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Author: Marvin L. Adams
Fission power has the potential to provide a large fraction of the world’s energy for centuries to come.
Increases in world population and per-capita energy demand in the next few centuries are expected to cause a substantial rise in world energy use. The World Energy Council predicts a doubling of consumption to 800 EJ/yr (EJ = exaJoule = 1018 J) by 2050 (WEC, 2002). For energy production to be sustainable, input requirements (including construction materials) must be available at reasonable financial and environmental cost, and the waste stream must have acceptably low economic and environmental impacts. No production option has yet demonstrated the ability to meet a substantial fraction of projected demand sustainably: every option needs further research and development. In this paper, I summarize options for sustainable energy production, discuss the need for a significant contribution from nuclear fission and its potential for providing such a contribution, and identify some challenges that must be met to achieve that potential.
In the following discussion of the relative merits of energy technologies hundreds of years into the future, we draw seemingly reasonable conclusions based on some fundamental truths. However, the possibility of unforeseen technological developments over such a long period of time introduces considerable uncertainty. With that caveat in mind, let us boldly proceed.
The vast majority of the world’s energy in the coming centuries will come from a few sources: fossil fuels, the sun, biomass, wind, geothermal sources, nuclear fission, and (potentially) nuclear fusion. Table 1 provides an overview of the general suitability of each of these sources for sustainable energy production. Because the anticipated demand is high and because different technologies are better for different applications, it is likely that all of these sources will be tapped.
Economically recoverable oil and natural gas will probably be depleted within a century or two, and both have attractive applications besides energy production. Thus, I will not consider them as sustainable energy sources. The burning of coal produces a great deal of carbon dioxide (CO2), a greenhouse gas. For coal to be a sustainable long-term energy option, either we must find a way to economically sequester the CO2 (300 kg/s from each 1-GWel power plant) or the world must decide that the addition of vast quantities of CO2 to the atmosphere is environmentally acceptable. The burning of biomass also releases CO2, but the same CO2 is then captured by the growth of new matter. Thus, biomass does not add to atmospheric CO2 levels. The greatest drawbacks of biomass are its large land use (according to the World Energy Council estimate, a sustainable 270 EJ/yr would require a crop area larger than the continental United States) and its high level of particulate and other emissions. Hydroelectric power is renewable and emission-free but limited to 30 EJ/yr, at most, by the availability of sites for dams. The sun seems to offer an almost limitless emission-free source of energy, and the role of solar power in the future energy supply will surely be significant, especially for small-scale local applications. However, solar energy is a diffuse source that requires large land areas, and its use of engineering materials is high per unit of energy produced. Technological advances will help, but the low energy density of solar radiation is a fundamental limitation that will be difficult to overcome for large-scale generation. Wind power is less material intensive than solar energy and will play an increasingly important role, but wind is also limited by its low energy density and relatively high land use. Other renewables, such as geothermal energy, have similar limitations. The technology for large-scale generation from nuclear fusion appears to be decades away, and its costs are uncertain. Although fusion has enormous potential, it is not yet clear how its advantages and disadvantages will compare to those of other options, including nuclear fission.
Each of these options has drawbacks, and each has positive features. Most of the drawbacks of any option can be overcome if we are willing to pay enough (in dollars, land, materials, etc.). It is reasonable to conclude that we should continue to develop all options and that all of them will be needed to some extent to meet the energy demand in the coming centuries.
Sustainable Fission Energy
The known economically recoverable 3.3 million metric tons of uranium (WEC, 2001) and 4 to 6 million metric tons of thorium (UNDP, 2000) could produce 250 ZJ (zettaJoule) and 350 to 500 ZJ, respectively, if used to their full potential. Thus, more than 600 ZJ of potential nuclear fission energy - 1,500 times the current total worldwide annual energy consumption - is readily available. Much more easily recoverable thorium will surely be found if a demand develops (Rubbia et al., 1995). An additional 7 million metric tons of uranium is estimated but not yet proven to be economically recoverable (WEC, 2001). Fission power uses little land and requires modest construction inputs (mainly concrete and steel) per unit of energy produced - lower than the construction inputs for wind and solar energy by factors of 10 and 100, respectively (AWE, 1998). Thus, as far as inputs are concerned, fission power has the potential to provide a large fraction of the world’s energy for many, many centuries. However, tapping the full potential energy of uranium and thorium resources will require changes from current fission-energy practice, including the use of "high-conversion" reactors and the recycling of fissionable isotopes.
The output from fission power includes modest amounts of chemical and low-level radioactive wastes, which are relatively easy to handle, as well as used ("spent") fuel, which is the main disposition challenge. Spent fuel from today’s power reactors contains approximately 5 percent fission products (atoms produced by splitting another atom or by radioactive decay of another fission product), 2 percent "fissile" material (including 235U, 239Pu, and 241Pu), and 1 percent other actinides (including 238Pu and 241Am), with 238U comprising most of the remaining mass. ("Fissile" means the atom is likely to fission after absorbing any neutron, including a low-energy neutron.) Note that fission power produces a very small volume of spent fuel. With current technology, six years of operation of a 1-GWel plant yields spent fuel that could fit inside a 4-meter cube, and the vast majority of this material is recyclable. If we recycle, which is essential for the substantial use of uranium and thorium resources, then much less material will require disposal, and most of it will have a much shorter half-life.
Tapping the Energy
In the current "once-through" fission fuel cycle in the United States, uranium fuel that is enriched to 3 to 5 percent in the 235Uisotope remains in a power reactor for four to six years, after which it is treated as waste. In the enrichment process, each kilogram (kg) of natural uranium (which is 0.7 percent 235U) is typically converted to 0.85 kg of "depleted" uranium (0.2 percent 235U) and 0.15 kg of low-enriched uranium (3.5 percent 235U). Thus, only 15 percent of the mined uranium reaches a reactor. During its four to six years in the reactor, approximately 5 percent of the atoms in the uranium fuel fission. Thus, current practice releases only 0.75 percent (5 percent of 15 percent) of the potential energy of the mined uranium; a great deal of the energy-storing material is treated as waste.
Essentially all natural thorium is 232Th, and 99.3 percent of natural uranium is 238U. These isotopes are not fissile - they do not fission when they absorb low-energy neutrons - and neither can fuel a reactor by itself. Both are fissionable (absorption of MeV-range neutron can cause fission) and fertile (capture of a neutron leads to a new fissile atom). To unlock the energy from uranium and thorium resources, the abundant fertile isotopes (238U and 232Th) must be converted into fissile isotopes (239Pu and 233U, respectively). This is simple: if either 238U or 232Th captures a neutron, it quickly converts via beta decay:
The conversion ratio of a reactor is the production rate of fissile atoms (by conversion of fertile atoms) divided by the consumption rate of fissile atoms (by fission or other neutron-induced destruction). A reactor with a conversion ratio greater than unity is sometimes called a breeder reactor. Unlocking all of the potential energy in 238U and 232Th will require high-conversion reactors.
High-conversion reactors have been demonstrated for both 239Pu/238U and 233U/238Th cycles; however, this technology is not as mature as the technology used in current commercial reactors. There are opportunities for innovation in many aspects of the design of high-conversion reactors, including materials (for higher temperature operation and improved resistance to corrosion and radiation damage), fuel form (e.g., oxide, metal, molten salt), and coolant (e.g., liquid metals, helium). Innovations will undoubtedly improve the technology, but even today it is clear that high-conversion reactors can be designed and operated for sustainable fission power.
To take advantage of high-conversion reactors that convert fertile material to fissile material, we must recycle the new fissile material into new fuel. In fact, to maximize the energy and minimize the waste from uranium and thorium, all actinides (not just fissile isotopes) should be recycled so that as many heavy atoms as possible can fission. This will require reprocessing spent fuel, separating out the desired elements, and using reactors that can cause most of the actinides to fission. Reprocessing is a commercial practice in England and France, as is highly efficient (> 99.8 percent) extraction of uranium and plutonium. Efficient extraction of other actinides has been demonstrated on a laboratory scale, and research and development are under way to improve these technologies and move them to commercial scale.
Because of the mix of fissile and fissionable isotopes that develops after several recycling steps, fuel from multiple recycling steps is best suited for fast-spectrum reactors. In a fast-spectrum reactor, neutrons retain relatively high energy from birth (via fission) to death (via absorption or escape). Fast-spectrum reactors can cause nonfissile actinides to fission, whereas other reactors have trouble doing so. High-conversion reactors can be fast-spectrum reactors, and thus could be both creators of fissile material and burners of nonfissile actinides.
Handling the Waste
If a substantial majority of the actinides are recycled and made to fission in fast-spectrum reactors, the remaining waste will be mainly fission products. Most fission products decay with half-lives of decades or shorter, and after 300 years the total fission-product inventory decays to a radiotoxicity level lower than that of the original ore (OECD, 1999). Fission products can be immobilized in glass; this is done commercially in England and France. The waste-containing borosilicate glass is so insoluble that if it were immersed in water, only 0.1 percent would dissolve in 10,000 years. Thus, it is not difficult to immobilize fission products for several hundred years, after which they are no more harmful than the original ore that nature placed underground. The waste-containing glass can be isolated, for example, in stable underground geologic formations.
If we wanted to reduce the radiotoxicity of fission products even further, we would need to address the fission products with long half-lives. The fission products that contribute most to long-term radiotoxicity are 99Tc and 129I, which have half-lives of 2.1?105 and 1.6?107 years, respectively. These isotopes could conceivably be separated from other fission products and transmuted (by absorption of neutrons from accelerators, reactors, or accelerator-driven subcritical assemblies) into shorter-lived isotopes (NRC, 1996; OECD, 2002). A recent study concludes that both of these isotopes could be transmuted so that their products would decay with 51-year half-lives (OECD, 2002). This would eliminate them as potential long-term hazards.
No separation technology is 100 percent efficient; thus, even with actinide recycling, some fraction of the actinides will remain with the fission products for disposal. The efficiency of the separations will determine the long-term (>1,000 years) radiotoxicity of the waste from fission power. For example, if 99.9 percent of the plutonium and 99 percent of the americium (Am), curium (Cm), and neptunium (Np) were separated, with all other actinides remaining with the waste, then it would take 10,000 years for radiotoxicity to decay to ore levels - a substantial increase over the 300 years required if 100 percent of all actinides were separated out (OECD, 1999). Increasing the separation efficiency to 99.9 percent of Am, Cm, and Np would reduce this to less than 1,000 years. Thus, the efficiency of separation of actinides has a significant impact on the waste-isolation time and thus on the difficulty of the waste-isolation problem. Economical separation of >99.8 percent of plutonium has been demonstrated on a commercial scale (Cogema’s La Hague plant), and 99.9 percent separation of Am has been demonstrated on a laboratory scale. Research and development continues to improve the technology for actinide separation; thus, it seems reasonable to expect that highly efficient (?99.9 percent) separation of key actinides will be economically achievable eventually. This raises the possibility of a relatively short waste-isolation time (a few hundred years), which would greatly simplify waste disposal.
The 233U/232Th cycle creates fewer transuranic atoms per unit of energy produced and, thus, in some sense creates less of a long-term waste problem (Rubbia et al., 1995). However, it also makes recycling somewhat more difficult, largely because the daughters of the recycled 233U are more highly radioactive than 239Pu and its daughters, which means hands-on operations must be replaced by remote-controlled operations.
In summary, if a substantial majority of actinides are recycled (which will be necessary for us to tap the majority of the potential energy of uranium and thorium resources), then the waste stream from fission power will consist of a very small volume of fission products along with a small fraction of lost actinides. This waste can be readily immobilized in an insoluble material, such as borosilicate glass. The efficiency of actinide-separation technology will determine whether this waste inventory will decay to ore-level radiotoxicity in hundreds of years or thousands of years. Thus, improvements in separation technology may have a significant impact on the waste-isolation time and on public acceptance of fission power.
One disadvantage of actinide recycling is that recycled fuel costs approximately 10 to 20 percent more than fresh fuel. Another is that recycling technology could conceivably enable countries or groups to develop nuclear weapons - the proliferation issue.
Nuclear weapons use highly concentrated fissile material, such as uranium that is highly enriched in 235U or plutonium that is mostly 239Pu. Highly enriched 235U can be obtained by the same process that produces low-enriched 235U for reactor fuel - the process is simply carried farther. (This is the process North Korea recently acknowledged using in its weapons program.) 239Pu is generated in every reactor that contains 238U (see Eq. (1)); however, if fuel stays in a reactor for more than a few months, significant quantities of other plutonium isotopes are also created, which makes the plutonium more difficult to use for weapon design and fabrication. Nevertheless, a National Research Council report has concluded that a credible weapon could be made from plutonium of almost any isotopic composition (NRC, 1995).
The 232Th/233U cycle may be more proliferation-resistant than the 238U/239Pu cycle, because 233U daughter products generate heat and radiation that could make it difficult to design, build, and maintain weapons. Nevertheless, it is technically possible to create weapons from separated 233U.
Every country that has developed nuclear weapons has obtained its concentrated fissile material from dedicated military programs, not by co-opting power- reactor technology. Nevertheless, there is a concern that if recycling technology is developed and widely used in the power industry, it could be used by some nations or groups to produce plutonium for a weapons program. A challenge, therefore, is to develop an economical, practical, proliferation-resistant fission-fuel cycle. This is one goal of the Generation-IV Reactor Development Program (Kotek, in press). One example of a proliferation-resistant technology that includes recycling of valuable actinides is the integral fast-reactor concept (Till et al., 1997). With this technology, Pu is never separated from U during reprocessing, and thus no weapons-usable material ever exists at any stage of the process.
Achieving sustainable fission energy will require changes in current practices, especially in the United States. Current U.S. policy is for all spent fuel to be shipped to a geologic repository (recently identified as Yucca Mountain, Nevada) with no reprocessing. This is a once-through, or "open," fuel cycle. There are several adverse consequences of not reprocessing spent fuel:
1. More than 99 percent of the potential energy in the uranium is lost.
2. Some actinides have long half-lives (24,000 years for 239Pu), which leads to stringent long-term requirements for disposal technologies.
3. Some actinides generate heat, which limits repository capacity.
The first consequence is significant for the very long term. The second affects public acceptance of nuclear power and public acceptance of any given repository site (people wonder how anyone can know that the material will remain sufficiently isolated for tens of thousands of years) and poses significant technical challenges. The third affects the capacity of a given repository. The latter two effects are very important for the near term (i.e., the next few decades).
From a technical point of view, repository space is not an issue. There appear to be more than enough suitable stable geologic formations in the world to handle waste from millennia of fission reactors, especially if fissionable materials (actinides) are recycled. However, because of the political picture today, repository space is a precious commodity. After 20 years of study, more than $4 billion of expenditures, and several political battles, the Yucca Mountain site has very recently been selected by the U.S. Department of Energy and Congress as the repository location for which the DOE will attempt to obtain a license. If licensing proceeds as quickly as possible, the first spent fuel will not be delivered until 2010 or later. Given the current once-through, no-reprocessing fuel strategy, the current U.S. fleet of reactors will produce enough spent fuel by 2040 to fill the Yucca Mountain repository to its estimated technical capacity (DOE, 2002). The addition of new reactors would of course hasten this date. Clearly, in the near term in the United States, repository capacity must be increased for fission power to achieve its potential. It seems equally clear that finding a second repository site would be challenging, especially politically.
The capacity of Yucca Mountain is limited by the thermal load it can accommodate (radioactive decay releases energy that heats the material). If the waste produced lower W/kg, more mass could be accommodated (DOE, 2002). With the current once-through fuel strategy, the main thermal load after a few decades will come from the decay of the isotopes 238Pu and 241Am, with half-lives of 88 and 432 years, respectively. If plutonium and americium were removed from the spent fuel, the heat load would be dominated by isotopes with 30-year half-lives; thus, the thermal load of a given mass of spent fuel would be halved every 30 years (P.F. Peterson, University of California-Berkeley, personal communication, June 2002). In other words, half of the repository capacity would effectively be regenerated every 30 years. It is easy to imagine that this might be more achievable politically than obtaining approval for another repository site.
Summary and Conclusions
Worldwide energy use is likely to increase in the foreseeable future, and sustainable energy sources are not abundant. It seems likely that many different sources will be tapped to meet energy needs. Nuclear fission could potentially provide a significant fraction of the world’s energy for millennia: its inputs (fuel and construction materials) are readily available and its waste stream (fission products and lost actinides) is very small and not technically difficult to handle. Realizing the potential of fission energy will require high-conversion reactors and the recycling of fissionable atoms, which in turn will require that some technical and political challenges be met.
In the short term, especially in the United States, waste-repository capacity is a significant issue. The long-term capacity of the Yucca Mountain repository could be increased significantly by separating plutonium and americium from spent reactor fuel.
I thank Per Peterson and Harold McFarlane for their helpful comments, information, and insights.
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TABLE 1 Comparison of Energy Sources