Management and Disposition of Excess Weapons Plutonium Reactor-Related Options (1995) / Chapter Skim
Currently Skimming:
Chapter 2: Background
Chapter 2: Background
Pages 26-58
The Chapter Skim interface presents what we've algorithmically identified as the most significant single chunk of text within every page in the chapter.
Select key terms on the right to highlight them within pages of the chapter.
From page 26... ...
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. ' 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)
|
From page 27... ...
(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)
|
From page 28... ...
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 thermalneutron 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 "missile," and all such isotopes can also sustain a chain reaction based on fast neutrons.
|
From page 30... ...
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 Waters 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.
|
From page 31... ...
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 subma . fines or ships.
|
From page 32... ...
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.
|
From page 33... ...
(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.
|
From page 34... ...
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)
|
From page 35... ...
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.~° 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)
|
From page 36... ...
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.
|
From page 37... ...
Fuel Preparation, Refueling, and Reprocessing The composition and configuration of nuclear fuel varies considerably across different types of reactors, not only in respect to the fractional content of fissile isotopes, as mentioned above, but also in terms of chemical composition (where the main possibilities include heavy-metal oxides and carbides, metallic fuels, and molten salts) and in the size, shape, and cladding of the individual fuel elements.
|
From page 38... ...
for production of WPu, technology was developed for chemical reprocessing of the irradiated fuel to extract the plutonium it contained. This technology involved chopping up the irradiated fuel, dissolving it in acid, and performing a series of chemical operations on the resulting radioactive brew in order to separate the heavy metal from the fission products and, within the heavy metal fraction, the uranium from the plutonium.
|
From page 39... ...
It is often argued, in favor of fuel reprocessing and plutonium recycle, that this technology will reduce significantly the problems associated with radioactive waste management at least by reducing the volume of high-level wastes and wastes from uranium-ore processing, and possibly also by permitting the separation of different waste components to enable transmutation or other special treatment of the most long-lived isotopes. If reduced costs of waste management were the result, reprocessing could become economically competitive with once-through fuel cycles sooner than would otherwise be the case.'5 |3 Most of these are, to be sure, under International Atomic Energy Agency safeguards, as discussed elsewhere in this report.
|
From page 40... ...
We will only say, therefore, that based on current evidence and arguments as we understand them, it would not be prudent to assume in thinking about options for WPu disposition that civilian nuclear fuel reprocessing will spread substantially beyond the few countries that now practice it in the time period extending 30-50 years from the present, with which we are mainly concerned. We do take up the question, later in this report, as to whether the deployment of fuel-reprocessing capacity explicitly dedicated to the mission of WPu disposition would be desirable.
|
From page 41... ...
Some Differences Between Plutonium and Uranium-Based Reactor Fuels An aspect of the physics of fission that is very important to the way the reaction is controlled in power reactors is the existence of"delayed" neutrons, which are produced not in the fission reaction itself but in the subsequent radioactive decay of some of the shorter-lived fission products. These delayed neutrons amount to about 0.7 percent of all of the neutrons associated with the thermal fission of U-235 and a fraction about three times smaller of the neutrons associated with the thermal fission of Pu-239.
|
From page 42... ...
Since plutonium fuel offers only comparable nuclear performance to uranium fuel in thermal reactors but superior performance in fast-breeder reactors, it might be argued that the latter represent the best use of plutonium for nuclear power. But because fast-breeder reactors seem unlikely to be economically competitive with thermal-convertor reactors for some decades to come, saving WPu until this happens would impose a severe time delay on its final disposition.
|
From page 43... ...
, rather than plutonium stored in separated form between now and then. Some Relevant Aspects of Weapon Science In the construction of the nominally 50,000 nuclear weapons in the world, U-235 and Pu-239 have been used as the principal fissionable materials to sustain the fast-neutron chain reaction essential to nuclear explosions.' Highly enriched uranium (in weapons, typically 90 percent U-235 or more)
|
From page 44... ...
Chief among these are the problems of coping with extra heat from the radioactive decay of the shorter-lived isotopes of plutonium that are more abundant in reactor-grade than in weapons-grade plutonium, and the problems of gamma radiation from extra americium-241, which is the daughter product of 14-year half-life Pu-241 and hence more abundant in reactor-grade plutonium than in weapons-grade plutonium. The magnitudes of the main differences between reactor-grade and weapons-grade plutonium are indicated in Table 2-2.
|
From page 45... ...
This panel did not evaluate the likelihood that a national government would decide to use one or another type of fissile material, but focused rather on the obstacles to use and therefore the security risks associated with the different forms of fissile materials. CISAC reached the general conclusion that the differences in security risk between different grades of separated plutonium that is, plutonium not mixed with radioactive fission products are smaller than the
|
From page 46... ...
(a) The spent fuel option employs a once-through fuel cycle to process WPu into spent reactor fuel similar in its radioactivity, and in the isotopic composition of the contained plutonium, to the spent fuel that already exists (in much larger quantities)
|
From page 47... ...
(With respect to "negative net destruction," it needs to be added that if the same reactor generated the same amount of energy without any input of WPu i.e., using only LEU fuel the amount of plutonium in the world would have increased even more: the new plutonium production from absorption of neutrons in U-238 throughout the core would not have been offset by consumption of any WPu. Clearly, the proper reference point for determining the net effect of reactor disposition of WPu on total plutonium inventories depends con whether the plutonium disposition occurs in a reactor that would have operated anyway, or in an additional reactor that displaces electricity from some non-nuclear source.
|
From page 48... ...
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.
|
From page 49... ...
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.
|
From page 50... ...
Military and civilian plutonium and HEU outside the categories above, including excess stocks, scrap, residues, and the like.
|
From page 51... ...
While the primary focus of this report is the excess WPu resulting from anns reductions which is initially in the fond 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.
|
From page 52... ...
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.
|
From page 53... ...
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 1 10 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.
|
From page 54... ...
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.
|
From page 55... ...
SOURCE: Nuclear News 1993. The present world civilian reprocessing capacity (counting the recently opened British Thermal Oxide Reprocessing Plant tTHORP]
|
From page 56... ...
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.
|
From page 57... ...
"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.
|
From page 58... ...
The Economics of the Nuclear Fuel Cycle. Paris: OECD Publications, 1994.
|
Key Terms
This material may be derived from roughly machine-read images, and so is provided only to facilitate research.
More
information on Chapter Skim is available.