one chooses a relatively high U-235 concentration in the tails, which increases the natural uranium feed requirement but reduces the separative work. If natural uranium is expensive and enrichment work is cheap, one chooses a lower U-235 concentration in the tails. If we take the intermediate value of 0.3 percent for the amount of U-235 to be left in the tails, the isotope-balance approach shows that an input of 226 kilograms of natural uranium (containing 0.7 percent U-235) is required to produce an output of 1 kilogram of uranium enriched to weapon grade at 93 percent U-235, neglecting losses in the enrichment plant. A hypothetical gun-type bomb de-

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Monitoring Nuclear Weapons and Nuclear-Explosive Materials sign requiring 60 kilograms of this HEU, for example, would require an input of 13,560 kilograms of natural uranium (60 kg HEU×226 kg natural U per kg HEU = 13,560 kilograms of natural uranium).* (If the uranium comes from ore that contains 0.1 percent uranium, the corresponding ore requirement is 13,560 metric tons.) To produce an output of 1 kilogram of LEU at the 5 percent U-235 concentration typically used in a modern light-water power reactor, by contrast, requires an input of only 11.5 kilograms of natural uranium. A 1,000 megawatt nuclear reactor of this type will require an input of about 20 tons of fuel of this enrichment per year, so the uranium input to the enrichment plant supporting this reactor must be 20,000 kg LEU×11.5 kg natural U per kg LEU = 230,000 kilograms of natural uranium, or 230 tons, and the corresponding mining requirement is 230,000 ton of ore containing 0.1 percent uranium. The quantitative measure of how difficult it is to separate isotopes of different atomic masses is the “separative work unit” (SWU). A formula derived from thermodynamics enables calculation of the number of SWU needed to produce a kilogram of uranium enriched to any specified concentration of U-235, given the starting concentration and the concentration desired in the tails. Application of this formula reveals that producing 1 kilogram of HEU with 93 percent U-235, starting from 226 kilograms of natural uranium and leaving behind 225 kilograms of uranium tails containing 0.3 percent U-235, requires 200 SWU. Thus the enrichment requirement for a hypothetical gun-type weapon containing 60 kilograms of this HEU would be 60 kg HEU×200 SWU per kg of HEU = 12,000 SWU. Producing 1 kilogram of LEU with 5.0 percent U-235 starting from 11.5 kilograms of natural uranium, leaving behind 10.5 kilograms of tails containing 0.3 percent U-235, requires 7.2 SWU. Thus the annual separative work requirement to enrich the uranium fuel for the 1,000 megawatt light-water reactor mentioned above is 20,000 kg of LEU×7.2 SWU per kg of LEU = 144,000 SWU. One sees from this comparison that the amount of enrichment capacity needed to support one large power reactor could, alternatively, perform the enrichment for something like a dozen hypothetical gun-type nuclear weapons per year. Commercially, one SWU costs about $100. The electric power requirements for uranium enrichment plants range from 100-150 kilowatt-hours per SWU in a centrifuge plants to 2,000-3,000 kilowatt-hours per SWU in gaseous diffusion plants to something like 4,000 kilowatt-hours per SWU for the noz-

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Monitoring Nuclear Weapons and Nuclear-Explosive Materials zle/aerodynamic technologies. Laser enrichment technologies are expected to be in the 100-200 kilowatt-hour per SWU range. The electricity requirement for enriching, by means of gaseous diffusion, the uranium for one hypothetical gun-type bomb using 60 kilograms of 93 percent U-235 HEU would therefore be in the range of 12,000 SWU×2,500 kilowatt-hours per SWU = 30,000,000 kilowatt-hours. At typical U.S. electricity costs of 7 cents per kilowatt-hour, this is $2 million worth of electricity. This electricity requirement likewise means that a gaseous diffusion complex big enough to enrich the uranium for, say, a dozen of these hypothetical gun-type HEU bombs per year would require the full annual output of a 50 megawatt power plant (which is a size adequate to meet the needs of a town of 50,000 people). Using a gas centrifuge plant at 125 kilowatt-hours per SWU, on the other hand, would entail electricity requirements 20 times smaller, worth about $100,000 per bomb, and needing only 2.5 megawatts of dedicated electricity generating capacity to make a dozen or so hypothetical gun-type HEU bombs per year. *   See Chapter 3, page 114, for the basis for the figure of 60 kilograms used in the calculations in this box.

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Monitoring Nuclear Weapons and Nuclear-Explosive Materials BOX A-2 Reactor Types and Terminology Nuclear reactors fall mainly into three categories: (1) power reactors, which are designed and operated to produce electric power; (2) production reactors, whose purpose is to produce particular nuclides for nuclear-explosive, industrial, or medical purposes; and (3) research reactors, which are used for studying nuclear physics and materials science, and for teaching. Sometimes reactors are used in a dual-purpose mode (e.g., generating power and producing nuclear weapon material, or research and medical isotope production), and a few have been designed from the outset for such dual-purpose use. Nearly all of the reactors that have been built to date for electric power generation, as well as most of those that have been built for producing weapon material, rely primarily on the fissile uranium isotope U-235 to sustain their fission chain reaction; and most of them do so by exploiting the especially high fission probability of U-235 when exposed to slow neutrons (i.e., those whose speeds are not much higher than those of neutrons in thermal equilibrium with their surroundings). Such reactors are called “slow-neutron,” or “thermal” reactors. Relying on slow neutrons, with their high probability of causing a fission in any U-235 nucleus they encounter, allows maintaining a chain reaction in fuel with a lower concentration of U-235 than would be needed if one were trying to sustain the chain reaction with fast neutrons. (A thermal reactor could similarly rely on a low concentration of one of the other fissile nuclides, U-233 or Pu-239, if desired, as these also have high fission probabilities at low neutron energies.) Use of fuel with a low concentration of its fissile nuclide(s) has a number of advantages over fast-neutron reactors by being able to operate at a lower power density (watts per cubic centimeter in the reactor core), which reduces the engineering challenges and increases the safety margin, and including (in the case of fuel based on U-235) reduced enrichment requirements. Fast-neutron reactors must compensate for the lower fission probability at high neutron energies by increasing the concentration of fissile nuclei and hence the power density with resulting increased U-235 enrichment costs. Because fission neutrons are “born” with energies much higher than the energy corresponding to the temperature of their surroundings, a thermal reactor must arrange for the neutrons to slow down to near-

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Monitoring Nuclear Weapons and Nuclear-Explosive Materials thermal velocities (where their probability of causing a fission is high) before they are captured in a nonfission reaction or escape from the reactor. This requires the use of a moderator, a substance in the reactor core that is efficient in slowing down neutrons without absorbing very many of them. (Fast-neutron reactors, by contrast, are designed to minimize presence of moderating materials in the core.) The best moderator materials are very pure graphite (the purity being required because graphite’s impurities would absorb too many neutrons) and heavy water (H2O in which ordinary hydrogen has been replaced by the heavier hydrogen isotope, deuterium). Ordinary water is a decent moderator but not as good as heavy water because the no-neutron isotope of hydrogen that most ordinary water molecules contain is much more likely to absorb a neutron than is deuterium, which already has one. Graphite and heavy water are such good moderators, in fact, that a suitably designed reactor using one or the other (or both) is able to sustain a chain reaction using natural uranium, despite its very low U-235 concentration of 0.7 percent. The CANDU (standing for Canada Deuterium Uranium) power reactor is an example; its development enabled Canada and other countries that bought them to generate electricity from nuclear energy without building a uranium enrichment plant or having to buy enriched fuel from someone else. Because of the desirability of minimizing unproductive absorption of neutrons when trying to make as much plutonium as possible, graphite-and/or heavy-water moderated designs have generally been the reactors of choice for producing plutonium for weapons in the countries that have done so. Many of these reactors were designed to be continuously refuelable, which means the reactor does not need to be shut down in order to remove some of its fuel elements for extraction of their plutonium. As well as being characterized by its moderator (or lack of one), a reactor type is characterized by its coolant. The function of the coolant is to remove the nuclear generated heat from the core so that the solid fuel and structure do not melt. In power reactors, the coolant also serves to carry this energy to adjacent equipment for conversion to electricity. Some graphite-moderated thermal reactors are gas cooled (employing carbon dioxide, helium, or air); others are cooled with heavy water or ordinary water (which is called light water in this context). In some reactor designs, heavy water or light water serves as both moderator and coolant. About 85 percent of the world’s power reactors are so-called light-water reactors, in which ordinary water plays both roles. These require uranium fuel enriched to 3 to 5 percent in U-235 or similar concentrations of U-233 or Pu-239. They cannot use natural uranium. Recycling the plutonium and unfissioned U-235 from their spent fuel could reduce

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Monitoring Nuclear Weapons and Nuclear-Explosive Materials their raw uranium and enrichment requirements by 25 to 30 percent. This does not pay at current prices for uranium enrichment and fuel reprocessing/recycling. The separation of plutonium from spent fuel increases proliferation risks but a few countries are doing it anyway. Fast-neutron reactors (usually but somewhat confusingly called just “fast reactors”) cannot be cooled with water because its moderator properties would result in too much slowing down of the neutrons. The attractions of fast reactors are the compactness of their fission cores (which is valuable in some applications but not generally in electricity generation), the energy and intensity of the neutron fluxes they generate (a useful property for certain research and industrial applications), and the high rate at which they can produce plutonium from U-238. The possibility of producing more plutonium than does a thermal-neutron reactor arises because fissions induced by fast neutrons release, on the average, more neutrons per fission than fissions induced by slow neutrons, and these extra neutrons are potentially available for plutonium-producing absorption by U-238. Gas and liquid metals are the main possibilities for cooling fast reactors. Liquid metals have been the pre-dominant choice so far, because of their greater capability for heat removal. The sodium-cooled liquid metal fast breeder reactor (LMFBR) is the fast-reactor type that has attracted the most interest, including prototype and pilot plant development in a number of countries. But it has proven, however, to be a very demanding technology whose principal potential advantage—the capacity to conserve uranium by breeding U-238 into Pu-239 at rate sufficient to refuel itself with some left over—has not paid off in a world where uranium continues to be very cheap and reprocessing fuel to recover bred plutonium for recycling continues to be very expensive. If it is desired to minimize rather than to maximize the production of plutonium—as might be sought in circumstances where the potential for diversion of the plutonium for use in weapons is of particular concern—it is necessary to avoid having very much U-238 in the reactor. One reactor design that can achieve this is the high-temperature gas-cooled reactor (HTGR), a thermal reactor that can operate using a combination of U-235 and U-233 as its fissile material. The remaining heavy nuclides present in the fuel are a mixture of thorium-232 and a modest amount of U-238. U-233 is produced in this fuel cycle as a result of the absorption of fission neutrons in the Th-232.

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Monitoring Nuclear Weapons and Nuclear-Explosive Materials BOX A-3 Reactor Size and Performance Reactors can be of different sizes and types. Size is an important characteristic in determining the potential production of nuclear-explosive materials of which a given reactor is capable. The most relevant measure of size is the “rated thermal capacity,” which is the rate of release of nuclear energy in the reactor core for which the reactor has been designed and at which it is authorized to operate. The usual units for rated capacity are megawatts of thermal energy flow. An energy flow of a megawatt sustained over a day adds up to 1 million joules per second multiplied by the 86,400 seconds in a day, or 86.4 billion joules. This unit of energy is called a megawatt-day. The fission of one gram of uranium or plutonium leads to the deposition in the reactor of about 82 billion joules of fission energy, which corresponds to about 0.95 of a megawatt-day. Rounding this off to one megawatt-day of thermal energy release per gram of heavy nuclei fissioned gives a rule of thumb that is often used for making estimates of nuclear fuel consumption rates in reactors, based on their rated capacity and the fraction of the time that they achieve it. The theoretical maximum amount of thermal energy that a reactor can generate in a year is given by its rated capacity in megawatts multiplied by the number of days in a year, hence 365 megawatt-days of energy per year per megawatt of rated capacity. The actual output of energy that a reactor achieves in a year, divided by this theoretical maximum that it would have generated if it had operated at 100 percent of its rated capacity for 100 percent of the time, is called its “capacity factor” for the year. This measure of fission energy extracted from fuel is called the “irradiation” or burnup; its units are megawatt-days per kilogram of heavy metal (uranium or plutonium) loaded into the reactor (MWd/kgHM). The burnup in today’s large commercial electric

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Monitoring Nuclear Weapons and Nuclear-Explosive Materials power reactors is typically between 30 and 50 MWd/kgHM, but in reactors being operated to produce plutonium for weapons, the figure has been much lower, on the order of 1.0 MWd/kgHM. Large light-water reactors built for electricity generation have rated thermal capacities in the range of 3,000 megawatts (at 33 percent electrical generation efficiency, corresponding to about 1,000 megawatts of electrical capacity). The smallest plutonium production reactors likely to be of interest would be around 20 megawatts. Using the rule of thumb of one gram of heavy nuclei fissioned per megawatt-day of thermal output indicates that a large power or production reactor rated at 3,000 thermal megawatts will fission about 3 kilograms of heavy nuclei per full-power day of operation. (Since the mass of the radioactive fission products is very nearly the same as the mass of the nuclei whose fission produced them, such a reactor generates about 3 kilograms per full-power day of radioactive fission products.) At the other end of the size range, a production reactor rated at 20 thermal megawatts will fission about 20 grams of heavy nuclei per day of full-power operation, yielding 20 grams of fission products and about 20 grams of Pu-239.

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Monitoring Nuclear Weapons and Nuclear-Explosive Materials BOX A-4 History of Plutonium Production Reactors United States: 9 graphite-moderated, light-water-cooled production reactors deployed at the Hanford site and 5 heavy-water-moderated production reactors deployed at Savannah River (none still operating). Former Soviet Union/Russia: 13 graphite-moderated, light-water-cooled production reactors at Chelyabinsk, Tomsk, Krasnoyarsk (of which 3 dual-purpose reactors are still operating to supply heat and electricity in the Krasnoyarsk and Tomsk regions). United Kingdom: a total of 10 graphite-moderated, gas-cooled production reactors at Windscale, Calder Hall, and Chapel Cross (none still operating). France: 9 graphite-moderated, gas-cooled reactors at Marcoule and two heavy-water moderated reactors at Celestin (none still operating); the prototype liquid-metal-cooled, fast-neutron breeder reactor (the Phénix) was shut down for maintenance between 1998 and 2003 and has reportedly returned to operation. China: 2 graphite-moderated, light-water-cooled reactors, one at Jiuquan and one at Guangyuan (none still operating). Israel: a heavy-water-moderated, air- and heavy-water-cooled production reactor at Dimona. India: 2 heavy-water-moderated production reactors near Bombay. North Korea: a graphite-moderated, gas-cooled production reactor at Yonbyon.