Billie Mauss
USDOE

David J. Swanberg
SAIC

Joan Young
BDM

This paper provides information on promising technologies for volume reduction of waste from storage and treatment of hazardous, radioactive tank waste at U.S. Department of Energy (DOE) sites. The information was compiled by DOE's Environmental Management (EM) Tanks Focus Area (TFA) to support development of the Accelerated Cleanup Plans (2006 Plans) for the following DOE sites: Hanford, Savannah River Site (SRS), Oak Ridge, and Idaho National Engineering and Environmental Laboratory (INEEL). Rough Order-of-Magnitude (ROM) estimates of cost savings are also presented. Cost savings in this paper refers to the cost avoidance associated with implementing a particular technology and are based upon average unit costs. The unit costs are not intended to account for site to site differences. The information presented here is useful for DOE High Level Waste (HLW) program managers and decision makers for targeting volume reduction efforts and comparing potential for cost savings. It may also be useful to anyone challenged with treatment and disposal of large volumes of radioactive liquid wastes.

There is a large potential for volume reduction and cost savings at DOE sites during the treatment of HLW in tanks. The volume of important waste streams from remediation of waste in tanks across the DOE Complex is shown in Figure 1. The magnitude of the remediation challenge is illustrated in Table I which shows what overall treatment costs would be if typical unit costs are applied to each stream. Total estimated treatment cost, for comparison purposes, is on the order of $16 billion.

DOE's Tanks Focus Area (TFA), in collaboration with EM programs at the tank sites is developing and demonstrating technologies for remediation of tank waste, Many of these technologies can achieve substantial volume reductions and cost savings when implemented. A list of selected technologies appears in Table II. The combined impact of these technologies represents a volume reduction of more than 350,000 m³ and avoidance on the order of $8 billion in waste treatment and disposal costs.

Brief descriptions of the remedial technologies evaluated, the baselines replaced by the technologies, and the potential volume reduction and cost avoidance impacts are provided below. Note that the technologies span a variety of process steps in the site baselines and that the combined impact is much greater than the impact of the individual technologies.

Determining the volume of waste in an underground waste tank is a major component of decision-making regarding retrieval and closure of DOE tanks. The current baseline for assessing the volume of waste in a tank includes the use of historical tank data; single point waste height measurements; and volumetric estimates from in-tank videos. All of these methods lack the accuracy needed to meet sluicing and tank closure requirements stipulated by regulators and stakeholders. Without the ability to accurately measure the volume of residual waste in each tank it will be necessary to over-sluice the tanks to ensure the remaining volume meet regulatory or compliance agreement criteria. Since waste heels are often highly insoluble and immobile, this requires large volumes of water to break up and suspend the waste. This results in increased waste volume and waste treatment costs.

Volume reduction and cost estimates are based on deployment of the TMS in Hanford's HLW waste tanks to provide data on residual waste volume for tank closure determinations. The TMS utilizes a remote dimensional measurement system that produces highly accurate topographic maps of complex surfaces. For estimating purposes, it is assumed that sluicing times are shortened by one week for each HLW tank as a direct result of more accurate residual waste volume measurements provided by the TMS. This represents a savings of 680 m³ of sluice water per tank. At $200/m³ to evaporate, the cost savings is $135,000/tank or $24 million for the 177 tanks at Hanford.

Carbon steel tanks used for HLW storage at SRS and Hanford are susceptible to corrosion which can be minimized by maintaining optimal concentrations of sodium hydroxide, sodium nitrate, and sodium nitrite in the waste. Corrosion is controlled by sampling and analysis of the waste with periodic addition of raw chemicals to minimize corrosive conditions. Typically, a substantial excess of these chemicals is added as a margin of safety.

Corrosion monitoring, based upon the electrochemical noise (EN) technique has been developed to the point where it is now feasible to detect and monitor the forms of corrosion most likely to affect carbon steel tanks. First generation EN probes are being tested in Hanford tanks. The TFA is also developing online monitors for OH/NO₃/NO₂ concentrations within the tanks that will minimize the need to sample and allow operators to fine tune additions of corrosion inhibiting chemicals.

With on-line corrosion monitoring instrumentation, the amount of corrosion inhibitor chemical additions can be reduced by as much as 2/3 by adding chemicals only when corrosive conditions exist and not adding additional chemicals as a contingency. For Hanford Double Shell Tanks (DSTs), the volume of corrosion inhibiting chemicals added and ultimately treated for disposal could be reduced by 25 m³ per year or 750 m³ over a 30-year lifetime. Total life cycle cost for wastes added to the DST system is conservatively estimated at $10,000 per m³ (estimated cost to treat and dispose of low activity waste (LAW)). If corrosion probes are deployed in 28 DSTs at Hanford, the life cycle cost avoidance is expected to be $7.5 million at Hanford. Similarly, the cost avoidance at Savannah River is assumed to be approximately $7 million for 25 tanks.

Skid-mounted, modular evaporator and cesium ion exchange systems have been demonstrated at Oak Ridge National Laboratory for the volume reduction of waste from the Melton Valley Storage Tanks (MVSTs). The evaporator system achieved a boil-off rate of 340 liters per hour operating at a partial vacuum to significantly reduce boiling temperature. An overall decontamination factor (DF) in excess of 10⁶ was achieved with actual waste feed (3).

Follow-on evaporator campaigns are planned at Oak Ridge that will remove up to 600 m³ of water from the waste stored in the MVSTs. This in turn will reduce the volume of immobilized LAW produced. A modular evaporator system will also be deployed at SRS to reduce the volume of Consolidated Incinerator Facility (CIF) offgas scrubber blowdown. An 80% waste volume reduction for this stream will reduce annual flow by about 150 m³. Assuming a 20 year operating period for the CIF, the overall waste volume reduction for both evaporators is 2,100 m³ and the estimated cost savings is $21 million.

Savannah River's DWPF produces a process condensate stream from the melter offgas scrub system. This stream is about 11,000 m³ per year and contains solids, dissolved solids, cesium, and mercury. The average radionuclide concentrations are high enough that the liquid must be recycled to the HLW tanks. DOE's Office of Science and Technology (OST) has developed techniques to remove the contaminants listed above but a continuous flow system combining all necessary unit operations has not been demonstrated. An on-site demonstration is planned that, if successful, will allow the decontaminated condensate stream to be routed directly to the SRS Effluent Treatment Facility. This would save the cost of neutralizing, storing and re-processing this stream as HLW. There would be incremental volume reductions of both HLW and LAW products as well as cost avoidance from not having to evaporate about 11,000 m³ per year of excess liquid. For this analysis, the annual volume reduction and waste evaporator cost avoidance over an estimated 20 years of operation is the basis for the overall volume reduction and cost avoidance shown in Table II.

Large quantities of sodium hydroxide (caustic) are present in the low activity liquid waste stream resulting from In-Tank Precipitation at SRS. This stream is a concentrated salt solution (5 molar Na) normally disposed of in a cement waste form referred to as saltstone. Caustic could be recovered from this stream by and used to neutralize newly generated waste from the Separations canyons, DWPF, and the Effluent Treatment Facility (ETF). Recycled caustic could be used as a corrosion inhibitor in the Tank Farms, and used to dissolve aluminum in Extended Sludge Processing (ESP). Significant savings in LAW disposal costs could be realized if the caustic were recovered and recycled.

An electrochemical salt splitting process can recover sodium from the waste as caustic solution. The total on-site caustic usage identified for Separations, Tank Farms, ETF, and DWPF is 2,300 m³/yr. Maximum possible recovery of Na from saltstone feed is 42% of total Na at average flowsheet composition and 80% if both nitrate and nitrites are converted to hydroxide before the Na recovery step. At 42% recovery, 2,050 m³/yr of caustic solution would be produced. Estimated capital cost is $9.5 million and annual operating costs excluding labor are $280,000 (4). Potential cost savings arise from eliminating the need to purchase fresh caustic and avoiding the cost of eventual disposal of the caustic in Saltstone. These amount to $6.5 M/yr in potential savings. If labor costs are estimated at $2.8 million/yr (7 staff/shift, 3-shift operation) the overall cost savings over a 20-year period would be about $70 million or $1,700/m³.

DOE has approximately 330 underground storage tanks containing 350,000 m³ of HLW. In reality, a large fraction of the waste is sodium nitrate formed by neutralizing nitric acid-based solutions with caustic soda (to allow extended storage in carbon-steel tanks). The clean salt process removes pure sodium nitrate from tank waste supernate by selective crystallization, a process used routinely at large scale in the chemical industry. The sodium nitrate product can then be re-used as a commercial chemical product.

The clean salt process can reduce the volume of immobilized LAW at Hanford by up to 75%. This would eliminate vitrification and disposal of approximately 204,000 cubic meters of liquid LAW. Estimating LAW vitrification cost at approximately $10,000 per cubic meter, this represents a cost avoidance of $2 billion. However, the application of this technology to HLW has not been demonstrated on a large scale and there must be a bona fide market for the clean salt product to satisfy regulatory requirements. While the capital and operating costs for the clean salt process are estimated to be $100 to $200 million, these estimates are quite uncertain. Contributing factors include requirements for greater radionuclide removal from the concentrated LAW stream and uncertainty in unit costs for treatment during Phase II of privatization (5,6).

A thermal denitration process will be demonstrated at the pilot-scale for treating INEEL LAW from separations processes for treating tank farm and calcine wastes. In addition, LAW will be generated by facility decontamination and process equipment effluents. The current baseline for treating LAW waste at INEEL is grouting. High nitrate levels in the feed require the development of a denitration process. The denitration process reduces the waste-to-grout volume by 3 to 8 times which will save in LAW waste storage costs. Actual volume reduction is not known, but INEEL reports a cost savings of $10.5 million over 15 years assuming annual operating costs of $3.2 million for a denitration facility (7).

INEEL has committed in a settlement agreement with the State and regulators to remove all liquid waste from the tanks by the year 2012 and to make the HLW ready for shipment off-site by the year 2035. The site has about 7,000 m³ of liquid waste in tanks but most of the HLW has already been calcined. The baseline is to remove the remaining waste from the tanks in the near term, then treat all of the waste for disposal. Final waste treatment will involve removal of radionuclides (Cs, Sr, Tc, & TRU) from dissolved calcine thereby separating the waste into high activity and low activity fractions for treatment and disposal.

A combination of ion exchange and solvent extraction is being demonstrated by the TFA and EM-30 at INEEL that will greatly minimize the overall HLW volume, reducing it from about 12,000 m³ to 700 m³. Estimated life cycle cost reduction corresponding to this volume reduction is about $1.1 billion and is considered to be conservative (7). Actual cost savings will be dependent upon costs for final disposal in the HLW repository.

Alkaline sludge in Hanford tank waste will be the primary source of immobilized HLW from Hanford tanks. Enhanced sludge washing is the baseline technology for pretreatment of the waste prior to vitrification. Simple water washing of HLW sludge removes only the interstitial liquid. Enhanced sludge washing includes a caustic leaching step to reduce the amount of constituents that limit the waste loading of borosilicate glass. The major sludge constituents to be removed by enhanced sludge washing are aluminum salts, phosphates, chromium, and sulfates. Certain Hanford sludge types contain high levels of insoluble chromium. Oxidative treatments are being developed to selectively oxidize the chromium to Cr+6, which is quite soluble in the alkaline leach solution.

Simple water washing of Hanford sludge would produce about 16,000 m³ of HLW glass. Enhanced Sludge Washing with Chromium Oxidation would reduce this volume to 9,000 m³ yielding a 7,000 m³ waste volume reduction (7). HLW treatment and repository costs are estimated at $600,000/m³ for total estimated cost avoidance of about $4 billion. However, both the estimated volume of HLW to be produced from Hanford sludge and the cost to treat and dispose of the waste are subject to large uncertainties. For example, total HLW glass produced could be as high as 32,000 m³ due to high levels of intractable chromium. However, even if repository costs were not significantly impacted by higher waste volumes, avoiding the increased size and extended operation of HLW treatment facilities would result in a comparable cost avoidance to the value presented here.

The total volume of immobilized HLW generated by vitrifying all of DOE's current and future HLW inventory can be reduced by increasing the percentage of waste oxides incorporated into each cubic meter of HLW glass. Current waste loadings are developed using models that predict glass viscosity, durability, and solubility, etc. and relate these properties to waste form performance and glass processing constraints. The uncertainty of these models results in conservative waste loading limits. The baseline for waste loading at Savannah River is 26%. Optimization of waste loading to 28% would decrease overall HLW volume by 7% or about 250 m³ (400 DWPF canisters) and reduce treatment facility operation by 1.3 years. Total life-cycle cost savings, a combination of facility operations and repository emplacement savings, is estimated to be $633 million.

The Hanford Tank Waste Remediation System process technical baseline (8 ) indicates a maximum allowable waste loading of 45 wt%. This seems very high but accounts for the fact that the aluminum and silicon in Hanford waste act as glass formers when vitrified and also incorporates considerations for optimizing waste loading. The total volume of HLW glass produced from Hanford waste after Enhanced Sludge Washing with Chromium Oxidation is about 9,000 m³ (see above). This corresponds to 45% waste oxide loading but also represents a 5,000 m³ reduction over the planning basis of 14,000 m³. At $600,000/m³ for HLW treatment and repository fees, the cost avoidance is approximately $3 billion.

Substantial opportunities exist, using innovative technology applications, to reduce the volume of waste for disposal and the cost of remediation of waste in tanks at DOE sites. Inserting new technologies into existing site baselines will also accelerate cleanup by reducing the overall time required to treat the waste.

In most cases, the technologies described in this paper represent opportunities for participation by private industry. Many of the basic technologies are available from commercial sources and can be used, with adaptation, for radioactive waste cleanup.

There has been reluctance in the past to utilize new technology partly due to a lack of economic (budget) pressure to reduce costs and a conservative regulatory climate that does not foster innovative technology. DOE's Environmental Management program has taken steps to remove these barriers and provide incentives to use new technology. The Accelerating Cleanup: 2006 Plan is establishing efficiency goals and requiring DOE field offices to identify innovative technologies that will be used to meet those goals.

The DOE is also initiating procurement strategy changes which tie contractor award fees to project acceleration and less costly performance. Finally, DOE has made a strategic commitment to the enhance performance, increase efficiency, and reduce costs through the use of fixed price contracting, recycling and waste minimization, privatization, and other activities (10). New technologies for remediation of tank waste will be a key part of that strategy.

The authors wish to thank TFA Program Managers David Geiser (HQ) and Jeff Frey (RL) for supporting this work. The TFA also wishes to thank the User Steering Group, TFA Site Reps, and EM Program Managers at the tank sites for their support of TFA initiatives to deploy new technologies that benefit tank waste remediation activities at DOE sites.

1. U.S. Department of Energy, "Integrated Data Base Report--1995: High Level Waste Inventories, Projections, and Characteristics," DOE/RW-0006, Rev. 12, Oak Ridge National Laboratory, Oak Ridge, Tennessee (September 1996).
2. Slaathaug, E. J., Tri-Party Agreement Alternative Engineering Data Package for the Tank Waste Remediation System Environmental Impact Statement. WHC-SD-WM-EV-104, Rev 0. Westinghouse Hanford Company, Richland, Washington., (July 1995).
3. U.S. Department of Energy, "Modular Evaporator and Ion Exchange Systems for Waste Reduction in Tanks: Accelerated Technology Deployment Plan," U.S. Department of Energy, Oak Ridge Operations, Oak Ridge, Tennessee (November 28, 1997).
4. Brooks, K.P., Hobbs, D.T., Klem, M.J., and D.E. Kurath, "Preliminary Evaluation of Applications of a Salt Splitting Process Using Ceramic Membranes," Pacific Northwest National Laboratory, Richland, Washington (June 1996).
5. McCown, A. "Cost Effectiveness Analysis of the Clean Salt Process." Los Alamos National Laboratory, Los Alamos, New Mexico (October 1997).
6. Swanberg, D. J., Nguyen, P. M., Young, J. K., "The Clean Salt Process (CSP) for Minimization of Hanford Tank Waste," Waste Management 97, Tucson, Arizona (March 3-7, 1997).
7. INEEL Site Technology Coordination Group. FY 99 Technology Need Statement No. ID-2.1.14. December 1997.
8. S. DeMuth and D. Williams, "Cost Effectiveness of Crystalline Silico-Titanate and Resorcinol-Formaldehyde Ion Exchange Resins, and Enhanced Sludge Washing with and without Chromium Oxidation" LA-UR-97-3903 Rev 1., Los Alamos National Laboratory, Los Alamos, New Mexico ( October, 1997).
9. R.M. Orme, A.F. Manuel, L.W. Shelton, and E.J. Slaathaug, TWRS Privatization Process Technical Baseline, WHC-SD-WM-TI-774, Westinghouse Hanford Company, Richland, Washington (September, 1996).
10. U.S. Department of Energy, U.S. Department of Energy Strategic Plan. DOE/PO-0053. U.S. Department of Energy Washington, DC (September 1997).

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