1
Introduction

The Committee on Technologies for Cleanup of High-Level Waste in Tanks was one of several committees formed at the request of the Assistant Secretary for Environmental Management, U.S. Department of Energy (DOE). The task of this committee was to provide independent review and recommendations to the office of Environmental Management (EM) on scientific and technical issues relevant to technology development to accomplish the environmental management of the DOE nuclear weapons complex (Appendix C). It is the purpose of this committee to provide findings, conclusions, and recommendations on an approach for identifying technology development needs for remediating the radioactive waste, especially the high-level waste (HLW) in large underground tanks in the DOE nuclear weapons complex. This report is based on information gathered through mid-1998. The scope of the study includes the wastes in the tanks and the final disposition of the tanks. Much of the waste is from development and implementation of separation processes associated with plutonium and tritium production.

The thrust of the committee's report is primarily to propose a methodology for identifying technology requirements to remediate stored tank wastes, and a limited example based on the HLW in the Hanford Site tanks (located near Richland, Washington) is presented to illustrate the approach. Currently, technology needs are formulated at each of the four DOE sites through their respective Site Technology Coordination Groups.1 While the technology needs are carefully screened, there is little evidence that the process is based on a clear-cut systems approach. The committee recommends a systematic and transparent generic approach that is considered applicable to any of the waste tank farms throughout the DOE nuclear weapons complex. In fact, the approach is widely applicable to many EM problems when there are significant uncertainties in the end state of the waste streams and technology development is needed for remediation. The approach involves the analysis of end state based remediation scenarios to highlight the technology needs to achieve specified goals. For this report, a scenario is defined as a qualitative description of the transition path of waste from its initial state to a specified end state.2

1  

The Site Technology Coordination Group (STCG) is a group consisting of stakeholders, technology users, and DOE representatives at each specific site. The group is responsible for coordinating regulatory and stakeholder interactions at each tank site and facilitating interactions among these groups and the Tank Focus Area.

2  

This definition of a scenario is more general than the one used in the probabilistic risk assessment (PRA) field. In particular, PRA models generally consider a scenario as a single path (initiating event to an end state) through a multi-branched event tree.



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--> 1 Introduction The Committee on Technologies for Cleanup of High-Level Waste in Tanks was one of several committees formed at the request of the Assistant Secretary for Environmental Management, U.S. Department of Energy (DOE). The task of this committee was to provide independent review and recommendations to the office of Environmental Management (EM) on scientific and technical issues relevant to technology development to accomplish the environmental management of the DOE nuclear weapons complex (Appendix C). It is the purpose of this committee to provide findings, conclusions, and recommendations on an approach for identifying technology development needs for remediating the radioactive waste, especially the high-level waste (HLW) in large underground tanks in the DOE nuclear weapons complex. This report is based on information gathered through mid-1998. The scope of the study includes the wastes in the tanks and the final disposition of the tanks. Much of the waste is from development and implementation of separation processes associated with plutonium and tritium production. The thrust of the committee's report is primarily to propose a methodology for identifying technology requirements to remediate stored tank wastes, and a limited example based on the HLW in the Hanford Site tanks (located near Richland, Washington) is presented to illustrate the approach. Currently, technology needs are formulated at each of the four DOE sites through their respective Site Technology Coordination Groups.1 While the technology needs are carefully screened, there is little evidence that the process is based on a clear-cut systems approach. The committee recommends a systematic and transparent generic approach that is considered applicable to any of the waste tank farms throughout the DOE nuclear weapons complex. In fact, the approach is widely applicable to many EM problems when there are significant uncertainties in the end state of the waste streams and technology development is needed for remediation. The approach involves the analysis of end state based remediation scenarios to highlight the technology needs to achieve specified goals. For this report, a scenario is defined as a qualitative description of the transition path of waste from its initial state to a specified end state.2 1   The Site Technology Coordination Group (STCG) is a group consisting of stakeholders, technology users, and DOE representatives at each specific site. The group is responsible for coordinating regulatory and stakeholder interactions at each tank site and facilitating interactions among these groups and the Tank Focus Area. 2   This definition of a scenario is more general than the one used in the probabilistic risk assessment (PRA) field. In particular, PRA models generally consider a scenario as a single path (initiating event to an end state) through a multi-branched event tree.

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--> The approach has been termed end state based because it is driven by the desired final state of the waste stream under consideration. An end state is defined as the final product of a waste processing, remediation, or management scenario characterized well enough in terms of chemical, physical, and radioactive attributes to allow details of scenarios to be specified. For example, in the case of waste in large underground tanks there are three principal potential categories of final waste end states; (1) immobilized HLW, (2) immobilized low-activity waste (LAW), and (3) residual radioactive materials contained and stabilized in the tank farms (i.e., the tanks themselves and surrounding contaminated soils). The committee believes that scenarios to achieve predefined end states for the waste are an effective framework in which to consider technology development needs for the implementation of cleanup operations. End states are defined in terms of planned composition, configuration, performance, and location of a particular waste at the completion of a major phase of processing and management activities. Consequently, end states play a major role in the findings, conclusions, and recommendations of this study. High-Level Waste Tanks The tanks considered in this study are large underground storage tanks that contain over 100 million gallons (380,000 m3) of radioactive wastes in 278 individual tanks distributed among four DOE sites. These sites are the Hanford Site near Richland, Washington; the Savannah River Site near Aiken, South Carolina; the Idaho National Engineering and Environmental Laboratory (INEEL) near Idaho Falls, Idaho; and the Oak Ridge National Laboratory (ORNL) near Oak Ridge, Tennessee (Figure 1). Tanks at the demonstration project site in West Valley, New York, are owned by the state of New York and, thus, are not discussed in this report. Of the 278 tanks in the complex, 177 are at Hanford and contain over 60 percent by volume of the DOE tank waste (U.S. Department of Energy, 1995). By comparison, the Savannah River Site tanks contain about 36 percent by volume of the waste, while the INEEL and ORNL tanks together contain less than 3 percent. Not all the tanks contain waste designated as HLW (e.g., the tanks at Oak Ridge), but all contain radioactive wastes of varying characteristics and amounts. The tanks also vary in design, but they can generally be described as single-shell or double-shell, the double-shell tanks having an annular space between an inner and outer steel tank shell (see Figure 2). Both single-shell and double-shell tanks are found at Hanford. Table 1 describes the tanks by site. Not all underground waste storage tanks are included in this study. For example, not listed in Table 1 under the Hanford Site are 63 tanks classified as miscellaneous underground storage tanks (MUSTs), either used in the past or currently being used for a variety of purposes. Eighteen of these tanks are in active use and 45 are inactive (Hanlon, 1998). These tanks vary in capacity from 900 gallons (3.4 m3) to 50,000 gallons (190 m3) and are part of the Hanford tank waste system. The tank wastes at the four DOE sites differ in quantity, radioactivity level, storage mode, originating process, chemical composition, and physical attributes. The non-sodium-bearing liquid wastes in the INEEL tanks remained acidic prior to being calcined and stored as solid granules, whereas most of the tanks at other sites contain wastes that were originally acidic solutions but were stored in the tanks in a strong caustic (i.e., high pH) medium. The caustic wastes are composed of solids associated with the supernatant liquid. At some sites, wastes were processed to concentrate solutions and reduce volumes, resulting in precipitated solid components of the waste also being present in tanks. Processing of the original waste at some

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--> Figure 1. DOE Tank Sites Table 1 Waste Tanks in the DOE EM Remediation Area Program by Siteb Site Number Of Tanks Type Of Tanks Waste Volume, million gal (m3) Activity, million Ci Hanford 177 Single and double-shell 54 (203,000) 198 INEEL 11 Single-shell 2 (7,600) 2 {calcine bin sets} {7}   {1 (3,800)} {50} ORNL 34 Single and double-shell 0.6 (2,300) 0.2 Savannah River Site 49a Double-shell 33 (125,000) 534 Total 278   92 (350,000) 784 NOTES: INEEL = Idaho National Engineering and Environmental Laboratory; ORNL = Oak Ridge National Laboratory. a Two tanks at the Savannah River Site were closed recently, leaving 49 tanks to be remediated and the wastes vitrified. b List does not include many small underground storage and process tanks at the sites. Sources: Pacific Northwest National Laboratory (1998); for Hanford Site, Hanlon (1998).

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--> Figure 2. Two Basic Types of Hanford Tanks—Above, a Single-Shell Tank; Below, a Double-Shell Tank

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--> sites removed selected components by processes different from those that produced the original waste.3 The resulting tank wastes are a heterogeneous mixture of solutions, sludges, saltcakes, and other phases. Many of the tanks have exceeded their design life, and some have leaked, contaminating the soil surrounding the tanks, and possibly the ground water. Since the Hanford tanks have been selected for an illustrative application of the committee's proposed methodology for identifying technology development needs, a brief description of these tanks follows. The Hanford tanks contain components of the waste streams from the chemical separations processes and various radionuclide recovery processes that took place between 1943 and 1989. The wastes are currently stored in 177 underground tanks in the 200 Areas of the Hanford Site. There are 149 single-shell tanks and 28 double-shell tanks (Figure 2). The 149 single-shell tanks constructed between 1943 and 1964 are made from reinforced concrete with carbon steel liners. The single-shell tanks vary in capacity from 210 to 3,800 m3 (55,000 to 1 million gallons) and currently contain approximately 134,000 m3 (35 million gallons) of saltcake, sludge, and liquid. Since 1956, 67 single-shell tanks have leaked waste into the surrounding soil, or are suspected to have leaked. Sixty-three have been interim stabilized (i.e., supernatant and interstitial liquid has been pumped from the tank solids), and the remaining four have had most of the supernate removed to limit further leakage. It is estimated that a total of about 3,800 m3 (1 million gallons) of tank waste has leaked to the surrounding soil. No waste has been added to the single-shell tanks since 1980 (Hanlon, 1998). The 28 double-shell tanks were constructed between 1968 and 1986, with the first being placed in service in 1971. Double-shell tanks consist of a carbon steel primary inner tank, an annular space, and a secondary steel outer tank encased in reinforced concrete. These tanks have a capacity of 3,800 m3 to 4,400 m3 (1 to 1.16 million gallons). The double-shell tanks currently contain about 69,000 m 3 (18 million gallons) of waste, mostly in liquid (slurry) form, although there are some sludges and saltcakes. There is no evidence that any of the double-shell tanks have leaked from the primary inner tank. The total radioactivity content of the Hanford tanks is approximately 198 million curies, of which two-thirds are in the tank solids. The main sources of radioactivity of the waste are cesium-137 and strontium-90 and their decay products. The chemical constituents of the solids are mostly precipitated hydrated oxides of iron, aluminum, and other metals. Saltcake is primarily sodium nitrate and nitrite, and the supernatant liquid contains large amounts of dissolved sodium salts, especially nitrates, nitrites and hydroxides. Tank Waste Remediation Technology Development Program Beginning in fiscal year 1994, EM reorganized its program on technology development for the remediation of tank waste. This reorganization was to provide enhanced coordination of technology development under the EM Office of Science and Technology (EM-OST) and to be relevant to all DOE sites (Hanford, ORNL, Savannah River, and INEEL) that are charged with remediation of tank waste. The new program management system involved the Richland Operations Office as the lead field office to coordinate activities at the sites through the Tank Focus Area (TFA) management team and the Site Technology Coordination Groups (STCG). Representatives from several national laboratories, academia, and industry participate in TFA technology development activities. The thrust of the technology development program is to 3   For example, at Hanford in the 1960s and 1970s, cesium and strontium were recovered from the tank waste using organic compounds (e.g., salts of citric acid); the waste was then returned to the tanks (Gephart and Lundgren, 1997).

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--> provide needed technologies to overcome the technical obstacles that jeopardize tank waste remediation performance and compliance and to reduce costs. Briefly, the TFA program development process is initiated by formulation of technology needs and local priorities at each of the four sites through their STCGs. The technology needs are screened, justified, evaluated, reviewed, and cast into a problem element structure similar to a work breakdown structure commonly used for large projects. The problem element structure has been applied to a generic tank waste remediation flowsheet (Pacific Northwest National Laboratory, 1997b) that is broadly applicable to the four sites. The relevance of proposed site-specific technology development to the problem element structure allows coordination of technology development among the DOE sites and with participating programs from industry and universities, along with EM cross-cutting programs and other user programs. The committee did not see evidence of a focus on end states as the EM technology development programs were constructed or justified. In particular, while the technology development program has led to such projects as the Hanford Tank Initiative (HTI) that involve some coordination of technology development, there is still little evidence that the technology needs assessment is based on a transparent systems approach. The TFA has also enlisted other parts of the DOE EM organization in its program development activities. At the behest of Congress (Public Law 104-46, 1995), DOE established a basic science program, called the Environmental Management Science Program (EMSP), to support the technology development activities. The program was initiated to provide basic information needed to reduce the cost of implementing waste remediation. For example, EMSP is supporting research to understand the movement of contaminants in the vadose zone. Layout of the Report The report layout generally conforms to the end state based approach for identifying technology development needs. The end state based approach, a principal product of the committee's study, is presented in Chapter 2. This is followed in Chapter 3 by consideration of the Hanford Site tank waste remediation in terms of developing illustrative end states and functional flowsheets. These functional flowsheets and end states become the basis for assessing technology development needs of important function process steps selected by the committee. In Chapter 4, technology needs for selected functions of the chosen scenarios are briefly assessed, using the Hanford tanks as an example. Conclusions and recommendations are provided in Chapter 5.