C. R. Cooley
DOE/EM-50

Selection of final waste forms to be used for disposal of DOE’s mixed low-level waste (MLLW) depends on the waste form characteristics and total life cycle cost. In this paper the various cost factors associated with production and disposal of the final waste form are discussed and combined to develop life-cycle costs associated with several waste stabilization options. Cost factors used in this paper are based on a series of treatment system studies in which cost and mass balance analyses were performed for several mixed low-level waste treatment systems and various waste stabilization methods including vitrification, grout, phosphate bonded ceramic and polymer. Major cost elements include waste form production, final waste form volume, unit disposal cost, and system availability.

Production of grout costs less than the production of a vitrified waste form if each treatment process has equal operating time (availability) each year; however, because of the lower volume of a high temperature slag, certification and handling costs and disposal costs of the final waste form are less. Both the total treatment cost and life cycle costs are higher for a system producing grout than for a system producing high temperature slag, assuming equal system availability. The treatment costs decrease with increasing availability regardless of the waste form produced. If the availability of a system producing grout is sufficiently greater than a system producing slag, then the cost of treatment for the grout system will be less than the cost for the slag system, and the life cycle cost (including disposal) may be less depending on the unit disposal cost. Treatment and disposal costs will determine the return on investment in improved system availability. Waste volume reduction through the use of high waste loadings and/or high density final waste forms typically reduce disposal costs; however, if by reducing the volume the radionuclide concentration is increased to the point where the waste acceptance criteria for disposal is exceeded, or the unit disposal cost is increased, then volume reduction may not be an economically viable option.

During 1993 to 1997, the Department of Energy (DOE) Office of Science and Technology (OST) sponsored a series of analyses of various waste treatment systems to guide its future research and development programs on mixed low-level waste (MLLW). MLLW consists of organic and inorganic solids and liquids which contain both radioactive materials and hazardous material regulated under RCRA. Treatment of MLLW is required to destroy organic hazardous materials and to immobilize inorganic hazardous materials. Treatment qualifies the waste for disposal under the Environmental Protection Agency (EPA) promulgated RCRA requirements for land disposal, or disposal as low-level radioactive waste if the waste is delisted or granted other exceptions after treatment. The studies that evaluated thermal systems are termed the Integrated Thermal Treatment Systems (ITTS) studies^1,2. Following these studies nonthermal³ and enhanced nonthermal⁴ systems were evaluated, a detailed comparison of selected thermal and nonthermal systems was performed⁵, and a sensitivity analysis of assumptions made in the original studies was performed to determine the effect of these assumptions on cost⁶.

Several conclusions can be drawn from this previous work including the following:

In addition to slag, grout and polymer evaluated in the previous studies, phosphate bonded ceramic was evaluated for this study as a final waste form. Phosphate bonded ceramic is a fast setting ceramic material produced at low temperature from a blend of MgO and phosphoric acid or sodium phosphate and water. These materials are blended with the treated waste and allowed to cure into a high strength, low porosity waste form using equipment similar to that used to produce grout. Because this is a process similar to that used to produce grout, production costs equal to those of a grout system were assumed. However, tests have indicated that phosphate bonded ceramic may be capable of 50 to 70 wt% waste loadings compared to approximately 30 wt% for grout.

The selection of a waste form based on cost will depend on production and treatment costs, waste form volume, unit disposal cost, and the availability of the treatment system and the equipment used to produce the final waste form. The interactions between these factors and their effect on system life cycle cost are discussed in the following sections.

Production and Treatment Costs: In the context used in this paper, waste form production costs include all costs associated with treating the incoming waste and producing the final waste form. These costs are insensitive to the volume of the final waste form but are a function of the waste volume to be treated and stabilized. Since all treatment subsystems, except the stabilization subsystem, are identical for the systems considered here, the differences in production costs are due only to the capital and operating costs of the stabilization process. These costs, and the volume of stabilized waste for the rotary kiln systems evaluated in the ITTS studies are shown in Table I.

Thus, the final waste form volume impacts total treatment costs through the functions of certifying and handling the waste prior to shipment to disposal. For the assumptions used in this study, the baseline cost of stabilization by high-temperature vitrification is approximately $140 million greater than the cost of grouting the same amount of treated waste. However, certifying and handling the higher volume of grout prior to shipment to disposal costs $200 million more than for vitrified waste. Thus, the total treatment cost using grout stabilization is $60 million more than the treatment cost using high-temperature vitrification.

Changing the vitrified waste form from a high temperature slag to borosilicate glass with 33 wt% waste loading, and changing the low-temperature waste form from grout to phosphate bonded ceramic, will have little effect on the respective cost of stabilization (e.g., capital and operating cost). However, the differences in waste loading will cause the volume of vitrified waste to increase to about 50,000 m³ and the volume of non-vitrified waste to decrease to 41,000 m³ thereby changing the relative total treatment cost to $2,280 million and $2,090 million, respectively.

Waste Loading and Unit Disposal Cost: As indicated above, waste form volume affects total treatment cost through the certification and handling subsystem, and it affects total life cycle cost through the cost of disposal. Waste form volume, in turn, is a function of the waste volume to be stabilized, the waste loading, and the density of the waste form. Approximate densities of the waste forms considered in this paper are as follows: slag/glass monolith, 2996 kg/m³; phosphate bonded ceramic, 2100 kg/m³; slag/glass marbles, 1794 kg/m³; grout, 2035 kg/m³; and polymer, 1282 kg/m³. The density of vitrified marbles is the effective density in a container and based on a 60% packing factor for randomly packed spheres of equal size⁹.

The volume and therefore the disposal cost decrease with increasing waste loading. The waste form volume as a function of waste mass to be stabilized and waste form density is given by Equation 1 where

The results shown in these figures indicate how waste loading and unit disposal costs affect the relative economics of the waste forms and final disposition. At a disposal cost of $706/m³ ($20/ft³) the TLCC of borosilicate glass and grout (at 30 wt% waste loading) are essentially the same within the accuracy of this analysis and for the same availability, whereas the TLCC for PBC is significantly less than that for high temperature slag (at 70 wt% waste loading). At a disposal cost of $8475/m³ ($240/ft³) the TLCC of PBC and slag are identical at 70 wt% loading and the TLCC of borosilicate glass is less than that of grout at 30 wt% loading. In all cases the TLCC for vitrified marbles is significantly greater than the other waste forms evaluated, and the TLCC for grout or glass (at 30 wt% loading) is greater than slag or PBC (at 70 wt% loading). Thus, for the same availability, higher waste loading implies lower TLCC; however, the TLCC of the waste form at a given loading depends on the unit disposal cost.

Disincentives for Volume Reduction: The waste volume is typically reduced during the treatment process, and the resulting treated waste volume may be reduced or increased as a result of stabilization. Conventional wisdom indicates that significant effort should be expended to reduce the waste volume thereby decreasing handling and disposal volume and cost. However, this may not always be the optimum cost option. As the waste form volume is reduced, the concentration of radionuclides increases, although this increase is less for higher density waste forms. If the concentration of radionuclides, and therefore the activity per unit mass of the waste form, increases due to volume reduction the waste form may exceed the waste acceptance criteria for the disposal site. This may cause the unit disposal cost to rise significantly as the class of low level waste changes from Class A to Class B or Class C.

Availability: In addition to waste form volume, treatment costs are also affected by system availability determined by the reliability and maintainability of the system and equipment, and the ability to consistently maintain the quality of the final waste form. The treatment costs in Table I assume equal availability of 46% (i.e., 4032 hours of operation per year). Assuming all other things are equal, increasing the reliability of one of the stabilization technologies, and therefore system availability, can significantly change the relative treatment cost of the systems being compared as shown in Figure 3. For example, increasing the availability of the grout system from 46% to 70% decreases the time required to treat the 107 million kgs of MLLW from 20 to 13 years and decreases the treatment cost by approximately $460 million. Thus, a grout system with 70% availability would cost $460 million less than a grout system with 46% availability, and $400 million less than a slag system with 46% availability.

Unit disposal costs will also affect the choice of waste form based on system availability and the investment decisions to improve system availability. Increasing availability for vitrification or grout systems from 50% to 75% decreases treatment cost and therefore TLCC for either system by ~$400 million regardless of disposal cost as indicated in Figure 3. This illustrates the importance of system availability. However, since production of grout or PBC uses simpler equipment that may be considered more reliable than vitrifiers, the grout and PBC systems might be expected to have a higher availability. As shown in Figure 4, for an availability of 70%, the grout system has a lower TLCC than a vitrification system with a 50% availability for unit disposal costs less than about $7415/m³ ($210/ft³).

Estimates of system availability may be based on past experience or the maturity of a technology, and improved availability can provide significant savings. However, costs associated with increasing system availability include investment in additional testing to identify flaws in equipment and improvements in equipment design and/or operations, or investing in backup equipment. The latter option will increase capital costs for the additional equipment and increased facility space to house the equipment.

The unit disposal cost affects the cost incentive to improve the availability of vitrification systems to be at least equal to that of grout systems. For example, for an availability of 70% and a disposal cost of 706/m³ ($20/ft³) the slag system TLCC is $1,780 million and the grout system TLCC is $1,820 million. Thus, the cost saving incentive to improve the availability of the slag system to 70% is $40 million. However, at a disposal cost of $7062/m³ ($200/ft³) the slag system TLCC is $1,980 million and the grout system TLCC is $2,270 million at 70% availability indicating a cost saving incentive of $290 million to improve the availability of the slag system. The higher the unit disposal cost the greater is the incentive for investment to improve the availability of a system producing a slag final waste form. However, when compared to a phosphate bonded ceramic system operating at 70% availability, there appears to be no economic incentive to improve or use a vitrification system.

Delisting: The possibility of delisting certain final waste forms offers the opportunity for cost savings. The assumption in the ITTS studies is that the disposal cost is $8580/m³ ($243/ft³) based on an average size RCRA-permitted engineered disposal facility using $2,366/m³ ($67/ft³) for receiving and $6,215/m³ ($176/ft³) for engineered disposal¹⁰. However, if vitrified waste produced in the thermal treatment processes can be delisted for the RCRA constituents, the waste form could be managed and disposed in a manner consistent with the requirements of the Atomic Energy Act of 1954 as implemented by DOE Order 5820.2A, Radioactive Waste Management. Delisting the vitrified final waste form would allow disposal of low-level radioactive waste at an existing shallow land disposal facility, such as the Hanford facility, at a cost of $1,412/m³ ($40/ft³)¹¹. Applying the shallow land disposal cost to the volume of slag from the vitrification process (approximately 0.23m³/hr or 8 ft³/hr), and the disposal cost of a RCRA engineered disposal facility for the remaining stabilized waste, the total estimated cost of disposal for the systems employing vitrification for stabilization is reduced by as much as $130 million, or 6% of the TLCC.

Packaging: As indicated in Figures 1 and 2, vitrified marbles have the highest TLCC due to the high production cost associated with vitrification, and the high volume associated with packaging uniform size marbles at a 60% packing factor. The cost of a vitrified monolith may also be penalized if the monolith is formed in a container that, after being subjected to high heat loads, cannot meet the Department of Transportation (DOT) shipping container requirements or the disposal site waste acceptance criteria. In this case an overpack would be required thereby increasing the packaging and disposal volume. If the monolith were formed in a 30-gallon container and a 55-gallon drum were used as an overpack, the packing factor would be about 55% producing a slightly greater penalty in terms of volume than vitrified marbles. However, if an overpack is not required for disposal, then there is no volume penalty associated with disposal of a vitrified monolith.

In this paper we have attempted to point out the various factors that should be considered in determining a cost effective waste form for treated MLLW. One of the most important factors affecting total life cycle cost is the overall system availability. This emphasizes the need to use reliable and easily maintained equipment, and to insure the complete system is designed with availability in mind. Other factors associated with the waste form that affect the system life cycle cost include waste form volume and associated certification, handling and disposal cost; volume reduction and associated radionuclide content of the waste form; unit disposal cost; and the potential for delisting.

This work was performed for the Department of Energy, office of Environmental Management, office of Science and Technology (DOE/EM-50) under DOE Idaho contract DE-AC07-94ID13223.