PERFORMANCE ASSESSMENT FOR THE
CLASS L-II DISPOSAL FACILITY
ORNL/TM-13401
The PA has been prepared in accordance with the guidance provided by the DOE Peer Review Panel that outlines the format and content for a radiological performance assessment.
The proposed CIIDF will be located about 2.5 km (1.5 miles) north of Oak Ridge National Laboratory (ORNL) and about 5 km (3 miles) southwest of the Y-12 Plant on ORR in Bear Creek Valley. The proposed facility is located on a 10-ha (24-acre) tract of land on the slopes of a midvalley knoll. The site is to be graded, with the addition of fill material to provide capacity for the construction of 20 tumulus pads. Each pad will have the capacity for 30,000 ft3 of waste contained in 330 concrete vaults. Waste is to be packaged in ST-5 waste containers (B-25 boxes), which will be emplaced in the concrete vaults. The annular space between each ST-5 container and concrete vault is to be filled with grout material and the vault sealed with butyl rope seal material. The vaults will be stacked in three layers and ultimately covered with an engineered earthen cover. CIIDF is to be constructed with diversion channels on the north, west, and east sides. These channels are designed to intercept surface water run-on, collect run-off, and locally depress the water table. Drains are to be installed in existing ephemeral creeks that transect the tumulus pad layout. The pads are to be sloped to a drainage channel that is connected to a drainage gallery. Underpad drainage is also piped into the drainage gallery. Gallery discharges are to be monitored prior to discharge to the National Pollutant Discharge Elimination System (NPDES) discharge point. Contaminated gallery discharges are to be collected and treated prior to release to the environment.
Approximately 10,000 m3/year of LLW is managed each year on ORR. Of this quantity, a relatively small volume is to be disposed of at CIIDF. These wastes are expected to include a large number of radionuclides, most of which are expected to have radionuclides with half-lives less than 30 years. Wastes disposed of at CIIDF will be characterized and certified by waste generators using waste acceptance criteria that are to be developed from the results of this PA.
This PA considered radionuclides likely to be present in wastes to be disposed of at CIIDF, and considered in detail those radionuclides with half-lives greater than 5 years. Radionuclides with half- lives less than 5 years were excluded from detailed consideration based on the integrity of CIIDF, which would allow decay to render them insignificant in contributing to exposures of individuals. Each radionuclide considered in detail was analyzed to determine the maximum allowable inventory that could be present on a tumulus pad and meet the performance objectives of DOE Order 5820.2A. Mixtures of radionuclides are addressed by combining the contributions of each radionuclide to ensure that the sum is less than the allowable inventories.
The maximum allowable inventories for each pad that meet the performance objectives of DOE Order 5820.2A were determined using a series of models to represent the site-specific behavior of CIIDF. Transport of contamination to points of public exposure was determined to be controlled by groundwater and surface water transport, to the exclusion of all other exposure pathways. Inadvertent intrusion was considered by analysis of the agriculture, resident, and post-drilling scenarios. Other scenarios were discussed but shown to be less significant in the analysis of CIIDF.
The long-term performance of CIIDF was conducted a connected suite of models The water budget for the facility was determined by the Unified Transport Model (UTM), which used hourly inputs to calculate the monthly seepage into CIIDF, the annual recharge into the groundwater, and the annual surface and shallow subsurface water fluxes. Seepage estimates into the CIIDF were used to calculate releases of contamination from the facility using the SOURCE1 model. The PADSIM model divided these releases between the annual lateral transport of contamination to the shallow subsurface and groundwater recharge. Recharge of contaminated water was used as input to the FTWORK model to determine the groundwater transport of contamination and the subsequent release to surface water. Through iteration of the problem, the results from the computer modeling were used to determine the maximum concentration of each radionuclide in waste corresponding to the performance objective for water resource protection of 4 mrem/year. The calculated maximum concentration was then converted to the maximum allowable inventory for each pad at CIIDF.
Estimates of the maximum allowable inventory on each pad which satisfies the performance objectives for inadvertent intrusion were calculated for identified scenarios that are the most restrictive for the CIIDF. The maximum allowable inventory for each radionuclide was then set based on the most restrictive limit calculated from the inadvertent intruder and environmental transport calculations.
The analysis for CIIDF required the use of assumptions to supplement the available site characterization data. Assumptions were made to define the physical and chemical properties of engineered and geologic materials, the closure of the disposal facility, and the application of annual averages of data representing processes and events of short duration. These assumptions were selected to provide a reasonable yet conservative representation of facility performance and were based on the limited information available.
The methodology used to analyze the performance of CIIDF was based on the available data about the waste anticipated to be disposed of at CIIDF, the disposal methods to be used at CIIDF, and the CIIDF site characteristics. Since the results were calculated to ensure that the performance objectives were satisfied for each radionuclide, operation of the facility with waste acceptance criteria consistent with the maximum allowable inventories ensures that the performance objectives are satisfied. A qualitative sensitivity and uncertainty analysis was conducted to identify parameters and mechanisms that contribute to uncertainty in the results. A quantitative uncertainty analysis will be prepared as part of the revision of this PA prior to operation of CIIDF.
The results determined in this PA indicate that of the 60 radionuclides analyzed, 35 were limited by the surface water or the groundwater pathway. The results were calculated to the time of maximum dose with the understanding that a time of 10,000 years would be appropriate for establishing a time of compliance. Only one radionuclide had a peak after the 10,000-year time of compliance (10Be), and all but four radionuclides had reached the their peak by 1,000 years. The analysis of direct intrusion indicated that extended periods of institutional control would increase the maximum allowable inventories of 9 of the 25 radionuclides limited by intrusion.
The analysis of the performance of CIIDF did not consider sources of contamination present in Bear Creek Valley associated with historical disposals or plant operations on ORR. These sources of contamination will be considered in a separate composite analysis report that will be prepared in conjunction with the revision of this PA.
The results presented in this PA provide a reasonable basis for evaluating the performance of CIIDF and provide reasonable assurance that the performance objectives of DOE Order 5820.2A will be met for the disposal of LLW at CIIDF. Continued work to reduce uncertainties and improve the methodology used to analyze the performance of CIIDF are likely to relax the allowable limits on inventory determined in the analysis.