Appendix A Nuclear Materials Production in the DOE Complex
Nuclear weapons production in the United States was a complex series of integrated activities at multiple sites across the country. To provide a context for the nuclear materials and spent fuel challenges and research opportunities described in this report, these activities can be grouped into seven major processes:
mining, milling, and refining of uranium;
isotope separation of uranium, lithium, boron, and heavy water;
fuel and target fabrication for production reactors;
reactor operations to irradiate fuel and targets to produce nuclear materials;
chemical separations of plutonium, uranium, and tritium from irradiated fuel and target elements;
component fabrication of both nuclear and nonnuclear components; and
weapon operations, including assembly, maintenance, modification, and dismantlement of nuclear weapons.
Uranium Processing and Enrichment
Uranium production began with mining and milling to extract uranium ore from the Earth and chemically processing it to prepare uranium concentrate (U3O8), sometimes called uranium octaoxide or yellowcake. Because natural uranium consists mainly of the mass 238 isotope (U-238) and only about 0.7 percent of the fissile isotope, U-235, the next step was to concentrate (enrich) the U-235 content in a portion of the uranium. The process began with natural uranium and resulted in enriched uranium and depleted uranium. The first U.S.
uranium enrichment facilities were located in Oak Ridge, Tennessee. Additional enrichment plants were later built in Piketon, Ohio, and Paducah, Kentucky.
Highly enriched uranium (HEU) contains 20 weight percent or more of U-235; it was fashioned into weapon components and also used as reactor fuel. Low enriched uranium (LEU), which contains less than 20 weight percent of U-235, and natural uranium were used as reactor fuel for plutonium production. Depleted uranium was used in weapon components and for Pu-239 production. All the uranium enriched during the Manhattan Project was HEU for weapon components. However, as early as 1950, LEU was used for reactor fuel.
Uranium enrichment has resulted in the accumulation of about 700,000 metric tons of depleted uranium hexafluoride (DUF6), most of which was stored in large carbon steel cylinders at the enrichment facilities. The DUF6 comprises the largest quantity of separated material in the DOE complex. Research opportunities that might lead to improved options for management, reuse, or disposal of this material are discussed in Chapter 6.
Nuclear Fuel and Reactor Operations
The focus of the Department of Energy’s (DOE’s) nuclear materials production activities was to produce plutonium for nuclear weapons.1 Enriched uranium served as fuel in production reactors, and excess neutrons from the nuclear chain reaction bred Pu-239 and other isotopes in “targets” made of U-238. Fuel and target fabrication consisted of the foundry and machine shop operations required to convert uranium feed material, principally metal, into fuel and target elements. Some later production reactors used separate fuel and target elements, while early production reactors used the same elements for both fuel and targets. Uranium ingots were extruded, rolled, drawn, swaged, straightened, and outgassed to produce rods and plates. The rods were machined, ground, cleaned, coated, clad, and assembled into finished fuel.
Reactor fuel and target fabrication was initially carried out by private contractors and at the Hanford, Washington, and the Savannah River, South Carolina, production reactor sites. Within a decade, government-owned plants in Fernald, Ohio, and Weldon Spring, Missouri, took over part of this mission, supplying the fuel manufacturing plants at Hanford and the Savannah River Site (SRS). At SRS, fuel rods were made by extrusion of an alloy of aluminum and HEU to form thin-walled, aluminum-clad fuel tubes.
Reactor operations include loading and removal of fuel and target elements, reactor maintenance, and the operation of the reactor itself. Early experimental reactors were built at Oak Ridge, Hanford, and in the Chicago, Illinois, area. Nine full-scale production reactors were located at Hanford, and five others were built at the SRS.
Reactor operations created essentially all the nuclear materials used in the DOE complex. Except for a few special cases, such as research reactor fuel, the highly radioactive spent fuel and targets were reprocessed to recover plutonium, uranium, and other isotopes and to separate waste materials. However, when the United States stopped its plutonium production in 1992, some spent fuels, including targets, were left unreprocessed. Currently, DOE’s inventory of spent nuclear fuels (SNF) amounts to about 2,500 metric tons of heavy metal (U and Pu), most of which are stored at Hanford, SRS, Oak Ridge, and the Idaho National Engineering and Environmental Laboratory (INEEL). Chapter 4 describes research needs and opportunities for improving DOE’s ability to manage and dispose of its SNF in view of their potential radiation and security risks.
Chemical separation involved dissolving SNF and targets and isolating and concentrating the plutonium, uranium, and other nuclear materials they contained. Three basic chemical separation processes were used on a production scale in the United States: bismuth phosphate, reduction oxidation, and plutonium uranium extraction (PUREX). Chemical separation plants were located at Hanford, SRS (see Sidebar A.1), and INEEL.
Chemical separation of spent fuel and target elements produced large volumes of highly radioactive waste (high-level waste), and large quantities of low-level radioactive wastewater, solid low-level waste, and mixed low-level waste. Dealing with these waste materials is a central part of the DOE Office of Environmental Management’s cleanup mission. Previous National Academies’ reports have provided advice to the Environmental Management Science Program on research to improve management of these wastes. Separated nuclear materials from reprocessing that are dealt with in this report include plutonium (Chapter 3), cesium and strontium (Chapter 5), and special isotopes (Chapter 7).
Weapons operations include the assembly, maintenance, and dismantlement of nuclear weapons. Weapons operations were chiefly
done at the Pantex Plant near Amarillo, Texas, the Iowa Army Ordnance Plant in Burlington, Technical Area 2 of Sandia National Laboratories, New Mexico, and the Clarksville, Tennessee, and Medina, Texas, modification centers.
Assembly is the process of joining together separately manufactured components and major parts into complete, functional, and certified nuclear warheads for delivery to the Department of Defense. Maintenance includes the modification and upkeep of a nuclear weapon during its life cycle. Dismantlement involves the reduction of retired warheads to a nonfunctional state and the disposition of their component parts. The dismantlement process yields parts containing special nuclear materials, high explosives, hazardous materials, and other components with hazardous and nonhazardous properties. Some parts are returned to the facility where they were originally produced. Other parts are maintained in storage (e.g., plutonium pits) or are dispositioned on site. With respect to the excess plutonium, a major step toward disposition will be conversion to mixed oxide fuel for commercial power reactors at a new facility to be built at SRS (see Chapter 3).
SIDEBAR A.1NUCLEAR MATERIALS PRODUCTION AT THE SAVANNAH RIVER SITE
The primary processing facilities at SRS are the F- and H-Canyons and B Lines (finishing facilities), with F-Canyon starting into production in late 1954 and H-Canyon starting in mid-1955. The two canyons were similar when first constructed but were modified over the years to provide separate capabilities, though many operations can be done in either, but at different rates. Originally, both utilized the PUREX solvent extraction process to separate plutonium from irradiated natural uranium. The original B-Lines were based on the plutonium peroxide, plutonium tetrafluoride, calcium reduction route to metal. The installation also incorporated recovery facilities for slag and crucible, out of specification material, and other residues, because an original goal was that no backlog of recoverable plutonium was to be accumulated. From 1957 to 1959, F-Canyon was shut down for the installation of higher-capacity equipment for solvent extraction and a new plutonium finishing line based on a plutonium fluoride precipitation route to metal. Later, more recovery capacity was added. Meanwhile, H-Canyon continued in plutonium production. During this period, reactor operation changed to driver elements of HEU and targets of DU metal for plutonium production and of lithium-aluminum alloy for tritium production.
Operation of F-Canyon restarted in 1959, and H-Canyon was shut down and modified to maintain nuclear safety while processing HEU driver elements. Changes included dissolver inserts to provide safe geometry, lowered concentration of the tributylphosphate extractant, and instruments to monitor and control concentrations of the uranium in the liquid phases. Only a few months were required for production in H-Canyon to resume.
A number of functions and capabilities were added to the separations facilities for special programs. Recovery of Np-237, fabrication of reactor targets, and separation and recovery of neptunium and
Pu-238 from the targets were provided by canyon installations and finishing facilities in H-Canyon. Special dissolver inserts allowed wide varieties of fuels to be processed, including enriched fuels being returned from domestic and foreign research reactors. An electrolytic dissolver was utilized for some stainless steel- and zirconium-clad fuels. The ability to remotely rearrange flow routes and equipment was utilized in processing irradiated thorium to recover U-233.
Many separate campaigns were involved in the program to produce transplutonium elements such as curium-244 and californium, which required repeated recoveries, target fabrications, and reirradiations of plutonium fractions. For the californium program, a special section was carved out of the far end of the F-Canyon for the installation of the Multi-Purpose Processing Facility. This consisted of a group of small racks containing capabilities for dissolving, chromatic ion exchange, precipitation, and calcining operations. A legacy from that program is the Am/Cm solution discussed inChapter 7. Special plutonium irradiation campaigns were made to produce various isotopic compositions of plutonium that would be approached in a plutonium breeder economy where plutonium would be recycled back into fuel. These materials went to tests to determine reactor neutronic characteristics at different stages of plutonium recycle operation. The H-Canyon B-Line can process Np-237, Pu-238, and Pu-239. The F-Canyon B-Line recovery can process slags and crucibles from metal production and miscellaneous scrap.
As of the summer of 2002, the last plutonium metal has been produced in F B-Line, the liquid system has been flushed, and preparations are under way to put F-Canyon on standby. The F B-Line dry mechanical line will be used to calcine plutonium returns to meet specifications on moisture and volatile materials, utilizing new high-temperature furnaces that can reach the specified firing temperature of 1000 °C. Products are to be packaged in both inner and outer containers to meet the 3013 Standard for storage containers (seeChapter 3).
H-Canyon will continue to process the backlog of aluminum-clad enriched fuels for some years and has the capability to process some plutonium materials. The present primary route for disposition of enriched uranium fuels is to process them for purification and blend the uranium down to nominally 4 percent enrichment for transfer to the Tennessee Valley Authority and to reactor fuel. Other enriched uranium fuels would be sent to a geological repository. A variety of plutonium scrap and mixed plutonium-uranium material will be treated in H B-Line with some plutonium going to mixed oxide fuel, some to waste and then the Defense Waste Processing Facility, and some to storage to await decisions on eventual disposition.