Fig. 1. Flow diagram
of the waste form initiative strategy.
Jenya Macheret
DOE-ID
Dieter A. Knecht and Dirk Gombert
LMITCO
Robert Waters
SNL
Ron Nakaoka
LANL
ABSTRACT
The U.S. Department of Energy (DOE) has responsibility for treating and disposing of its mixed hazardous and low-level radioactive Waste (MLLW). A total volume of 130,000 m3 is expected to require treatment to produce final waste forms for disposal. Treatment and disposal requirements for MLLW have not been established by DOE on a national basis. The performance assessment of disposal facilities, technology development, and waste form testing efforts by DOE have been separate, disjointed activities which may not lead to producing disposable waste forms. A Waste Form Initiative (WFI) is established to ensure that treatment technologies produce waste forms for MLLW that meet the waste acceptance requirements of potential disposal facilities. The Waste Form Initiative integrates in one program results of previous performance evaluations and assessments of selected disposal facilities, performance characteristics of waste forms, and development of waste treatment technologies. This paper summarizes the technical basis, strategy, and plans of the Waste Form Initiative.
INTRODUCTION
The U.S. Department of Energy has responsibility for treating and disposing of its mixed hazardous and low-level radioactive Waste (MLLW). A total volume of 130,000 m3 is expected to require treatment to produce final waste forms for disposal.(1) In the past, waste treatment and generation of final waste forms, characterization of waste form performance, and performance assessments of disposal facilities were conducted as independent efforts. A vast amount of literature documents performance characteristics of various waste forms. Most studies on performance testing of waste forms are obtained for generic, rather than site specific disposal conditions. Such results are typically not used in the performance assessment of the DOE disposal facilities.(2)
The Mixed Waste Focus Area (MWFA) is funding development of treatment technologies that produce waste forms for disposal. In March 1996, the MWFA was requested by the Office of Science and Technology of DOE to lead a program with a goal of ensuring that the resulting waste forms are acceptable for disposal. This program, the Waste Form Initiative (WFI), integrates the characterization of waste form performance, disposal requirements for selected disposal facilities and development of waste treatment technology. The purpose of this paper is to present the technical basis, strategy, and plans of the Waste Form Initiative.
REGULATORY BACKGROUND
Treatment and disposal requirements for MLLW have not been established by DOE on a national basis. The radioactive components of MLLW of DOE are regulated under DOE Order 5820.2A, and the hazardous components are regulated by Environmental Protection Agency or States with delegated authority under the Resource Conservation and Recovery Act (RCRA).
DOE Low Level Waste (LLW) disposal performance objectives contained in 5820.2A involve conducting site-specific performance assessments. Waste acceptance criteria (WAC) must be formulated for each disposal facility to ensure compliance with performance objectives and establish performance standards for final waste forms. In addition, DOE allows disposal of certain wastes at commercial facilities through exemptions granted on a case-by-case basis. Waste acceptance criteria for commercially disposed LLW are developed under NRC regulation 10 CFR 61 and enforced and licensed by the authorized State.
DOE waste is considered hazardous under RCRA if it contains any waste listed in 40 CFR 261 or if it exhibits the characteristics of reactivity, corrosivity, ignitability or toxicity. In general, RCRA hazardous waste must be treated in compliance with the RCRA Land Disposal Restrictions (LDR) standards before it can be land disposed, and hazardous waste management facilities must meet technical standards under 40 CFR 264. Many hazardous constituents in the waste are covered by universal treatment standards in 40 CFR 268.48 which include Toxic Characteristic Leaching Procedure (TCLP) test limits or concentration limits in the waste form.
PAST ACTIVITIES
The Federal Facility Compliance Act (FFC Act) of 1992 requires DOE to work with state and federal regulatory authorities and public to establish plans for treatment of MLLW. The FFCAct does not specifically address disposal of treated MLLW. However, all involved parties recognize that disposal issues are an integral part of the treatment discussions. The FFCAct Disposal Workgroup (DWG) was established by DOE in 1993 to work with the States in identifying, from among sites storing or potentially generating MLLW, those suitable for disposing of MLLW. The technical capabilities of fifteen DOE facilities were evaluated in performance evaluations (PE)(1) to determine allowable concentration limits for disposal of the radionuclides. A subsequent residuals analysis (RA)(3) estimated radionuclide concentrations in the final waste forms using the Site Treatment Plans (STP). The STP specified treatment processes for the mixed waste streams under consideration. The RA further compared the STP- derived concentrations in the final waste forms with the PE-derived concentration limits for each of the 15 sites. This comparison allowed identification of potentially problematic streams which after treatment will result in waste forms with radionuclide concentrations exceeding allowable limits at the considered disposal facilities.
WASTE FORM INITIATIVE STRATEGY
The WFI strategy continues the previous efforts of the DWG to achieve a solution for disposal of wastes in the potentially problematic waste streams. A flow diagram of the WFI strategy is shown in Fig. 1.
The initial efforts refine the modeling data and assumptions which lead to the identification of the potentially problematic waste streams for selected arid sites. These waste streams are identified in the residuals analysis by comparing the PE- derived radionuclide concentration limits of 15 candidate disposal facilities with the concentrations in waste forms from the treatment process. The PEs are simplified representations of site specific performance assessments. They use a set of conservative modeling assumptions regarding performance of the final waste forms and mechanisms of radionuclides migration to derive conservative estimates of allowable radionuclide concentrations in the disposed waste. The assumptions of radionuclide migration include sufficient details to capture major site-specific characteristics but are general enough to allow consistent application at all facilities. The assumptions of the waste form performance are introduced because of insufficient available data. A search for such data will be conducted, and, if necessary, a request for information will be issued to obtain these data. Availability of such data may lead to higher allowable radionuclide concentration limits in WAC at the selected facilities, and, consequently, to reducing the number of potentially problematic waste streams.
As a further refinement of the analysis, the potentially problematic waste streams will be evaluated against WAC for existing DOE and commercial facilities, such as Hanford and Envirocare of Utah, Inc. These facilities were recommended for disposal of the MLLW by the DWG.(4) Because the WAC were developed specifically for each site, they serve as more realistic screening tools in determining potentially problematic waste streams. The WAC for Hanford MLLW have not been finalized, therefore LLW WAC will be used, since they are expected to be similar in terms of radiological limits to the MLLW WAC.
By applying more realistic site-specific WAC as compared to the PE-derived limits, the number and volumes of potentially problematic streams are expected to decrease. The remaining waste streams which will not pass those WAC will remain problematic, and a waste stream-specific evaluation will be conducted to determine options that will ensure disposal of final waste forms.
In addition, a cost-benefit analysis for selecting the most cost effective waste forms will be performed. A computer decision-making tool will be developed for selecting most economic waste forms. That tool will allow estimating transportation, packaging, and disposal costs, as well as benefits of the improved waste form performance.
Fig. 1. Flow diagram
of the waste form initiative strategy.
DISCUSSION
The RA compares the estimated radionuclide concentrations in treated MLLW streams with the PE-derived disposal limits. Based on DOE's current and five-year projected MLLW inventory of 130,300 m3 evaluated in the RA, 111,700 m3 of treated waste will result from treatment due to an estimated volume reduction of 18,600 m3. Of the treated waste volume, the RA identifies approximately 92,000 m3 requiring disposal as MLLW, 5,100 m3 as transuranic TRU waste, and 6,000 m3 as LLW. Not considered in the RA are 8,600 m3 of treated MLLW because a treatment process has not been assigned in the STP. Of the 92,000 m3 MLLW, a volume of approximately 65,000 m3 is sufficiently characterized with respect to radionuclides to allow comparison with the PE-derived limits for disposal and identification of the potentially problematic waste streams. The remaining volume of 27,000 m3 of MLLW is not characterized for either the radionuclide composition or concentration in the waste. Thus a total treated waste volume of 34,600 m3, consisting of 27,000 m3 that is not characterized and 8,600 m3 that is not considered in the RA, will require additional evaluation to determine viable alternatives for disposal.
The computer based cost-benefit analysis tool will provide waste generators with a useful method to make cost-effective decisions related to MLLW management. This tool will allow evaluations of economic benefits of various assumptions including use of volume reduction and improved waste forms. The assumption that volume reduction is universally desirable, for example, may not always be valid, since restrictions due to Department of Transportation regulations or intruder scenarios in the WAC for disposal facilities may limit weight or radionuclide concentration of waste forms. In addition, costly treatment or use of exotic technology may more than offset the benefits of shipping and disposing of a lesser volume. The cost-benefit analysis will provide guidelines for waste generators on the options available for treatment of any class of wastes based on life-cycle costs.
The proposed strategy is based on recommendations of site waste managers of DOE to dispose the MLLW at the Hanford and Envirocare facilities. If other disposal facilities are recommended for disposal, different waste streams may be identified as problematic. The same approach based on the combination of the PE and RA of the new/additional sites with the cost-benefit analysis tool will ensure that all generated waste forms will meet the disposal regulations and be optimized with respect to cost.
CONCLUSION.
The WFI strategy provides a mechanism for ensuring that the treatment of MLLW will result in the waste forms that meet disposal regulations. Selection of waste forms for disposal of MLLW are based upon cost-benefit analysis which ensures that the most cost effective waste forms are produced. The proposed approach is applicable to selection of disposable and cost-effective waste forms if other new and/or additional disposal facilities are recommended.
REFERENCES