People in the United States currently get about 20 percent of their power from the approximately 100 operating commercial nuclear reactors in this country.
There are more than 400 commercial nuclear reactors supplying energy around the world. Nuclear power is economically competitive and has several advantages over fossil-fuel-burning plants, including no carbon monoxide and nitrous oxide emissions.
However, among the disadvantages, the "waste product" from nuclear powerÑnamely spent nuclear fuel-represents the remaining significant technical challenge to the nuclear-power industry.
Los Alamos is assisting in an international effort to concentrate the toxins from spent nuclear fuel into a special fuel form and to burn that fuel. The project aims to develop ways to extract the toxins and to make the fuel. A number of methods for dealing with spent nuclear fuel have been advanced. Two prominent methods under consideration are direct disposal in a geological repository after a once-through burn and a waste-minimization approach known as accelerator transmutation of waste (ATW).
Both disposal paths will require some type of repository. The repository for the first disposal path will need to provide isolation for 10,000 to one million years, while that of the second will need to provide isolation for a few thousand years.
In the near future, approximately 87,000 tons of spent nuclear fuel will exist in the United States alone; the worldwide inventory will be closer to 250,000 tons.
Under the current disposal scenario, 60,000 of the 87,000 tons in the United States are slated for disposal at Yucca Mountain in Nevada. However, current conservative projections estimate that by 2050 nearly one million tons of spent fuel could exist throughout the world.
Those levels of discharge would require the construction and commission of a repository on the scale of Yucca Mountain every three or four years somewhere around the globe.
Nearly all health-related risks arising from long-term disposal of spent nuclear fuel are attributable to about 1 percent of the fuelÕs content. This 1 percent primarily consists of plutonium, neptunium, americium, and curium (called transuranic elements), and long-lived isotopes of iodine and technetium created as products from the fission process in power reactors.
When the transuranics are removed from discharged fuel destined for disposal, within a period of several hundred years the toxic nature of the spent fuel drops below that of uranium ore, which occurs naturally in EarthÕs crust. In addition to a lower toxicity, removing the transuranics eliminates concerns related to the need for centuries-long heat management within geologic environments.
Removing neptunium, technetium, and iodine renders negligible the possibility of radioactive materials penetrating into the biosphere far in the future. Finally, removing plutonium negates any incentive for future intrusion into repositories driven by overt or covert recovery of material for nuclear proliferation.
The potential payoff of ATW is huge. Instead of requiring a geologic repository that must stay intact for one million years (about the time homo sapiens has existed), ATW requires a repository that must last a few thousand years (or as long as some of the buildings currently on EarthÑthe Egyptian pyramids and the Colosseum in Rome, for exampleÑhave lasted).
In transmutation, the nucleus of an atom undergoes a change to form either a new element or a new isotope. Some of the processes that cause transmutation include natural radioactive decay, nuclear fission, nuclear fusion, and neutron capture.
Transmutation uses neutrons, produced either in a nuclear reactor or in an accelerator-based system, to transmute the plutonium, long-lived fission products, and minor actinides.
One current method of transmutation under development involves a two-stage waste program. Spent nuclear fuel would be reprocessed to separate out the plutonium portion and some of the long-lived fission products, as well as the minor actinides (neptunium, americium, and curium) and other long-lived fission products.
The plutonium-bearing portion could be efficiently transmuted in a nuclear power reactor, while the minor-actinide portion would be transmuted in an accelerator-based system.
The vital aspects to the accelerator system are the operating conditionsÑnamely a subcritical system with a fast neutron flux. This flux will change how transmutation occurs and the fission products generated.
There are a number of fuel options being considered that could meet transmutation program objectives.
One fuel form could be chosen for transmuting plutonium or plutonium with minor actinides in commercial power reactors (called Tier I transmutation). This fuel form may be either an oxide pellet or a tri-isotropic (TRISO) particle fuel and could be made with or without depleted uranium oxide.
The fuel made without depleted uranium oxide will use another matrix diluent such as zirconium oxide because depleted uranium produces more plutonium while under irradiation (fertile) and zirconium is inert (nonfertile).
Another fuel form could be chosen for transmutation of plutonium and minor actinides in the accelerator-driven system (called Tier II).
Tier II systems are fast (high-energy) neutron systems and there are several potential fuel forms under consideration: metal alloy fuel segments, nitride or oxide pellets, or oxide or nitride particle fuel in a metal matrix (composite fuel).
The diluent for each fuel form will likely be zirconium metal or the appropriate zirconium ceramic. Since much of the technology for commercial fuel already exists, most of the research and development effort for transmutation fuel is being directed toward the accelerator-driven systems.
Nuclear Materials Technology (NMT) Division and its predecessors have conducted research into advanced nuclear fuels for many years. The research began in the 1950s with the development of very high temperature particle fuels for the Rover space reactor program. Since then, Los Alamos has continued to be on the cutting edge of research into nuclear fuel.
The most recent major fuel program, SP-100, was a development and production program for a space nuclear fuel. Uranium nitride was the obvious choice for the project because of its favorable properties: a high melting point, excellent thermal conductivity, high fissile density, lower fission gas release, and good radiation tolerance.
Several years of development culminated in the production of a fully qualified core of advanced nuclear fuel. However, the choice for a fuel form for accelerator transmutation of waste has not been decided.
While nitrides have a number of favorable qualities, making them leading choices for the ATW fuel form, some yet-to-be-determined factors come into play, such as the fuel temperature in the accelerator-driven system, the required fissile density, and the degree of fuel burnup.
Until a better determination of the operating conditions of the accelerator is made, the best fuel type cannot be decided upon. Therefore, the research and development effort will consider the four fuel types in parallel, including nitride, metal alloy, oxide, and dispersion (ceramic particles in a metal matrix). NMT Division will focus its efforts on development of ceramic-based and dispersion fuels.
The Advanced Accelerator Application (AAA) ProgramÑconsisting of Accelerator Production of Tritium (APT) and Accelerator Transmutation of Waste (ATW)-is truly a national and international effort.
While the program director, Edward Arthur, is headquartered at Los Alamos, many of the project leaders are located at other institutions.
Los Alamos is working in close collaboration with Argonne National Laboratory (East and West) to determine a viable fuel form.
Experimental fuel pellets fabricated at Los Alamos and metal segments fabricated at Argonne are scheduled to be inserted into the Advanced Test Reactor (a thermal-spectrum research reactor at Argonne-West) late next summer.
Internationally, the U.S. program is interacting with the European and Japanese transmutation programs.
The near-term goal of the fuel development effort is the insertion of a final (or near-final) fuel type into the Phenix Facility in France sometime after 2004. Exposure to Phenix's fast neutron spectrum will help determine a given fuel typeÕs ultimate performance.
Many challenges to fully implementing ATW remain. They range from political (addressing reprocessing of spent nuclear fuel) to social (public acceptance of nuclear technology) to technical (fabricating fuel from very radioactive minor actinides).
But the consequence of not pursuing ATW is significant because spent fuel buried in geological repositories will require safe containment for tens of thousands of years.
However, if ATW is implemented, the nuclear fuel cycle will be nearly complete, and nuclear power can retain its position-and potentially increase its appeal-as a means for power production for many years to come.
For more on spent nuclear fuel reprocessing, see "The Actinide Research Quarterly," 2nd and 3rd Quarter, 2000.
This article was contributed by Robert W. Margevicius (NMT-15).
NMT |
LANL |
DOE
Phone Book |
Search |
Help/Info
L O S A L A M O S
N A T I O N A L
L A B O R A T O R Y
Operated by the University of California for the US Department of
Energy
Questions? -
Copyright © UC
1998-2000
-
For conditions of use, see Disclaimer