5
Characterization of Mixed Waste Forms

The possibility for effects on human health and the environment from mixed waste disposal depends on the rate at which the waste constituents are released from the waste form and the concentration of these materials as they move into the environment. Quantitative information about the mechanisms and kinetics of waste form degradation and contaminant release during very long time periods under conditions expected for the disposal facility can provide a scientific basis for estimating these possible effects. Characterizing the chemical and physical properties of waste forms through laboratory and field testing is an essential first step in understanding their ability to control the release of contaminants.

During the past 15 years extensive research has been conducted in the U.S. and worldwide to develop methods to determine the physical and chemical integrity of waste forms, contaminant release mechanisms, and waste form degradation rates (Cunnane, 1994). Much of this research has been sponsored by the U.S. Department of Energy (DOE) in support of its high- and low-level radioactive waste management programs and by the U.S. Environmental Protection Agency (EPA) to ensure that waste forms comply with Resource Conservation and Recovery Act (RCRA) requirements. This chapter describes methods that have been used to characterize the physical and chemical behavior of stabilized waste forms. This information is needed to determine a waste form's suitability for disposal and its long-term behavior in the disposal environment.



The National Academies | 500 Fifth St. N.W. | Washington, D.C. 20001
Copyright © National Academy of Sciences. All rights reserved.
Terms of Use and Privacy Statement



Below are the first 10 and last 10 pages of uncorrected machine-read text (when available) of this chapter, followed by the top 30 algorithmically extracted key phrases from the chapter as a whole.
Intended to provide our own search engines and external engines with highly rich, chapter-representative searchable text on the opening pages of each chapter. Because it is UNCORRECTED material, please consider the following text as a useful but insufficient proxy for the authoritative book pages.

Do not use for reproduction, copying, pasting, or reading; exclusively for search engines.

OCR for page 70
--> 5 Characterization of Mixed Waste Forms The possibility for effects on human health and the environment from mixed waste disposal depends on the rate at which the waste constituents are released from the waste form and the concentration of these materials as they move into the environment. Quantitative information about the mechanisms and kinetics of waste form degradation and contaminant release during very long time periods under conditions expected for the disposal facility can provide a scientific basis for estimating these possible effects. Characterizing the chemical and physical properties of waste forms through laboratory and field testing is an essential first step in understanding their ability to control the release of contaminants. During the past 15 years extensive research has been conducted in the U.S. and worldwide to develop methods to determine the physical and chemical integrity of waste forms, contaminant release mechanisms, and waste form degradation rates (Cunnane, 1994). Much of this research has been sponsored by the U.S. Department of Energy (DOE) in support of its high- and low-level radioactive waste management programs and by the U.S. Environmental Protection Agency (EPA) to ensure that waste forms comply with Resource Conservation and Recovery Act (RCRA) requirements. This chapter describes methods that have been used to characterize the physical and chemical behavior of stabilized waste forms. This information is needed to determine a waste form's suitability for disposal and its long-term behavior in the disposal environment.

OCR for page 70
--> Although there are no long-term stability criteria for stabilized mixed wastes, U.S. Nuclear Regulatory Commission (USNRC) regulations require the evaluation of disposal facility performance for periods up to 10,000 years, as described in Chapter 3. From a waste management perspective, the fundamental question is not whether a waste form will decompose during such a long time period, but when and at what rate. Short-term failure could result in a high concentration (pulse) of contaminants reaching the environment, whereas slow failure occurring over hundreds or thousands of years would produce insignificant effects. Determination of performance of the waste form by tests lasting generally a few months and then extrapolated to estimate performance over centuries or millennia introduces considerable uncertainty into predicting long-term performance. The possibility of early failure of the waste form with subsequent generation of contaminated leachates in the disposal facility places an extra burden on design of the engineered and natural barriers and emphasizes the selection of disposal sites offering favorable geology, hydrology, and climatic conditions. In addition to estimating long-term performance, there are a number of reasons for determining the chemical and physical characteristics of waste forms. These include (Franz, et al., 1994): Regulatory compliance: Show that the waste meets criteria established by state and federal regulatory agencies prior to disposal. Waste form development: Testing to measure and compare the effects of the many possible modifications of waste form composition and fabrication to identify the best formulation to satisfy specified criteria. Waste form comparison: Testing to compare the performance of alternative waste forms to assist in selecting a suitable matrix for a particular application. Data for site performance assessments: Tests to produce data for use in predicting the long-term performance of a disposal facility. and Quality control: Rapid, routine testing during waste processing.

OCR for page 70
--> The procedures and data needed to satisfy each of these needs can be quite different. For example, testing procedures for a quality control program must be relatively inexpensive, suitable for repeated application, and provide rapid results. In contrast, testing procedures used to develop data for a waste form qualification or for a site performance assessment (PA) involve estimating long-term stability of the waste form as described earlier. Such procedures may involve lengthy tests and be very expensive. Four factors are important when selecting tests to characterize a waste form (Poon, 1989): representation (of actual field conditions); compatibility (same test applicable to different types of wastes); reproducibility (consistent results from repeated testing); and test stability (consistent results as a function of the duration of testing).1 The most common approach to estimating long-term behavior of a waste form is to evaluate its performance under severe conditions over a short period. For cement-based waste forms, the most common means of accelerating testing is through use of higher water flow rates and more aggressive leaching solutions (Quillin, et al., 1994). As will be seen in the next section, this approach has led to the design and implementation of leaching tests that bear little resemblance to the environmental conditions experienced by the disposed waste. There is an increasing recognition of the need for tests that elucidate reaction mechanisms and that determine reaction rates over a range of repository-relevant conditions, and, if possible, to confirm results of laboratory investigations with data from analogues, for example ancient natural or man-made glasses (NRC, 1996a). Franz, et al. (1994) proposed that tests used to evaluate the performance of solidified mixed low-level waste (MLLW) be organized 1   The committee recognizes that there are difficulties in practice with applying each of these factors. For example actual field conditions may not be predictable for the long-term, and poorly designed experiments can appear to give consistent results.

OCR for page 70
--> into two tiers, which are summarized in Table 6. In the first, the leachability of a waste form is measured, because low leachability is the most important single disposal criterion. If a waste form cannot sufficiently limit the leachability of hazardous or radioactive constituents, it will almost certainly not be considered suitable for disposal in an MLLW facility.2 Only wastes that meet the specifications of appropriate leach tests should be subject to the second tier of testing. The second tier of testing is designed to determine if conditions in a disposal facility will affect the integrity of the waste form and, therefore, its ability to retain hazardous and radioactive contaminants. These two testing tiers, tests for leachability and the physical durability of waste forms, are discussed in this chapter. Along with waste form characterization for directly quantifiable parameters (e.g., leach rate, corrosion rate, and mechanical properties) much effort has been spent in the past on more fundamental properties such as physical structure, diffusion phenomena, impact of radiation and biochemical phenomena. The latter information enhances understanding and interpretation of directly measured characteristics. Leachability The potential for leaching waste components from a waste form depends on external factors and the intrinsic characteristics of the waste form. These include the following: chemistry of the leaching fluid (lixiviant) and the near-field barriers; hydrodynamic regime near the waste (i.e., the rate of lixiviant flow past the waste); chemical properties of the waste form matrix and the waste constituents; physical properties of the waste form, particularly its porosity and specific surface area; and 2   In the special case of mixed transuranic (MTRU) waste destined for disposal in the Waste Isolation Pilot Plant, leachability is not a consideration.

OCR for page 70
--> TABLE 6 Summary of Testing Procedures Suggested for MLLW Waste Forms First Tier—Applicable to all candidate waste forms Leachability   Release of radionuclides   Release of RCRA constituents Second Tier—Applicable to waste forms that pass leachability tests Compressive strength/nondestructive testing (NDT) Stability in water   Dimensional changes   Compressive strength/NDT Stability after irradiation   Dimensional changes   Leachability Stability after freeze/thaw cycling   Dimensional changes   Leachability Stability after wet/dry cycling   Dimensional changes   Compressive strength/NDT   SOURCE: Franz, et al., 1994. biochemical properties of the waste form and the disposal environment. The leaching rate is important since it will determine the concentrations of hazardous or radioactive constituents that can potentially move into the environment. A summary of some of the leaching tests that are most relevant to mixed waste forms is presented in Table 7.

OCR for page 70
--> TABLE 7 Important Leach Tests for Hazardous and Radioactive Waste Forms Test Leaching Solution Duration Comments Toxicity Characteristic Leaching Procedure (TCLP) Acetic acid 18 hours Leaching of crushed sample used to define characteristic hazardous wastes ANS-16.1 Deionized water Up to 90 days Measures leach rate from monolithic waste form Accelerated Leach Test (ALT) Deionized water 11 days Elevated temperature and leach solution changes used to measure leach rates   SOURCE: 40CFR 261, Appendix II, Method 1311; ANS, 1986; Fuhrmann, et al., 1990. The Toxicity Characteristic Leaching Procedure (TCLP) is required by EPA for hazardous wastes regulated by RCRA.3 The procedure was developed to simulate typical conditions in a municipal solid waste landfill with a high concentration of organic acids resulting from decomposition of organic waste materials. The TCLP uses a buffered acetic acid solution mixed with crushed waste at a liquid-to-solid ratio of 20:1. After the sample is agitated in a rotary tumbler for 18 hours, the leachate is filtered and analyzed for regulated constituents. By requiring the waste form to be crushed and then leached with an acidic lixiviant, the TCLP subjects the waste form to more severe conditions than are likely in a properly designed disposal facility. Although recognizing that data from this single-point test are not sufficient for estimating the waste form's long-term performance in an actual disposal facility, the EPA considers the procedure to be a practical way that the many different waste types and forms subject to RCRA can be qualified for disposal. As a means for demonstrating acceptable leach resistance of 3   RCRA and the requirement for use of the TCLP are discussed in Chapter 3.

OCR for page 70
--> solidified Class B and C radioactive wastes, the USNRC recognizes a test developed by the American Nuclear Society (ANS, 1986), the ANS-16.1 procedure. Rather than using crushed waste as is done in the TCLP, the ANS-16.1 procedure specifies that a monolithic cylinder be leached with deionized water at a volume-to-surface-area ratio of 10 cm. The water is sampled and replaced at 2 hours and 7 hours, and then at 1, 2, 3, 5, 14, 28, 43, and 90 days. This test therefore measures leaching as a function of time, in contrast to the single measurement that is made with the TCLP. The ANS test includes calculation of a "leachability index" that can be related to an effective diffusion coefficient for waste forms that leach mainly by diffusion (rather than by dissolution of the waste form matrix itself) during the test period. The ANS-16.1 procedure also is intended to provide quality assurance information during the production of waste forms and to make rapid intercomparisons of waste forms in laboratory testing. This information is provided by chemical analysis of the first few samples (e.g., after 2 and 7 hours), which will reasonably quickly determine whether the quality of a tested waste form is inferior to that of others tested previously. For quality assurance and product intercomparison purposes the test does not require any assumptions about the leaching mechanism. Investigators at Brookhaven National Laboratory (Fuhrmann, et al., 1990) have developed an Accelerated Leach Test (ALT) intended for higher leach-rate materials, such as portland cement, in which leaching is controlled by diffusion in the porous medium. It is similar to the ANS-16.1 procedure because the leaching solution is sampled and replaced periodically. Elevated temperatures, large volumes of lixiviant, frequent lixiviant change, and small specimen size are used to accelerate contaminant release. The test can be completed in 11 days, and the results extrapolated to 20 °C to allow determination of an effective diffusion coefficient. Analysis of the data also can indicate whether the contaminant release is controlled by diffusion or some other process, such as dissolution of the waste form itself. Both the ANS-16.1 and the ALT methods yield effective diffusion coefficients, which make their methods more appropriate for estimating long-term behavior of the waste form and for input into a PA.

OCR for page 70
--> The American Society for Testing and Materials (ASTM) has developed a number of tests that can be used to characterize waste forms: ASTM Method D 4874–95, "Standard Test Method for Leaching Solid Material (Waste) in a Column Apparatus;" ASTM Method D 5233–92, "Standard Test Method for Single Batch Extraction Method for Wastes;" ASTM Method D 5284–93, "Standard Test Method for Sequential Batch Extraction of Waste with Acidic Extraction Fluid," for wastes with at least 5% solids; and ASTM Method D 5369–D, "Standard Practice for the Extraction of Solid Waste Samples for Chemical Analysis Using Soxhlet Extraction" at elevated temperature with cycling organic solvent for the extraction of non-volatile and semi-volatile organic compounds. A more recent ASTM method, C1308–95, is an "Accelerated Leach Test for Diffusive Releases from Solidified Waste and a Computer Program to Model Diffusive, Fractional Leaching from Cylindrical Waste Forms." This method accelerates the leach rate and determines if leaching is diffusion controlled. The resultant data may be appropriate for use in a PA. Many other characterization tests were developed in other laboratories in the U.S. and abroad. They all tend to meet one of the two major objectives: provide information for performance analysis prove conformity with quality standards, regulations and waste acceptance criteria. Only a few of the presently available leach tests provide a basis for evaluating long-term behavior. Solution compositions are determined, but the identification of the phases that form as a result of the alteration generally is not required in the test. The latter information is essential to determining reaction progress and evaluating long-term behavior. With regard to characterization for performance assessment, increased atten-

OCR for page 70
--> tion is being given to testing in realistic conditions (e.g., in site-specific ground water). Durability This section summarizes methods of estimating the long-term durability of proposed waste forms. There is an extensive body of literature on this topic, much of it generated in support of high-level nuclear waste disposal programs in the United States, Europe, and Japan (Lutze and Ewing, 1988). The following discussion is limited to the two waste forms most frequently considered for MLLW and MTRU: cement-based grouts and glass. Four activities for predicting the long-term durability of waste forms are laboratory testing, field testing, analogue studies,4 and modeling (Means, et al., 1995). Laboratory testing involves developing procedures for accelerating the effects of degradation processes. These processes include thermal cycling, radiation damage, biological degradation for wastes containing biodegradable compounds, dissolution, and structural failure (Mayberry, et al., 1993). Means, et al. (1995) provide a brief review of laboratory testing. Although a successful waste form must maintain its physical integrity over long time periods, the short-term laboratory procedures available to assess durability have not been demonstrated to replicate field behavior (Kirk, 1996). According to Kirk, research must be undertaken to: provide equivalent test parameters to the physical and chemical forces in the field; and calibrate the laboratory time scale to long-term field exposures, including cyclic conditions, elevated temperature, and 4   In its survey of methods to characterize waste forms, the committee did not review studies of analogues. These studies of ancient concretes and glasses as well as similar natural analogues have been conducted to better understand the long-term behavior of these materials in the environment. Several studies of natural glasses are presented in NRC, 1996a.

OCR for page 70
--> chemical conditioning that would be equivalent to long-term repository conditions. Franz, et al. (1994) identified needs in the existing tests that currently are employed to characterize MLLW forms and the absence of tests they feel are required for the evaluation of MLLW forms, especially when predicting long-term behavior. For grouted wastes these needs include: an improved thermal cycling test; a means to judge pass or fail for wet and dry testing; a procedure for irradiation of waste forms, including definition of test conditions (e.g., dose rate, temperature, and moisture); a nondestructive test to replace compressive strength measurements so that the same sample can be tested repeatedly; and pass or fail criteria for dimensional changes and loss of material by spalling. An example of the application of models is the work by Atkins, et al. (1994), who investigated the performance of cementitious waste forms by considering the chemistry of the calcium-aluminum-silica phases present in ordinary portland cement. Using this approach, they were able to investigate the effects of elevated temperature, pH, and cement and ground water interactions. In particular, they were able to evaluate the effects of waters containing high concentrations of sulfate, chloride, and carbonate on a cementitious waste form. Lee, et al. (1995) adopted an integrated approach to modeling the long-term durability of concrete in a disposal facility that included a model of the concrete and calculation of pore fluid speciation, coupled with mass transport in and near the concrete. The models were used to simulate degradation of concrete over a period of 300 years. They found that the alkali elements (sodium and potassium) control the pore chemistry of the concrete for much longer than most previous barrier degradation studies assumed. There are few field studies of actual waste form performance principally because of the lack of facilities in which stabilized waste could be placed. Means, et al. (1995) cite only one study in which

OCR for page 70
--> grouted waste was sampled after 9 and 18 months of burial. Testing showed that lead and other metals remained immobilized, while the physical properties and the porosity of the waste form decreased slightly. The organic contaminants, however, were not immobilized effectively. Extensive testing protocols for vitrified waste forms have been developed to support high-level waste disposal initiatives. Long-term glass stability depends in large part on maintenance of a silica-saturated solution around the waste form. The effects of variations in lixiviant composition can readily be measured in lab tests. Usually these tests must be run over a long period of time, making them expensive to conduct. Laboratory simulation of long-term glass stability frequently includes studies at elevated temperature. Higher temperatures affect the solubility of glasses and the kinetics of release mechanisms. These higher temperature effects must then be correlated to leaching and degradation rates at temperatures likely to be encountered in a repository. A second factor that has been investigated extensively in glasses is the effect of radiation damage (NRC, 1996a; Weber, 1997). Findings, Discussion, and Recommendations To determine if current waste forms are sufficiently developed to stabilize EM's inventory of mixed waste, test methods to characterize the waste forms must be available. The committee reviewed the methods available to characterize the chemical and physical stability of waste forms for mixed waste. The committee found that no test methods are accepted by the technical and regulatory authorities to demonstrate the long-term (greater than a few hundred years) behavior of a waste form in the disposal environment. Available test methods can be used to measure the short-term stability of the waste. Because some mixed wastes contain long-lived radionuclides and chemically hazardous constituents, knowledge of the long-term behavior of waste forms is necessary if credit is to be taken for the waste form in assessing the long-term performance of disposal facilities.

OCR for page 70
--> Committee findings include the following: The different regulatory approaches taken by EPA and USNRC, as discussed in Chapter 3, lead to different approaches to waste form testing. This is manifested by pass-fail waste form tests, such as the TCLP that are required by EPA, and tests that provide data for performance assessment of the disposal system, as required by USNRC. The long-term behavior of a waste form in a disposal facility is difficult to assess because there is no agreed way to extrapolate laboratory tests conducted for periods of days to years to behavior over hundreds or thousands of years. It is difficult to elucidate the reaction mechanisms pertinent to waste form degradation that can be used to estimate long-term performance. Laboratory methods to accelerate natural processes that govern waste form degradation usually subject waste forms to unrealistic conditions. The best that can be expected of any disposal facility is that it will release its inventory of very-long-lived radionuclides and hazardous materials slowly over long times so that their effects in the environment are inconsequential and within regulatory limits. The available waste forms discussed earlier in this report can play a very beneficial role in retarding the release of waste constituents. After they have come into contact with ground water, most good quality waste forms can be counted on to retard the release of waste constituents for long periods of time, typically hundreds of years. The relevant question regarding the behavior of a waste form in a disposal facility is not if it will fail, but when and over what period of time. The committee therefore recognizes the value of efforts to characterize waste forms in terms of fundamental physical and chemical factors that govern their stability and eventual degradation, including physical structure, diffusion phenomena and, where appropriate, effects of radiation and biological activity. Because the committee considered the problem of characterizing the long-term behavior of waste forms to be relevant for all of DOE's wastes, not only mixed waste, the committee's recommendations are

OCR for page 70
--> directed toward OST, which is primarily responsible for research and development in EM. The committee recommends the following: OST should continue to support programs aimed at fundamental understanding of waste form durability and degradation processes. These programs should lead to a better representation of the waste form in PA modeling. OST should work with EPA and the USNRC to develop criteria for rapid testing protocols that can be used to determine whether a stabilized waste is suitable for disposal and to assist in quality assurance and quality control in the waste treatment and stabilization process. The objective of this rapid testing protocol would be to reduce the need for performing TCLP analyses on every batch of waste prior to its disposal. OST should work to promote consensus among EPA, USNRC, DOE, and the scientific community on waste form testing protocols that are generally acceptable for providing at least a qualitative evaluation of long-term waste form performance in disposal environments.