Research Opportunities for Managing the Department of Energy's Transuranic and Mixed Wastes (2002)

The accumulation of radioactive wastes began in the 1940s with the development of the atomic bomb and continued with the large-scale production of fissile materials such as uranium and plutonium during the Cold War period. Manufacturing processes involved the production of plutonium and its separation from irradiated fuel elements, the development and application of methods for isotopic enrichment, and the production and fabrication (casting, machining, plating) of metal at Hanford, Washington; Rocky Flats, Colorado; Oak Ridge, Tennessee, and other supporting sites. The lower-activity wastes from these operations ranged from trash contaminated with plutonium, to process wastes (e.g., organic sludges or waste contaminated with metallic compounds) from liquid-liquid extraction employed in product purification. During this period, emphasis was placed on production and little attention was given to the types or quantities of waste generated. Wastes were managed using practices analogous to those found in other process industries, which involved the use of on-site disposal in landfills for process waste and the use of ponds and lagoons to control large volumes of wastewater. Wastes were frequently contaminated with both radioactive and chemical substances.

The radioactive wastes from these processes have been categorized into two types, transuranic waste (TRU)¹ and low-level waste (LLW).²

Mixtures of TRU or LLW with toxic or hazardous substances, defined by the Resource Conservation and Recovery Act (RCRA) as well as the Toxic Substances Control Act (TSCA) and any applicable state regulations, are defined as mixed wastes and identified as MTRU or MLLW.

Exposure to radioactivity was recognized as a human health hazard at the onset of nuclear operations, and standards for health protection were established early on by the Atomic Energy Commission, forerunner to the Department of Energy (DOE) and the U.S. Nuclear Regulatory Commission (USNRC). These standards constrained the manner in which radioactive materials were handled to minimize human exposure. In 1970, new standards were established for the burial of transuranic wastes.

Human health hazards resulting from exposure to chemicals were not fully recognized until 1970, with the formation of the Environmental Protection Agency (EPA) and the subsequent establishment of air and water pollution standards. Substantial control of chemical waste disposal began in 1976 with the authorization of RCRA, which directed EPA to set standards for the land disposal of toxic and hazardous substances. Prior to this time, little attention was given to controlling chemical wastes. Compounds of lead, mercury, cadmium, and chromium along with commonly used industrial solvents were allowed to enter the environment with little control.

The application of USNRC and EPA regulations has further subdivided the waste. TRU wastes generated after 1970 have been placed in retrievable storage. In response to RCRA regulations, ponds and lagoons were closed and chemically contaminated trash was no longer buried but instead placed in retrievable storage. The waste materials have several attributes summarized below and discussed further in succeeding paragraphs:

• The volume of waste is substantial.

• The waste is incompletely characterized as to its physical state and its radiological and chemical components.

• The waste is highly heterogeneous on both the total volume and the individual waste container scale.

• The chemical or radiological contents of many waste streams contain individual substances or mixtures of components that complicate the selection and application of treatment systems.

Present estimates of the volumes of waste represented by the categories of TRU and MLLW are found in the DOE’s Summary Data on the Radioactive Waste, Spent Nuclear Fuel, and Contaminated Media Managed by the U.S. Department of Energy (DOE, 2001a). Table B.1, which is excerpted from this document, illustrates the magnitude of the TRU and MLLW waste volume reported at the end of fiscal year 1999 or 2000. DOE does not distinguish between TRU and MTRU in the summary, essentially because all of DOE’s TRU, including MTRU, is destined for disposal in the Waste Isolation Pilot Plant (WIPP; see Chapter 2).

The chemical composition of the previously buried waste is largely unknown. Because production processes did not change with the 1970 limitation on burial, it probably has similar composition to waste in retrievable storage. The fate of previously buried waste is yet to be determined, but it is expected that some of this material will require retrieval.

The summary contains no information about the physical or chemical characteristics of the waste materials. Information about radioactivity is reflected only by the materials’ classification as TRU or LLW. Other sources of information are necessary to complete the description of the inventory.

For the purpose of developing site treatment plans in the early 1990s, the Department of Energy directed all sites to evaluate the inventory of accumulated TRU and mixed waste based on the best information

available (DOE, 1997). This inventory was very detailed and based on the best available knowledge, including available sampling and analysis, history of the process that generated the waste, or the recollections of persons involved in manufacturing operations. Information was gathered for each waste stream, including radioactive materials, chemical constituents, a text description of the waste, and a physical description of the matrix (e.g., solid, liquid, debris). Because of the uncertainty in knowledge, a degree of confidence (high, medium, or low) was also assigned to the data established for each waste stream in the categories of matrix or physical state, chemical composition, and radioactivity. Although the inventory lacks high confidence in characterization of all waste streams, it does allow study of the potential problems of treatment and disposal confronting DOE. The following discussion and tables are based on information in this database and apply to wastes that were inventoried in 1995.

Information about the wastes is crucial to the design of treatment processes and the generation of data supporting disposal requirements. Table B.2 illustrates overall knowledge about the waste as expressed by the confidence in the data. There is reasonable confidence in the chemical composition for only about a third of the MLLW and pond residue waste volume. The chemical characterization of the MTRU waste is even poorer, with only a sixth of the volume meeting the criteria of medium to high confidence. In addition to the need for better data to support treatment and disposal requirements, some reclassification of MTRU and MLLW wastes is expected, as the composition is better determined.

The cover of this report shows cross sections of waste drums as examples of the typical heterogeneity of the wastes. Sidebar B.1, taken from a previous National Research Council (NRC) study, illustrates the

diversity of materials that are found in mixed waste. Table B.3 shows the relative magnitude of retrievably stored and pond residue wastes in each of the five categories described in the sidebar.

The retrievably stored MLLW, the pond residues, and the previously buried MLLW or pond waste that is excavated must be treated to meet RCRA Land Disposal Restrictions and the requirements of the USNRC for disposal of radioactive materials. Chemical and radiological composition becomes a significant issue in the selection, design, and operation of treatment systems.

The retrievably stored TRU and MTRU waste and any previously buried TRU waste that is recovered are destined for disposal at the WIPP site and must meet the special requirements for shipment and acceptance at that site. Treatment to meet RCRA Land Disposal Restrictions is not required. However the TRU and MTRU waste must meet the requirements of TSCA for the treatment of polychlorinated biphenyls (PCBs).

These wastes contain chemicals designated as hazardous by the Environmental Protection Agency under the Resource Conservation and Recovery Act as well as low levels of radioactive fission products.

Table B.4 identifies the fractions of the waste volume in each category known or suspected to be contaminated with various classes of hazardous and toxic materials. These classes are chosen to reflect chemical contaminants commonly found in mixed waste and represent major processes used for the treatment and separation of chemical wastes prior to disposal in a RCRA facility.

• Mercury: Mercury occurs in several forms including the metal, amalgams with other metals, inorganic compounds, and organic compounds. Each form of mercury requires a different approach to treatment. Because of its high vapor pressure, mercury poses a problem when exposed to high temperatures such as those encountered in incineration where it vaporizes and enters the off-gas, from which it is difficult to trap and remove. Grouting techniques useful for other heavy metals are generally not effective to control mercury and its compounds. Retorting is used in the chemical industry for the removal of mercury from waste and to prepare it for recycling. The applicability of this technique to separate mercury from radioactive materials is unknown.

• Metallic compounds (metals): Compounds containing elements such as chromium, cadmium, and lead are commonly treated by grouting, which converts these materials to an insoluble form, resistant to leaching and acceptable for land disposal.

• Toxic organic materials and solvents (organics and solvents): This category includes halogenated and nonhalogenated solvents commonly found in industry and declared by EPA to be either toxic or hazardous. Incineration is commonly employed to remove and destroy these materials. Encapsulation and grouting are generally not effective treatments for this type of contaminant.

• PCBs: These compounds are highly resistant to natural degradation and are known to accumulate in the fatty tissue of animals and humans. TSCA requires that PCBs be destroyed to an acceptable level. Incineration is a common treatment method.

• Waste from electroplating and metal treatment (plating waste): In addition to metallic ions, these wastes commonly contain cyanides, which must be destroyed either chemically or by incineration.

• Hazardous characteristics: Materials designated as ignitable, corrosive, or reactive by RCRA regulations are unacceptable for land disposal without treatment. The unique waste category contains a high percentage of reactive materials such as sodium-potassium alloy, pyrophoric materials, explosives, and compressed gases that pose special treatment problems.

Metals, mercury, and organics or solvents are selected as major chemical classes in Table B.4. In addition to presenting total data, the table shows the breakdown of commingled waste in each class. The data represent only that one or more chemicals in the particular class exceed the level allowed for land disposal under RCRA Land Disposal Restrictions (LDRs). No information is available regarding quantitative levels of contamination. In some instances, a waste stream is classified as MLLW, but there is no specific information as to the contaminating material.

Examination of Table B.4 illustrates the difficulties inherent in the selection and application of treatment systems. There are few waste streams that might be considered “pure” (i.e., other metals without mercury or organics and solvents, organics and solvents without mercury). For the most part, waste streams are heterogeneous and contain some of everything. For simplicity, the following discussion considers only the debris category (57 percent of the total MLLW volume). However, the conclusions apply to all categories. It must also be stressed that the data in the table do not represent a quantitative measure of the amounts of contaminating material in the wastes, only that the materials are probably present at levels requiring treatment.

Incineration is a common method for destroying RCRA organic materials. Control of air emissions from incineration or other thermal treatment methods is a major consideration in the employment of this technology. As noted above, 77 percent of the debris category contains total organics and solvents above levels acceptable for land disposal. However, 16 percent of the debris volume contains both organic materials and mercury. The ability of a thermal treatment system to accept this fraction will depend on the quantitative amounts of mercury and the efficiency of the system for the removal of mercury from discharged gases.

Retorting is commonly used for the separation of mercury from waste materials. It is carried out at lower temperatures than incineration and usually involves indirect heating to avoid mixing combustion gases with mercury vapor. In the debris category, 20 percent of the volume is contaminated with enough mercury to require treatment. However, 80 percent of this quantity is co-contaminated with organics and solvents, which will evolve with the mercury and significantly complicate the separation of the materials.

Alkaline grouts are common for stabilizing heavy metals, but they are not generally effective for controlling significant quantities of accompanying mercury or organic materials. The effectiveness of grouting depends on the quantities of contaminating materials. Wastes containing only small amounts of mercury or organic materials, only marginally above the required treatment levels, might be grouted successfully. Referring again to the debris group, 70 percent of the volume in this category is contaminated with other metals requiring treatment. However, 90 percent of the metal-contaminated waste in this category is co-contaminated with organics and/or mercury.

Hence, no single treatment system is likely to meet the requirements of all waste streams containing similar materials. Quantitative measurement of the constituents of the waste streams is key to the selection and operation of treatment systems.

Meeting the requirements for RCRA land disposal is only one treatment consideration. Another consideration involves the levels and types of radioactivity associated with the waste. Knowledge about its radioactivity is necessary to specify the design of treatment systems that may be operated and maintained safely. In addition, specific isotopes and radioactivity levels in the waste will govern the disposal of treated MLLW. The USNRC specifies three classes of radioactivity for low-level waste—A, B, and C—each containing higher levels of radioactivity (see Chapter 2, Table 2.4). The permitting of disposal facilities requires a performance assessment of proposed sites to ensure adequate containment of the radioactivity.

Table B.5 shows levels of radioactivity associated with the various classes of waste. Generally, wastes at the lower end of the spectrum, <10 nanocuries of alpha-emitting TRU materials per gram, may qualify as Class A. Those at the higher end of the spectrum, approaching 100 nanocuries of alpha-emitting TRU materials per gram, are most likely Class C. Wastes that contain >100 nanocuries of alpha-emitting isotopes per gram are classified as TRU wastes and cannot be disposed as MLLW.

Certain actinides such as Pu-238 have attributes that may affect the choice of treatment processes. Oxides of Pu-238 are known to form extremely finely divided particles that are dispersed easily. The high radioactivity of this actinide and the toxicity of plutonium place added emphasis on selecting treatment systems that can manage this material properly. The presence of Pu-238 is not a quantitative measure but only an indicator that it is present in the waste and should be considered. Quantitative measurements of the waste must be made to understand

the significance of this contaminant and its effect on treatment system operation.

Wastes in the MLLW aqueous, organic, and unique categories and in pond residue solids have relatively low levels of alpha activity. They are candidates for disposal after treatment at facilities having both a RCRA and a USNRC Class A permit, such as Envirocare in Utah. However, some combinations of chemical and radioactive isotopes will require highly specialized treatment schemes. DOE has estimated that 10 to 15 percent of the mixed waste will fall into this category (DOE, 2001a).

Although free from RCRA treatment requirements, TRU and MTRU waste must meet transportation and waste acceptance criteria for WIPP. Shipments received at WIPP must also be characterized for RCRA components even though treatment is not required. Considerations for shipment and acceptance at WIPP follow:

• The generation of hydrogen gas during shipment must be controlled. Hydrogen generation may result from radiolysis of organic materials (e.g., wood, paper, cloth, plastics, solvents), biological activity, or corrosion. In the event of a transportation accident, an explosion of accumulated hydrogen might result in the dispersal of both hazardous and radioactive materials.

• Free liquids exceeding 1 volume percent of the outside container are prohibited. Wastes classified as aqueous or organic liquids are not acceptable.

• Most of the material comprising the unique waste category is prohibited. This includes explosives and compressed gases as well as pyrophoric materials.

• Corrosive wastes (defined by RCRA) are prohibited. However, corrosive waste usually can be treated easily.

• Flammable volatile organic compounds (VOCs) are limited to 500 parts per million (ppm) in the headspace of any payload container. PCBs are currently prohibited from disposal by the TSCA requirement that specifies destruction of this material. An effort is currently in progress to obtain administrative relief and allow disposal of PCBs at WIPP without treatment.

• Highly radioactive materials designated as remote-handled TRU (RH-TRU) are currently prohibited from disposal at WIPP. Work is in progress to define methods for safe shipment and handling of RH-TRU.

Some properties of TRU and MTRU wastes are listed in Table B.6.

Organic material content is one measure of the ability of the waste to generate hydrogen, others being the type and energy of associated radioactivity and possible microbiological activity.³ A high percentage of both TRU and MTRU wastes contains organic material as well as Pu-238. Significant work is in progress to understand the mechanism of hydrogen formation resulting from radiolysis and methods to control the accumulation of hydrogen during shipment to WIPP. Ongoing research is currently focused on developing materials that will absorb (“getter”) the hydrogen. Large volumes of air are circulated throughout the underground disposal areas at WIPP, and generation of hydrogen after placement at WIPP is not regarded as a problem.

As noted in Table B.2, confidence in the chemical composition of the MTRU waste is very low, 16 percent. The reason for the designation of 13 percent of the MTRU volume as reactive and 35 percent as corrosive is unclear. Direct examination and analysis are needed to make a positive determination of these parameters as well as to determine which containers may exceed headspace flammability limits or contain free liquids. In addition, waste characterization to confirm “acceptable knowledge” is required to quantify waste for WIPP disposal (see Sidebar 3.1).