Two full years of oil imports-the untapped energy in the spent nuclear fuel currently stored in the United States-is the potential-energy equivalent of a staggering six-billion barrels of oil. Such is the bounty-and the challenge-facing team members of the Advanced Fuel Cycle Initiative (AFCI). AFCI, a national program directed by the DOE Office of Nuclear Energy, Science and Technology, teams Los Alamos; Idaho National Engineering and Environmental Laboratory; Savannah River Technology Center; and Oak Ridge, Argonne, and Lawrence Livermore National Laboratories.
AFCI's stated goal is to enable the future of nuclear power by developing and implementing spent-fuel treatment and transmutation technologies to enhance the performance of the proposed high-level waste repository and reduce the cost of geologic disposal for the United States.
The Los Alamos team is currently centered in AFC-PO (Advanced Fuel Cycle Project Office), with Michael Cappiello as the Los Alamos program manager, Kemal Pasamehmetoglu the project's national director for fuels, and Cappiello its national director for transmutation technology.
The Los Alamos AFCI work uses the skills of personnel from many divisions, including Nuclear Materials Technology (NMT), Physics (P), Chemistry (C), Decision Applications (D), Los Alamos Neutron Science Center (LANSCE), Materials Science and Technology (MST), Applied Physics (X), Theoretical (T), Engineering Sciences and Applications (ESA), and Nonproliferation and International Security (NIS).
Spent nuclear fuel comprises the "waste" or byproduct of typical light-water reactors. It consists largely of uranium dioxide (about 96 percent) and a hodgepodge of elements produced by the fission and neutron absorption processes within the fuel, including plutonium, neptunium, and other higher actinides (americium, curium, etc.), as well as fission products such as the lanthanides and lighter elements like strontium, krypton, and cesium, among others.
The geologic repository referred to in AFCI's goal is the proposed Yucca Mountain site in Nevada. At the current 2,000-metric-ton annual rate of spent fuel production by electricity-generating nuclear power plants, its statutory capacity will be reached in 2015. A decision for a second repository will be made in the 2007 to 2010 time frame.
Commercial light-water reactors (LWRs) account for one-fifth of the electricity production in the United States, one-sixth globally. In addition to the obvious implication for independence from fossil-fuel electricity generation, their great advantage lies in their minimal generation of the greenhouse gas, carbon dioxide, and therefore, their positive impact on mitigating global climate change.
Moreover, in the face of dwindling oil supplies and an overdependence on the oil of volatile Middle Eastern nations, nuclear power offers the possibility of greatly assisting a hydrogen-based domestic economy, for example, by providing hydrogen for automotive fuel cells without the release of additional carbon into the atmosphere. Nonetheless, many scientists believe that light-water reactors can remain a viable alternative to greenhouse-gas- producing (largely coal-fired) electric-power plants, only if researchers can ultimately close the nuclear fuel cycle.
In general, this means treating spent nuclear fuel to reduce its volume, its radiotoxicity, and its decay-heat load, thereby greatly altering what remains for geologic storage. In the process, additional energy is extracted. This is akin to burning trash, in the sense that it changes the form of matter in the waste (molecular rearrangement in trash burning, elemental transmutation in the case of the closed fuel cycle) and liberates energy.
However, there are consequences in each case: Trash burning produces undesirable carbon dioxide, and a closed fuel cycle using today's technology yields separated streams of fission products and recycled plutonium. The minor actinides are customarily sent to waste (an outcome that would be altered if they could be reintroduced into a transmuter).
The recycled plutonium is potentially a concern for theft or diversion to clandestine weapons development if the technology were deployed in countries that did not already possess nuclear weapons. The minor actinides greatly increase the radiotoxicity (i.e., cancer risk) of waste requiring geologic disposal.
It should be noted that the mixed-oxide (MOX) fuel cycle (see ARQ Spring 1996) is already being used in other countries. In France, for example, recycled plutonium in MOX fuel is partly transmuted in power reactors and the high-level nuclear waste is reduced in volume by a factor of six.
Given these complex issues, closing the fuel cycle entails a number of complementary approaches. In general, AFCI project activities encompass improved nuclear fuels, actinide separations, separate management of fission product elements, advanced safeguards, and transmutation capability-the ability to consume transuranic actinides in the spent nuclear fuel, such that a smaller quantity of lower-radiotoxicity, reduced-decay-heat waste products will result.
The actinide transmutation activity parallels the age-old alchemist's goal of "lead into gold." In this instance, the operant descriptor might be "waste into dollars," given that, overall, the project could result in a net savings of between $35 billion and $50 billion, a combination of revenue from rescued-fuel energy production and savings from the delay or elimination of the need for a second geologic repository beyond Yucca Mountain.
The latter goal is readily attainable, given the projected reduction in waste volume to about one percent of what is currently targeted for geologic storage. "Hopefully, if we're successful, we'll get by with one repository forever," commented AFCI national director for separations, James Laidler of Argonne. Another benefit of closed fuel cycles is what can be characterized as the fissionable resource advantage. In the face of long-term dwindling global supplies of uranium, the ability to separate and recycle fissionable materials from spent fuel is viewed as an important plus.
These images show various stages in the preparation of small beads that form the core of tri-isotropic (TRISO) fuels for high-temperature gas-cooled reactors. TRISO fuels are kernels of uranium oxide or plutonium oxide that are coated to prevent the escape of fission products. Hafnium and zirconium dioxide beads are being used as stand-ins for uranium in development tests at Oak Ridge National Laboratory. The hafnium beads are about 500 micrometers in diameter before being dried and heated to produce the final oxide. In the top photo, the small size of the hafnium beads is illustrated next to a penny. The oxide beads are then coated with multiple layers of carbon and a layer of silicon carbide. The middle photo shows both uncoated (white) and coated (black) zirconia beads. In the photo immediately above, the multiple layers of the coating are visible in the microscope image of a coated zirconium oxide bead that has been cut open to examine the thickness and uniformity of the various layers.
A long-term initiative in two parallel phases
Currently in the first five-year period, during which the focus is the development of proliferation-resistant fuels and of separations processes for current light-water reactor spent fuels, the research is geared toward evaluating technologies for the deployment of a spent-fuel treatment facility in 2015.
In addition, researchers must identify candidate transmutation systems, whose deployment will likely not occur until at least 2022. This sort of timeline illustrates the program's complexly interdigitated activity matrix.
Termed "Series One" and "Series Two" to designate intermediate and long-term project phases, respectively, the project timeline currently extends to 2040. This is not an unexpected duration, because if the Secretary of Energy's 2010 Initiative is successful, the United States will have operational nuclear power plants at least through 2070, and advanced reactors beyond that date.
Series One and Two activities are essentially executed in parallel, with Series One research focusing on the current generation of reactors and their near-term successors. Series Two research and development is directed toward the development of fuel and chemical processing technologies needed to support a sustainable nuclear energy system in this country. The potential growth requirements for the U.S. nuclear energy system, needed to curtail greenhouse gas emissions and to provide the means for hydrogen recovery from water, demand a closed fuel cycle for the maximum efficient resource utilization. Together with the Generation IV program of DOE, which is aimed at developing advanced reactor technologies, the AFCI program strives to accomplish this turnaround in the production of energy for homes, industry, and transportation.
Deployed in 2015, the Series One spent-fuel treatment facility will process spent commercial reactor fuel to recover plutonium and neptunium for incorporation in proliferation- resistant fuels for burning in current Light-Water Reactors (LWRs) or intermediate-term Generation IV reactors. The minor actinides and certain heat-generating fission products will also be extracted from the commercial spent fuel and stored, either for eventual disposal (in the case of fission products) or for minor actinide transmutation in fast reactors or in subcritical accelerator-driven systems.
The intermediate-term prototype Generation IV reactor is slated for deployment in 2015, and will be focused on efficient hydrogen production and plutonium destruction. The longer-term Generation IV reactor will be deployed after 2030 and will be focused on efficient uranium usage and waste minimization. The Accelerator Driven System provides an efficient option for destruction of the Series Two plutonium and minor actinides without the creation of additional actinide waste.
The fast-neutron versions of the Generation IV reactors as well as Accelerator Driven Systems will be capable of fissioning the minor actinides (primarily americium and curium) down to low levels, something that is not possible to accomplish efficiently in LWRs.
Burning of these highly radiotoxic elements is crucial to decreasing both the radiotoxicity and long-term decay-heat burden of spent nuclear fuel. Many scientists believe that the demonstration of the feasibility of this transmutation system is the key to transitioning to a nuclear- energy economy.
Uranium dixoide beads shrink in diameter during drying and sintering. These images, from top to bottom, show 960-micron wet beads, 610-micron dry beads, and 300-micron sintered beads. After sintering, layers of carbon and silicon carbide are added to the uranium dioxide beads. Many of the beads containing the enriched uranium fuel are packed together into spherical or cylindrical fuel elements to be used in the core of high-temperature gas-cooled reactors.
Series One project activities in the next several years can be viewed as precursors to crucial milestones that occur in 2006. In that year, separations and fuels technologies must be selected, and those selections will directly impact aspects of the design of the spent fuel treatment facility.
In the following year, a major decision point arrives when the government is slated to make a decision on whether or not to commit to deployment of the advanced fuel cycle system. If the decision is positive, it could lead to the initiation of construction of the large spent-fuel treatment facility in 2010.
Based on the opinion of some experts, it appears that the long duration of AFCI may be entirely consonant with other constraints, particularly economic ones. For example, Ernest Moniz of the Massachusetts Institute of Technology opined in a recent talk here that on economic and nonproliferation grounds, open-fuel-cycle reactors will remain the choice through midcentury if we are to use nuclear power to help in meeting the increased global demand for electricity over the next 50 years. (See story on Page 12.)
"Open fuel cycles will dominate for a long time," predicts Moniz, citing economic and proliferation constraints while at the same time admitting that "Yucca Mountain isn't the answer for growth scenarios," and that tapping the "unlimited uranium resource" in seawater would be "expensive."
Moniz claims that the spent fuel from the open fuel cycle could be stored for 50 to 100 years before disposal in a repository. This would allow time for development and evaluation of advanced fuel cycles for which this spent fuel becomes a resource.
Moniz does raise the interesting question of whether "reducing that long actinide tail . . . matters"-i.e., whether there has been enough discussion on the advantage of transmuting the long-radiotoxicity actinides in spent fuel. For those researchers intimately involved in the rigorous and demanding activities of AFCI, the answer would appear to be a resounding "yes."
This article was contributed by Luis A. Morales of Nuclear Materials Science (NMT-16), and John L. Sarrao and Joe D. Thompson of Condensed Matter and Thermal Physics (MST-10)
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