Also, significant quantities of plutonium separated from discharged fuel have been placed in long-term storage. Prospects for future reprocessing, whether for MOX fuel for conventional reactors or for breeder reactors, depend on future demand for nuclear power and on the availability and cost of uranium fuel. Recent economic studies indicate that widespread breeder implementation is not likely to occur until well past the middle of the 21st century.
Thus, discharged fuel and its plutonium will continue to accumulate. The current global inventory of plutonium in discharged fuel is about 1, metric tons. Various projections indicate that by , the inventory could increase to 5, metric tons if nuclear power becomes widely used in developing countries. Even if global nuclear power generation remains at present levels, the plutonium accumulation by will total 3, metric tons. The plutonium in discharged fuel is a central concern for two reasons.
The second concern is the proliferation risk of plutonium. Plutonium at work in a reactor or present in freshly discharged fuel is in effect guarded by the intense radiation field that the fission products mixed with it produce.
The radioactive discharged fuel must be handled very carefully, with cumbersome equipment, and the plutonium must then be separated in special facilities in order to be fabricated into weapons. Over several decades, as the radioactivity of the fission products decays, the radiation barrier is significantly reduced.
But plutonium already separated out of discharged fuel by reprocessing, and thus not protected by a radiation barrier, would be easier for terrorists or criminals to steal or for nations to divert for weapons.
This difference in ease of theft or diversion is one of many factors involved in assessing the proliferation risks of nuclear power. There are widely disparate views about these risks. Underlying the disparities often are differing assumptions about world security environments over the next century and the proliferation scenarios that might be associated with them. Such inherent unpredictabilities argue for creating new options for the nuclear power fuel cycle that would be robust over a wide range of possible futures.
A better fuel cycle would fulfill several long-term goals by having the following features. It would greatly reduce inventories of discharged fuel while recovering a portion of their remaining energy value, keep as much plutonium as possible protected by a high radiation barrier during all fuel cycle operations, reduce the amount of plutonium in waste that must go to a geologic repository, and eventually reduce the global inventory of plutonium in all forms.
We propose a nuclear fuel cycle architecture that we believe can achieve these goals. It differs significantly from the current architecture in three ways. Interim storage facilities. Facilities for consolidated, secure, interim storage of discharged fuel should be built in several locations around the world. The facilities would accept fuel newly discharged from reactors, as well as discharged fuel now stored at utilities, and store it for periods ranging from decades at first to a few years later.
These facilities could be similar to the Internationally Monitored Retrievable Storage System concept that is currently being discussed in the United States and elsewhere. Plutonium conversion facilities. A facility of a new type—the Integrated Actinide Conversion System IACS —would process fuel discharged from power reactors into fresh fuel of a new type and use that fuel in its own fission system to generate electricity.
Throughout this integrated process, the plutonium would be continuously guarded by a high radiation barrier. All discharged fuel that exists now or will exist-whether just generated, in the interim storage facilities, or in utility stockpiles-would eventually pass through an IACS. Each IACS could process fuel discharged from 5 to 10 power reactors on a steady basis. In comparison to a power reactor, an IACS would discharge waste that is smaller in volume and nearly free of plutonium.
Waste repositories. The residual waste finally exiting an IACS would be ready for final disposal. Because it would be smaller in volume than the initial amount of fuel discharged from power reactors and have greatly reduced levels of plutonium and other long-lived isotopes, this waste could be deposited in permanent geologic repositories that could be less expensive than the repositories required for the current waste stream.
There would also be greater confidence that the material could be isolated from the environment. In this architecture, most of the power will be generated by reactors whose designs will continue to be improved for safety and economical operation. These could evolve from current designs or they could be new. Some new designs, such as the high-temperature gas reactor, produce less plutonium that can be used for weapons in their operation. This could reduce the number of IACS needed for the fuel cycle architecture.
The safety and protection of discharged fuel, plutonium, and radioactive waste during transportation are important considerations in any fuel cycle. Quantities and distances of shipments of discharged fuel would be about the same in our architecture as in projections of current architectures.
This means that they can be split by both slow ideally zero-energy and fast neutrons into two new nuclei with the concomitant release of energy and more neutrons. Each fission of plutonium resulting from a slow neutron absorption results in the production of a little more than two neutrons on the average. If at least one of these neutrons, on average, splits another plutonium nucleus, a sustained chain reaction is achieved.
The even isotopes, plutonium, , and are not fissile but yet are fissionable—that is, they can only be split by high energy neutrons. Generally, fissionable but non-fissile isotopes cannot sustain chain reactions; plutonium is an exception to that rule.
The minimum amount of material necessary to sustain a chain reaction is called the critical mass. A supercritical mass is bigger than a critical mass, and is capable of achieving a growing chain reaction where the amount of energy released increases with time.
The amount of material necessary to achieve a critical mass depends on the geometry and the density of the material, among other factors.
The critical mass of a bare sphere of plutonium metal is about 10 kilograms. It can be considerably lowered in various ways. The amount of plutonium used in fission weapons is in the 3 to 5 kilograms range.
According to a recent Natural Resources Defense Council report 1 , nuclear weapons with a destructive power of 1 kiloton can be built with as little as 1 kilogram of weapon grade plutonium 2.
The smallest theoretical critical mass of plutonium is only a few hundred grams. In contrast to nuclear weapons, nuclear reactors are designed to release energy in a sustained fashion over a long period of time. This means that the chain reaction must be controlled—that is, the number of neutrons produced needs to equal the number of neutrons absorbed.
This balance is achieved by ensuring that each fission produces exactly one other fission. All isotopes of plutonium are radioactive, but they have widely varying half-lives.
The half-life is the time it takes for half the atoms of an element to decay. For instance, plutonium has a half-life of 24, years while plutonium has a half-life of The various isotopes also have different principal decay modes. The isotopes present in commercial or military plutonium are plutonium, , and Table 2 shows a summary of the radiological properties of five plutonium isotopes. The isotopes of plutonium that are relevant to the nuclear and commercial industries decay by the emission of alpha particles, beta particles, or spontaneous fission.
Gamma radiation , which is penetrating electromagnetic radiation, is often associated with alpha and beta decays. Various sources give slightly different figures for half-lives and energies. Table 3 describes the chemical properties of plutonium in air. These properties are important because they affect the safety of storage and of operation during processing of plutonium.
These complex systems and regulation make for very long build times. In addition, public opinions on nuclear energy tend to be more negative than with other energy sources. The over-estimation of the dangers associated with releases of radioactive material is a significant issue, as large-scale nuclear incidents are rare.
Fossil Fuels. Nuclear Fuels. Acid Rain. Climate Change. Climate Feedback. Ocean Acidification. Rising Sea Level. An enriched nuclear fuel pellet.
June 17, Fuel Pellet [Online]. July 6, Nuclear Fuels [Online]. Nuclear Energy , 6th ed. Nuclear Fuel Fabrication [Online]. Fuel Rod [Online].
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