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Management and Disposition of Excess Weapons Plutonium: Reactor-Related Options 2 Background Consideration of the options for disposition of weapons plutonium (WPu) by irradiating it in nuclear reactors or mixing it with radioactive waste streams from reactors, which is the task of this report, requires reference to the physics and technology of nuclear fission as applied to both nuclear power production and nuclear weapons. Limitations of time and space preclude presentation of a thorough review to these topics here. In the first part of this chapter, we cover in very abbreviated form just the aspects of fission physics and technology most germane to technical arguments made elsewhere in this report. 1 The remainder of the chapter elaborates on the ways in which reactors or waste streams from reactors could be used in the disposition of WPu, and surveys the numbers, capacities, and distribution of the extant nuclear facilities potentially relevant to such an enterprise. 1 For accessible but much more thorough introductions to nuclear fission in the nuclear power context, see the report of the American Physical Society study group on the nuclear fuel cycle (APS 1978) or any good introductory text on nuclear reactors (e.g., Nero 1979). On nuclear weapons physics and technology, see Cochran et al. (1987), Serber (1992), and Mark (1993).
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Management and Disposition of Excess Weapons Plutonium: Reactor-Related Options PHYSICS AND TECHNOLOGY OF NUCLEAR FISSION Fission and Chain Reactions Fission arises because nuclei of atomic number 92 and higher are so large and rich in protons that they are unstable if set into vibration of large amplitude, from which one of the modes of decay is division into two smaller nuclei and two to four free neutrons. Vibration of an amplitude sufficient to cause this fission can be induced in a heavy nucleus by the absorption of a suitably energetic neutron. In the case of heavy nuclei with an odd number of neutrons to start with—such as uranium-233 (U-233), U-235, plutonium-239 (Pu-239), and Pu-241—the absorption of a neutron with the very low energy associated with thermal motion at room temperature is sufficient to induce fission. For heavy nuclei with an even number of neutrons to start with, fission can only be induced if the absorbed neutron carries an energy of a million electron volts (MeV) or more. Depending on the number of neutrons released per fission, the energy distribution arrived at by the neutrons, the densities of heavy nuclei in the vicinity and their probabilities of fissioning as a function of the energy of an incident neutron, and the probabilities of nonproductive absorption of neutrons or their escape from the vicinity (which depend on the geometry and composition of the materials at hand), it may happen that, for each and every nuclei that fissions, exactly one of the resulting neutrons induces yet another fission. This situation corresponds to a chain reaction that is just "critical," in which the fission rate and thus the rate of nuclear-energy release do not change with time. (This would be the case, for example, in a nuclear reactor operating at constant power level.) If the circumstances are such, on the other hand, that the neutrons released by each fission succeed in inducing more than one additional fission, the chain reaction is "supercritical," and the fission rate and rate of nuclear-energy release grow with time; this growth can be gradual, as in a nuclear reactor during the startup phase, when its power is being increased from zero up to the reactor's rated output, or it can be extremely rapid, as in a nuclear bomb.2 Similarly, an unintended chain reaction (as sometimes occurs when a sufficient quantity of plutonium or enriched uranium is brought together in a geometry favorable for a chain reaction) is known as a "criticality accident." Nuclear-reaction probabilities are expressed in terms of "cross-sections," with dimensions of area, such that the rates of fission or of nonfission capture associated with a flux of neutrons of N neutrons per square centimeter per sec- 2 The doubling time of the energy release rate in a nuclear reactor in startup typically would be measured in seconds, minutes, or even hours; the doubling time in a nuclear bomb is a small fraction of a microsecond.
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Management and Disposition of Excess Weapons Plutonium: Reactor-Related Options ond in a material containing n of a given type of transuranic nucleus per cubic centimeter are fission rate = N (neutrons/cm2-sec) × n (nuclei/cm3) × σf (cm2), nonfission capture rate = N (neutrons/cm2-sec) × n (nuclei/cm3) × σc (cm2), where σf and σc are the reaction cross-sections corresponding to fission and nonfission capture, respectively, for the indicated type of heavy nucleus and the specified incident neutron energy. The cross-sections depend quite strongly on this energy, so that, for the usual case in which the neutron energies are distributed over a range of values, an integration of flux times cross-section over this range of neutron energies is required in order to obtain the reaction rates.3 The heavy isotopes that are most important in both the civilian and military applications of nuclear energy to date are U-235 and -238 and Pu-239, -240, and -241. Uranium-235 and -238 are the two main naturally occurring isotopes of uranium, constituting, respectively, 0.715 and 99.285 percent of natural uranium by weight. Plutonium is virtually nonexistent in nature, but is manufactured in nuclear reactors by means of the nonfission capture of neutrons in U-238. 4 If the plutonium is removed from the reactor at short intervals, it consists almost entirely of Pu-239; if it remains in the reactor for longer periods, larger quantities of the higher plutonium isotopes are formed by successive absorption of further neutrons. Key characteristics of these isotopes as regards their interactions with neutrons—hence their performance in nuclear reactors and nuclear bombs—are summarized in Table 2-1. (Also shown in the table is another isotope of uranium, U-233, which can be manufactured in nuclear reactors containing thorium by a process, analogous to the production of Pu-239 from U-238, beginning with absorption of a neutron in thorium-232. U-233 has not been important in nuclear energy or nuclear weaponry to date, but has properties that would permit its use in either in the future.) In consequence of these properties and the energy distributions of the neutrons emitted in fission, it can be shown that U-233, U-235, Pu-239, and Pu-241 all are capable of sustaining chain reactions in either a fast-neutron or a thermal-neutron environment. Thus they can serve as fuels in either "fast" or "thermal" reactors, as well as in nuclear bombs.5 Isotopes that can sustain a chain reaction based on thermal neutrons are called "fissile," and all such isotopes can also sustain a chain reaction based on fast neutrons. 3 This integration is often performed in a very approximate way by representing the neutron energy distribution in terms of a modest number of groups of neutrons with energies in different ranges, whereupon the neutron flux in each group is multiplied by an average cross-section for the group's energy range and the results are summed. 4 The absorption of a neutron in U-238 gives U-239, which undergoes beta decay with a half-life of 23.5 minutes to yield neptunium-239, which in turn undergoes beta decay with a half-life of 2.3 days to yield Pu-239. 5 Here the terms "fast" and "thermal" refer to the average neutron energies in the reactor core; "thermal" means the neutrons are in thermal equilibrium with the surroundings.
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Management and Disposition of Excess Weapons Plutonium: Reactor-Related Options TABLE 2-1 Nuclear Properties of Key Uranium and Plutonium Isotopes Thermal Incident Neutronsa Fast-Spectrum Neutrons LMFBRb Generic PWR Spectrumc ABC (graphite-salt)d Isotope σf (barns) σc (barns) v η sf (barns) σc (barns) v η sf (barns) σc (barns) v η sf (barns) σc (barns) v η U-233 529 46 2.49 2.30 2.63 0.26 2.53 2.30 62.3 7.6 2.50 2.23 203 21.4 2.51 2.28 U-235 583 98 2.43 2.08 1.82 0.53 2.49 1.93 46.1 10.3 2.42 1.98 195 36.9 2.45 2.06 U-238 negl 2.7 negl negl 0.05e 0.26 2.47 0.40 0.1 0.9 2.80 0.28 0.02 5.6 2.43 0.01 Pu-239 748 269 2.88 2.12 1.79 0.45 2.94 2.35 110 61.3 2.88 1.85 545 311 2.90 1.85 Pu-240 negl 286 negl negl 0.39 0.52 2.88 1.23 0.58 127 3.14 0.01 0.15 225 2.82 0.002 Pu-241 1.011 358 2.94 2.17 2.39 0.42 2.99 2.54 119 39.1 2.93 2.21 566 211 2.97 2.21 ABBREVIATIONS: LMFBR = liquid-metal fast breeder reactor. PWR = pressurized-water reactor. ABC = accelerator-based conversion reactor. σf = fission cross-section (barns); I barn = 10-24 cm2. σc = nonfission capture cross-section (barns). v = neutrons produced per fission. η neutrons produced per neutron absorbed in fuel. negl = negligible. a 0.025 electron volts or 2.200 m/sec. compared with 14.000 km/sec at I MeV. b MCNP calculations at Los Alamos National Laboratory (Venneri 1994). c In commercial light-water reactors, the ratio of water to fuel is not sufficient to fully thermalize the neutrons, so that the very high “thermal" cross-sections shown are not achieved. Furthermore, the Pu-239 resonance prevents its cross-sections from diminishing as much with increasing energy as those for U-235. The neutron spectrum in a light-water reactor also changes with irradiation. The data here are for the spectrum in a generic PWR, with fuel initially enriched to 3.5 percent U-235. after 22 megawatt-days per kilogram of heavy metal (MWd/kgHM) exposure (Schnitzler 1994). d Graphite-moderated molten-salt accelerator-driven system of 500 megawatt-thermal (MWt) nominal power, with flux of 2.1 × 1014 neutrons per square centimeter per second (n/cm2 sec), after five years (Venneri 1994). e At an incident neutron energy of 2 MeV, however, U-238 has a fission cross-section of about 0.5 barn and a v of 2.46. Thus the fission of U-238 by neutrons in the high-energy "tail" of the neutron-energy distribution contributes some energy in fission reactors and in fission explosives, even though U-238 by itself cannot sustain a chain reaction (see text).
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Management and Disposition of Excess Weapons Plutonium: Reactor-Related Options Plutonium-240 (and the other even-numbered isotopes of plutonium) can sustain a chain reaction only in a fast-neutron environment; these isotopes are not fissile, but they are usable fuels for fast-neutron reactors and for bombs. Uranium-238 cannot sustain a chain reaction even in a fast-neutron environment, because, on the average, fewer than one of the neutrons produced by the fission of a U-238 nucleus retains enough of its energy for long enough to induce another such nucleus to fission. Nuclear bombs must rely on fast-neutron chain reactions, which may seem paradoxical at first because the fission-reaction probability (cross-section) is so much larger for slow neutrons than for fast ones. But, in a growing chain reaction that depends on the high reaction probability of thermal neutrons, time is required for the fast neutrons produced in fission to become thermalized through nonfission/noncapture collisions with nuclei; this introduces a sufficient lag in the growth of the chain reaction to prevent its reaching explosive proportions before thermal expansion reduces the fuel to subcritical density. Nuclear bombs and fast reactors compensate for the low fission cross-section for fast neutrons by means of a high density of the chain-reacting nuclei and a low density of neutron-absorbing materials. Thermal reactors, by contrast, make use of neutron moderators—materials whose combination of low atomic number and low propensity to absorb neutrons permits rapid slowing of the neutrons with low losses to absorption—in order to establish a thermal-neutron spectrum in the reactor core and thus take advantage of the high fission cross-sections at thermal-neutron energies. This allows maintaining a chain reaction in fuel with much lower concentrations of U-233, U-235, or Pu-239 (and lower power density, hence greater flexibility with respect to cooling arrangements) than are required in a fast reactor. The best of all moderator materials are very pure graphite (impurities absorb neutrons) and heavy water (deuterium oxide). The advantage of heavy water compared to ordinary water is that deuterium—the one-neutron isotope of hydrogen—is much less likely to absorb a neutron than is the no-neutron isotope of hydrogen that most ordinary water molecules contain. Reactors of suitable design using either graphite or heavy water as a moderator can sustain a chain reaction in natural uranium, despite its mere 0.7 percent of U-235. The use of ordinary water6 as the moderator, which is the most common choice in the world's power reactors, entails enrichment of the uranium fuel to a U-235 concentration typically between 2 and 5 percent. Typical fast-reactor designs would require U-235 concentrations of 20 percent or so in order to perform satisfactorily with U-235/U-238 fuel. Similarly, using 6 Ordinary water is called "light-water" in nuclear jargon, to contrast it with heavy water which, as noted, contains one-neutron deuterium in place of the common no-neutron isotope of hydrogen. The two main types of reactor designs that are moderated with light-water—boiling-water reactors (BWRs) and pressurized-water reactors (PWRs)-together constitute the class called light-water reactors (LWRs).
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Management and Disposition of Excess Weapons Plutonium: Reactor-Related Options U-235/U-238 mixtures containing less than 20 percent U-235 for practical nuclear bombs would be extremely difficult, and at the 2-5 percent U-235 concentrations typical of today's commercial-reactor fuel it is impossible. (From the standpoint of bomb design, the higher the U-235 concentration the better, and high-enriched uranium [HEU] produced for weapon purposes typically contains over 90 percent U-235.) Because different isotopes of uranium are essentially identical chemically, the enrichment process for increasing the concentration of U-235 above its value in natural uranium depends on physical rather than chemical means of separation, most of which exploit the 1.3-percent difference in mass between U-235 and U-238 atoms. Such approaches typically involve first converting the natural uranium to uranium-hexafluoride gas, followed by separation of the isotopes using the difference in their diffusion rates through a "cascade" of thousands of porous barriers (gaseous diffusion plants) or using the effects of their differing inertial masses in other devices, such as very high-speed centrifuges.7 In general, enrichment technologies that can achieve the 2-5 percent U-235 concentration required for light-water reactor fuel can also be used, at additional cost, to produce the much higher U-235 concentrations required for nuclear weapons. But nearly all enrichment techniques demand sophisticated technology in large and expensive facilities, and lack of widespread access to this enrichment technology up to the present has been considered one of the primary technical barriers to the spread of nuclear weapons capability. This barrier, as with the similar one applying to reprocessing technology (see below), was of course never absolute, and it has eroded somewhat over time with the diffusion of relevant knowledge and, to some extent, hardware. Reactor Cooling and Electricity Generation Most of the energy of the fission reactions is deposited in the nuclear fuel within a short distance of the site of the fission's occurrence. The rate of such energy deposition in a typical reactor is such that the fuel would overheat (losing its structural integrity) and then melt within a very short time unless provision is made for removal of the thermal energy by a coolant. From the time of the conception of the neutron chain reaction, it was realized that, by restricting the rate of extraction of heat, the coolant could be maintained at a steady elevated temperature and used to run a heat engine of some kind for the generation of mechanical work—as in an ordinary steam engine for propelling submarines or ships. Most commercial reactors drive steam turbines coupled to generators for the production of electrical power. Attainment of satisfactory efficiencies—30 7 A more recently developed technique exploits differences in the excited atomic states of U-235 and U-238 to permit selective excitation by tuned lasers, followed by selective ionization and separation by an electric field.
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Management and Disposition of Excess Weapons Plutonium: Reactor-Related Options percent or more—in the conversion of thermal energy to electricity in this way requires steam temperatures above 550° F (290° C), which implies, for effective heat transfer, a somewhat higher temperature at the surface of the fuel. The fluid chosen as coolant must have satisfactory heat-transfer properties at these elevated temperatures, must be chemically compatible—at these temperatures and the corresponding pressures—with the fuel or cladding with which it is in contact, and must have neutronic properties (i.e., propensities to slow down and to absorb neutrons) consistent with establishing in the reactor core an intensity and average energy of neutron flux that permits criticality in nuclear fuel of the chosen type. The light-water reactors that dominate world nuclear electricity generation today employ ordinary water as both neutron moderator and coolant. Graphite-moderated reactors may be cooled with water or with gas, which was carbon dioxide in early gas-cooled reactors and generally is helium in more recent designs. In fast reactors, neither the coolant nor other materials in the reactor core may contain high densities of low-atomic-number elements, since the elastic collisions of neutrons with such elements rapidly slow the neutrons to energies where U-238 no longer fissions but does strongly absorb neutrons, with consequent reduction of reactivity. For this reason, water is an unsatisfactory coolant for fast reactors. Most contemporary fast-reactor designs employ liquid-metal coolants, such as sodium or lead. The requirement for cooling the nuclear-reactor core is not restricted to the times when the chain reaction is underway, but extends afterwards because of the phenomenon of "afterheat." This refers to the energy released by the radioactive decay of fission products, which process continues—albeit at a rate that declines with time—after the chain reaction has been shut down. In a reactor that has been operating at its rated power level for some time and then is quickly shut down, the afterheat power amounts initially to about 7 percent of the operating level. Thus a reactor that operates at a rated thermal power of 3,600 megawatts-thermal (MWt)—corresponding, at 33 percent thermal-to-electric conversion efficiency, to about 1,200 megawatts-electric (MWe)—would have an initial afterheat power of about 250 MWt. This would be more than enough power, in most reactor types, to quickly overheat and then melt the fuel if cooling were not maintained following shutdown of the chain reaction. Since overheating and, to an even greater extent, melting, poses the threat both of significant damage to the reactor and of release from the fuel of some of the highly radioactive fission products, reactors must be provided with an emergency core-cooling capability that can be relied on if the normal cooling system becomes disabled. In most contemporary power reactors, the emergency core-cooling system involves "active" elements such as pumps, valves, and sensors that would work to provide and circulate coolant if an accident such as a pipe break allowed the normal coolant to escape. A number of newer reactor types—including some modified light-water reactor (LWR)
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Management and Disposition of Excess Weapons Plutonium: Reactor-Related Options designs, some liquid-metal reactors (LMRs), and the modular high-temperature gas-cooled reactor (MHTGR)—are designed to be able to avoid significant damage or radioactivity release, from any plausible loss-of-coolant accident, based on "passive" heat-removal mechanisms alone, with no or minimal reliance on the proper functioning of any pumps, valves, sensors, or switches. Nuclear reactors intended not for electricity generation but only for the production of WPu have no need for the high temperatures associated with efficient thermal-to-electric energy conversion. Such "production" reactors can therefore be designed to operate at lower power densities and temperatures, which alleviates a number of design problems, greatly eases the problem of cooling the reactor core in both normal operation and emergencies, and, thus reduces costs as well as increasing safety. The same advantages would be available in a reactor with the sole function of burning plutonium, without electricity generation, as a means of disposition of this material. (Whether these advantages are worth the extra costs and delays of building such a reactor is discussed in Chapter 6.) Energetics and Fuel Consumption Of the energy released by the fission of a uranium or plutonium nucleus, about 80 percent is in the kinetic energy of the two main fission fragments, some 3 percent is in the form of "prompt" gamma emissions from the excited fission-product nuclei, about 2.5 percent is carried by the neutrons resulting from fission, another 2.5 percent is gamma emissions resulting from the capture of these neutrons in surrounding materials, and about 12 percent materializes subsequently as the energy of gamma, beta, and neutrino emissions from the radioactive fission products. In characterizing the energy made available in a nuclear-fission chain reaction, it is customary to include, in the case of a nuclear reactor, all of the energy forms just mentioned except the neutrinos (since these escape the reactor carrying their energy with them). The available energy, by this definition, is about 200 MeV per uranium nucleus and about 210 MeV for a plutonium nucleus. Converting to everyday units gives 82 gigajoules (GJ), or 0.95 megawatt-days (MWd) of thermal energy per gram (g) of U-235 that is fissioned; the figure for thermal fission of Pu-239 is 0.98 MWd/g. In calculating the yields of nuclear weapons in terms of the equivalent quantity of high explosives, neither the neutrinos nor the delayed forms of energy release are counted, which gives about 175 MeV per nucleus or 72 GJ/g of U-235. With the customary value of 1,000 kilocalories per kilogram (kcal/kg) or 4,184 kilojoules per kilogram (kJ/kg) for the explosive energy of TNT, this makes the complete fission of 1 kg of U-235 in a bomb equivalent to 17 kilotons of TNT.8 8 Since fission bombs are not 100-percent efficient-that is, not all of the fissionable material they contain undergoes fission-actual yields are less than 17 kilotons per contained kilogram of fissionable material.
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Management and Disposition of Excess Weapons Plutonium: Reactor-Related Options The foregoing numbers lead to the widely used approximate rule of thumb that all reactors, irrespective of details of type and fuel, fission about one gram per day of heavy nuclei (and produce about one gram per day of fission products) per megawatt of average thermal output. This ratio—one megawatt-day of output per gram fissioned—means that a large modern power reactor of nominal 1,200-MWe capacity, thermal-to-electric generating efficiency of 33 percent, and annual average capacity factor of 75 percent—providing 1,200 MWe × 365 days × 0.75 / 0.33 MWe/MWt = 1 million MWd of thermal energy per year—will fission 1 million grams or 1 metric ton (MT) of heavy nuclei per year. A reactor of this size that derived all of its energy from plutonium, then, would fission a ton of plutonium per year, and this relation provides a helpful metric for the WPu quantities addressed in this report: 100 tons of WPu represents the amount of fissionable material consumed by 100 large power reactors in a year. In mid-1993, world nuclear power capacity was equivalent to more than 270 such reactors (see the latter part of this chapter). The preceding figure does not mean, however, that it is feasible to destroy 100 tons of surplus WPu in the space of a year using a third or so of the world's power reactors. The technical reasons this is not possible (leaving aside logistic and institutional obstacles to such an approach) are that (1) most reactors unavoidably produce plutonium at the same time as, although in most reactor types at a slower rate than, they burn it; and (2) it is not possible, in general, to burnup all of the plutonium (or other fissionable material) that is in a reactor at any one time. The production of plutonium occurs, as described above, in consequence of the absorption of fission neutrons in the principal isotope of natural uranium, U-238. This process is called "fertile-to-fissile conversion," with U-238 correspondingly termed a "fertile" material. The customary index of the rate of production is the "conversion ratio," CR, defined as CR = (fissile atoms produced) / (fissile atoms destroyed), which can be calculated as a ratio of instantaneous rates or, more practically, in terms of total production and destruction over a period of time.9 The LWRs that dominate world nuclear-energy generation today have conversion ratios around 0.6, while the heavy-water reactors in commercial use in Canada and a few other countries have conversion ratios in the range of 0.7 to 0.8. Any reactor with a conversion ratio greater than unity is called a "breeder reactor," and the conversion ratio is then called the "breeding ratio"; reactors with a conversion ratio less than unity are called "burners" or "convertors.” Of the new fissile material produced in a reactor by the conversion of U-238 to Pu-239 (or, in the 9 The definition's generality allows it to account for production of fissile Pu-241 by neutron absorption in Pu-240 as well as production of Pu-239 by neutron absorption in U-238, and it is applicable as well to the "thorium cycle," in which U-233 is produced by neutron absorption in thorium-232.
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Management and Disposition of Excess Weapons Plutonium: Reactor-Related Options thorium cycle, by conversion of thorium-232 (Th-232) to U-233), part gets fissioned while still in the reactor—contributing to energy output and reducing the quantity of fissile material that otherwise would need to be provided in fresh fuel—and part is discharged from the reactor in the spent fuel. The only way to achieve conversion ratios near zero, as would be essential if the aim were to burnup surplus WPu without generating new fissile plutonium or U-233 in the process, would be to use fuel containing no U-238 or Th-232. Such "nonfertile" fuels are possible and have served as the basis for some high-temperature gas-cooled reactor (HTGR) designs. But, as we discuss later, their development for use in existing light-water, heavy-water, or fast reactors would require considerable effort and a corresponding investment of time and money. While nonfertile fuels for HTGRs are closer to availability, more development and higher costs would be involved in using these reactors than in using existing reactor types with fertile fuels. Even with nonfertile fuels in hand, it would not be easy to burn an initial stock of WPu down to zero, for reasons to which we now turn. An important part of the difficulty of "burning up" fissile material completely is that, for fertile and nonfertile fuels alike, the fuel tends to lose either its structural integrity or its capacity to sustain a chain reaction long before its fissile content is exhausted. It tends to lose its structural integrity because of the combined effects of cyclic thermal stresses, corrosion, and the structural damage caused by fission neutrons and the fission-product tracks, as well as the problems posed by the pressure from gaseous fission products; when these take too high a toll, the result is excessive leakage of fission products into the reactor coolant, generating problems in maintenance and compliance with environmental standards. 10 Or, before the fuel starts to lose structural integrity, its reactivity may fall below the level required, because of the combination of diminishing density of fuel nuclei as these are burned up and growing density of fission products, some of which are strong neutron poisons (absorbers). The amount of fission energy derivable from fuel before this happens can be increased-within the limits of fuel structural integrity—by increasing the initial concentration of fissile nuclei. This measure may require the addition to the fuel of "burnable poisons" to offset the high reactivity that would otherwise be associated with the high initial fuel density.11 It may be attractive for economic reasons (to reduce the amount of fuel that must be fabricated for a given energy output)-or to 10 Of course it is always possible to manufacture the fuel to be tougher, but the extra structural material entailed in doing so tends to degrade both the neutronic and the heat-transfer properties of the fuel, at the same time that the extra material and extra care in manufacturing increase the costs. 11 Burnable poisons absorb neutrons, and thus hold down reactivity. early in the fuel's operating life when the density of fuel nuclei is high and that of fission-product neutron poisons is low: as time goes on, the absorbing capacity of the burnable poisons is used up, ideally at a rate that just compensates for the buildup of fission products and burndown of fuel nuclei.
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Management and Disposition of Excess Weapons Plutonium: Reactor-Related Options increase the rate at which WPu can be loaded into reactors of a given size—but it does not reduce the amount of plutonium still remaining in the fuel when it is finally spent. The quantity of energy generated by a given batch of fuel before it is considered spent is called the "discharge burnup" (or, alternatively, the "discharge exposure" or "discharge irradiation”). This is customarily measured in megawatt-days of thermal energy per metric ton of initial heavy metal (MTIHM or just MTHM), where "initial heavy metal" refers to the quantity of uranium, plutonium, and (sometimes) thorium and heavier elements in the fuel at the time it is first loaded into the reactor. Typical average discharge burnups for commercial LWRs are in the range of 25,000 to 40,000 MWd/MTHM, with the most recent fuel designs achieving the higher figures.12 Canadian heavy-water moderated reactors (called CANDU, for Canadian deuterium-uranium) using natural uranium fuel achieve discharge burnups of about 7,000 MWd/MTHM; LMRs with fuel enrichments of 20-30 percent U-235 or plutonium achieve figures in the 100,000-MWd/MTHM range; and HTGRs are being designed to use fuel with enrichments above 90 percent to achieve discharge burnups of 500,000 MWd/MTHM and higher. If every gram in an initial metric ton of heavy metal in fuel were actually fissioned, at 1 MWd/g the total burnup would be 1,000,000 MWd/MTHM. Correspondingly, each 10,000 MWd/MTHM of burnup represents the fission of about 1 percent of the heavy metal atoms initially present. Thus, for example, low-enriched uranium (LEU) fuel that achieves a discharge burnup of 33,000 MWd/MTHM starting with a U-235 content of 3.3 percent has fissioned altogether about 33 kg or 3.3 percent of the heavy atoms initially present in each ton of fuel, much of the fission occurring in the initial 33 kg stock of U-235, but a significant contribution coming from the fission of plutonium produced during the reactor's operation by the absorption of neutrons in U-238. Thus, for example, a 1,200-MWe LWR fueled with LEU at 3.3 percent U-235, and running at a capacity factor of 75 percent with a discharge burnup of 33,000 MWd/MTHM and thermal efficiency of 33 percent, would load annually about 30 MTHM (containing 1,000 kg of U-235), and would discharge, in spent fuel (after three years), about 250 kg/yr of U-235 and 300 kg/yr of plutonium, two-thirds of it the fissile Pu-239 and Pu-241 isotopes. A breeder reactor with the same electrical output would discharge 1,200-2,000 kg of plutonium per year, depending on fuel characteristics and operating mode. If the fuel is not LEU but plutonium-based to start with, and the burnup and the conversion ratio are about the same, the plutonium content of the spent fuel necessarily will be higher. Even in a plutonium fuel that contains no fertile material from which new plutonium is produced in operation, the spent fuel will 12 Increasing burnups attained by commercial LWRs to 50,000 MWd/MTHM is undoubtedly feasible and may be economic.
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Management and Disposition of Excess Weapons Plutonium: Reactor-Related Options in the spent fuel option. If the ultimate aim of disposition is to make the WPu at least as inaccessible for weapons purposes as plutonium in spent fuel from typical civilian power reactors, the reactor-spiking option cannot stand alone but must be followed by processing in either the "spent fuel" mode (as just described) or the "elimination mode" (described next). The elimination option employs fission and transmutation in a nuclear reactor to convert nearly all of the plutonium processed into other elements. The purpose of this approach is to eliminate the plutonium essentially completely from human access. 24 Destruction fractions as high as 80 percent may be achievable without fuel reprocessing through the use of nonfertile fuels. Destruction fractions much above 80 percent are only achievable in practical systems through the use of fuel reprocessing and plutonium recycle. As described in the remainder of this report, the time required for such options, the technical uncertainties, and the costs involved are all very much larger than in the case of the spent fuel or spiking options. A fourth approach, which we denote the "waste-spiking" option, achieves a result similar to those of the "spent fuel" or "reactor-spiking" options by processing the plutonium in waste management facilities rather than in reactors: The waste-spiking option mixes the WPu with radioactive wastes from previous reactor operations—most probably with military reactor wastes—for storage and subsequent ultimate disposal by one of the schemes that will need to be selected for such wastes in any case. The purpose of this approach is similar to that of the spent fuel option: to create substantial physical, chemical, and radiological barriers to further use of the WPu in nuclear explosives. This way of doing so, unlike the spent fuel approach, does not change the isotopic characteristics of the plutonium. Each of these categories has many variants—reactor types, operating schemes, waste types, and so on—and the different approaches and variants have different combinations of international security advantages and liabilities, as well as different combinations of advantages and liabilities with respect to technological readiness, economics, institutional requirements, and other factors. Characterizations of—and comparisons among—the different approaches and variants in these respects are presented in Chapters 3-7. 24 Destruction is reckoned on a "net" basis, i.e., consumption minus production of all plutonium isotopes. An alternative criterion to the complete elimination of the WPu is to require that the total plutonium residue from the disposition campaign be just equal to the total plutonium residue that would have resulted from generating the same quantity of electricity from the same reactors without WPu disposition (see Garwin 1995).
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Management and Disposition of Excess Weapons Plutonium: Reactor-Related Options PRESENT AND FUTURE FISSILE MATERIAL STOCKPILES Excess Weapons Plutonium Stockpiles How much excess WPu is there likely to be? As noted, current nuclear arms reduction agreements and pledges, if successfully implemented, would mean the retirement of tens of thousands of nuclear weapons. The U.S. Department of Energy has recently stated publicly that "up to approximately 50 metric tons of plutonium will (or may) become available by about 2005 . . . [for] civil (unclassified) purposes," from both weapons dismantlement and other sources.25 Similarly, the Russian government has indicated that it expects to have 50 tons of plutonium and 500 tons of HEU that are excess to its military needs. We will use 50 tons on each side as a nominal figure for the amount of plutonium requiring disposition. But it should be remembered that this number could grow depending on further disarmament initiatives and decisions concerning how much material should be retained in military reserves. The schedule on which this material may become available for disposition depends on the planned schedule of arms reductions and the rate of dismantlement activities. Some WPu is already available, in principle. The United States and Russia have each indicated that they have already dismantled thousands of nuclear weapons over the last several years; the plutonium components of these weapons, containing many tons of plutonium, are currently in storage. The currently agreed arms reductions are to be carried out on an uneven schedule stretching to the year 2003; dismantlement of the retired weapons is ongoing, but may lag behind the retirements. In any case, it appears that none of the disposition options could be implemented quickly enough for the dismantlement schedule to be a major limiting factor in determining when disposition could be carried out. Total World Plutonium and HEU Stockpiles The plutonium and HEU resulting from arms reductions are only part of the world's stocks of these materials, which include: Military plutonium and HEU in operational nuclear weapons and their logistics pipeline. Military plutonium and HEU held in reserve for military purposes, in assembled weapons or in other forms. Military plutonium and HEU withdrawn from dismantled weapons and considered excess. 25 Willett (1993, p. 2). The uncertainty implied by the parenthetical "(or may)" reflects continuing debate within the U.S. government over how much of these materials should be kept as military reserves.
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Management and Disposition of Excess Weapons Plutonium: Reactor-Related Options FIGURE 2-1 World plutonium stockpiles * Includes weapons and weapon components from weapons already dismantled. SOURCE: Committee estimates, based on IAEA (1993), Albright et al. (1993) and USDOE (1993). Separated plutonium and HEU in storage in preparation for use in military or civilian reactors. Plutonium and HEU currently in reactors. Irradiated plutonium and HEU in spent fuel from reactors. Military and civilian plutonium and HEU outside the categories above, including excess stocks, scrap, residues, and the like. The problem of management and disposition of excess WPu (category 3) is the focus of this report, but policy for it must take into account the large stocks of plutonium and HEU in the other categories, since with varying degrees of difficulty they can all be used in nuclear weapons (see Figure 2-1.) Official figures on total stockpiles of plutonium and HEU are generally not available. Quantities in military stockpiles are considered military secrets, and
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Management and Disposition of Excess Weapons Plutonium: Reactor-Related Options quantities in civil stockpiles are often considered commercial secrets. Unofficial public estimates suggest that, at this writing, the total world stockpile of plutonium amounts to roughly 1,100 tons.26 Military Plutonium Stockpiles Roughly one-quarter of the world's plutonium stock is held in military stockpiles. The U.S. Department of Energy recently declassified information concerning the U.S. stock, indicating that the United States had produced 102 tons of military-related plutonium (of which 13 tons is fuel-grade, rather than weapons-grade).27 Of this stock, 33.5 tons is in scrap, residues, and other forms at several sites in the nuclear weapons complex, leaving 68.5 tons that is either in intact weapons or in components from dismantled weapons now stored at Pantex near Amarillo, Texas. Russia is believed to have a somewhat larger total stock of military plutonium, but has not released comparable figures. Stocks held by the other declared nuclear-weapon powers (Britain, France, and China), and the threshold states (Israel, India, Pakistan, and North Korea) are small by comparison. While the primary focus of this report is the excess WPu resulting from arms reductions—which is initially in the form of weapons components from dismantlement—both the United States and Russia also have large quantities of military plutonium in scrap and residues from past operations of their nuclear weapons complexes, most of which is also likely to be considered excess. While the amount of plutonium in these forms is smaller than the amount in pits that will result from arms reductions, the volume of the materials is much greater, the material is in many different forms, and for some of these the environment, safety, and health risks are substantial. Even characterizing the constituents of these materials accurately is difficult. Some of them can be processed readily to plutonium metal or oxide that could then be fed into many of the disposition options described in subsequent sections. Some reactor options (typically the more advanced ones that would take longer to bring on line) are more capable than others of handling variations in the form of the initial fuel feed, though there are materials that none of the reactor options could plausibly handle. Processing some of these materials into more tractable forms, moreover, would entail additional environment, safety, and health hazards. The vitrification option described in this report may be a particularly promising approach for stabilizing and ultimately disposing of the plutonium in these less tractable forms. 26 These figures are adapted from Albright et al. (1993). 27 This figure for the amount produced in the United States does not necessarily correspond to the amount in the current stock, as there have been transfers of plutonium from abroad.
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Management and Disposition of Excess Weapons Plutonium: Reactor-Related Options Civilian Plutonium Stockpiles The remaining three-quarters of the world's plutonium is held in civilian stocks. Recent International Atomic Energy Agency (IAEA) estimates indicate that as of late 1992, some 86 tons of this plutonium was in separated form, awaiting use in civilian reactors; in this form, if diverted, it would be directly usable in nuclear weapons (see below).28 The remainder of the world's civilian plutonium stock, over 600 tons, is in spent reactor fuel or in reactors in some 26 countries. Not all spent fuel is equally inaccessible, however. When the fuel is in the reactor it would be difficult to divert, and when it first leaves the reactor its very high levels of radioactivity offer significant protection against diversion. Over the decades after leaving the reactor, however, the fuel's radioactivity will decay substantially, allowing it to be handled more readily and making it potentially more susceptible to diversion. Roughly 140 tons of the plutonium in the world's spent fuel was discharged prior to 1980, and is thus well over a decade old. During the first decade after discharge, the radioactivity of the fuel declines rapidly as the shorter-lived radionuclides decay; for about a century thereafter, the radioactivity of the spent fuel is dominated by the contributions of 30-year half-life cesium-137 and 29-year half-life strontium-90, and hence declines by about 50 percent every 30 years. Projected Plutonium Stocks in the Year 2000 Plutonium production for the U.S. military stockpile has ceased, and the United States has proposed a global convention banning further production of plutonium and HEU for weapons. Military production in Russia has declined drastically, and continues at a low level only because the three reactors still operating provide needed heat and power to the surrounding areas. U.S.-Russian discussions of how best to terminate this production are underway. Thus, world military stockpiles in the year 2000 will be quite similar to the stockpiles today, except that many tens of tons of plutonium will have been made available if the dismantlement of nuclear weapons is carried out on schedule. Future civil stockpiles involve somewhat greater uncertainties. Plutonium production in reactors can be projected with some confidence, but the future of both reprocessing of spent fuel and the use of the resulting plutonium in reactors 28 See IAEA (1993). More than half of this accumulated plutonium belonged to Great Britain and Russia; while other reprocessing countries have decided to use plutonium in light-water reactors to reduce the buildup of excess stocks, neither of these countries has yet taken this route. Because of these civilian plutonium programs, an infrastructure of existing and planned civilian facilities exists to store many tons of plutonium, fabricate it into reactor fuel, and use it in reactors. These facilities, however, are already burdened with managing civilian plutonium; using them to handle excess military plutonium would require substantially expanding them or displacing the civilian plutonium in some way—an option discussed further in subsequent sections.
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Management and Disposition of Excess Weapons Plutonium: Reactor-Related Options are currently the subjects of considerable controversy. The cumulative amount of plutonium discharged from the world's civilian power reactors will nearly double, from over 700 tons today to nearly 1,400 tons by the year 2000. As the rate of plutonium reprocessing continues to outpace the rate at which plutonium fuels are used in reactors, the civilian stock of separated plutonium will grow. Recent IAEA estimates suggest that the stock of separated civilian plutonium in storage may increase to between 110 and 170 tons by the latter part of this decade or early in the next century, depending on the scale of reprocessing and plutonium use over the intervening period. 29 In other words, it is very likely that in the early years of the next century the amount of separated plutonium in civil stocks will in fact be equal to or larger than the stocks of military separated plutonium freed from weapons as a result of arms reductions. HEU Stockpiles In addition to plutonium, there are large stocks of another directly weapon-usable material, highly-enriched uranium.30 The total world stockpile probably amounts to roughly 1,500 tons. All but about 20 tons of this is in military stocks, either incorporated in weapons, held in reserve, or intended for use as naval reactor fuel. Over 95 percent of these military stocks are held by the United States and Russia. The United States and Russia have ceased production of HEU, and only very limited, if any, production continues elsewhere. The only likely changes in the world stockpile by the year 2000, therefore, will be decreases resulting from the use of existing stocks in naval and research reactors and the "blending down" of HEU to low-enriched reactor fuel. WORLD NUCLEAR-ENERGY SYSTEMS RELEVANT TO PLUTONIUM DISPOSITION Nuclear-Power Plant Types and Numbers Table 2-3 summarizes the types and numbers of nuclear power reactors worldwide as of the end of 1993.31 As indicated in the table, nearly 80 percent of the 419 power reactors operating at that time were light-water reactors 29 IAEA (1993). Albright et al. (1993) provide roughly similar estimates. 30 The security risks posed by this material before it is diluted to low-enriched uranium are even greater, in some respects, than those of plutonium: as noted earlier, HEU can be used in relatively simple, gun-type bomb designs, while plutonium cannot; and HEU is easier to handle and to conceal than plutonium. The emphasis in the 1994 CISAC study on the security risks of plutonium as opposed to those of HEU is based on the assumption that the easily accomplished prescription for "denaturing" excess HEU by dilution with natural or depleted uranium will in fact be promptly carried out in both the United States and Russia. 31 Adapted from Nuclear News (1994).
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Management and Disposition of Excess Weapons Plutonium: Reactor-Related Options (LWRs), 9 percent were gas-cooled reactors (GCRs), and 8 percent were heavy-water reactors (HWRs). Of the 87 power reactors in various stages of partial completion, three-quarters were LWRs. Only 4 liquid-metal reactors (LMRs) were in operation at the end of 1993, with another 5 partly completed. For approximately 30 of the reactors of all types listed as partly completed, no construction is currently underway and plans for completion are indefinite. In the United States, there were 109 operating commercial nuclear power plants at the end of 1993, with a capacity of 99,400 MWe. Of these, 72 were pressurized-water reactors (PWRs) and 37 were boiling-water reactors. There are also 2 experimental LMRs in the United States: the EBR-II (20 MWe) and the Fast Flux Test Facility (FFTF) (130 MWe equivalent). Operation of both FFTF and EBR-II has recently been discontinued. Of the 7 U.S. LWRs listed as partly completed, none is actively under construction at this time. Five U.S. LWRs and one GCR that were shut down within the past five years have been decommissioned or are awaiting decommissioning.32 Forty-two nuclear power plants are operating in the former Soviet Union, with a total capacity of 34,000 MWe. Of these, 25 are PWRs, 15 are light-water-cooled, graphite-moderated reactors (LGRs in Table 2-3, widely known as RBMKs), and 2 are LMRs. As indicated in Table 2-3, 25 of these reactors (13 PWRs, 11 RBMKs, and 1 LMR) are in Russia, where, under current plans, all of the dismantlement of former Soviet nuclear weapons will take place, and therefore all of the excess plutonium from weapons will arise. Twenty-one additional nuclear plants are under construction in the former Soviet Union, but at this time construction work has been suspended on all but five. Civilian Plutonium Separation and Use Nearly all of the reactors just described use low-enriched or natural uranium as their fuel. Nevertheless, as noted earlier, several countries are reprocessing plutonium from spent fuel for use as fresh fuel in nuclear reactors. In most cases, the original plan was to use this plutonium as fuel for liquid-metal "breeder" reactors. But delays in the commercialization of breeder reactors have led a number of countries to pursue major programs to use plutonium as mixed-oxide (MOX) fuel in LWRs, to limit the buildup of excess stocks of separated plutonium.33 As just noted, despite these recycling programs, some 86 tons of separated plutonium has built up, and reprocessing continues to outpace the use of the resulting plutonium. 32 These are the Rancho Seco (California), San Onofre (California), Yankee (Massachusetts), and Trojan (Oregon) pressurized-water reactors, the Shoreham (New York) boiling-water reactor, and the Fort St. Vrain (Colorado) gas-cooled reactor. 33 For a discussion of the major civilian plutonium programs in the world, see NAS (1994, Appendix B).
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Management and Disposition of Excess Weapons Plutonium: Reactor-Related Options TABLE 2-3 Nuclear-Power Reactors of the World Number of Reactors Country LWR LGR HWR LMR GCR Total Capacity (GWe) United States 109/7 -- -- -- -- 109/7 99.4/8.5 France 53/5 -- -- 1/1 1/0 55/6 56.5/8.3 Japan 44/7 -- 1/0 0/1 1/0 46/8 35.9/7.8 Germany 21/0 -- -- -- -- 21/0 22.6/- Russia 13/11 11/1 -- 1/3 -- 25/15 19.8/13.6 Canada -- -- 22/0 -- -- 22/0 15.4/- Ukraine 12/6 2/0 -- -- -- 14/6 12.1/5.7 Great Britain 0/1 -- -- 1/0 34/0 35/1 11.7/1.2 Sweden 12/0 -- -- -- -- 12/0 10.0/- Korea 8/4 -- 1/3 -- -- 9/7 7.2/5.9 Spain 9/6 -- -- -- -- 9/6 7.1/5.7 Belgium 7/0 -- -- -- -- 7/0 5.5/- All Othersa 42/18 2/- 10/13 1/- -- 55/31 28.8/17.5 Totals 330/65 15/1 34/16 4/5 36/0 419/87 332/74 NOTES: Figures before the slash refer to operating reactors, figures after the slash to partially completed ones. Countries are arranged in descending order of net capacity in operating reactors. ABBREVIATIONS: LWR = light-water reactor. LGR = light-water-cooled, graphite-moderated reactor. HWR = heavy-water reactor. LMR = liquid-metal reactor. GCR = gas-cooled reactor. GWe = gigawatt-electric. a Of the countries not listed separately, none has as much as 5 GWe of operating nuclear capacity. SOURCE: Nuclear News 1993. The present world civilian reprocessing capacity (counting the recently opened British Thermal Oxide Reprocessing Plant [THORP]) amounts to roughly 5,600 MTHM/yr (IAEA 1992). The major centers of civilian reprocessing are in France, Britain, and Russia, though Japan has a small facility and is building a large one, and several other countries have small-scale capabilities. All of these plants use aqueous homogeneous processing (PUREX; plutonium and uranium recovery by extraction). If planned facilities in Japan and Russia are completed, roughly an additional 2,000 MTHM/yr will be added by the year 2005. During the 1990s, these facilities are expected to produce some 20 tons of separated plutonium each year.
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Management and Disposition of Excess Weapons Plutonium: Reactor-Related Options MOX fabrication capacity worldwide currently amounts to roughly 75 MTHM/yr (Berkhout et al. 1993). Substantial additional capacity is under construction. Since most of this fuel is being used in LWRs at loadings of the order of 5 percent, rather than in LMR fuel with typical loadings of roughly 20 percent, the existing MOX fabrication capacity is substantially less than required to handle the 20 tons of civilian separated plutonium likely to be produced by reprocessing each year over the next decade. Hence it now appears inevitable that the substantial current excess stocks of civilian plutonium will continue to increase. The existing and planned MOX fabrication facilities would not be able to fabricate WPu into fuel unless their capacity were substantially expanded or civilian plutonium were displaced. Neither the United States nor Russia currently have operating MOX fabrication capacities on any significant scale, though both have incomplete facilities that could be completed in order to facilitate the disposition of excess WPu. In short, the existing world reactor capacity is more than sufficient to handle the projected quantities of excess WPu (as well as the civilian plutonium being separated), but the available plutonium fuel fabrication capability is insufficient (taking into account continuing civilian reprocessing). Thus if disposition options involving the use of WPu as reactor fuel are chosen, provision of additional plutonium fuel fabrication capability may be required.
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Management and Disposition of Excess Weapons Plutonium: Reactor-Related Options REFERENCES Albright et al. 1993: David Albright, Frans Berkhout, and William Walker. World Inventory of Plutonium and Highly Enriched Uranium 1992. Oxford: Oxford University Press for the Stockholm International Peace Research Institute, 1993. APS 1978: American Physical Society, Study Group on Nuclear Fuel Cycles and Waste Management. “Report to the APS." Reviews of Modern Physics 50(1), Part II, January 1978. Berkhout et al. 1993: Frans Berkhout, Anatoli Diakov, Harold Feiveson, Helen Hunt, Edwin Lyman, Marvin Miller, and Frank von Hippel. "Disposition of Separated Plutonium." Science and Global Security 3:161-213, 1993. Cochran et al. 1987: Thomas B. Cochran, William M. Arkin, Robert S. Norris, Milton M. Hoenig. U.S. Nuclear Warhead Production. Cambridge, Mass.: Ballinger, 1987. Comptroller General 1978: Comptroller General. Quick and Secret Construction of Plutonium Reprocessing Plants: A Way to Nuclear Weapons Proliferation? EMD-78-104. Washington, D.C.: U.S. General Accounting Office, October 6, 1978. Garwin 1995: Richard L. Garwin. "Beyond the 'Spent Fuel Standard': Two Interpretations of 'Elimination' of Excess Weapons Plutonium." Unpublished paper available from the Committee on International Security and Arms Control, National Academy of Sciences, Washington, D.C., 1995. IAEA 1992: J.L. Zhu and N. Oi. "Inventory of Plutonium in Civilian Nuclear Programs." International Atomic Energy Agency, Division of Nuclear Fuel Cycle and Waste Management, Vienna, December 1992. IAEA 1993: J.S. Finucane. "Summary: Advisory Group Meeting on Problems Concerning the Accumulation of Separated Plutonium." International Atomic Energy Agency, Division of Nuclear Fuel Cycle and Waste Management, Vienna, September 21, 1993. Mark 1993: J. Carson Mark. "Explosive Properties of Reactor-Grade Plutonium." Science and Global Security 4: 1 1-128, 1993. NAS 1994: National Academy of Sciences, Committee on International Security and Arms Control. Management and Disposition of Excess Weapons Plutonium. Washington, D.C.: National Academy Press, 1994. Nero 1979: Anthony V. Nero, Jr. A Guidebook to Nuclear Reactors. Berkeley: University of California Press, 1979. National Research Council 1992: National Research Council, Committee on Separations Technology and Transmutation Systems. Interim Report of the Committee on Separations Technology and Transmutation Systems. Washington, D.C., May 1992. Nuclear News 1993: "World List of Nuclear Power Plants." Nuclear News, September 1993.
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Management and Disposition of Excess Weapons Plutonium: Reactor-Related Options Nuclear News 1994: "World List of Nuclear Power Plants." Nuclear News, March 1994. OECD 1994: Organization for Economic Co-operation and Development, Nuclear Energy Agency. The Economics of the Nuclear Fuel Cycle. Paris: OECD Publications, 1994. OFR 1992: Office of the Federal Register. U.S. Code of Federal Regulations: Title 10 (Energy). Chapter I (Nuclear Regulatory Commission). Washington, D.C.: U.S. Government Printing Office, 1992. OTA 1977: Office of Technology Assessment. Nuclear Proliferation and Safeguards. OTA-E-48. Washington, D.C.: U.S. Government Printing Office, June 1977. Ramspott et al. 1992: Lawrence D. Ramspott, Jor-Shan Choi, William Halsey, Alan Pasternak, Thomas Cotton, John Burns, Amy McCabe, William Colglazier, and William W. M. Lee. Impacts of New Developments in Partitioning and Transmutation on the Disposal of High-Level Nuclear Waste in a Mined Repository . UCRL ID-109203. Livermore, Calif.: Lawrence Livermore National Laboratory, March 1992. Schnitzler 1994: Bruce Schnitzler, Idaho National Engineering Laboratory. Letter to Richard L. Garwin, May 24, 1994. Serber 1992: Robert Serber. The Los Alamos Primer. Berkeley: University of California Press, 1992. USDOE 1993: U.S. Department of Energy. Press release, December 7, 1993. Venneri 1994: F. Venneri. Personal communication to Richard L. Garwin, October 2, 1994. Willett 1993: Louis R. Willett, Deputy Director, Office of Weapons and Materials Planning, Defense Programs, U.S. Department of Energy. "Excess Weapons Materials." Paper presented at the annual meeting of the American Power Conference, Chicago, Illinois, April 13-15, 1993.
Representative terms from entire chapter: