Nuclear weapons production in the United States was a complex series of integrated activities carried out at 16 major sites and over 100 smaller ones. Production stopped abruptly in 1992 at the end of the Cold War leaving a legacy of radioactive wastes, contaminated media and buildings, and surplus nuclear materials. Focusing on the last of these categories, the statement of task for this report directed the committee1 to provide recommendations on a research agenda that would improve the scientific basis for the Department of Energy’s (DOE’s) management of its inventory of high-volume, high-cost, or high-risk spent fuel and nuclear materials. To this end the committee focused its attention on the following:
Plutonium-239. About 50 metric tons of this isotope, a principal component in nuclear weapons, have been declared excess. DOE intends to convert most excess Pu-239 into mixed oxide fuel for use in commercial reactors. About 17 metric tons of the excess are in the form of impure scraps and residues for which conversion may be difficult.
Spent nuclear fuel. DOE manages a wide variety of fuel types, which total approximately 2,500 metric tons. Many fuels are corroding, and their processing or disposal is many years away.
Cesium-137 and strontium-90 capsules. Approximately 2,000 capsules stored at the Hanford, Washington, site contain a total of 67 million curies2 of radioactivity within a volume of only
about 5 cubic meters. These capsules represent almost 40 percent of the radioactivity at the Hanford site and have been described as the most lethal source of radiation in the United States, except for the core of an operating nuclear reactor.
Depleted uranium. A residue from uranium enrichment operations, DOE’s inventory includes over 700,000 metric tons of uranium hexafluoride (UF6), which can produce toxic gases by reacting with moisture and air. Most is stored at three sites in 14-ton carbon steel canisters, many of which are badly corroded, and some have leaked. DOE intends to convert the UF6 to a more stable oxide. Disposition3 plans for the oxide have not yet been determined.
Higher actinides. Including neptunium-237, americium-243, and curium-244, these are materials that can no longer be produced in the United States in the kilogram quantities now available. Continued storage is expensive and presents potential health risks; discarding them may prove to be an irrevocable loss of a unique asset.
Cleaning up the Cold War legacy is the mission of DOE’s Office of Environmental Management (EM). In 1995, Congress chartered the Environmental Management Science Program (EMSP) to bring the nation’s scientific capability to bear on the difficult, long-term cleanup challenges facing DOE. To fulfill its charter, the EMSP solicits proposals and selectively funds research on problems relevant to the needs of EM. This report completes the fifth in a series of studies requested by the EMSP to assist in developing its calls for proposals and evaluating proposals. The previous studies (NRC, 2000, 2001a, 2001b, 2002) dealt with waste and site cleanup. A significant difference with the excess nuclear materials dealt with in this report is that most have not been declared as waste. The statement of task for this study accordingly directed the committee to identify research opportunities for storage, recycle, or reuse as well as disposal of these materials.
Findings and Recommendations
The overarching theme throughout this study is that scientific research beginning now can inform DOE’s future decisions for permanent disposition of surplus nuclear materials. A salient characteristic of
nuclear materials is their potential for unforeseen, beneficial future uses. DOE should avoid decisions today that foreclose future options.
The EMSP should emphasize research toward stabilizing DOE’s excess nuclear materials and discovering beneficial uses for these materials.
There is a tension between the needs of today’s milestone-driven decisions and the planning of longer-term research. Meeting programmatic milestones is a primary objective for EM. Research priorities have been tied to these milestones. Such a narrow focus may foreclose research that can lead to fundamentally new concepts and opportunities.
The committee was guided in its deliberations by considering a different role for research, namely, preparing to make more informed programmatic decisions in the future. This is a better approach than trying to settle all decision making now, for all time, in light of substantial uncertainties (see also NRC, 2003). This approach implies a program of research that is not restricted by current milestones or assumptions about future needs.
The nuclear materials dealt with in this report have been available for only a few decades. Basic physical and chemical principles guarantee that there will be no simple, shortcut ways to replace the currently available quantities of nuclear materials that resulted from 50 years of intense effort in the United States’ massive nuclear complex. The next few decades may bring unforeseen beneficial uses so that these materials are recognized as valuable and irreplaceable resources.
Making the plutonium isotope of mass 239 (Pu-239) was a principal objective of nuclear materials production in the United States from the 1940s through the late 1980s. Approximately 100 metric tons of Pu-239 were obtained from the nuclear reactors and separations facilities at the Hanford, Washington, and the Savannah River, South Carolina, sites for use in nuclear weapons (see Chapter 3 and Appendix A). According to current U.S. policy, about half of this product has been declared as surplus. The surplus inventory includes clean metal— mainly from disassembly of weapons—oxide, and plutonium combined with a variety of other materials in reactor fuels, targets, and miscellaneous forms.
DOE’s disposition options for surplus Pu-239 include:
storage according to the DOE 3013 Standard for up to 50 years;
fabrication into mixed oxide (MOX) fuel;
disposal as transuranic (TRU) waste in the Waste Isolation Pilot Plant (WIPP); and
disposal along with high-level waste and spent fuels, e.g., in the planned Yucca Mountain, Nevada repository.
A key element in DOE’s strategy for eventual disposal of its inventory is the conversion of as much of the excess Pu-239 as is technically and economically feasible into MOX fuel for commercial power reactors.4 The spent MOX fuel would be co-disposed with other spent nuclear fuels. However, approximately 17 metric tons of excess Pu-239 are in the form of scraps and residues, including very impure materials. The disposition of this material is uncertain and will present technical challenges for MOX operations.
The EMSP should support research to help maximize the portion of DOE’s excess Pu-239 inventory that can be used as MOX fuel and that will support the scientific basis for disposal of impure plutonium not suitable for MOX fuel. Research should include fundamental chemistries for storing and purifying plutonium, modeling of MOX fuel performance to help ensure reactor safety, and devising high- integrity, theft-resistant forms for disposal.
Research opportunities for storage include study of long-term corrosion and gas generation in the sealed 3013 canisters (see Chapter 3), process analytical chemistry and materials characterization for MOX fabrication, and improved moisture analysis and nondestructive assay techniques for use in high-radiation environments. For less pure materials that may not be directly suitable for MOX fabrication, research is needed to improve the characterization and separation of undesirable impurities to make more material available for MOX and potentially to allow greater flexibility in incorporation of a wider range of materials into MOX than current specifications allow.
The committee believes there will likely be impure Pu-239 materials that cannot be converted to MOX, but nevertheless are too rich for disposal as TRU waste in the WIPP. Further research into alternate ways of immobilizing this material, for example, in ceramic matrices, to meet criteria for co-disposal with high-level waste and spent fuel is needed. In addition, there are potential crosscutting research topics on stabilization of spent fuel and plutonium residues for storage and disposal.
Spent DOE Nuclear Fuel
DOE manages an assortment of over 250 spent nuclear fuel (SNF) types that altogether comprise about 2,500 metric tons of heavy metal (MTHM).5 DOE spent fuel was generated in military and civilian reactor development, research, and fuel testing programs. The inventory also includes irradiated fuel and target6 assemblies that were placed in storage when DOE stopped reprocessing nuclear fuel for production purposes in 1992. DOE plans to dispose of its SNF along with commercial SNF and vitrified high-level waste in a repository at Yucca Mountain. Because DOE has only recently begun to prepare a license application for Yucca Mountain, uncertainty exists in the future waste acceptance criteria for the various types of DOE spent fuel.
Most types of DOE spent fuel have important characteristics that are different from commercial spent fuel, which will comprise most of the waste disposed in Yucca Mountain, if licensed and constructed. These are primarily differences in the chemical forms of the fuel and the cladding materials that encase it, and the isotopic composition of the fuel. The different characteristics affect the spent fuel’s chemical stability and potential for gas generation, decay heat generation and potential for thermal damage under different storage and accident conditions, potential for inadvertent nuclear criticality, and attractiveness of the material for theft.
The EMSP should support research to help ensure safe and secure storage and disposal of DOE SNF. Research should emphasize materi als characterization and stabilization, including developing a better understanding of corrosion, radiolytic effects, and accumulated stresses. This research should be directed toward determining a lim ited number of basic parameters that can be used to evaluate the long-term stability of each of the types of DOE SNF in realistic storage or repository environments.
The primary research challenge and opportunity in characterization is nondestructive assay of plutonium and other isotopes in the high-radiation environment that is typical of most spent fuels. Interim storage
MTHM refers to the mass of uranium and/or plutonium used to fabricate the fuel. It does not include the mass of the fuel cladding or ancillary components.
Most of DOE’s nuclear materials were created in nuclear reactors through the capture of neutrons by various target isotopes, e.g., U-238 (see Appendix A). Using separate fuel (driver) and target assemblies increased production efficiency. DOE manages irradiated targets as SNF. The committee does not distinguish between fuels and targets when referring to SNF.
requires conditioning methods that are inexpensive but provide sufficient stability to meet safety requirements for several decades. For spent fuels of relatively low chemical stability, such as DOE aluminum-clad spent fuels, a wide variety of potential degradation mechanisms exist: radiolytic gas generation, biocorrosion, pitting corrosion, interactions with other materials in storage containers, oxidation, matrix dissolution, and hydriding. Stresses can accumulate from the fuel’s thermal history and from other effects such as swelling due to oxidation or radiolytic displacements and transmutations. There are opportunities for research to better understand these degradation mechanisms and to identify inexpensive approaches to arrest them.
Because disposal criteria are uncertain, research is needed to provide bases for a variety of conditioning methods. Minimal conditioning may prove to be problematic for highly enriched uranium fuels, due to criticality issues, and for aluminum-clad fuels, due to chemical stability issues. Research to further develop reprocessing options where the spent fuel is dissolved in a molten salt or an aqueous solution and separate streams of well-characterized materials are created may help to address the specific issues of high enrichment and cladding stability. There are opportunities for collaboration with the new DOE Advanced Fuel Cycle Initiative to identify research that would make the reprocessing approach viable for some DOE spent fuels that would otherwise have difficulty meeting repository waste acceptance criteria.
Cesium-137 and Strontium-90 Capsules
In the early 1970s operators at the Hanford site removed a large fraction of the Cs-137 and Sr-90 from the site’s high-level tank waste in order to reduce the requirements for cooling the tanks. The cesium and strontium were concentrated and sealed in stainless steel capsules for potential uses, for example, thermoelectric generators or sterilizers. The expected applications for the Hanford capsules did not materialize, and ceased entirely in 1988 after a capsule being used in the commercial sector was found to be leaking. The almost 2,000 capsules, stored underwater at the Waste Encapsulation and Storage Facility (WESF), contain a total of 67 million curies of radioactivity—approximately 37 percent of the total radioactivity at the Hanford site (see cover photograph). The disposition of these capsules has not been decided; options include:
continued underwater storage at the WESF facility,
passive storage in air at a new facility,
overpacking and disposal of the capsules in a geologic repository, and
incorporating the isotopes into a glass or crystalline matrix for disposal in a geologic repository.
The EMSP should support research that will help ensure continued safe storage and potential use or eventual disposal of the Hanford Cs- 137 and Sr-90 capsules. Research should lead to understanding poten tial failure mechanisms of the present capsules, ways to convert the isotopes to stable glass or ceramic forms, and understanding long- term hazards of disposition options.
There are opportunities for fundamental research to understand the chemical and physical alterations of CsCl and SrF2 under intense radiation, localized heating, and change of valence states accompanying radioactive decay. CsCl and SrF2 are susceptible to partial radiolytic decomposition to colloidal metal particles and evolvable halogen gas in the temperature range 100–200 °C after accumulated ionization doses in the dose region 108–1010 Gy. Cesium-137 (monovalent) decays into barium-137 (divalent), and strontium-90 (divalent) decays into zirconium-90 (normally tetravalent) via a short-lived yttrium-90 intermediate. These transmutations lead to very different physical and chemical properties, such as melting and phase-transition points, bulk volume changes, and changes in the ionic radii. Ionization due to the intense radiation fields is likely to induce other changes.
Capsule integrity is essential for interim storage. Twenty-three cesium capsules have been placed in overpacks because they have swollen or otherwise been damaged. Reasons for the swelling are not well understood. There are opportunities for research toward understanding the possible failure mechanisms and predicting incipient failures.
Because the materials in the capsules are concentrated and relatively pure, they are good candidates for incorporation into crystalline matrices that could be developed to be robust against heat, radiation, and transmutations. For vitrification, research is needed to ensure that the isotopes can be sufficiently dispersed in a glass matrix to avoid detrimental effects of heat and radiation in long-term storage or disposal.
Most depleted uranium (DU) is in the chemical form of uranium hexafluoride (DUF6) amounting to 450,000, 198,000, and 56,000 metric tons, stored at DOE sites near Paducah, Kentucky; Portsmouth, Ohio; and Oak Ridge, Tennessee, respectively. The DUF6 is stored in cylinders stacked in open-air storage yards. Each contains about 14 tons
of DUF6 (see cover photograph). The Oak Ridge Reservation has the oldest of these cylinders, some dating back to the Manhattan Project. The most immediate risk posed by the DUF6 is its potential to react with moisture to form hydrogen fluoride, a highly corrosive and chemically toxic gas.
DOE has recently taken a first step toward dispositioning its DUF6 by awarding an 8-year contract to Uranium Disposition Services to build and operate facilities at Paducah and at Portsmouth to convert it to the stable oxide U3O8. The Portsmouth plant will also convert the Oak Ridge DUF6. The contractor will store the oxide at the two conversion facilities. Options for future disposition of the DU, once converted to oxide, are continued storage, reuse, or disposal as waste. Recent concerns over the health effects of DU have led to a resurgence of research on its health effects, but significant gaps remain. Beneficial ways to reuse large amounts of uranium have not been identified.
The EMSP should support near-term (1–5-year) research to help ensure safety of the DUF6during storage, transportation, and conver sion. The EMSP should also support longer-term research that might lead to new, beneficial uses for uranium or that would provide a sci entific basis for selecting a disposal option.
The way the cylinders are stacked in the storage yards restricts the workspace between cylinders and in some cases precludes workers from being able to examine the entire outer surface of each cylinder. Nor is it possible to confidently move and hoist all cylinders because corrosion may have weakened some to the point that they could be damaged by the available handling techniques and equipment—a problem that will increase as time passes. There is need and opportunity for near-term research that will support DOE’s plans for converting its DUF6 to oxide. For example, robotic or remotely operated methods to assess the integrity of the cylinders, extract DUF6 from those that cannot be moved safely, and measure radioactive contaminants (some contain low levels of fission products from recycled uranium) would enhance worker safety.
Research to exploit the special chemical and metallurgical properties of uranium for new uses could convert this large amount of material from a disposal problem to an asset. There are opportunities to use recent advances in biology to develop a better understanding of the potential health effects of uranium metal, oxide, and typical compounds. This research can help establish a scientific basis both for new uses of DU or for its eventual disposal. For disposal, research to develop a scientific basis for returning the material to a former uranium mine or mined cavity is recommended.
The Higher Actinides
With the closure of its production reactors and separations facilities, DOE no longer has the capability for large-scale production of higher actinide isotopes,7 most of which were made in special campaigns that involved multiple irradiation and separation steps (see Chapter 7 and Appendix A). Currently there is little or no use foreseen for the kilogram quantities of these isotopes that are in storage, and for the most part they are considered a liability by EM. The facilities for handling and storing these isotopes are being closed as part of site cleanup. Consequently, EM plans to dispose of many unique materials as waste, e.g., by mixing with high-level tank waste. This route would foreclose all other options and risks future regret of an irrevocable action.
The EMSP should support research to preserve and stabilize the inventory of higher actinide isotopes, identify beneficial new uses, and develop a better understanding of their radiological and chemical health effects.
The higher actinides in the DOE inventory represent material that may be useful in its present form, may be suitable for target material, or may be essential for research into developing new materials. The committee concluded that there are three principal challenges to preserving the inventory:
Facilities capable of handling or storing the materials are being closed.
Few new nuclear scientists are being trained.
Accumulated knowledge, both documentation and personal expertise, is being lost.
The Office of Science has an opportunity to lead other DOE offices and industrial partners in establishing a center of excellence to ensure that the United States has a continuing capability to handle and store large inventories of higher actinides for research, beneficial use, or as feedstock. EMSP-funded research directed at both fundamental science and new uses of the higher actinides can be an important step toward preserving the inventory.
The EMSP’s congressional charter calls for long-term, path-breaking research. In addition, opportunities for research that provides a high potential payoff in addressing urgent, near-term needs may arise. As a practical matter, the EMSP may well encounter a range of research opportunities that span short- and long-term needs.
Opportunities for research that might provide shorter-term (1-5 year) payoffs are generally in the area of stabilizing the inventory for storage. Specific examples include stabilizing Pu-239 for 50 years of storage according to the DOE 3013 Standard, arresting the cladding degradation on some DOE spent fuels and preparing them for decades of storage before eventual disposal, and supporting DOE’s plans to convert its DUF6 to a stable oxide.
Begun now, longer-term research would feed a continuously growing body of scientific information to support decision making and have the potential of providing scientific breakthroughs. Longer-term research should be directed toward beneficial new uses for DOE’s nuclear materials or their disposal.
This report is the last in a series of five National Academies’ studies requested by the EMSP to assist in providing an agenda for research to support and enhance DOE’s site cleanup program. The previous reports dealt exclusively with environmental contamination and waste issues. Most of the excess nuclear materials that are the subject of this report have not been declared as waste, and according to its statement of task the committee emphasized research directed toward preserving and reusing the materials.
Nevertheless, there is a broad consistency among the recommendations in all five studies. Three areas stand out as offering opportunities for the EMSP to support scientific research that crosscuts most of DOE’s cleanup challenges:
characterization of fundamental chemical and physical, and biological properties of the materials, wastes, or contaminated media;
treatment to ensure near- and long-term stability, including understanding the fundamental parameters that affect stability; and
assessment of health or environmental risks.
By focusing its limited funds in these crosscutting areas and by leveraging funding by cooperative research with other DOE offices or the private sector, the EMSP is most likely to achieve the scientific breakthroughs intended by its congressional charter.