K.2.0 UNCERTAINTIES IN ALTERNATIVES
A full range of representative alternatives was developed for detailed analysis in the EIS. Upper, lower, and intermediate bounding alternatives were developed in terms of cost, risk, and technologies for the two primary decisions that affect environmental impacts: the amount of waste to be retrieved from the tanks and the degree of separations of retrieved waste into high-level waste (HLW) and low-activity waste (LAW).
The alternatives developed were chosen to be representative of many possible variations of the alternatives. The design information for all alternatives is at an early planning stage, and the details of the alternative ultimately selected and implemented are likely to change as the design process matures.
Each alternative developed for analysis in the EIS consists of a set of technologies, or building blocks, that have been engineered to work together, forming complete systems for accomplishing the remediation of the tank waste.
Engineering data were developed for each alternative in support of the environmental impact analysis. These data included the following major components:
- Conceptual design of the type and size of facilities required for waste treatment;
- Schedules and staffing requirements (radiological and nonradiological workers) for the construction and operation of waste treatment facilities;
- Resource requirements for the construction and operation of the waste treatment facilities;
- Air emissions for routine tank farm operations, waste treatment operations, and post remediation;
- Contamination releases to the soil during waste retrieval and during the post- remediation phase; and
- Land use requirements, both temporary and permanent, for the construction and operation of waste treatment facilities.
These major components were developed based on certain assumptions, general engineering information, and previous development work. The uncertainties associated with engineering assumptions for each alternative are presented in Section K.2.2, and the uncertainties related to general information such as schedule projection, staffing and resource prediction, and cost estimation are discussed in Sections K.2.3 through K.2.6, respectively.
K.2.1 OVERVIEW
There are many uncertainties associated with the alternatives for remediating the tank waste. These uncertainties involve the types of waste contained in the tanks, the effectiveness of the proposed retrieval techniques, and the processes used to separate and treat the waste. These uncertainties exist because some of the technologies that would be implemented are first-of-a-kind and have not previously been applied to the Hanford Site tank waste, or they have not been applied at the scale required for the tank waste.
K.2.2 UNCERTAINTIES FOR MAJOR ASSUMPTIONS
To develop the engineering data required to perform impact analyses for each of the alternatives discussed in the EIS, assumptions were made regarding the technologies that create a remediation alternative. These assumptions were based on either the best information available, applications of a similar technology, or engineering judgement. When an assumption is made, there is some level of uncertainty associated with it that can be expressed as a range that reasonably could be expected for the assumed value. This section identifies the major assumptions used for the alternatives, describes uncertainties associated with the assumptions, and presents the results of a waste loading sensitivity analysis for the Ex Situ Intermediate Separations alternative.
K.2.2.1 Long-Term Management and In Situ Alternatives
Tank Leakage
It was assumed that there would be no leaks from the single-shell tanks (SSTs) or double-shell tanks (DSTs) during the administrative control period for the No Action, Long-Term Management, or In Situ Fill and Cap alternatives because the ongoing process of removing the pumpable liquids from SSTs was assumed to be completed, and leaks would be recovered from the space between the inner and outer liners of the DSTs. The SSTs and DSTs were assumed to maintain their structural integrity throughout the administrative control period under the No Action and Long-Term Management alternatives. For the Long-Term Management alternative, replacement of the DSTs was assumed to be necessary to prevent leaks.
The uncertainty with this assumption is that a leak could develop or a structure failure could occur, resulting in a release of contaminants during the administrative control period. It is likely that corrective actions would be taken in the event of a leak or signs of structural deterioration. Corrective actions could include waste retrieval and retanking activities to minimize environmental releases. If these activities were to occur, increases in the release of contaminants of the air and vadose zone would be expected.
In Situ Vitrification
The In Situ Vitrification alternative is more conceptual in design and development than the ex situ vitrification alternatives and thus has a higher degree of uncertainty associated with the data developed for impact assessments. The in situ vitrification system was assumed to be capable of vitrifying each of the tanks to the required depth, resulting in a consistent waste form. It also was assumed that the variation in waste composition and inventory from tank to tank would not impact the ability to produce an acceptable waste form.
There is considerable uncertainty about the ability of the in situ vitrification system to vitrify the large volume required for the SSTs and DSTs. This uncertainty could be reduced through the use of smaller vitrification systems and the development of depth-enhancing techniques. This likely would result in increased staffing requirements and longer operating durations.
The air emissions estimates developed for the In Situ Vitrification alternative assumed that the entire inventory of iodine-129 (I-129) would be released to the atmosphere during the operating period. Off-gas treatment systems could be expected to remove part of the I-129 and reduce these emissions.
The long-term waste form performance for the vitrified waste was based on the assumption of a homogeneous waste form with properties similar to the glass produced by the Ex Situ No Separations alternative. Inspecting the final waste form to verify that all of the wastes were vitrified would be difficult and could result in undetected waste form variations. Variability in the waste form or fracturing of the waste form during cooling would be expected to result in increased contaminant release rates to the vadose zone.
The safety of drying some of the waste types is uncertain. Further evaluation of this issue could result in some tanks not being suitable for in situ vitrification.
In Situ Fill and Cap
Under the In Situ Fill and Cap alternative, the DST liquids would be concentrated using the 242-A Evaporator to remove as much water from the waste as possible, but the waste still would contain substantial volumes of liquid. It was estimated that concentration by the 242-A Evaporator would reduce the current liquid volumes contained in the tanks by approximately one-third (WHC 1995f). The concentrated liquid waste contained in the DSTs was assumed to be acceptable for gravel filling.
Additional development of this alternative could result in a requirement for additional liquid removal and drying of the waste in the tanks. If this were to occur, development of an in situ drying technology would be required and its use would result in increased volatile radionuclide and chemical emissions from the tanks, in addition to increases in staffing levels and operating schedule.
K.2.2.2 Ex Situ Alternatives
Waste Retrieval Efficiency
The waste retrieval function described for the ex situ alternatives was assumed to remove 99 percent of the waste volume contained in each tank during waste retrieval. Under this assumption, 1 percent of the tank volume would be left in-tank as residual. It was further assumed that the 1 percent waste volume represented 1 percent of the waste inventory on a chemical and radiological basis including soluble waste constituents. This assumption is conservative and will bound the impact from the tank residuals.
The amount and type of waste that would remain in the tanks after retrieval is uncertain. The Hanford Federal Facility Agreement and Consent Order (Tri-Party Agreement) (Ecology et al. 1994) set a goal for the SSTs that no more than 1 percent of the tank inventory would remain as residual following waste retrieval activities to the extent technically practicable. The engineering data for the waste retrieval and transfer function common to all ex situ alternatives was developed using 99 percent retrieval from SSTs and DSTs as an assumption.
It would be expected that the residual contaminants left in the tanks either would be insoluble and hardened on the tank walls and bottom or be of a size that could not be broken up and removed from the tanks without extraordinary measures. In either case, the residual waste would have low solubility because the retrieval technologies proposed would use substantial quantities of liquid to dissolve or suspend the waste during retrieval.
The effect of retrieving less than 99 percent of the waste volumes from the tanks during retrieval would be an increase in the amount of waste left in the tanks and corresponding increases in long-term contaminant releases. The in situ and combination alternatives would leave substantially more waste onsite for disposal and provide an upper bound on the impacts associated with the amount and type of waste that is disposed of onsite. Retrieval of more than 99 percent of the waste would reduce the impacts associated with residual waste.
A nominal case tank residual inventory was developed to evaluate the impacts that would result from a more nominal residual inventory. The nominal residual inventory was developed by accounting for the solubility of the mobile constituents of concern. The mobile constituents of concern were evaluated because of their contribution to post-remediation risk. The isotopes carbon-14 (C-14), technetium-99 (Tc-99), and I-129 were reduced for the nominal case to 10 percent of the bounding residual inventory. For additional information, refer to Volume Two, Appendix B.
The U.S. Department of Energy (DOE) currently is developing the Hanford Tanks Initiative (HTI) program that will provide information on the characteristics of the tank residuals and the capability of retrieval systems to handle difficult-to-remove SST wastes. This program will reduce the uncertainties associated with residual waste, demonstrate the capability to quantify residual waste volume, and demonstrate technologies for sampling and characterizing the residual waste.
Assumptions Affecting HLW Volume
The major factors that affect the volume of HLW produced by any of the ex situ alternatives include waste inventory, waste loading (glass specifications), blending, and the efficiency of the separations processes. The waste inventory that has been used for all alternatives is provided in Volume Two, Appendix A along with a discussion on data accuracy and uncertainty.
Waste loading is the mass fraction of the nonvolatile waste oxides in the vitrified waste. The waste oxide loading would be controlled by the amount of glass formers added during the vitrification process. The higher the waste loading, the more waste contained in the vitrified glass and the lower the waste volume.
Blending is the mixing of the waste from different tanks during retrieval to obtain an average waste feed stream for treatment. Because there are 177 tanks that contain waste, and the waste composition varies from tank to tank, it would be difficult to achieve a completely uniform blending of the waste during retrieval.
Separating the waste into HLW and LAW streams for treatment would involve various processes to physically or chemically separate specific constituents in the waste stream. The separations efficiency would be a measure of how well these processes work and would define the amount of each constituent that would be processed in the HLW and LAW treatment facilities.
The assumptions used for each of the previously described factors and their combined effect on the overall volume of HLW and LAW are discussed in the following sections.
Waste Loading
The waste loading for all ex situ treatment alternatives, except for the Ex Situ No Separations alternative was assumed to be 20 weight percent waste oxides for the HLW and 15 weight percent sodium oxide (Na2O) for the LAW. The waste loading for the Ex Situ No Separations alternative was assumed to be 20 weight percent Na2O.
Waste loading would affect the final volume produced from an initial amount of waste. This volume, along with the operating schedule and the assumed operating efficiency, would determine the size of the processing facilities and operating resources required to support the process. A decrease in waste loading would translate into a larger volume of vitrified waste, larger treatment facilities or longer operating schedules, increased resource requirements, and higher disposal cost.
Waste loading typically ranges from 20 to 40 weight percent waste oxides, with 30 to 35 weight percent loading used as a target value. The Savannah River Site Defense Waste Processing Facility glass has a design basis waste loading of 25 weight percent and a maximum waste loading of 38 percent (DOE 1995s).
The waste loading for all alternatives that would produce LAW was assumed to be 15 weight percent Na2O. The volume of LAW produced would affect the size and number of LAW disposal vaults built onsite.
Waste Blending
Each of the ex situ alternatives that would use vitrification as an immobilization technology assumed a waste blending factor of 1.2 for the HLW to account for variations in the composition of the waste during retrieval operations. Variations in the waste feed composition would not affect the calcined product that would be produced by the Ex Situ No Separations (Calcination) alternative. Uniform blending would require simultaneous retrieval from specific groups of tanks to deliver a uniform average feed stream to the treatment facilities. The blending factor would be multiplied by the volume of HLW produced under uniform blending conditions to calculate the waste volume expected due to variation in the waste feed. One of the major sources of uncertainty associated with developing a retrieval sequence that would achieve a uniform blending is the lack of accepted tank-by-tank inventory data. Preliminary studies on retrieval sequences, waste blending, and the effects on HLW volume show that the volume of vitrified HLW with no blending would be approximately twice that with total blending (WHC 1995p).
The volume of HLW produced combined with the size of the HLW canisters would directly impact the number of HLW packages requiring disposal at the potential geologic repository, which in turn would affect the cost associated with disposal. The number of HLW packages produced would also determine the number of offsite shipments required to transport the immobilized HLW to the potential geologic repository. The waste loading would also determine the concentration of radiological contaminants in the waste form. There is a relationship between the waste loading, number of shipments (cumulative probability of an accident), and the concentration of contaminants in the waste form (consequence of an accident). As the waste loading increased, the cumulative probability of an accident would decrease because there would be fewer trips required to transport the waste. The consequences of an accident would increase because there would be a higher concentration of contaminants in the waste form (see Volume Four, Section E.16.0 for a discussion of accident uncertainties).
Releases to the Soil During Retrieval
Retrieval operations under each ex situ alternative was assumed to result in the release of 15,000 L (4,000 gal) of waste at full solution strength from each SST to the surrounding soil. No leakage from the DSTs was assumed to occur during retrieval operations. This assumption was based on the 67 known or suspected SSTs that have leaked in the past (Hanlon 1995) and no known or suspected leaks from DSTs to date. Most of the SSTs were built in the 1940's and now are about 50 years old. The leakage volume estimate assumed that the average leakage from an SST would be one order of magnitude lower than the maximum release estimated for tank 241-C-106 during sluicing operations. The maximum leak estimated from tank 241-C-106 during sluicing operations was 150,000 L (40,000 gal). This estimate also assumed that the leak occurred early in the sluicing operation, leak detection devices and controls failed, sluicing operations proceeded without these leak detection devices, the leak(s) occurred at the bottom of the tank, and the remaining sludge did not plug any leaks (DOE 1995d).
The most probable occurrence of a leak during sluicing would involve the sluicers opening a plugged leak in the tank wall. The waste leakage during sluicing would be any free-standing liquid above the level of the leak point and the sluicing stream as it impacted the tank wall.
A nominal retrieval release inventory was developed by assuming that the waste would be diluted by one-third by adding water during waste retrieval. Possible dilution ratios that would be used during waste retrieval range from 3:1 to 10:1, depending on waste type. The nominal retrieval release inventory accounts for partial dilution of the tank contents while retrieval operations are underway. The volume of waste released during retrieval was assumed to be the same for the nominal and bounding cases. There currently is insufficient basis to support a lower nominal case leakage estimate. DOE currently is developing criteria and technologies to identify leaks and limit releases during retrieval.
Sensitivity Analysis
A sensitivity analysis was performed for the Ex Situ Intermediate Separations alternative to show the range in the data expected if the volume of HLW and LAW produced were to increase or decrease based on waste loading assumptions. The following sensitivity parameters were assumed for analysis:
- HLW loading at 15 weight percent and 40 weight percent waste oxides;
- LAW loading at 10 weight percent and 25 weight percent Na2O; and
- No variation in the separations efficiencies or the blending factor.
The results of the sensitivity analysis performed for the Ex Situ Intermediate Separations alternative are shown in Table K.2.2.1. Lower waste loading would require increased resources, land commitments, transportation, and cost. The facility sizes were held constant for the sensitivity analysis, resulting in constant capital cost and staffing levels and variable operating schedules. If the treatment schedule were held constant, the required treatment facilities, capital cost, and staffing levels would change.
K.2.3 SCHEDULE
Schedules for construction, operation, and closure were developed for each of the alternatives within the constraints of the Tri-Party Agreement (Ecology et al. 1994). Schedule constraints would affect the size of the treatment facilities required to process the waste. Following design and construction of a waste treatment facility, the major schedule uncertainty would be the operating duration.
Each of the ex situ alternatives was developed using 60 percent overall operating efficiency, except for Phase 2 of Phased Implementation, which used 70 percent overall operating efficiency. Operating at higher efficiencies would reduce the operating duration, and conversely, lower operating efficiencies would increase the operating duration. For the alternatives that would have multiple treatment components, such as retrieval, pretreatment, HLW treatment, and LAW treatment, the overall operating schedule would depend on the operating efficiency for each component.
Uncertainties in the operating schedule would be expected to result in longer operating durations. Previous analysis has shown that the operating duration for the ex situ alternatives would be sensitive to the rate at which waste can be retrieved from the SSTs. A low SST sludge retrieval rate could increase the operating duration by 50 percent (WHC 1995r).
K.2.4 STAFFING
Staffing estimates were developed for each alternative in support of risk, accident, and socioeconomic impact analysis. These staffing estimates were developed using conservative assumptions for both construction and operating staffing levels. The major uncertainty in overall staffing requirements would be associated with the operating schedule uncertainty. Staffing requirements would be affected by operating efficiencies because operating efficiency changes would increase or decrease the operating duration and the overall staffing requirements.
Table K.2.2.1 Ex Situ Intermediate Separations Sensitivity Summary
K.2.5 RESOURCES
The resources required to construct and operate waste treatment facilities were estimated for each alternative using a consistent methodology and common assumptions. The ex situ alternatives and the In Situ Vitrification alternative would have the largest uncertainty for estimated resources. The major uncertainties associated with the estimated resource requirements for the ex situ alternatives include the size and type of facilities required and the volume of LAW and HLW produced. Variations in operating resource requirements as a function of waste loading for the Ex Situ Intermediate Separations alternative are shown in Table K.2.2.1.
K.2.6 COST
Cost uncertainty for all of the tank waste alternatives has been evaluated and is discussed in Volume 2, Section B.8. Upper and lower ranges were estimated for the major cost components of each alternative. Upper and lower cost ranges were based on the technology, level of development, and degree of complexity. These cost ranges along with confidence levels were used as input to Decision Science Corporation's Range Estimating Program for personal computers to model the treatment cost range and total cost range including repository fee.
The cost uncertainty results in a cost range within which the alternative cost would be expected to fall. The cost range is the highest for the In Situ Vitrification alternative at 3.3 percent below to 66.5 percent above the target cost based on the uncertainties associated with implementing this technology for remediation of the tank waste. Cost ranges for the ex situ alternatives are generally 3 to 8 percent below to 20 percent above the target cost. The Ex Situ Extensive Separations alternative results in an upper cost range of 35 percent above the target cost based on the application of many first-of-a-kind technologies and the complexity of the separations process.
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