REALITY: New recycling technologies
Governments of approximately a dozen nations possess, officially or unofficially, the technology to make atomic weapons from spent nuclear fuel. As described in Tom Clancy's best-seller The Sum of All Fears, one of the greatest concerns of the international community is the possibility that such knowledge might fall into the hands of outlaw nations and terrorist groups seeking to make a "dirty bomb" capable of spreading radioactive contamination over large urban populations. The very real risk of weapons proliferation has contributed to a reluctance, beginning with President Jimmy Carter's decision in 1977 to halt efforts to reprocess spent nuclear fuel, to look at other options for using and storing the by-products of nuclear power.
Three decades later, concerns about global warming and the desire to reduce oil imports from politically volatile regions have led to a renewed interest in the role of carbon-free nuclear power. Researchers at Department of Energy national labs, including ORNL, are helping to facilitate the growth of nuclear power by developing new technologies that reduce the risk of proliferation and lessen the demand for permanent storage of spent fuel.
Of particular importance, ORNL research engineers are developing ways to recover and reuse valuable components of spent nuclear fuel, such as plutonium, neptunium and uranium. Their goal is to provide, for the first time, a sustainable method for managing and reusing the waste generated by the production of nuclear power. The plutonium, recycled without ever being isolated, is removed along with neptunium and some of the uranium to make a mixed-oxide fuel that is unsuitable for nuclear weapons but ready once again for fueling reactors. Simply stated, the technology repeatedly squeezes energy out of plutonium without building up an inventory of separated plutonium that could be used to build a dirty bomb.
"There is no near-term solution to our energy security, climate change and economic competitiveness challenges that does not involve a concerted expansion of nuclear power," says Nuclear Technology Programs Director Sherrell Greene, who oversees ORNL's nuclear energy R&D activities. "For that expansion to become a reality, we must develop technologies for more efficiently using and reusing the uranium resource, managing the used nuclear fuel inventory, recapturing and reusing valuable constituents of used nuclear fuel and managing the waste streams."
Uranium occurs in nature, but only seven-tenths of a percent of natural uranium is fissionable U-235; the remainder is nonfissionable U-238. Plutonium is produced in nuclear reactors when U-238 absorbs neutrons from the fissioning U-235 fuel. In 1943 Oak Ridge researchers, including Enrico Fermi, demonstrated at ORNL's Graphite Reactor that plutonium could be produced in a reactor and separated from uranium and fission products. In thermal reactors, more potential fuel is created when U-238 nuclei each capture a neutron to become U-239 nuclei that then become Np-239, which then decays to turn into Pu-239. Without recycling, the inventory of plutonium would continue to accumulate in the spent fuel. Through recycling, the plutonium can be reused in mixed-oxide fuel for reactors, thus extending the fuel cycle and ultimately reducing the net inventory of plutonium.
"It's troubling to think that we can protect forever a continuously increasing inventory of spent nuclear fuel, which contains plutonium," says Dana Christensen, associate laboratory director for energy and engineering sciences at ORNL. "Rather, if we decide we are going to employ recycling—extract the plutonium and then reformulate it into a fuel that goes into a reactor for transmuting—we begin down the path of making substantial reductions in the volume and availability of plutonium. This approach would produce a corresponding reduction in the risk of nuclear arms proliferation."
The two principal fissile isotopes that provide energy for reactors and weapons are U-235 and Pu-239. In reactors, the nucleus of a heavy element splits, or fissions, into nuclei of lighter atoms, releasing neutrons and electromagnetic energy. Further fission produces light elements that nearly cover the entire periodic chart, whereas heavy transuranic elements, such as neptunium, plutonium, americium and curium result from neutron capture, not fission.
"In the reactor we make some new plutonium, but if we design and run the reactors properly, we can eventually destroy more plutonium than we make," Christensen says.
Storing what's left
With funding from DOE's Office of Nuclear Energy, ORNL scientists are working with research quantities of commercial spent nuclear fuel to develop and demonstrate new technologies for both recycling and storage.
"We are trying to develop evolutionary technologies that simultaneously reduce proliferation concerns about separated plutonium, lower the cost of production and achieve better managed waste streams," says ORNL's Jeff Binder, senior program manager for the Coupled-End-to-End (CETE) Demonstration.
At two ORNL facilities—the Irradiated Fuels Examination Laboratory and the Radiochemical Engineering Development Center—researchers strive to improve the multiple steps associated with recycling fuel rods, from their receipt and characterization to their chopping and processing. Volatile fission product species are removed, the fuel is dissolved in nitric acid and uranium, neptunium and plutonium are co-extracted and oxidized to create a solid mixed-oxide fuel pellet for powering nuclear reactors.
"We extract the plutonium along with the neptunium and some of the uranium so that the plutonium is never isolated," Binder says. "Co-extraction is not a bulletproof solution, but it is a step in the right direction." Neptunium provides added proliferation resistance benefits by emitting a strong, distinctive gamma ray that makes the material easier to detect if diverted. Moreover, neptunium forms Pu-238 under irradiation in the reactor. The reconstituted plutonium isotopic mix is less adaptable for weapons use.
CETE researchers are motivated by the goal of linking several recycling processes. "We don't know if the process can work on an industrial scale until we connect all of the steps together," Binder says. One immediate challenge is removal of volatile fission species prior to co-extraction. "We want to understand how to remove radioactive fission products like tritium, krypton and iodine from the fuel early in the process so they will not create problems during subsequent steps in the recycling process," Binder says.
The program also addresses the controversial issue of how best to dispose of spent fuel from commercial reactors. The Department of Energy in 1998 committed to accept spent fuel for permanent storage in Nevada's Yucca Mountain Repository, scheduled for construction in 2013. The repository's fate resides ultimately in the hands of the next president and Congress. While the debate over a permanent storage site remains unresolved, a growing volume of spent nuclear waste continues to be stored on site at the nation's nuclear power plants.
Recycling of spent nuclear fuel would fundamentally alter the nature of the debate. Changes in the characteristics of the spent fuel would reduce the net volume of waste that requires permanent storage and arguably makes storage in a repository easier and safer to manage, Binder says. Because Pu-239 has a half-life of about 24,000 years, opponents question the viability of permanent storage. "However, the long-lived isotopes could be removed, recycled and transmuted in the reactor to shorter-lived isotopes," he explains. "Instead of putting waste in a geologic repository with the need to isolate it for 10,000 years or more, we are left with the manageable problem of engineering a system designed to safely store the materials for only two to three hundred years."
Enthusiastic about these new technologies, Christensen envisions a nuclear renaissance between now and 2050. By then, several factors should make it economically worthwhile to extract the remaining energy value of spent fuel. "We will have a huge amount of fuel value sitting in spent fuel pools," he says. "The cost of new uranium will rise, along with the cost of enriching it. At some point reusing the fuel in your pool will be cheaper than purchasing and enriching new uranium ore. To be prepared to recycle spent fuel by mid-century, we have got to be doing the research today."
Working with ORNL and Idaho National Laboratory, the technical integrator for the DOE program, are Argonne National Laboratory, whose researchers conducted some early proof-of-principle chemistry, and Los Alamos National Laboratory, where development is under way on a mixed-oxide fuel that could be used in either a thermal reactor or a fast reactor, providing flexibility in managing the resource.
Christensen sums up the opportunity and the challenge: "If we had a sustainable nuclear cycle that could recycle spent fuel into either thermal or fast-spectrum reactors, we could lower carbon emissions, enhance America's security and provide clean electricity for many decades into the future." —Dawn Levy
Web site provided by Oak Ridge National Laboratory's Communications and External Relations