TEARING down the Berlin Wall in 1989 symbolized the end of the Cold War. However, the real work of ending the Cold War--sharply reducing the number of nuclear weapons--remains to be done.|
The governments of the United States and Russia have taken the first steps toward nuclear arms reduction by negotiating the Strategic Arms Reduction Treaties. Under START I, which was ratified in 1991, both countries agreed to reduce their large nuclear weapons arsenals to approximately 6,000 warheads and have already begun to do so by dismantling between 1,300 to 2,000 weapons each year. START II, when it is ratified, will reduce the numbers further to between 3,000 and 3,500.
The dismantling of weapons and the cessation of new nuclear weapons manufacturing, while positive for world peace, have raised a problem: what to do about the fissile materials recovered from the weapons or in inventories that will remain unused. These materials--primarily plutonium and highly enriched uranium--are environmental, safety, and health concerns. But of more urgency is the threat they pose to national and international security if they fall into the hands of terrorists or rogue nations. As arms reduction continues and amounts of surplus fissile materials increase, the potential for such security breaches will increase.
As part of bilateral nuclear nonproliferation work, both the U.S. and Russia have initiated scientific studies to find a way to dispose of surplus fissile materials. In the U.S., the Department of Energy is the technical lead for the disposition studies, acting as a member of the Interagency Working Group of the White House Office of Science and Technology. In this capacity, DOE has mandated separate studies for disposing of plutonium and highly enriched uranium, because of their different chemical characteristics. Lawrence Livermore is focusing primarily on the study of plutonium disposition.
Recovery and Conversion
Producing Plutonium Oxide|
To be suitable for most of the disposition methods, plutonium must first be converted into plutonium oxide, the job of ARIES's second module. Lawrence Livermore has been developing pyrochemical techniques to accomplish this conversion using various hydride/oxidation (HYDOX) reactions. Three such processes are being researched, all based on reactions in which pure hydrogen gas is used to remove plutonium from a pit by forming a plutonium hydride. The formation of the hydride causes the plutonium to break up into small particles and separate from the other pit materials. The plutonium hydride is collected and then converted to plutonium oxide either directly or after conversion to plutonium nitride.
The experiments on the HYDOX processes seek to minimize production cycle times and maintain safety while producing oxide particles to the required disposition specifications, particularly the more stringent specifications for oxide fuels used in reactors.
A prototype HYDOX furnace has been designed, assembled, and installed and is being used to test the various process options. An additional unit (Figure 2) is being assembled in a glovebox and will be installed and operated at Los Alamos as part of the ARIES demonstration.
The Spent Fuel Standard|
Because most nations and even some terrorist groups are technically capable of converting surplus plutonium into nuclear weapons, the ideal disposition method eliminates the possibility of surplus plutonium being used for weapons. If a disposition method is not available within a reasonable time frame, the growing volume of plutonium surplus will make proliferation easier and render arms-reduction agreements meaningless.
Because total elimination is not a practical objective, a National Academy of Sciences study, commissioned by DOE's Office of Nuclear Energy, proposed the next best thing: minimized accessibility. Dubbed the "spent fuel standard" and accepted as the goal of plutonium disposition efforts by the U.S., Russia, and the seven other stakeholder nations, it defines "minimized accessibility" as equivalent to the accessibility of the plutonium found in spent reactor fuel. The spent fuel standard is a reasonable goal because the technology to accomplish it appears achievable within 10 years and implementation can be completed within 25 years. It is also a practical goal because, by definition, it excludes spent fuel plutonium--which comprises the larger part of the surpluses--from disposition and concentrates on weapons-grade plutonium.
The Immobilization Task
Three Glass Variations
Variation 2: Internal Radiation Barrier. This two-stage process is similar to Variation 1 but would use existing, modified facilities. The first-stage melt of plutonium oxide and borosilicate frit (containing a neutron absorber) is made in an existing facility at Savannah River, and the second-stage melt (Figure 3c), which incorporates the cesium radiological barrier, will be done at a new melter to be built next to Savannah River's Defense Waste Processing Facility. The high-level-waste fission product cesium-137 will come from the Savannah River tank farms.|
Variation 3: External Radiation Barrier. This is a "can-in-canister" concept in which plutonium is immobilized in borosilicate glass that contains a neutron absorber. Then the mixture is poured into cans, which are in turn placed in canisters into which molten high-level-waste glass is poured (Figure 4). The high-level-waste glass comes from the Defense Waste Processing Facility at Savannah River.
Two Ceramic Variations|
Variation 4: Internal Radiation Barrier. Plutonium oxide is first converted to plutonium nitrate and then blended with mineral-forming oxides (ceramic precursors), a neutron absorber, and a titanate that contains cesium. The mixture is calcined (heated but not fused), loaded into bellows, and hot pressed into a dense form (Figure 5). Twenty of these forms are loaded into a canister and packed with titanium oxide granules. The canisters are stored until they can be sent to a high-level-waste repository.
|Variation 5: External Radiation Barrier. This is a can-in-canister approach similar to Variation 3. The ceramic form is made by blending plutonium oxide with ceramic precursor materials and a neutron absorber. The mixture is calcined, cold pressed, and sintered (heated but not melted) into a dense form that is loaded into small cans. The small cans are put inside a storage canister, where they are surrounded by glass made with high-level waste (Figure 6).|
For the five process variations, the task team developed process flowsheets and preconceptual plant designs; gathered the required environmental data; and determined the workforce, cost, and schedule requirements for implementing them.
At the end of these tasks, the team recommended the can-in-canister concept to DOE and has proceeded to the research and development stage to determine whether glass or ceramic should be the immobilization form. Research on vitrification forms is being done with Savannah River, Pacific Northwest, and Argonne laboratories, while the Australian Nuclear Science and Technology Organisation (ANSTO) and Savannah River are Lawrence Livermore's partners in ceramic form research.
Ceramic Forms and Processes
Science to End Cold War
Key Words: ceramics, deep boreholes, fissile materials, immobilization, nuclear waste repository, plutonium disposition, plutonium oxide processes, spent fuel standard, Synroc, vitrification, waste forms, weapon pits.
Contact: For further information contact Leonard Gray (510) 422-1554 (email@example.com).
Members of Livermore's Fissile Materials Disposition Program team (clockwise from lower left):
MARK BRONSON holds a B.S. in metallurgical engineering and an M.S. in metallurgy from the University of Utah. In addition to being leader of the defense-related projects in the Isotope Separation and Advanced Manufacturing Program at Livermore, he leads the plutonium pyrochemistry work of the Fissile Materials Disposition Program. Particular accomplishments are development of the pit splitter for recovering excess plutonium from the cores of nuclear weapons and the hydride/oxidation process that converts plutonium to plutonium oxide prior to immobilization. He came to the Laboratory in 1988 by way of DOE's Rocky Flats facility in Colorado, where he concentrated on research and development in the field of plutonium pyrochemical technology.
BARTLEY EBBINGHAUS joined the Laboratory in 1991 after earning his doctorate in chemistry at the University of California, Berkeley. He is currently task leader for Livermore's ceramic immobilization work on DOE's Fissile Materials Disposition Program. He co-designed the formula and fabrication process for the proposed ceramic form (a variation of a material called Synroc) that is able to incorporate and immobilize excess plutonium. He has also demonstrated the successful preparation of a large plutonium-bearing ceramic pellet that meets preliminary design expectations.
GUY ARMANTROUT joined the Laboratory in 1965. He holds a doctorate in electrical engineering and physics from Purdue University. He is a project leader in the Fissile Materials Disposition Program responsible for the development and demonstration of production-scale processing systems for the immobilization of plutonium in glass and ceramic in preparation for disposal in a geologic repository.
LEONARD GRAY (Ph.D., University of South Carolina) has been a part of DOE's Fissile Materials Disposition Program since its inception in 1990, when he was asked to organize and lead an international team responsible for developing the immobilization portion of the program. After a 20-year career as a staff chemist at DOE's Savannah River Site, he joined the Laboratory in 1988 as a section leader for plutonium process development in the Special Isotope Separation Program. He is currently chief scientist for Livermore's contributions to the Fissile Materials Disposition Program.