Fuel Reprocessing and Isotope Separation Methods for ENU4930/6937: Elements of Nuclear Safeguards, Non-Proliferation, and Security Presented by Glenn E. Sjoden, Ph.D., P.E. Associate Professor and FP&L Endowed Term Professor -- 2007.2010 2010 Florida Institute of Nuclear Detection and Security Nuclear & Radiological Engineering University of Florida
Introduction Overview Discussion i of Fissile il Materials French Pub Nuclear Fuel Cycle Front End / Back End Reactor Centric Conversion Enrichment Reprocessing Summary
Basic Power Reactor Schematic From Benedict, et al, Nuc. Chem. Engineering
Nuclear Power Fission Chain Reactors Power is produced using nuclear fission to generate heat Neutrons are the fission chain carrier Criticality is the precise balance of leakage, absorption, production of neutrons (from fission) in a system called a nuclear reactor Subcritical/critical/supercritical High power/low power in reactors Neutrons must be managed From Benedict, et al, Nuc. Chem. Engineering
Discussion of Fissile Materials French Pub Detailed Information on all fissile materials Criticality potential Critical Masses Bare, shielded d Isotopic Nuclear Data Packaging limitations
The Nuclear Fuel Cycle: Overview Front End vs Back End Centered around reactor irradiation Recovered material through reprocessing High level waste volume is <5% of fuel conversion From Reilly, et al, Passive NDA of Nuclear Materials, NRC Press, March 1991
The Nuclear Fuel Cycle: Step by Step Application and use of uranium in different chemical and physical forms. As illustrated below, this cycle typically includes the following stages: Uranium recovery to extract (or mine) uranium ore, and concentrate (or mill) the ore to produce "yellowcake" Conversion of yellowcake into uranium hexafluoride (UF6) Enrichment to increase the concentration of uranium- 235 (U235) in UF6 Deconversion to reduce the hazards associated with the depleted uranium hexafluoride (DUF6), or tailings, produced in earlier stages of the fuel cycle Fuel fabrication to convert enriched UF6 into fuel for nuclear reactors Use of the fuel in reactors (nuclear power, research, or naval propulsion) Interim storage of spent nuclear fuel Recycling (or reprocessing) of high-level waste (currently not done in the U.S.) Final disposition (disposal) of high-level waste From USNRC, April 2010
The Nuclear Fuel Cycle: Uranium Conversion Yellowcake is produced at the mill Yellowcake converted into pure uranium hexafluoride (UF6) gas impurities are removed; uranium is combined with fluorine to create the UF6 gas. UF6 is then pressurized and cooled to a liquid. In liquid state UF6 is drained into 14-ton cylinders where it solidifies after cooling for approximately five days. The UF6 in the cylinder, now in the solid form, is then shipped to an enrichment plant UF6 is the only uranium compound that exists as a gas at a suitable temperature. One example of a conversion plant is operating in the United States: Honeywell International ti Inc. in Metropolis, Illinois. i Canada, France, United Kingdom, China, and Russia also have conversion plants. Primary risks associated with conversion are chemical and radiological. Strong acids and alkalis are used in the conversion process Converting the yellowcake (uranium oxide, U3O8 ) powder to very soluble forms, leading to possible inhalation of uranium. Conversion produces extremely corrosive chemicals that could cause chemical, fire and explosion hazards From USNRC, April 2010
The Nuclear Fuel Cycle: Uranium Enrichment Most nuclear reactors need higher concentrations of U235 than found in natural uranium U235 is "fissionable," meaning that it starts a nuclear reaction and keeps it going. Normally, the amount of the U235 isotope is enriched from 0.7% of the uranium mass to about 5%, as illustrated in this diagram of the enrichment process. The three processes often used to enrich uranium are Gaseous diffusion (the only process currently in the United States for commercially enrichment) Gas centrifuges (as often reported in Iran) and Becker Nozzle (South Africa) AVLIS (Atomic Vapor Laser Isotope Separation) From USNRC, April 2010
The Nuclear Fuel Cycle: Uranium Enrichment Enriching uranium increases proportion of uranium atoms that can be "split" by fission Not all uranium atoms are the same. Mined uranium is typically 99.3% uranium-238 or U-238 (U238), 0.7% uranium-235 or U-235 (U235), < 0.01% uranium-234 or U-234 (U234). These are the different isotopes of uranium: U234, U235, U238 While they all contain 92 protons in the atom s center, or nucleus (which is what makes it uranium), the U238 atoms contain 146 neutrons, the U235 atoms contain 143 neutrons, and the U234 atoms contain only 142 neutrons. (The total number of protons plus neutrons gives the atomic mass of each isotope that is, 238, 235, or 234, respectively.) Under the Atomic Energy Act, as amended, NRC must license a uranium enrichment plant under 10 CFR Parts 40 (source material) and 70 (special nuclear material). Before an applicant can begin construction of a plant, NRC must issue a license for construction and operation. To issue a license, the NRC must prepare an Environmental Impact Statement (EIS) and a Safety Evaluation Report for the project. NRC must also conduct a formal hearing before issuing a license, and members of the public may request status as intervenors in order to raise important safety or environmental issues about the proposed plant. From USNRC, April 2010
Types of Enrichment: Electromagnetic E-M Separation ( Calutron Method ) uses mass spectrometry Charged particles are deflected in a magnetic field The amount of deflection depends upon the particle's mass The most expensive enrichment method for the quantity (mass) produced d Has an extremely low throughput Enables very high purities to be achieved Often used for processing small amounts of pure isotopes for research or specific use (such as isotopic tracers) Impractical for industrial use based on throughout and cost Historical fact: At Oak Ridge and University of California, Berkeley, Ernest O. Lawrence developed electromagnetic separation for much of the uranium used in the first United States atomic bomb (see Manhattan Project). Devices using his principle are named calutrons. From USNRC and Wiki, April 2010
Types of Enrichment: Gaseous Diffusion Uranium Hexafluoride (UF6) gas slowly fed in plant s pipelines Pumped through special filters called barriers or porous membranes UF6 gas strikes porous membrane (barrier) GD Process uses molecular diffusion to separate a gas Holes in barriers are very small; just enough room for UF6 gas molecules to pass through uses the different molecular velocities of the two isotopes from a two-gas mixture Enrichment occurs when the lighter UF6 gas molecules (with the U234 and U235 atoms) tend to diffuse faster through the barriers than the heavier UF6 gas molecules containing U238 One barrier isn t enough, though. It takes many hundreds of barriers, one after the other, before the UF6 gas contains enough U235 to be used in reactors. At the end of the process, enriched UF6 gas is withdrawn from the pipelines Condensed back into a liquid and poured into containers. Allowed to cool and solidify before transport to fuel fabrication facilities From USNRC, April 2010
Types of Enrichment: Gaseous Diffusion Hazards: The primary hazard in gaseous diffusion plants include the chemical and radiological hazard of a UF6 release and the potential for mishandling the enriched uranium, which could create a criticality accident (an inadvertent nuclear chain reaction). Plants: The only gaseous diffusion plant in operation in the United States is in Paducah, Kentucky. Portsmouth GDP in Piketon, Ohio, shut down in March 2001. Both plants are leased to the United States Enrichment Corporation (USEC) from the U.S. Department of Energy and have been regulated by the NRC since March 4, 1997 From USNRC, April 2010
Types of Enrichment: Gaseous Diffusion K-25 GDP, Oak Ridge, TN From Benedict, et al, Nuc. Chem. Engineering
Types of Enrichment: Gaseous Centrifuge Gas Centrifuge uranium enrichment process Large collective of rotating cylinders containing UF6 gas are ganged in series and parallel formations Centrifuge machines are interconnected to form trains and cascades UF6 gas is placed in a cylinder and rotated at a high speed. This rotation ti creates a strong centrifugal force Mass is conserved, but heavier gas molecules (containing U238) move toward the outside of the cylinder, and lighter gas molecules (containing U235) collect closer to the center The enriched and the depleted gases in each centrifuge are removed by scoops A stream slightly enriched in U235 is withdrawn and fed into the next higher stage A slightly depleted stream is recycled back into the next lower stage. Significantly more U235 enrichment can be obtained from a single-unit gas centrifuge than from a single-unit gaseous diffusion stage. From USNRC, April 2010
Types of Enrichment: Gaseous Centrifuge No gas centrifuge commercial production plants are currently operating in the United States. Louisiana Energy Services (LES) and USEC Inc. have recently received licenses to construct and operate commercial enrichment facilities. USEC Inc. was granted a license in February 2004 for a demonstration and test gas centrifuge plant, which is currently under construction. Both of these commercial facilities are now under construction. December 30, 2008, AREVA Enrichment Services, LLC (a subsidiary of AREVA NC, Inc.), submitted an application to the NRC Seeking a license to construct and operate a gas centrifuge facility in Bonneville County, Idaho. This proposed plant is known as the Eagle Rock Enrichment Facility. From USNRC, April 2010 From Benedict, et al, Nuc. Chem. Engineering
Types of Enrichment: Becker Nozzle Becker Nozzle Process was perfected in South Africa A dilute mixture of (fmole fraction) UF6 in hydrogen at upstream pressure p is expanded through a convergentdivergent slit with throat spacing s into curved groove of radius a. After being deflected through 180 o by the wall of the curved groove, the gas stream at lower pressure p' traveling at high speed is separated by a flow divider set at radius c into an outer heavy fraction depleted in UF6 + hydrogen, and an inner light fraction enriched in these components. The separation factor alpha is higher the higher the speed attained by the gas, which is higher the higher the pressure ratio p/p' and the lower the UF6 content of the feed gas From Benedict, et al, Nuc. Chem. Engineering
Types of Enrichment: AVLIS Atomic Vapor Laser Isotope Separation (AVLIS), Molecular Laser Isotope Separation (MLIS), and Separation of Isotopes by Laser Excitation (SILEX) all inviolve isotopic separation of uranium based on photoexcitation principles p Exciting the molecules using laser light Three major systems are required Laser system, Optical system, Separation module system Tunable lasers can be developed to deliver monochromatic radiation (light of a single-color) The radiation from these lasers can photoionize a specific isotopic species while not affecting other isotopic species. The affected species is then physically or chemically changed, which enables the material to be separated. AVLIS used a uranium-iron (U-Fe) metal alloy as feed, while SILEX and MLIS use UF6 No laser separation uranium enrichment plants are currently operating in the United States. In 2007, General Electric - Hitachi submitted a license amendment request to the NRC, seeking approval for R&D associated with laser enrichment at GNF in Wilmington, NC. The NRC approved the amendment on May 12, 2008, and GE-Hitachi is currently constructing the test loop with the intention of beginning operations in the near future. June 2009, GE-Hitachi license application for commercial laser enrichment plant From USNRC, April 2010
Fuel Fabrication Fuel fabrication facilities Convert enriched UF6 into fuel for nuclear reactors Fabrication also can involve mixed oxide (MOX) fuel Combination of uranium and plutonium components NRC regulates several different types of nuclear fuel fabrication operations From USNRC, April 2010
Fuel Fabrication LWR Fuel Light Water Reactor (LWR) Low-Enriched Uranium (LEU) Fuel Typically begins with receipt of low-enriched uranium (LEU) hexafluoride (UF6) from an enrichment plant. UF6 solid in cylinders is heated to gaseous form UF6 gas is chemically processed to form LEU uranium dioxide (UO 2 ) powder Powder is then pressed into pellets Pellets are sintered into ceramic form and loaded into Zircaloy tubes Tubes filled with pellets are constructed into fuel assemblies. Depending on the type of light water reactor, a fuel assembly may contain up to 264 fuel rods and have dimensions of 5 to 9 inches square by about 12 feet long. From USNRC, April 2010
Fuel Cycle Facilities in the US by NRC Region From USNRC, April 2010
Irradiated fuel is highly radioactive Burnup in MW*Days/MT(hm) Typical 33,000 MWD/Mtu When spent fuel is discharged, it contains substantial amounts of fissile and fertile material Because of the fission i products, spent fuel is intensely radioactive -- Activities of 10 Ci/g are common. 1 Ci is 3.7E10 dis/s Spent fuel lis usually held ldin cooled storage basins (Spent Fuel Pools) at the reactor site for 150 days or more to allow some of the radioactivity to decay. If to be reprocessed, spent fuel would be shipped in cooled, heavily shielded casks, strong enough to remain intact in a shipping accident. Irradiated or Used Fuel From Reilly, et al, Passive NDA of Nuclear Materials, NRC Press, March 1991
Reprocessing of Used Fuel Nuclear fuel reprocessing - the recovery and separation of fissile fuel, actinides, and fission products from fuel burned in a reactor Reprocessing is essential to provide a stable nuclear fuel supply to meet current and future energy demands, while minimizing spent-fuel waste streams and the associated need for high level waste storage facilities. PUREX Plant in Hanford, WA From Benedict, et al, Nuc. Chem. Engineering
Reprocessing Need to extract Pu for weapons drove development as part of Manhattan Project Reprocessing separates components of spent nuclear fuel Recycling all actinides for reactor fuel Closes the nuclear fuel cycle Multiplies the energy extracted from natural uranium by more than 60 Many processes investigated around WWII PUREX process most efficient and produces separated plutonium that was used for nuclear weapons October 1976: Proliferation fears President Gerald Ford to issue a Presidential directive to indefinitely suspend the commercial reprocessing and recycling of plutonium in the U.S. April 1977: President Jimmy Carter banned the reprocessing of commercial reactor spent nuclear fuel President Reagan lifted the ban in 1981, but did not provide the substantial subsidy that would have been necessary to start up commercial reprocessing. March 1999: DOE reversed its own policy and signed a contract with a consortium of Duke Energy, COGEMA, and Stone & Webster (DCS) to design and operate a Mixed Oxide (MOX) fuel fabrication facility. Site preparation at the Savannah River Site (South Carolina) began in October 2005. From Multiple Sources: NCE, Reilly, Wiki
General Closed Fuel Cycle From Benedict, et al, Nuc. Chem. Engineering
PUREX From Benedict, et al, Nuc. Chem. Engineering PUREX (Plutonium URanium EXtraction) aqueous process flowsheet Reprocessing of nuclear fuel involves several distinct processes, including isotope separation, solvent extraction, ti as well as the separation and purification of intensely radioactive fission products and materials. The organic solvent used is typically up to 30% tri-butyl phosphate (TBP) mixed with kerosene. Extraction takes place in banks of centrifugal contactors or pulsed columns. PUREX is an excellent process when it comes to delivering separated uranium and plutonium from spent fuel; however, it results in a direct separation of plutonium [Long]. Under GNEP, proliferation resistance was viewed as pervasive and reprocessing operations must actively prevent explicit separation of plutonium to avoid its diversion to weapons by state and non-state actors.
CETE UREX Demonstration at ORNL From UT-Battelle, ORNL UREX process enables reprocessing without direct separation of Pu Offers a pathway for proliferation resistance
UREX+1a Flowsheet UREX+1a flowsheet UREX is a new solvent extraction reprocessing method under development as part of the DOE R&D ; it has never been developed beyond the laboratory experimental scale Existing models assume ideal operation conditions not duplicated in practice, and there are deviations in the predictions from experimental data--those those involving dilute and/or multiple species. The head end of UREX+1a begins with spent power fuel in cooling ponds. Following years of cooled storage, and pre-processing via mechanical decladding operations, burned oxide fuel rods are dissolved in nitric acid. The dissolved fuel is then contacted with a series of solvents that sequentially extract tkey components to complex and disolate them from the remaining i mixture The UREX (left) process is a modified PUREX process (above) where Pu is prevented from extraction. This can be done by adding a plutonium reductant before the first metal extraction step. In the UREX process, ~99.9% of the Uranium and >95% of Technetium are separated from each other and other fission products & actinides. The key is the addition of acetohydroxamic acid (AHA) to the extraction and scrub sections of the process. Use of AHA greatly diminishes the extractability of Pu and Np, providing greater proliferation resistance than with the plutonium extraction stage of the PUREX process. From UT-Battelle, ORNL
Summary Fuel Cycle and related Enrichment and Reprocessing are complex subjects We touched on each here in a brief overview More on these will be covered in exercises through the course
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