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6 Tank Closure and Other Long-Term issues It has long been considered impractical to dismantle and remove HLW tanks completely once they have been emptied because of the radiation exposure incurred by workers from radioactive residues and because of the overall prohibitive costs. Therefore, the tanks must be closed and left on site. To do so, the DOE must demonstrate, according to state and federal regulations (see Sidebar 6.1 ), that waste residues in the tanks can be declassified from HLW to "waste incidental to repro- cessing." Before proceeding to tank closure, the DOE must adequately remove HLW from the tanks and from connecting pipelines, and must characterize and then properly immobilize residues within the tanks. Of the 239 tanks distributed among DOE HLW sites, only two tanks have been closed so far at the SRS. After tank closure, it is necessary to ensure that residual waste does not leave the tank boundaries and pose unacceptable environmental risks in the future. The area surrounding the tanks (the near field) must be monitored in the long term after tank closure to allow immediate remedial action in case residual waste releases threaten plants, animals, or humans. Radionuclides of primary concern for the environment, because of their long half-lives and their mobility, are technetium-99, selenium-79, iodine-129, carbon-14, and the actinides uranium, neptunium, and plutonium. Migration of conta- minants into the environment can be controlled with the use of engi- neered barriers around the tanks. In the committee's opinion, HLW cleanup problems have been too rigidly compartmentalized into tank problems and subsurface contami- nation problems, with the critical transition from the tank wall to the far field being ignored. Therefore, the committee addresses both tank clo- sure issues and subsurface contamination problems in the near-field of the tanks, although the latter is not in the committee's statement of task. T a n k C I o s u r e a n d 0 t h e r L o n 9 - T e r m I s s u e s 65

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SIDEBAR 6.1 WHAT IS ADEQUATE RETRIEVAL OF HLW? High-level radioactive waste is defined by its origin from the reprocessing of spent reactor fuel.When HLW is retrieved from any storage system for disposal, there will be some radioactive residue in the storage system. Criteria have been developed for use in determining that these residues are not HLW, but Wl R.The WIR criteria are presented in DOE G 435.1-1, Implementation Guide for use with DOE M 435.1 (DOE, 1999). Briefly stated, the WIR criteria are as follows: 1. The waste has been processed (or will be further processed) to remove key radionuclides to the maximum extent that is technically and economically practical. 2. The waste is to be managed, pursuant to DOE's authority under the US Atomic Energy Act, so that safety requirements comparable to the performance objectives in 10 CFR Part 61, Subpart C, are satisfied.* 3. The waste will be incorporated in a solid form at a concentration that does not exceed the applic- able concentration limits for Class C LLW as established in 10 CFR Part 61.55.t The action guided by these criteria is to determine when HLW has been removed adequately so that residues may be treated as other than HLW and be disposed of on-site. Further details can be found in a previous NRC report (NRC, 2000b). *Title 10 CFR Part 61 describes licensing requirements for classification of LLW sent to land disposal facili- ties. Subpart C describes the performance objectives of such waste. ITitle 10 CFR Part 61.55 describes technical requirements for land disposal facilities.The regulation lists the upper limits of concentration for long-lived radionuclides and for other radionuclides,which constitute the limits for Class C LLW. ·eeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeee The issues discussed in this chapter are the following: removal of H LW from the tan ks; removal of H LW from piper i nes; characterization of residual waste; immobilization of residual waste within the tank; near-field monitoring after tank closure; and near-field containment barriers. The issues of near-field monitoring and subsurface barriers are also addressed (often in greater detail) in other NRC reports (NRC, 1 996c, 1 997b, 2000a, 2000b, 2000d). H ~ G H - L E V E E W A S T E 66

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Tank Closure and Other Long-Term issues During its tour of the Hanford Site and of the SRS, and after discus- sion with site personnel, the committee identified research needs relat- ed to tank closure and other long-term issues. The objective of the rec- ommended long-term basic research for tank closure (and other long- term issues) is to provide the scientific basis to develop innovative methods to achieve tank closure and non-invasive monitoring of the near-field areas, as well as improved containment barriers. Removal of H[W from the Tanks Because most of the HLW is retrieved from the tanks, the residual waste will likely consist of solids firmly attached to the tank surfaces as a "crust" or "hard-heel." These residual wastes present a more difficult challenge to retrieve than the bulk of the waste. First, hard-heel materi- als require very aggressive techniques for removal and transport, such as pulsed-action sprays, chemical dissolution approaches, and mechan- ical scraping technologies. The use of these processes could damage the tanks, allowing their contents to leak into the subsurface. Second, it is necessary to minimize water usage to limit secondary waste and tank leakages. Third, residual waste in tanks can be very difficult to access, since the interiors of some of the tanks are encumbered with pipes, in- tank equipment, or cooling coils. In some DSTs, waste has leaked into the "annulus," the confined space between the primary liners and the secondary containment pans. Access and removal of waste that has leaked into the annulus present a serious technological challenge (see Figure 6.1 ). tong-Term Research Needs Long-term basic research is needed to identify methods for the ade- quate removal of HLW from tank surfaces and from the annulus. New techniques should reduce the risk of secondary waste leakage and could use semiautonomous retrieval methods. The use of untethered robots is an example of a highly desirable basic research field that cou Id lead to effective sol utions where conventional engi neeri ng approaches often involve prohibitive costs (see Sidebar 6.2~. Such robots could assist in retrieval operations by removing blockages, so that areas can be pumped, chipping off hard patches of HLW or round- ing up pieces of equipment for possible retrieval and reuse. T a n k C I o s u r e a n d O t h e r L o n 9 - T e r m I s s u e s

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FIGURE 6. 1 HEW leak in the confined space between the primary liner and secondary pan, also called annulus, of Tank 16 at the SRS. The dis- tance between the primary liner and the secondary pan in this type of tank (type 11) varies between 2 and 6 feet. The secondarypan is only 5 feet high. SOURCE: DOE- Tanks Focus Area. Removal of H[W from Pipelines In addition to the waste in the tanks, the Hanford Site must also clean up waste from a maze of underground pipes. Some of the pipelines have also been plugged for a long time due to particle set- tling, phase changes, or reactions accompanied by precipitation or gel formation that occurred during transport. According to the TEA, there is also an increasing potential that the transfer lines currently in place will become plugged (DOE-TFA, 2000b). Currently, operations involving the cleanup of pipelines often involve manual intervention; hence they are extremely expensive. As recognized by the TEA, methods are needed to accurately locate pipelines and blockages and to unplug lines with devices that will not cause damage. Many of the activities related to pipeline cleanup belong to the province of applied research and engi- H ~ G H - L E V E E W A S T E 68

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SIDEBAR 6.2 EXAMPLE OF INNOVATIVE BASIC RESEARCH NEEDED TO AID IN TANK CLOSURE: UNTETHERED SEMIAUTONOMOUS METHODS A tether from a robot to a controller can be a severe limitation in areas with restricted access. For example, the National Aeronautics and Space Administration (NASA) has had considerable experience with robots in severe environments. In 1992, NASA tried to send a tethered robot into a volcano crater in Antarctica. It experienced numerous problems with the connecting cable. Eventually, the entire mis- sion was lost when the connecting fiber-optic cable broke (San Francisco Chronicle, 1 993).The environ- ment in an HEW tank is also severe, and access is also greatly restricted. An untethered robot, which could remove blockages in a tank or pipeline, retrieve lost equipment, and remove hot spots, could be of great value. Furthermore, an untethered robot would greatly simplify the decontamination require- ments after retrieval. Untethered semiautonomous robots could have many applications in the domain of HLW. For example, it may be possible to send such robots through piping or into processing vessels to monitor routine operations, locate problems in the walls of the pipe or vessel, and locate dangerous chemical condi- tions within the processing system. Another possible application for these robots is the investigation of soils surrounding the tanks. A burrowing untethered semiautonomous robot could sample soil chemistry and image the soil in front of it. The use of untethered semiautonomous devices in tanks could lead to a more complete sampling of the tank contents and to a significant decrease of radiation exposure to workers, costs, risk of unex- pected buildup of hazardous conditions, and turnaround time for analytical data. Some desirable features of untethered semiautonomous tank investigation and sampling are the fol- lowing: 1 ) relatively low-data-rate, low-frequency, electromagnetic or seismic telemetry system for con- trol of the robot and for transmitting processed data outside the tank to a distant operator; 2) wide variety of sensors installed for electrical, electromagnetic, acoustic imaging, and mea- surements of chemical and mechanical properties; 3) novel means of movement, including the ability to crawl along tank walls, move over the fluid surface, travel through the fluid, and possibly burrow through consolidated materials. For instance, such movement could be accomplished through innovative propulsion means on the robot or through manipulation of a surrounding magnetic field; 4) novel power supplies, including possibly exterior power supplied remotely to the robot via magnetic or electric fields without any wire link; and 5) small size and resistance to intense radiation and corrosive chemicals. Conventional engineering solutions for untethered semiautonomous tools involve budgets well beyond EMSP's budget. Therefore, this is an example where highly innovative basic research is needed to develop completely new and cost-effective solutions. ·eeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeee T a n k C I o s u r e a n d O t h e r L o n 9 - T e r m I s s u e s 69

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peering. However, basic research needs have been identified in two domains: (1 ) location of the pipelines, and (2) identification of block- ages and removal: 1. Location of pipelines. The exact location of underground o pipelines Is often u~tt~cuit to determine, particularly in the case of stacked pipelines. Currently, a severe limitation, also faced by other commercial organizations (e.g., utilities), is that there are no technologies available to accurately locate stacked pipelines. Of course, maps with the approximate location of each pipeline are available. However, at DOE sites, lines are still often located and cleared from the surrounding soil by hand digging. Automated digging is limited because oinelines cannot be locat- ed precisely enough. 2. Identification of blockages and removal. Removal of waste in plugged piper ines is a further chal lenge. First, it is necessary to locate the blockage. Then, the blockage must be removed with- out damaging the pipeline. Current methods to remove waste plugs involve chemical, pressure cycling, and vibration methods (DOE-TFA, 2000b). However, these methods require applying mechanical or corrosion stress to the equipment. tong-Term Research Needs Long-term basic research is needed in methods for remotely imaging the precise location of pipelines, particularly in soils, which significant- ly absorb or scatter the remote sensing fields. Because commercial companies also share interest in this problem, there is a great potential for dual-use technology. Seismic or electric techniques could be used for remotely locating pipeline blockages. Research on the use of unteth- ered robots or other semiautonomous devices to remove blockages is also highly desirable to reduce workers' exposure. Characterization of Residual Waste To meet closure requirements there must be a determination that residual waste is WIR (see Sidebar 6.1~. Characterization of the remain- ing radioactivity (i.e., radionuclides and their chemical form) in the tank is necessary to make this determination. Detection of residual alpha-emitting transuranic elements with half-lives greater than five years and a quantitative estimate of residual inventory are particularly important. Table 6.1 shows the radionuclide inventories in the two tanks that have been closed at the SRS (tanks 17 and 20~. The main radionuclides remaining in the tanks are strontium-90, cesium-137, technetium-99, and cobalt-60. The radioactivity levels for the main residual materials in an empty waste tank are in the order of tens to H ~ G H - L E V E E W A S T E

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TABLE 6.1 SRS Racionuc~ice Waste Inventories in Tanks 17 anc 20 Radionuclide SRS Tank 1 7 inventorya Curies per 1,000 gallons SRS Tank 20 inventoryb Curies per 1,000 gallons Strontium-90 379 190 Plutonium-238 29.6 8.3 Cesium-137 26.2 40.9 Americium-241 17.9 1.7 Plutonium-239 6.8 3.5 Technetium-99 1.6 0.8 Plutonium-240 1.5 0.8 Cobalt-60 1.0 0.7 NOTE: Curie inventories are estimated on the basis of 1,000 gallons of residual waste. 1 curie per 1,000 gallons = 1 curie per 3,785 liters or approximately 10 million becquerels per liter. aD'Entremont et al. (1 997a) bD'Entremont and Hester (1 997b) hundreds of curies per 1,000 gallons (108 to 109 becquerels per liter) of residual materials. The major challenge is that characterization methods must operate in high gamma fields. For instance, in the case of the WVDP tank cleanup operation, the gamma field is about 10 red per hour on tank walls and infrastructures and about 800 red per hour on the bottom of the tank after the water shield is removed (DOE-TFA, 2000b). tong-Term Research Needs Long-term basic research on methods for the characterization of tanks, crusts, or heels is necessary to determine their chemical and radionuclide (alpha, gamma, and beta emitters) contents. Research is needed on remote sensing and analytical methods to determine the chemical and radiological composition of residual waste (see also Chapter 3~. Research is particularly needed for gamma-ray spectrometry and neutron counting to survey in-tank transuranic elements and gross beta counting for strontium-90 in strong gamma fields. Research on methods that do not involve in situ sampling is desirable. Consideration should also be given to research on untethered semiautonomous char- acterization tools. Immobilization of Residual Waste within the Tank Grouting is the current baseline technique to fill the volume before closure. It provides isolation against water ingression from the surface T a n k C I o s u r e a n d 0 t h e r L o n 9 - T e r m I s s u e s

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and immobilization of residual waste. Grout consists of inert materials such as sand and gravel, with a cement-based binder. In the two closed tanks at the SRS, the three-layered backfill system used consists of a chemically reducing grout at the bottom of the tank, a controlled ~ow- strength material occupying most of the empty space, and a high- strength grout at the top of the tank. The primary function of the chemi- cally reducing grout is to reduce the mobility oftechnetium-99, the main environmental risk contributor at the SRS. Technetium-99 is much less mobile under reducing conditions. Other near-field containment techniques for immobilizing radionu- clides include in situ vitrification, solidification and stabilization, in situ redox manipulation, and bioremediation. These techniques are described in detail in a previous NRC report (NRC, 1 999d). . . . . . . . . . tong-Term Research Need After some discussion, the committee decided that this is not one of the most fertile topics for basic research because applied research and engineering activities undertaken within the TEA and the Subsurface Contaminants Focus Area are already addressing this technological need. Near-FieItl Monitoring In this report, the near field includes the tank boundaries and extends up to 100 meters into the subsurface. Monitoring techniques for the near-field and the far-field subsurface are similar. However, spe- cial requirements exist for the near-field monitoring of HEW tanks. For example, the area surrounding the tanks is a very disturbed environ- ment, because pumps and other rotating machinery produce significant seismic noise. Also, the presence of metal on the earth's surface leads to coupling interference with electrical geophysics survey methods. Near-field monitoring of the subsurface is important to identify movements of fluids that might indicate tank leakage. This is key during waste processing operations, as described in Chapter 3. However, after tank closure, long-term monitoring of the near-field is still important to verify that residual waste does not traverse tank boundaries and reach the subsurface. Therefore, tank boundaries must be monitored to detect corrosion, cracks, and other signs of potential loss of waste contain- ment. When tanks have leaked, near-field monitoring is particularly dif- ficult because of preexisting contamination in the subsurface. Drilling technologies are currently employed for near-field subsur- face monitoring. However, drilling wells in potentially contaminated sites is extraordinarily expensive. Current costs for drilling a monitoring well to a depth of 75 meters in low-to-moderately contaminated soils H ~ G H - L E V E E W A S T E 72

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are typically $210,000 per well.1 The cost for developing and drilling a specialized sampling well in a highly contaminated soil, such as a 55- meter slant well (30 degrees) under the leaking tank SX-108 in Hanford, . , , ^, .... . . . . v Is about >~.5 moon, wrack noes not include sample analysis.2 Moreover, drilling wells in a potentially contaminated region may cre- ate new pathways for the flow of contaminants and water intrusion, and generate large quantities of contaminated soil that must be treated. A further complication in the use of drilling technologies is the het- erogeneity of the earth's subsurface, as shown in Figure 6.2. To sample this heterogeneity adequately, numerous wells would have to be drilled close together, with the disadvantages cited above. Push technologies have recently become a popular alternative to core drilling. They con- sist of pushing a rod, equipped with various sensors, into the earth. The advantages of push technologies are that costs and chances of spread- ing the contamination are lower than with core drilling. However, push technologies do not penetrate to sufficient depth in soil containing large cobbles or rock formations. Furthermore, they supply only point mea- surements and do not provide the detail shown in Figure 6.2, obtained with alternative geophysical methods. Imaging techniques are also becoming increasingly popular to study movements of fluids in the subsurface. Currently, gamma surveys in dry boreholes as well as seismic and electrical borehole-to-borehole meth- ods are used to produce reasonably detailed images of the ground between two wel Is. However, non-i evasive h igh-resol ution methods for surveying the earth from the surface need further development. A dis- cussion of tomography approaches and other promising subsurface imaging techniques can be found in previous NRC reports (NRC, 1 996d, 2000a, 2000e). tong-Term Research Needs Long-term basic research is needed to develop non-invasive meth- ods to characterize the geohydrology and also the nature, movement, and projected fate of contaminants near the tanks. Research in imaging techniques is desirable in order to develop methods that penetrate soil in depth and can identify contaminants. For example, prompt gamma neutron activation is a promising technique but has a penetration range of only one meter or less, depending on the sensitivity required. Imaging tools can also be used to monitor the state of the tank bound- ~Personal communication between committee member B.K. Sternberg and G. Mitchem, Bechtel, Hanford, Washington. 2Personal communication between committee member B.K. Sternberg and T. Knepp, CH2M-HILL, Hanford, Washington. T a n k C I o s u r e a n d O t h e r L o n 9 - T e r m I s s u e s

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of ~= ~ cO ~ ~w ~ ED an ~0 go w ~ 1 Fin W ~ ~ W ~ ~ a3 1 Cad C" can 1 '''''' " to 2 4 6 8 10 set ~ 1 XS ~ ~ 1 31 0 2 4 B 8 10 12 :C.~ 83 ~2 I xS3, I X&i Can W Bed con cO ~3 12 300 4060 500t Velocily (IVUS) k~ 0 2 4 B 8 10 12 1 . . . . . . l!; a ~ . .. . . . .~ .se . . . . . . . .c" .~ w on c" 91 It on ~ ~ , , ,~. ~ 0 2 4 6 8 10 12 0.1 0.2 0.3 0A 0.5 Atlenualion (DBIM) FIGURE 6.2 Representative cross-section of the earth between two wells (TAN-3 1 and TAN-8) at the INEEL site, showing typical heterogeneity. The cross-section image was constructed by transmitting a seismic (acoustic) signal from a fixed depth in the left well and receiving the seismic signal at many depths in the right well. The transmitter was then lowered to a new depth, and the signal was again received at many depths in the other well. The process is repeated for many trans- mitter locations until a large number of ray paths are measured through the earth. Data are analyzed by calculating the velocity and attenuation of the seismic signal through each cell in a grid of cells between the two wells. The resulting pic- ture of the lateral and depth variations in velocity and attenuation can be interpreted as variations in soil (or rock) types, fracturing, and fluid movement. This cross-section image shows how complex typical soil variations are. In this black and white reproduction of the original color figure, the velocity and attenuation scales do not fully show the complete range of values (for example, black may indicate either high or low velocity/attenuation, while white represents the intermediate values). However, the purpose of this figure is only to demonstrate soil heterogeneity and the black and white scales are adequate for this purpose. The original color figure can be found in the source. SOURCE: Maker et al. (2000). H ~ G H - L E V E E W A S T E 74

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aries to identify possible damage. Investigators should consider imaging techniques used in other fields, such as medical imaging. Long-term research is also needed in modeling the transport of contaminants from the tank into the environment. Models should include the most accu- rate subsurface characterization parameters and estimation of the physicochemical form of the mobile contaminant. Therefore, modeling activities should be coordinated with the latest advances in subsurface contamination research and geochemical speciation studies. Because of the shared interest for remote monitoring in other fields, such as in oil and gas exploration or groundwater monitoring, there is also a great potential for science and technology transfers. Near-FieId Containment Barriers Engineered barriers are used to reduce the flow of fluids from a con- taminated area toward water supplies.3 Engineered barriers can be physical barriers or reactive barriers. The first type consists of an imper- meable wall surrounding the contaminated area; the second type con- sists of a permeable we'' mane of reactive material that allows grouno- water to flow through it while it immobilizes metal or radionuclide contaminants by sorption or precipitation. Engineered barriers can be . . .. . . . .. . .. / 1 1 1 made of injected grout, various resins, or clay liners, and can be cou- pled with chemical treatments, freezing, or pumping techniques. Subsurface engineered barriers are under consideration at Hanford to reduce the impact to the environment of leaked waste by containing subsurface contamination within the near-field. The major drawback of current barrier technologies is that engineered barriers tend to leak even after short times. However, research is in progress in this field. For instance, a promising type of engineered barrier is a reactive barrier containing a biological layer in which microorganisms are used to immobilize contaminated material. tong-Term Research Needs Because barriers have a tendency to leak, long-term basic research is needed to identify methods for remotely monitoring contaminant transport through barriers. Some examples of promising remote moni- toring methods are electrical and seismic geophysics methods. New materials with greatly reduced permeability or biomaterials using O , , 3Additional detail on barrier technologies can be found in three previous NRC reports: Barrier Technologies for Environmental Management (NRC, 1 997b), Groundwater and Soil Cleanup: Improving Management of Persistent Contaminants (NRC, 1 999d), and Long-Term Institutional Management of U.S. Department of Energy Legacy Waste Sites (NRC, 2000d). T a n k C I o s u r e a n d O t h e r L o n 9 - T e r m I s s u e s

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microorganisms to immobilize hazardous chemicals and radionuclides also offer research opportunities for engineered barriers. Moreover, future engineered barriers should protect all potential pathways, in par- ticular underneath the tank and not just the surface surrounding it. Basic research on innovative horizontal drilling methods that do not create new pathways for flow of contaminants and new injection mater- ial to create an impermeable barrier beneath the tank is also desirable. H ~ G H - L E V E E W A S T E 76