Phillip Pohl, Robert Waters, Wu-Ching Cheng, Marilyn Gruebel,
Brenda Langkopf and Timothy Wheeler
Sandia National Laboratories
Albuquerque, NM, 87185-720

A key issue for the U.S. Department of Energy’s Mixed Waste Focus Area’s Waste Form Initiative was identifying mixed low-level waste (MLLW) streams that may be problematic in terms of disposal. Previous reports have quantified and qualified the capabilities of fifteen DOE sites for disposal of MLLW and provided estimates of volumes and radionuclide concentrations of treated MLLW based on the DOE inventory. Scoping-level analysis indicated that 105 waste streams identified in this report (approximately 6400 m³ of the estimated total treated MLLW) had radionuclide concentrations that may make their disposal problematic. In the present work, the radionuclide concentrations of these waste streams were compared with the waste acceptance criteria (WAC) for a DOE disposal facility at Hanford and for Envirocare’s commercial disposal facility for MLLW in Utah. Of the volume of treated MLLW identified as potentially problematic, about 100m3 exceeds the WAC for disposal at Hanford, and about 4600 m3 exceeds the WAC for disposal at Envirocare. Approximately 7% of DOE’s total MLLW inventory has not been sufficiently characterized to identify a treatment process for the waste and was not included in the analysis. In addition, of the total treated MLLW volume, about 30% was associated with waste streams that did not have radionuclide concentration data and could not be included in the determination of potentially problematic waste streams.

In September 1996, the MWFA requested that Sandia National Laboratories conduct a refined residuals analysis project to evaluate the disposability of certain "potentially problematic" MLLW streams that they had recently identified through scoping analyses. This previous work on disposal issues has been conducted by Sandia for the FFCAct Disposal Workgroup. The group was established by DOE in 1993 to work with the States in identifying, from among the sites currently storing or expected to generate MLLW, those that might be suitable for the disposal of MLLW. The disposal capabilities of the fifteen sites selected through this process were quantified and qualified in the scoping-level performance evaluation (PE) project completed in early 1996. An additional scoping-level residuals analysis (RA) provided estimates of volumes and radionuclide concentrations for treated MLLW that were based on DOE’s current and five-year projected inventory of approximately 130,000 m³. The RA provided a means for identifying MLLW streams that may be potentially problematic in terms of disposal.

The 105 waste streams (approximately 6400 m³ of treated MLLW) that were considered in this refined residuals analysis as potentially problematic had total radionuclide concentrations, based on the scoping-level estimates used in the RA project, that were greater than 10 times the concentration limits derived from the PE project for the Hanford Reservation. In the refined analysis, the radionuclide concentrations of these waste streams were compared with the WAC for DOE’s low-level waste (LLW) disposal facility at Hanford (the WAC for a MLLW facility at Hanford are not yet complete) and with the WAC for Envirocare’s commercial disposal facility for MLLW in Utah. At Hanford, the WAC are based on the LLW performance assessment recently completed for its currently operating shallow land burial site. The WAC provide limiting activity concentrations for Category 1 and Category 3 wastes, which correspond to the performance-assessment results of the homesteader and post-drilling scenarios, respectively. Envirocare of Utah is a commercial site, and details about the development of their WAC are not available.

The details of the methodology, results, and conclusions of the Sandia study are contained in the report by Waters et al. (1) and summarized here.

The waste characterization data used in the analysis were obtained from the Mixed Waste Inventory Report (MWIR) database and augmented and modified during site reviews. While more detailed waste characterization data are expected to be available at the sites, the level of data contained in the MWIR database was expected to be sufficient for this scoping-level analysis.

The characterization data contained in the MWIR database are based on untreated waste. Prior to disposal, these waste streams must be treated to remove or stabilize the hazardous constituents contained in the waste. This treatment and stabilization will change the concentrations of radionuclides remaining in the residual wastes. These after-treatment radionuclide concentrations are the ones that need to be considered for waste disposal; therefore, estimates of the changes in concentrations due to treatment must be made.

The concentration in each waste stream after treatment for each radionuclide i, C_Fi, was estimated using the following Eq. 1:

where

The preliminary estimates for r _b-initial for the waste streams were based on the matrix parameter categories (MPC) associated with each waste stream in the MWIR database (2). The values for the AMR were based on work done at SRS (3). The sites reviewed and updated the estimates for all parameter values for each waste stream.

Radionuclides with half-lives less than 5 years were not included in the analysis due to their limited effect on the long-term risks from disposal.

The values that were assumed for C_Ii were the mean concentration values for a particular waste stream given in the MWIR database or were based on a given range.

The comparisons of radionuclide concentrations in the potentially problematic waste streams to the WAC at Hanford and Envirocare were made using the sum-of-fractions (SOF) method described in 10 CFR Part 61.55 (see Eq. 2):

where

Waste streams with SOF less than one were considered acceptable for disposal.

Results of these calculations are presented in Figure 1 for Hanford and Figure 2 for Envirocare. At Hanford, all but 97 m³ of waste are acceptable for disposal, most using Category 3 disposal. Of the 97 m³, 79 m³ is associated with one waste stream with a SOF of 2. With more refined analysis, this waste stream may be shown to be acceptable for disposal at Hanford. The remaining 18 m³ of waste that exceeded the Category 3 limits at Hanford represent a small inventory of radionuclides and may also be shown to be disposable at Hanford with more refined analysis.

Approximately 4600 m³of the 6400 m³ of waste exceeded the WAC limits at Envirocare, illustrating that this site has relatively restrictive radionuclide limits. More refined analyses may indicate that some of these wastes streams would be acceptable for disposal at this site.

Based on the results of the analyses summarized here and discussed more fully in the Sandia report, the following conclusions and recommendations can be made:

• Of the approximately 6400 m³ of treated MLLW that was identified as potentially problematic for disposal at Hanford based on the PE-derived limits, all but 97 m³ meets the WAC for either Category 1 or Category 3 wastes at Hanford. Additional documented justification for disposal of the 97 m³ may allow its disposal at Hanford, with possible additional disposal requirements. Disposal of this waste in concrete boxes (as is the current practice for Category 3 waste) may be sufficient, thus removing the need for additional measures.

• Of the approximately 6400 m³ of treated MLLW that was identified as potentially problematic for disposal, approximately 4600 m³ exceeds the WAC for disposal at the Envirocare of Utah facility. This result illustrates that the WAC for the Envirocare facility is relatively restrictive compared to the radionuclide concentrations in DOE MLLW.
• There appear to be adequate data to estimate waste form performance for both grouted wastes and vitrified wastes. However, the data with which to estimate performance of both polyethylene microencapsulated and macroencapsulated waste is sparse. Based on the data used in this analysis, polymer encapsulation comprises less than 1% of the volume of potentially problematic MLLW for disposal at Hanford. If this waste form is expected to be used more extensively for MLLW disposal, additional performance data should be collected.
• The evaluation analyzed only a portion of the MLLW for disposal. Approximately 8700 m³ of MLLW was insufficiently characterized to allow the sites to determine a treatment process for these waste streams. Some of this volume is associated with MLLW streams that have not been generated. Other insufficiently characterized waste streams may require advanced treatment processes or special waste forms to be acceptable for disposal.

• The evaluation indicated that Hanford could dispose of much of the problematic MLLW as Category 3 wastes. However, the cost for disposal of Category 3 wastes is approximately three times higher than for Category 1 wastes at Hanford. A cost/benefit analysis should be conducted to determine if the use of different or additional treatment processes or different waste forms will result in MLLW that can be disposed of as Category 1 wastes.
• If it is to be used significantly as a waste stabilization agent, more performance data should be collected for the polyethylene waste form for both microencapsulation and macroencapsulation. The data currently available are sparse and not radionuclide-specific, and they cannot be used to support decisions on waste form performance. Most of the work on polyethylene waste form performance has been conducted at Brookhaven National Laboratory.
• Further analysis of MLLW streams that do not currently have an assigned treatment process should be conducted.
• The analysis of performance of the individual waste streams in this evaluation was based on the waste forms selected by the sites. However, the basis for the selection of the waste forms by the sites was not always clear. The data should be reviewed to ensure that the types and characteristics of the waste are amenable to stabilization by the waste forms considered in this evaluation. If compatibility problems are identified, the use of alternative waste forms should be investigated.

1. WATERS, Robert D., Marilyn M. Gruebel, Brenda S. Langkopf, and Paul B. Kuehne, 1997. Analysis of the Technical Capabilities of DOE Sites for Disposal of Residuals from the Treatment of Mixed Low-Level Waste. SAND97-1098. Albuquerque, NM: Sandia National Laboratories.
2. KIRKPATRICK, T. D., 1995. DOE Waste Treatability Group Guidance, DOE/LLW-217, Rev. 0, Radioactive Waste Technical Support Program, Lockheed Idaho Technologies Company, Idaho Falls, Idaho.
3. WSRC (Westinghouse Savannah River Company), 1995. Conceptual Report on Onsite and Commercial Disposal Options for SRS Mixed Waste Streams (U), WSRC-RP-95-783, Rev. 0, Solid Waste Engineering, Westinghouse Savannah River Company, Aiken, South Carolina.

Sandia is a multiprogram laboratory operated by Sandia Corporation, a Lockheed Martin Company, for the United States Department of Energy under Contract DE-AC04-94AL85000.

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