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Blocking the Terrorist Pathway to the Bomb
Material Production
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Production of Nuclear Materials
This has been regarded as the principal barrier not only because the technologies for producing these materials are demanding and costly (about which more in a moment), but also because the steps needed to make a weapon once the material is in hand are not as difficult as producing the material is,[2] and because the plutonium and highly enriched uranium that have been produced until now by those possessing the requisite technologies have mostly been well guarded or have resided in forms awkward to steal and difficult to use in weapons (such as plutonium in spent reactor fuel[3]). So how difficult is the production of these materials? The subsequent subsections elaborate on this question.[4] Producing HEU The process of producing HEU begins with acquiring natural uranium. This element is quite widespread in the Earth’s crust. It is mined today principally from sandstone ores in which the uranium occurs at concentrations of 0.03% to 0.2% by weight, but it 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.[5] Once the ore is in hand, the rock is crushed and exposed to an acid bath which leaches the uranium into solution. It is then extracted as an oxide, U3O8. These steps by which uranium is extracted from its ore are collectively called uranium milling, and the quite voluminous, 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—even a small one—entail rather sizable operations, with distinctive observable characteristics. For example, to obtain enough raw uranium to feed the enrichment operation for a single "gun-type" nuclear weapon (about which more below) would require the mining of more than 13,000 metric tons of sandstone ore contain 0.1% U, as well as the disposal of about 13,000 metric tons of uranium-mill tailings.[6] This is not an operation likely to be manageable by terrorists. Even when conducted by a nation it is the sort of thing that would require some effort to successfully conceal. The actual process of enriching the U-235 concentration above its value of 0.7% in natural uranium is even more demanding. Because different isotopes of uranium behave almost identically chemically, most separation methods have relied on physical rather than chemical means of separation, based principally on the 1.3 percent difference in mass between U-235 and U-238 atoms. The main approaches to this task that have been used to date involve first converting the natural uranium to uranium hexafluoride gas (UF6), followed by physical separation of the lighter U235F6 molecules from the slightly heavier U238F6 molecules. The best-known technologies for accomplishing this separation are:
Gaseous diffusion plants involve large, complex piping arrangements, esoteric membranes (the characteristics of which remain classified in all countries that have developed them), and immense electric-power requirements for running 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 and closely controlled. Gaseous-diffusion plants are the size of factories, not of laboratories, and this plus their large electric-power requirements makes them difficult to conceal. Gas-centrifuge plants can be somewhat smaller, and they need less electric power; but they still need room for hundreds of centrifuges so concealment poses some challenges. Neither technology could be mastered by terrorists. Indeed, they are challenging even for countries other than the major industrial nations. Countries operating gaseous diffusion or gas-centrifuge enrichment plants for the support of commercial nuclear-power operations include the United States, the United Kingdom, France, Russia, China, Japan, Germany, and the Netherlands. These commercial uranium-enrichment plants are being used to enrich uranium only to the 3 to 5 percent U-235 level suitable for use in commercial nuclear power reactors of today’s dominant types. This material cannot sustain a nuclear explosion and so cannot be the driver of a nuclear weapon. In terms of the "enrichment work" needed to separate isotopes, however, it is half way or more toward the 90%+ enrichment levels desirable for nuclear weapons. (See Box 3: Uranium Enrichment: Inputs and Outputs.)
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. Commercial enrichment facilities in countries other than the "authorized" nuclear-weapon states[7] are subject to International Atomic Energy Agency safeguards designed to detect such activity if it occurs. It is not something likely to be accomplishable by terrorists or by agents of a proliferation-inclined country by taking over someone else’s enrichment plant for a few hours. All five of the "authorized" nuclear-weapon states used gaseous-diffusion and/or gas-centrifuge enrichment plants to produce HEU for their weapons. (None of these countries is producing HEU for weapons at this time.) In the past, these countries produced uranium at a range of enrichments above 20 percent not only for nuclear weapons but also for use in nuclear reactors for propulsion of submarines, other warships, and icebreakers; in research reactors; and in experimental power reactors of a variety of kinds. Smaller gaseous-diffusion and gas-centrifuge enrichment plants were operated in the past by Argentina and Brazil in connection with nuclear-weapon programs that have since been abandoned, and such plants are operating today in the "de facto" nuclear-weapon states India, Pakistan, and North Korea. (North Korea appears to have obtained the centrifuge technology from Pakistan.) Other approaches to uranium enrichment besides gaseous diffusion and centrifuges have been explored from time to time but have their own drawbacks. Some have even larger power requirements than those of gaseous diffusion; the aerodynamic-nozzle technology used by South Africa,[8] and the electromagnetic separation technology developed by the United States in World War II and subsequently tried by Iraq in the nuclear-weapon program unveiled by the Gulf War, 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 pursued by France and Japan – and also by Iraq – fall into this category. Technologies exploiting the capability of precisely tuned lasers to selectively excite uranium-235 atoms, allowing their separation from the uranium-238 items by electromagnetic or other means, appear to have the potential for low energy requirements and high separation factors, and they have been under investigation in a number of countries for decades. They have not yet been developed as practical options, however, and it remains unclear whether the worries of non-proliferation analysts about laser enrichment – that it might finally make possible the production of HEU with modest resources and easy concealment – will ever be realized. Producing plutonium As noted above, any nuclear reactor that contains U-238 in its fuel produces Pu-239 in the course of operation, as a result of the absorption of some of the fission neutrons by this uranium isotope. Some of the Pu-239 that is produced is invariably fissioned itself in the course of the continuing chain reaction, and some undergoes successive absorption of further neutrons to become the heavier plutonium isotopes—Pu-240, Pu-241, and Pu-242. Some of these 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 reactor and the characteristics of its fuel, its coolant, and its moderator. (See Box 4: Reactor Types and Terminology). The quantity and isotopic composition of the accumulated plutonium depend also on how the reactor is operated, particularly on how much fission occurs in each kilogram of fuel up until the time it is removed from the reactor, a parameter called the irradiation or burnup of the fuel. (See Box 5: Reactor Size and Performance).
High burnup is desirable for electricity production because it means more saleable energy from the fuel one has paid for fabricating, as well as less "down time" for refueling (in the case of reactor types that need to be shut down and partly disassembled in order to remove their 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 and because higher burnup means the spent fuel contains larger amounts of radioactive fission products in relation to the plutonium quantities present, making it more dangerous and difficult to separate out the plutonium. The reactors that countries determined to produce plutonium for weapons have built for this purpose have nearly all been fueled with natural (un-enriched) 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.[9] 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 is generating neutrons and power. 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.[10] Reactors that must be shut down and opened up for refueling—called "batch refuelable"—can lose considerable operating time in this process. Thus they cannot make as much plutonium in a year (and, in dual-purpose reactors, neither as much plutonium nor as much electricity) as a continuously refuelable reactor of the same rated thermal power. Regardless of the details, when operated at the very low burnup levels associated with production of weapon-grade plutonium (see Box 4), all the graphite-moderated and heavy-water-moderated production reactors deliver a net rate of plutonium production in the range of 0.9-1.0 grams 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 produce in a year, if it achieves the equivalent of 250 full-power days of operation, about 5.5 kilograms of weapon-grade plutonium – about one bomb’s worth.[11] Clearly, a production reactor l00 times larger, typical of those the United States operated at Hanford and Savannah River, could produce 100 bombs’ worth of plutonium per year. Building a plutonium-production reactor is a demanding task even for a nation of some technical capability. A number have achieved this, of course, but some of those who have done so have made use of help from more advanced nations. The problem of building a production reactor is not just a matter of having the needed knowledge plus the trained personnel and industrial equipment needed to translate the knowledge into hardware. It is also a matter of being able to acquire the unusual materials that are required: natural uranium, which while widespread in ore is not easy to get in the refined form needed for a reactor; and either heavy water or extremely pure graphite, to serve as the moderator. Producing any of these materials is itself a high-technology operation, and sales are subject to restrictions and tracking. The challenges of using a production reactor to acquire weapon plutonium are even greater if those trying to do so wish to conceal this activity. The process of building the reactor is not so easy to hide, all the less if help is being provided by another country. The reactor itself can be put underground to hide it from satellites, but this increases the cost and there still must be access points and ventilation shafts whose construction or use might be detected. Most problematic of all for concealment, it is in the nature of a reactor that the 20 or 200 or 2,000 megawatts of energy flow that it produces while operating must be discharged to the environment as heat. (In a dual-purpose reactor, some of the energy is converted to electricity and transmitted elsewhere, but two thirds or more of the energy still ends up in the reactor’s immediate environment as heat.) Heat sources of this magnitude are extremely difficult to hide from infrared sensors on satellites. Of course, as noted earlier even reactors designed for electricity production will automatically make significant quantities of plutonium, as long as their fuel contains substantial quantities of U-238. In a typical "light-water reactor"—a batch-refuelable reactor type designed for electricity generation—the net rate of plutonium production if the reactor is operated at the high burnups optimum for the electric-generating role is 0.22-0.27 grams of plutonium per megawatt-day. This means that a 3,000 thermal-megawatt light-water reactor that operates at full power for 330 days per year (hence a million megawatt-days per year) will discharge 220-270 kilograms of plutonium per year in its spent fuel[12]—enough plutonium to make something like 40 nuclear weapons.[13] If such a reactor were operated instead, for purposes of optimum production of weapon plutonium, at a burnup of 1 megawatt-day per kilogram of heavy metal in its fuel rather than the 30-50 megawatt-day per kilogram levels characteristic of commercial operation in reactors of this type (Box 4), the net plutonium production per megawatt-day would rise to about 0.5 grams of plutonium per megawatt-day. But in this mode the reactor would need to be shut down much more frequently for refueling, which means its capacity factor would fall substantially, perhaps to 60%, corresponding to 220 days per year of full-power operation per year. Then the output of plutonium would be 3,000 megawatts x 220 days per year x 0.5 gram per megawatt-day = 330 kilograms per year, and this plutonium would be of higher quality for weapon purposes than in the higher burnup case.[14] In order to use the plutonium produced in a nuclear reactor in a nuclear weapon, it must be chemically separated from the fission products produced along with it, and from the residual U-238, by reprocessing the nuclear fuel. Reprocessing, like uranium enrichment, is a technically demanding and costly operation; and 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 also much more hazardous than enrichment from the standpoint of health and safety. The approach to reprocessing that has been used virtually universally for military and civilian purposes alike—called the "Purex" process—was worked out in the United States in the Manhattan Project of World War II. It 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 the metal. The set of operations associated with reprocessing is made greatly more difficult than it would otherwise be by the intense radiation emanating from the fission products, much of it in the form of highly penetrating gamma rays. Standard practice is to allow the spent fuel to "cool" for a period months to years before subjecting it to reprocessing, so that some of the shorter-half-life radionuclides will have decayed away. Even after such cooling, the radiation hazards from spent fuel to those trying to work with it remain high. The dose rate at the surface of a spent fuel assembly from a modern light-water reactor, at typical commercial burn-up and after ten years' cooling time, is around 20,000 rem per hour, and at distance of a meter it is around 2,500 rem per hour. (Recall from Box 2 that a whole body dose of 1,000 rem delivered in a period of less than a week or so is certain to be fatal within weeks.) At the far lower burnups associated with production reactors operated to make weapon-grade plutonium, the dose rate and any given time after discharge from the reactor is lower, but impatience to get the plutonium out in order to get on with making weapons from it tends to reduce the length of time the fuel is allowed to cool. A fuel assembly from a light-water reactor that had a experienced a burnup level appropriate to weapon-plutonium production and then been allowed to cool for just two years would deliver a dose rate at its surface of nearly 40,000 rem per hour. In all cases, then, 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 possibi |
