6
Role of the Waste Form in Performance Assessment

The long-term behavior of a waste disposal facility is a function of the entire disposal system, including the waste form, engineered barriers, and surrounding environment. In order to assess the ability of a given disposal concept to meet regulatory requirements it is necessary to consider the influence of each of these system components on short-and long-term performance. This is accomplished through the performance assessment (PA) process. As discussed in Chapter 3, in order to permit a low-level waste (LLW) facility, both U.S. Department of Energy (DOE) and U.S. Nuclear Regulatory Commission (USNRC) regulations require completion of a PA. The PA is the only step in the mixed low-level waste (MLLW) management process the where long-term stability and performance of the waste form is part of the formal evaluation of a disposal system. Ascertaining the role of the waste form in current PA methodology was necessary for the committee to evaluate the adequacy of available waste forms to meet present regulatory criteria. This chapter gives an overview of current PA methodology, the role of the waste form in PA, and the committee's findings and recommendations for improving the waste form's representation in future PA methodology.

Performance assessment is based on a mathematical model of the proposed facility and its environment. Results of this modeling can be used to help demonstrate that a disposal facility will protect the health and safety of the public. The PA process addresses all potential exposure



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--> 6 Role of the Waste Form in Performance Assessment The long-term behavior of a waste disposal facility is a function of the entire disposal system, including the waste form, engineered barriers, and surrounding environment. In order to assess the ability of a given disposal concept to meet regulatory requirements it is necessary to consider the influence of each of these system components on short-and long-term performance. This is accomplished through the performance assessment (PA) process. As discussed in Chapter 3, in order to permit a low-level waste (LLW) facility, both U.S. Department of Energy (DOE) and U.S. Nuclear Regulatory Commission (USNRC) regulations require completion of a PA. The PA is the only step in the mixed low-level waste (MLLW) management process the where long-term stability and performance of the waste form is part of the formal evaluation of a disposal system. Ascertaining the role of the waste form in current PA methodology was necessary for the committee to evaluate the adequacy of available waste forms to meet present regulatory criteria. This chapter gives an overview of current PA methodology, the role of the waste form in PA, and the committee's findings and recommendations for improving the waste form's representation in future PA methodology. Performance assessment is based on a mathematical model of the proposed facility and its environment. Results of this modeling can be used to help demonstrate that a disposal facility will protect the health and safety of the public. The PA process addresses all potential exposure

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--> pathways that may occur at the site and allows comparison of the doses estimated from performance calculations with the performance objectives in DOE Order 5820.2A to determine whether the proposed facility will be in compliance. As noted in Chapter 3, DOE Order 5820.2A requires that a PA be prepared for each proposed DOE disposal facility for LLW and MLLW. No waste form performance criteria have been established solely for mixed waste, therefore, these wastes are subject to the requirements for LLW established by the USNRC and to the leachability criteria established for hazardous wastes by the U.S. Environmental Protection Agency (EPA). Recently, the USNRC has provided guidance for the preparation of PAs for LLW disposal that addresses the issue of waste form (USNRC, 1997). This chapter describes the role of the waste form in the PA methodology developed for LLW disposal facilities in the United States.1 Emphasis is placed on the rate of release of waste constituents from stabilized waste in conjunction with other physical and chemical processes that can affect future exposure to humans and the environment. It is important to recognize that a PA is only required for the radioactive constituents of a waste and not for the chemically hazardous materials, as defined by the Resource Conservation and Recovery Act (RCRA), that are present in mixed waste.2 Although substantial work is being done on developing probability based models for risk assessment of hazardous materials that consider variability and uncertainty, they generally do not extend over the long time periods required by the USNRC for its assessment of disposal facilities.3 Performance assessment encompasses the entire disposal facility as an integrated system. It involves constructing a conceptual simplification of the system (conceptual model) that can be represented with mathematical models of the process involved. The mathematical models 1   Performance and risk assessment methodologies differ among countries due to differing regulatory approaches. For example, in some European countries, the time horizon for PA may extend beyond that required in the U.S., with results for longer times becoming more qualitative. Instrumental regulations for disposal of radioactive and hazardous wastes are being harmonized, with PA likely to include disposal systems for both types of waste (Seitz, 1998). 2   See Chapter 3 for a discussion of RCRA. 3   See, for example, Amendola (1992) and Shevenell (1993).

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--> are then used to calculate the exposure to a critical group (a hypothetical group of maximally exposed persons at a specified location near the facility) as a result of releases from the facility. The waste form itself is considered to be one of several barriers that act to prevent or retard the release of radionuclides. Quantitative data are needed to describe the types of waste that will be received, the components of the disposal facility (especially the liner and cover designs), and the site characteristics. Because all of the needed data usually are not available, the models typically include estimated parameters that are derived from assumptions about the system that cannot be validated experimentally. In addition to long-term characteristics of the waste form, these parameters may include the following: (1) infiltration rates of water into the facility, (2) time to failure of the engineered barriers, (3) water flow through the unsaturated zone, (4) ground water flow rates, (5) interaction between the contaminants and soils, (6) atmospheric mixing, and (7) probability of inadvertent intrusion. Each of these parameters can have a significant effect on the results of the PA calculations. The USNRC (1997) notes that the goal of the PA analysis is not to predict the future but rather to test the robustness of the disposal facility against a reasonable range of future scenarios. For time periods extending to 1,000 years and beyond, there is considerable uncertainty associated with site conditions due to potential processes such as climate change, seismic events, and volcanic activity. The effects of these uncertainties are captured in the sensitivity analysis performed as part of the PA calculations. Sensitivity analysis, the identification of the parameters that, when changed, can have a significant effect on the conclusions of the assessment is an essential part of the PA process. Mayberry, et al. (1993) have noted that there has been little feedback between laboratory testing programs and development of PA methodologies for LLW disposal.4 Without detailed understanding of release mechanisms, laboratory data cannot be confidently extrapolated to the long time periods and environmental conditions modeled in PA. Thus, conservatism is often introduced into waste form-related calcu- 4   The committee recognizes that there have been some cooperative efforts among laboratory scientists and PA modelers at several DOE sites that have resulted in better representation of the waste form in PA, for example Sullivan, 1994.

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--> lations with the result that they may be overly conservative. There is very limited experience with actual field data from disposal facilities containing stabilized waste forms. The release of contaminants from a waste form is the initial step in any exposure pathway, and is a major factor in the performance of the disposal facility, although it is the overall system that is assessed for compliance with the performance objectives. The waste form is often referred to as the source term. There are three general pathways that may result in exposure to radioactive or hazardous compounds: (1) aqueous transport, (2) airborne transport, and (3) inadvertent intrusion into the facility. These are discussed in the following sections. Aqueous Transport Scenarios The waterborne pathway is the most widely recognized and generally the best quantified exposure pathway in considering disposal of solid waste materials, including hazardous and radioactive wastes. This is in large part because of the long experience with municipal solid waste landfills and with RCRA hazardous waste landfills. Instances of ground and surface water contamination from landfills are widely known and have been studied for many decades. Indeed, most of the site selection and design criteria for both sanitary and hazardous waste landfills are oriented towards preventing water contamination. Waterborne exposure pathways involve a multi-step process in which hazardous or radioactive constituents leach from a waste form are transported through an engineered barrier (e.g., a liner) if it is incorporated in the facility design, out of the disposal facility to the surrounding geologic strata, and finally through the unsaturated and saturated geologic formations around the site to a critical group. The principal role of the waste form in this sequence is to limit the rate of leaching of the contaminants. Radionuclide release from a waste form may result both from physical processes and chemical processes. These include simple wash-off and more complex processes such as dissolution, diffusion, and sorption/desorption. Wash-off is the result of aqueous contact with surface contamination on an insoluble surface, such as plastic, metal, or

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--> glass. Dissolution is primarily associated with long-term leaching of stable waste forms, whereas diffusion release occurs from porous waste forms over a shorter time. Desorption occurs when a contaminant is alternatively bound on a surface (e.g., a soil particle, the waste form, or degradation products from the waste form) or released into solution in response to a reversible, quasi-equilibrium chemical process. The equilibrium is modeled using a linear distribution coefficient, or Kd, which represents the equilibrium between the sorbed constituents and those in solution.5 Recognizing the different release mechanisms is important in developing a conceptual model for a PA scenario. In addition to these release mechanisms, possible limits on maximum aqueous concentrations of some constituents in the leachate may occur because of solubility constraints. Solubility will be determined by the complicated chemistry and biochemistry of the leaching solution and the waste form. For example, even though a contaminant might be rapidly released in laboratory experiments with grouted waste, the high pH associated with the grout might cause this contaminant to precipitate in the leachate in a disposal facility, thus, limiting the overall release rate. Identifying which mechanisms control the rate of contaminant release from a stabilized waste may require extensive chemical and theoretical analysis, hence, it is common to simply report an overall release rate that is specific to the waste form and leaching solution. With respect to the waste form's impact on the PA calculations, it is important to note that the release rates measured in laboratory tests are generally conservative in that they expose the waste form to far more water than would be experienced in a properly sited and constructed disposal facility. Furthermore, there is a much higher degree of mixing in laboratory tests than in a disposal facility, which acts to further increase effective diffusion coefficients. However, it should also be noted that release rates measured under laboratory conditions do not take into account heterogeneities that are likely to occur in actual waste forms produced by full-scale treatment and stabilization processes. The USNRC does not provide universal guidance on the length of time that a disposal facility can take credit for waste immobilization 5   Kd is referred to as the sorption coefficient in descriptions of ion exchange processes. It is the concentration of a given ion on a solid sorbent divided by the concentration of that ion in the ambient solution.

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--> by a particular waste form. However, in its guidance document (USNRC, 1997) the USNRC notes that the performance of the waste form has very little effect on the containment of long-lived radionuclides (i.e., isotopes with half-lives greater than about 30 years). Unless special features are incorporated in the facility design, the USNRC recommends assuming that engineered barriers (including waste forms and containers) will physically degrade after 500 years. In the degraded condition, a barrier can still function to limit migration of contaminants, however, its performance will be based more on the chemical characteristics of its components, and not on the engineered structure. Due to the lack of long-term performance data, the PAs developed for DOE LLW disposal sites give only short-term credit to waste form performance in the algorithms used to determine exposure (DOE, 1998b).6 This approach introduces a high degree of conservatism into PA exposure calculations by incorporating radionuclide release rates that are almost certainly greater than would actually occur in a disposal facility. This in turn leads to a higher calculated dose and a shorter travel time to persons exposed through the water pathway. The IT Corporation (1993) performed a set of PA calculations for two generic MLLW disposal facilities, one located in a humid climate and the other in an arid region. This analysis showed the waste form can be expected to do little to reduce the annual dose that a critical group far in the future might receive from radionuclides with half-lives greater than 1,000 years. The principal factors that determine the integral radiological dose from long-lived nuclides are the total waste inventory and the site's hydrogeological and geochemical characteristics. The IT study found that sites in arid regions would release radionuclides at sufficiently low rates to be below DOE dose limits. The conclusion that the leach rate of long-lived radionuclides from a waste form is less important than site hydrogeological conditions in limiting exposure was also reached in the performance evaluation study conducted by Waters, et al. (1996). This study investigated the technical feasibility of disposing grout-stabilized mixed waste (other 6   The lack of long-term performance data for waste forms was confirmed by a literature search of relevant reports from waste form developers. The reports provided no quantitative longevity data that could serve as input to PA models.

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--> waste forms were not considered) at 15 DOE sites throughout the country using a simplified PA process. The study found that waste disposal facilities in arid climates with low infiltration rates, large depths to ground water, and low ground water velocities could accept large amounts of MLLW without exceeding DOE dose limits. In contrast, waste facilities in humid climates with high infiltration rates, shallow depths to ground water, and high ground water velocities could only accept low amounts of long-lived radionuclides that are soluble in water (i.e., 99Tc). The performance of the waste form had little effect on the analysis because it was assumed that extensive leaching would occur during the 10,000 year period covered in the analysis. Although the waste form was assumed to have decomposed by the time the integrity of the facility was lost, a sensitivity analysis performed by Waters, et al. (1996) showed that the retention of radionuclides by the residual components of the grouted waste form (i.e., by sorption) and the annual infiltration rate were the parameters that had the most significant effect on the dose from the water exposure pathway. Both studies (Waters, et al., 1996; IT Corporation, 1993) concluded that limiting the long-lived soluble radionuclides in waste disposal facilities in humid regions of the country may be necessary to reduce the long-term dose that might occur through the water exposure pathway. It is worth noting that in spite of the high degree of conservatism in the analysis conducted by Waters, et al. (1996), the analysis showed that most of EM's MLLW waste could be disposed of at existing DOE sites. Another report (Waters, et al., 1998) considered 6,250 m3 of treated MLLW identified as potentially problematic (i.e., containing radionuclides at concentrations potentially too high to permit disposal). This study determined that this problematic waste could in fact be disposed safely at arid sites.7 The waste forms assumed in the study were based on treatment plans developed by individual DOE sites. In this context, about 95% of the waste was in grout form, about 4% was vitrified, and the remainder was macroencapsulated in polymer. These modeling studies tend to confirm that disposal options for EM's mixed waste are not 7   About 1,800 m3 of this waste could be disposed at the Envirocare MLLW facility in Utah, and all but 100 m3 of the waste met waste acceptance criteria for the Hanford LLW disposal facility.

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--> limited by a need for new or better waste forms. This conclusion was also reached in the Waste Form Initiative Close Out Report (DOE, 1998b). Although RCRA does not require knowledge of the long-term performance of stabilized hazardous wastes, Franz, et al. (1994) applied a PA methodology to calculate releases of hazardous metals, including lead (Pb), chromium (Cr), and mercury (Hg) from a hypothetical generic underground disposal facility. The parameters used in the calculations were chosen to simulate releases from one generic facility in a humid environment and from one in an arid environment. The objective was to determine potential leaching criteria that might be appropriate for the release rate of RCRA constituents from a stabilized waste. The principal focus of the study was to evaluate the effect of a waste form's fractional release rate on the contaminant concentration at the boundary of the disposal unit. The fractional release rate is the fraction of the total mass of contaminants in the waste released in one year. Fractional release rates ranging from 10-8/yr to 10-2 /yr were used in the calculations. A conservative modeling approach was used that neglected the effects of solubility on contaminant concentrations, and did not consider sorption reactions by soil constituents. This model is conservative in the sense that the neglected effects reduce contaminant concentrations at the boundary, thus, neglecting them gives a worst-case scenario. The results of this analysis were that a fractional release rate of 10-5/yr is appropriate for RCRA metals as well as for long-lived radionuclides (Franz, et al., 1994). This study is perhaps the first to establish a numerical leaching criteria based on the modeled performance of a disposal facility containing RCRA hazardous constituents. However, the fact that it is based on generic site parameters introduces considerable uncertainty to the resulting fractional release rate. Nevertheless, it does give an order-of-magnitude estimate of the performance that might be required for stabilized wastes containing both hazardous metals and long-lived radionuclides. The study helped confirm that the fractional release rate, a parameter that is typically measured in waste form characterization tests, is a key parameter for assessing long-term performance of waste forms that contain both hazardous and radioactive wastes.

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--> Airborne Exposure Scenarios There are only a few radionuclides in the DOE inventory that can exist in the vapor phase at normal temperatures, notably radon, carbon-14 as 14CO2, and tritium in the form of tritiated water. While many RCRA wastes (such as solvents) are volatile, they are expected to be removed during waste treatment. If gases are present after treatment, their low viscosity will allow them to move freely through most waste forms considered for MLLW disposal. For these reasons, most PAs do not evaluate the ability of typical waste forms, such as those described in Chapter 4, to retain volatile constituents. The USNRC (1997) has developed guidance on developing PA analysis for gaseous releases from LLW disposal facilities. They are predicated on the containment of gases in high integrity containers rather than in typical waste forms. Two release scenarios are proposed: (1) all the containers simultaneously fail, resulting in a puff release; and (2) the entire inventory of 14C, 3H, 85Kr, 222Rn, and 129I in the disposal facility is available for release during the time period that is considered in the assessment (e.g., 1 year). Waters, et al., 1996, conducted a performance evaluation (PE) study to consider site constraints for waste disposal at 15 DOE sites throughout the country. This PE considered only 3H and 14C. The analysis used the approach for the PA documents written for LLW disposal facilities at Hanford, Idaho National Engineering and Environmental Laboratory, Nevada Test Site, Oak Ridge Reservation, and Savannah River Site. Volatile radionuclides were assumed to be transported to the soil surface by diffusion in the vapor phase, and then transported and dispersed in the atmosphere according to an analytical Gaussian dispersion model. A grouted waste form was assumed in this analysis, and for conservatism, it was further assumed that its diffusive properties were similar to those of the native soil. Therefore, no credit was taken for the waste form or the disposal facility's ability to reduce emanation of the volatile constituents. Furthermore, 100 years of retention in the disposal facility were assumed prior to release of the radionuclides. This provided sufficient time for most of the 3H (with a half-life of about 12.1 years) to decay. Based on the PE analysis, permissible amounts of 14C to be disposed in the facility would be limited by the atmospheric pathway

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--> at eight of the 15 sites, but there would be no limit for 3H because of its decay. The eight sites are located mainly in arid regions where low water infiltration rates reduce potential waterborne exposure scenarios. One factor not considered in the PE analysis was the interaction between the 14CO2 and the grout used to stabilize the waste. Aqueous leachates from cementitious grouts have high pH values that would increase the solubility of 14CO2 in the liquid phase by orders of magnitude so that much less than the amount calculated in the PE could become airborne. Furthermore, portland cement-based grouts are hygroscopic and would likely scavenge much of the tritiated water preventing its release to the atmosphere. Waters, et al. (1996) noted that even if these retention mechanisms are ignored and hence the model is overly conservative, atmospheric exposure scenarios do not limit the permissible concentrations of tritium and 14C for disposal of most LLW waste constituents. Intrusion Scenarios Inadvertent intrusion scenarios are constructed hypothetically as a means to estimate the level of protection that the disposal system would provide if it should be breached in the future due to a loss of societal memory regarding the nature of the waste. Intrusion scenarios leading to exposure to radioactive wastes are notoriously difficult to quantify, because they depend on future activities under conditions that are not possible to foresee with any degree of certainty. Intrusion scenarios are generally assumed to be inadvertent since access to the waste disposal facility is unintentional and results from some other activity, such as construction, excavation, or drilling. The assumption of inadvertent intrusion into a waste disposal facility was used in developing the waste classification system for near-surface disposal of LLW contained in 10CFR61 (i.e., Class A, B, and C wastes), discussed in Chapter 3. Inadvertent intrusion scenarios assume that the integrity of the waste form is compromised by excavation or drilling activities, and therefore no credit for contaminant immobilization is claimed. Waste release is immediate and no attenuation occurs. There are two important assumptions required to evaluate inadvertent intrusion scenarios: (1) the nature of the applicable scenarios,

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--> and (2) the time at which intrusion occurs. The applicable scenarios will depend primarily on the site conditions and facility design. There are at least five exposure pathways for MLLW: (1) inhalation of contaminated dust, (2) ingestion of contaminated soil, (3) ingestion of contaminated agricultural products, (4) dermal contact with contaminated soil, and (5) exposure to radiation from the radioactive waste materials. The possibility of burying waste deeper than a homesteader's expected excavation represents one possibility for minimizing intrusion. Placing the waste in a concrete vault or tumulus is another. The time at which inadvertent intrusion is assumed to occur will affect the remaining activity of short-lived radionuclides. DOE Order 5820.2A specifies that MLLW disposal facilities will be controlled for 100 years after closure. For the homesteader scenario, Waters, et al. (1996) assumed an inadvertent intrusion at 300 years following closure for a RCRA-compliant trench and 500 years for a concrete tumulus. An inadvertent intrusion at 100 years following closure was assumed for the post-closure drilling scenario. Due to its great depth, the inadvertent human intrusion scenarios for the Waste Isolation Pilot Plant (WIPP) are all predicated on future drilling of a borehole while exploring for mineral resources (Rechard, 1995). In reviewing progress on the WIPP, a National Research Council (NRC) committee observed that it would be unfortunate if such intrusion scenarios, made without a scientific basis, resulted in disqualification of the site (NRC, 1996b). The role of the waste form in inadvertent intrusion scenarios has not been extensively considered to date since no credit is usually taken for it. Conventional scenarios rely more on the facility location and design to prevent future exposure than on the waste form itself. The PE analysis by Waters, et al. (1996) assumed that the waste form would be indistinguishable from the native soil at the time of inadvertent intrusion and therefore accidental post-closure intrusion scenarios took no credit for the waste form. While this may be reasonable for a grout stabilized waste form, it may not be likely for a vitrified waste form, which should be much more durable. In fact, one scenario has been proposed in which an intruder encounters MLLW waste that has been stabilized as vitrified glass beads. The beads are so attractive that they are mined and subsequently incorporated in jewelry, possibly resulting in large doses of gamma radiation to the wearers. As a specific example, Pohl, et al.

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--> (1996) developed a method of estimating the exposure risk associated with this pathway. Waters, et al. (1996) noted that inadvertent human intrusion scenarios for PA calculations at LLW disposal facilities tend to be the most restrictive of the standard exposure scenarios evaluated. This finding was confirmed in their PE analysis of potential mixed waste disposal at 15 DOE sites throughout the country. This is particularly important for long-term, chronic exposure since a homesteader may encounter the radioactive waste materials over a long period of time and through a variety of different mechanisms, including inhalation, ingestion, and external exposure. At many sites, particularly arid sites, the inadvertent intrusion scenarios provide the most restrictive permissible waste concentrations for most radionuclides, because airborne and waterborne pathways are not significant (Waters, et al., 1996). Findings, Discussion, and Recommendations PA is required by the DOE and the USNRC to evaluate the safety of radioactive waste disposal systems for future generations and the environment. The committee undertook to determine the role of the waste form in PA. The committee found that current PA models do not take significant credit for the waste form's ability to reduce the release rate of hazardous and radioactive constituents. This is mainly because of the lack of quantitative long-term release data that can be used in PAs and results in a conservative perspective with respect to the release of contaminants from a prospective disposal facility. More realistic assessments may allow more effective use of the capacity of disposal facilities by allowing them to accept a larger inventory of radionuclides or hazardous wastes. Committee findings include the following: EPA regulations that include only short-term performance of the waste form (e.g., passing the Toxicity Characteristic Leaching Procedure (TCLP)] and the USNRC requirement for PA to extend beyond the useful life of waste forms both de-emphasize

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--> the role of the waste form in limiting the long-term release of waste constituents. Radioactive waste inventories allowed in USNRC-licensed facilities are limited by PA calculation of exposures that result from future intrusions into disposal facilities (intrusion scenarios). These calculations generally do not take credit for the waste form as a barrier. Current EPA regulations require that the waste form meet short-term prescriptive criteria, such as the TCLP. USNRC regulations require PA evaluations that extend well beyond the expected useful life of any waste form. Taken together, these regulations de-emphasize the role of the waste form in limiting the long-term release of waste constituents. Between these extremes, however, the waste form can be expected to play a very important role, including near total confinement of the most common intermediate-lived radionuclides 137Cs and 90Sr, and very gradual release of long-lived radionuclides (such as 99Tc) and hazardous waste constituents.8 Performance assessment is part of the regulatory process. If a PA is to take credit for the waste form, the waste form's long-term performance must be described well enough that the PA can withstand regulatory review. As discussed in Chapter 5, current understanding of long-term performance of the waste form is not sufficient for extrapolations over thousands of years. The limited role of waste forms in PA is not necessarily a failure of waste form development, but rather that the present understanding of their long-term performance is inadequate. The committee believes that the credibility of performance assessments can be enhanced by better representation of the waste form's behavior in the disposal environment. Because performance assessment is a general requirement for disposal of essentially all types of DOE waste, not only mixed waste, the committee addresses the following recommendations to the Office of Science and Technology (OST, EM-50). 8   137Cs and 90Sr radioactive have half-lives of approximately 30 years, and their decay will effectively remove them from a waste inventory in 300–500 years. For practical purposes, long-lived nuclides like 99Tc and many hazardous waste constituents, such as the heavy metals, will never decay.

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--> OST should support efforts to obtain data that will allow a more realistic inclusion of waste forms in PA models, including intrusion scenarios. Without such data the waste form will never receive proper credit in PA with the resulting cost penalties for additional engineered barriers and possible restriction in site selection. OST should play a more significant role in promoting (funding) cooperation among investigators who are characterizing waste forms and those who are developing PA models. This will help ensure that characterization data are useful for PA models, and that PA models properly incorporate this data.