ORNL seeks to close the nuclear fuel cycle.
Two facts shape much of the discussion regarding America's nuclear power industry. First, increasing anxiety about the growing volume of greenhouse gas emissions from coal-fired power plants has made nuclear power a more palatable option for many policymakers. Second, if nuclear power is to occupy a larger portion of our future energy inventory, new technologies will be required to address increased amounts of spent nuclear fuel generated by additional reactors and reduce proliferation risks.
With these considerations in mind, in September 2007 U.S. Secretary of Energy Samuel Bodman and senior officials from 16 nations agreed to enhance international nuclear energy cooperation through the Global Nuclear Energy Partnership. The partnership's overreaching goal is to meet worldwide demands for more electricity while avoiding climate-altering carbon dioxide emissions and nuclear weapon proliferation.
Two major objectives of the GNEP concept encourage the safe expansion of nuclear power. One objective is the development of "grid-appropriate reactors" for use in developing nations whose electric grids cannot accommodate large, gigawatt nuclear plants. The other is the development of advanced spent-fuel recycling technologies that provide a reliable fuel source for fuel-supplier nations as well as all countries that operate power reactors. ORNL has leading roles within GNEP in both arenas.
Underscoring GNEP's international aspect, the deployment of new grid-appropriate reactors will require multinational partnerships that will develop new reactor technologies and designs to improve safety and reduce the risk that spent fuel will be used to make nuclear weapons. In addition to the technological and security challenges of nuclear power, GNEP will address the more fundamental issues of providing workforce and management infrastructures in a number of developing countries. Sherrell Greene, director of ORNL's Nuclear Technology Programs, says that ORNL has established relationships with several other DOE laboratories and a number of countries.
The Department of Energy's proposed permanent repository at Nevada's Yucca Mountain site for spent nuclear fuel from America's existing 104 reactors will have insufficient capacity to meet the needs of an expanded nuclear power industry. The two-part alternative to building additional repositories would involve processing the spent fuel to separate the components. A large fraction of the reprocessed fuel would be reused in other reactors. The remaining materials would be treated to reduce their radioactive toxicity and repository decay heat load.
Estimates indicate that the volume of waste sent to Yucca Mountain can be dramatically reduced, potentially allowing Yucca Mountain to meet U.S. repository needs throughout the 21st century.
Several studies have demonstrated that most of the spent fuel components can be recovered and recycled. Uranium represents about two-thirds of the U.S. spent fuel inventory, including residual fissile uranium-235 that can be recycled directly as fuel for heavy-water reactors or enriched again for use as fuel in lightwater reactors. The next largest component—roughly one-quarter of the total volume—is the zircaloy cladding. ORNL is working with an industrial consortium to explore recovery of zirconium from the cladding for reuse.
About 1% of spent fuel consists of plutonium and minor actinides. The radioactive toxicity of the disposed wastes sent to Yucca Mountain can be reduced significantly by transmuting long-lived, highly radioactive actinides, such as plutonium, into isotopes that would not require disposal in a geological repository. This task can be accomplished by separating the elements from the reprocessed fuel stream, fabricating them into new "transmutation fuels" (containing uranium, plutonium, neptunium, americium and curium), or separate "mixed-oxide fuels" (containing uranium, plutonium and neptunium) and "transmutation targets" (containing only americium and curium). Neptunium is coupled with plutonium primarily to enable neptunium recycle, but also to contribute a safeguards benefit. Protactinium-233, the decay product of neptunium, emits an easily detected gamma ray that could impede attempts to smuggle plutonium from a nuclear facility. These fuels and targets would then be re-irradiated in commercial reactors to accomplish the transmutation mission.
Fission products represent only about 4% of the spent fuel components. GNEP seeks to recover and immobilize long-lived iodine-129 and technetium-99 and explore possible recovery and reuse of xenon and platinum—group metals. ORNL researchers are developing new storage and disposal approaches for the remaining intermediate-half-life materials, such as tritium, krypton-85, strontium-90 and cesium-137.
A variety of options includes relying upon a protocol of processing fuel only after the fuel has "aged" for a few decades to allow time for decay of much of the short-lived fission product inventory. Commercial-scale facilities to accomplish these specialized recycle separations and fuel fabrication processes are still in the planning stage, although several of the key processes have been demonstrated at the laboratory scale.
"Previous advanced fuel-cycle R&D has been limited to a single portion of the overall process," says Jeff Binder, who leads the Coupled End-to-End (CETE) Demonstration to tie together all the process steps in support of GNEP. "ORNL's pilot project is a logical step toward creating a viable industrial-scale process, allowing us to identify any problems that could arise from linking together the individual process steps."
The CETE process originates at ORNL's Irradiated Fuels Examination Laboratory. There, commercial light-water reactor fuel rods are chopped into 1.5-inch segments. The fuel segments are heated in oxygen, which the uranium dioxide ceramic pellets bind with as they crumble into U3O8 powder. This "voloxidation" process drives off the fuel's volatile fission products, such as tritium, preventing contamination of the subsequent processing system.
The fuel powders are packaged and transported to the Laboratory's Radiochemical Engineering Development Center, where the powders are easily dissolved in nitric acid and the separations processes are tested using existing "mixer settler" contactors to perform several sequential solvent extraction steps. The initial step uses tri-butyl phosphate to perform one of the newly developed UREX+ uranium extraction processes. TRUEX and TALSPEAK processes are then used to separate americium and curium from the chemically similar lanthanide fission products. By changing the flow rate, acidity level, temperature and number of contactor stages, process performance can be optimized to produce the desired products.
"In the first CETE campaign, spent fuel from the Dresden-1 Boiling Water reactor in Illinois was processed, and we produced a uranium stream, a uranium-plutoniumneptunium stream, an americium and curium stream and a fission product stream," Binder says. All these streams containing nitric acid will be turned into a solid to enable production of a fuel, target or waste form.
The CETE team uses a process developed at ORNL, called modified direct denitration, to drive out the nitric acid and convert the uranium and uranium-plutonium-neptunium products to an oxide powder that can be pressed and sintered into a high-density recycle fuel for irradiation in a reactor. Researchers perform analytical work using X-ray diffraction and a variety of other measurements to determine whether the chemistry of the powders will allow the particles to be compressed and heated to form highquality fuel pellets. The goal is to process in one year 10 fuel pins, or 20 kilograms of heavy metal content of spent fuel.
Greene says the Department of Energy has engaged the commercial nuclear industry—General Electric, AREVA and General Atomics—and nuclear utilities such as the Tennessee Valley Authority to support development of spent-fuel recycle technologies and "advanced recycling reactors" that could produce electricity while transmuting the recycled actinides to less troublesome isotopes. "It's a multidecade program requiring a sustained effort," says Greene.
Developing technologies that can recycle spent fuel and greatly reduce the volume of nuclear waste requiring permanent disposal is among the most important scientific challenges facing a nation seeking long-term energy solutions. With a rich history of innovative nuclear reprocessing technologies and an array of unique capabilities, ORNL will continue to play a critical role in solving these challenges.—Carolyn Krause
Contact: Sherrell Greene
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