PERFORMANCE ASSESSMENT FOR
CONTINUING AND FUTURE OPERATIONS
AT SOLID WASTE STORAGE AREA 6
ORNL-6783/R1
This revised performance assessment (PA) for the continued disposal operations at Solid Waste Storage Area (SWSA) 6 on the Oak Ridge Reservation (ORR) has been prepared to demonstrate compliance with the performance objectives for low-level radioactive waste (LLW) disposal contained in U.S. Department of Energy (DOE) Order 5820.2A. This revised PA considers disposal operations conducted from September 26, 1988, through the projected lifetime of the disposal facility. The performance objectives of DOE O 5820.2A require the disposal facility be managed to accomplish the following:
This revised PA has been prepared in accordance with the guidance provided by DOE in the Interim Format and Content Guide and Standard Review Plan for U.S. Department of Energy Waste Disposal Facility Performance Assessments (DOE 1996c), which outlines the presentation of the analysis conducted for this revised PA. This revised PA has been prepared in response to the review of the initial PA for SWSA 6 (ORNL 1994) and is responsive to the commitments made by DOE to the Defense Nuclear Facilities Safety Board (DNFSB) in the Implementation Plan, Defense Nuclear Facilities Safety Board Recommendation 94-2 (DOE 1996b). The review of the initial PA required a revision to be prepared to address the changes made to waste management operations in response to the initial PA and to include the additional material developed in the review of the initial PA. This revised PA includes cross references to the initial PA, where appropriate, to indicate the changes that have been made to waste management operations and to many of the conclusions of the initial PA. Changes were also made to the methodology used in the analysis to provide results of greater utility to the development of revised waste acceptance criteria (WAC) and waste disposal operations. This revised PA along with the Composite Analysis for the SWSA 6 Facility (ORNL 1997b) provide the basis for the issuance of a Disposal Authorization Statement for the continued disposal operations at SWSA 6, providing these two documents are determined to be acceptable to DOE after their review by DOE.
SWSA 6 is located about 3 km (2 miles) south of Oak Ridge National Laboratory (ORNL) on the DOE ORR. The facility is located on a 27.5-ha (68-acre) tract of land with rolling terrain. Approximately 12 ha (30 acres) of the site are suitable for disposal operations. Most of the site's capacity was used before September 26, 1988. The facility is projected to continue operations until the currently operating Interim Waste Management Facility (IWMF) disposal unit is filled to capacity. The IWMF is currently filled to 50% capacity. Those portions of the facility associated with historical disposal operations are presently subject to remediation under the Comprehensive Environmental Response, Compensation, and Liability Act of 1980 (CERCLA). Following the guidance for preparation of this revised PA, the PA has been prepared without the consideration of disposal operations performed prior to the issuance of DOE O 5820.2A. This PA considers five different types of disposal units: unlined trenches, concrete wells in silos, concrete silos, lined wells, and tumulus units. Each disposal unit and the wastes placed in the disposal unit or planned to be placed in the disposal unit are considered separately. The analysis integrates the results of all of the disposal units to evaluate compliance with DOE O 5820.2A for existing disposals which are subject to the order and to provide limits on future disposals that will meet the performance objectives of DOE O 5820.2A. The analysis of each disposal unit assumes normal or expected performance. Accidental releases or abnormal operations are considered in the safety documentation for the facility and are not part of the PA.
In applying the performance objectives of DOE O 5820.2A, performance measures or performance criteria about points of compliance, times of compliance, and dose objectives for groundwater resource protection were developed that are consistent with the guidance on the preparation of performance assessments (DOE 1996c). The all-pathways performance objective of 25 mrem per year, excluding doses from radon and progeny, was applied at the land use boundary for the SWSA 6 facility, which is considered to be the fence surrounding SWSA 6. Current land use planning on the ORR is being conducted with the expectation that some portions of the ORR will be released to the public by DOE. As a conservative measure to provide defense in depth, the land use boundary was selected as the buffer zone boundary for SWSA 6 until formal land use plans for the ORR are completed. The performance objective for the air pathway of 10 mrem per year was also applied beyond the edge of the buffer zone for SWSA 6. For the consideration of radon and progeny the average flux of 20 pCi/m2/s was used as a performance criterion and was applied at the disposal facility surface. The performance criteria for the protection of the hypothetical inadvertent intruder of 100 mrem per year from chronic exposure and 500 mrem from a single event were applied to the SWSA 6 facility. For groundwater protection, the performance criterion of 4 mrem per year effective dose equivalent (EDE) was selected based on the lack of other rules or requirements for groundwater protection on the ORR; this dose is consistent with the U.S. Environmental Protection Agency proposed revision of the drinking water standards (EPA 1991). This groundwater protection performance criterion was applied outside of a 100-m buffer zone around each disposal unit, which was modified to include a small tract of land within SWSA 6. The groundwater performance measure was extended to surface water because of the rapid communication of contaminated groundwater to surface water and the presence of ephemeral streams within the SWSA 6 facility. This surface water performance criterion was applied at the White Oak Dam (WOD), which is located adjacent to the SWSA 6 facility boundary, upstream of the White Oak Creek (WOC) Embayment and the Clinch River. The results of the PA indicate that the groundwater and surface water performance criteria are more restrictive than the all-pathways performance criterion, and the air-pathway performance criterion is not important for evaluating acceptable disposals at SWSA 6.
Calculations of contaminant transport were performed to determine the magnitude and timing of the peak dose at the point of compliance. The time of occurrence of the peak dose was then examined against the 1000-year time of compliance specified in the guidance for preparing PAs. While several radionuclides have peak doses at some time in excess of 1000 years, the determination of compliance for all radionuclides was based on doses occurring at 1000 years, and potential long-term impacts beyond 1000 years were addressed. For the analysis of the inadvertent intruder, the time of compliance was specified as greater than 100 years but less than 1000 years for all radionuclides. For radionuclides with radiologically significant long-lived decay products (e.g., isotopes of uranium), potential long-term impacts beyond 1000 years also were addressed, but these results were not used in determinations of compliance. This is consistent with the guidance for preparing PAs.
Approximately 2000 m3/year (75,000 ft3/year) of LLW is managed at ORNL, with the IWMF being the only operating disposal unit. Waste not acceptable for disposal at the IWMF is stored for disposal at some other facility. Waste accepted for disposal at the IWMF is contact-handled (CH) waste (<200 mrem/h dose rate at the surface). This waste includes debris from ORNL operations, research and development activities, environmental restoration, and decommissioning and demolition activities. Waste disposed of in low-range silos and tumulus disposal units included CH waste, while remotely handled (RH) waste (>200 mrem/h at the surface) was disposed of in high-range silos and concrete wells. RH waste included debris from reactors and hot cell operations. RH waste with <1 rem/h was disposed of in silos, and RH waste with >1 rem/h was disposed in metal lined wells with concrete plugs. Fissile waste from research and development activities using enriched uranium was disposed of in separate metal lined wells with concrete plugs. Biological waste consisting of excrement and animal carcasses was disposed of in biological trenches. Asbestos waste generated from demolition and maintenance was disposed of in concrete silos. All below-grade disposal operations at SWSA 6 were discontinued in 1994.
Wastes generated at ORNL are subject to the ORNL Waste Certification Program, which requires the wastes to be characterized prior to their acceptance by ORNL Waste Management. Wastes are accepted for disposal at the IWMF providing they meet the WAC for the IWMF. Wastes not acceptable for disposal are stored for disposal at some later date and location. The Waste Certification Program provides procedures for radiological characterization, procedures for meeting WAC for the IWMF, and the structure and waste certification methods to be used by waste generators. Characterization data are also recorded and maintained as part of the waste manifesting system at ORNL. Prior to disposal, most wastes are subject to volume reduction including compaction, incineration, and metal melting. Wastes disposed of at the IWMF are generally packaged in metal boxes of approximately 90 ft3 capacity prior to placement inside of a concrete vault. Other metal containers are allowed on a case by case basis.
The waste characteristics used for assessing the performance of SWSA 6 were defined using the existing data records from the ORNL Waste Management Program and did not use the most probable activity factors (MPA), which were applied to the reported inventories in the initial PA. While the data from these records are imperfect, they represent the best source of information on waste disposals. These records have been reviewed and corrected where obvious errors were identified. The uncertainty associated with the disposed inventory was analyzed as part of the PA. The radionuclides considered in detail in this revised PA include all those identified from waste management records and all radionuclides which can be reasonably expected to be present in ORNL wastes. Radionuclides with half-lives less than 5 years were not considered in this revised PA because the potential contribution to dose could not be significant. A total of 63 radionuclides was considered in detail to fulfill three objectives: (1) to demonstrate that the disposals that have occurred at SWSA 6 since September 26, 1988, in currently closed disposal units are in compliance with the performance objectives of DOE O 5820.2A; (2) to demonstrate that the wastes previously disposed of at the currently operating IWMF disposal unit are in compliance with the performance objectives of DOE O 5820.2A; and (3) to determine the disposal inventory limits to be applied to future waste disposals at the IWMF disposal unit that will demonstrate compliance with the performance objectives of DOE O 5820.2A.
The methodology used to analyze the performance of SWSA was based on the available data on the waste disposed of at SWSA 6, the disposal methods used at SWSA 6, and SWSA 6 site characteristics. In analyzing the site performance, the results of the source-term modeling (which provide estimates of releases of contamination from the disposal units) were used as input to the shallow subsurface model. The contamination released to the shallow subsurface was diluted with upslope shallow subsurface drainage, which was estimated from terrain analysis and moisture transport through soils to disposal units. The shallow subsurface model provided estimates of the transport of contamination in soils to surface water, the percolation of contaminants to groundwater, and estimates of uncontaminated recharge to groundwater. The groundwater model used the contaminated and uncontaminated water fluxes as input to estimate the transport of contamination to a well at the edge of the expanded buffer zone surrounding the waste disposal units in SWSA 6 and to estimate the discharge of contaminated groundwater to surface water. The resulting concentrations in groundwater and surface water were used to estimate the dose from the use of water resources from existing disposals at SWSA 6 and to establish inventory limits on future disposals at IWMF that would meet the performance criteria for SWSA 6.
The analysis for SWSA 6 required the use of assumptions to supplement the available site data when the available data were incomplete for the purposes of analysis. Assumptions were made to define the partitioning of recharge to runoff from each disposal unit, the aquifer properties, the sorption characteristics of disposal units and the site, the geometry of waste configurations, and the degradation and cracking of concrete. These assumptions were selected to provide a reasonable yet conservative representation of facility performance and were based on the limited information available for the disposal units.
The long-term performance of SWSA 6 was analyzed using a series of linked models to provide estimates of the doses attributable to LLW disposals from closed disposal units as well as the evaluation of allowable inventories for future IWMF disposals that are consistent with meeting the performance objectives. The long-term performance of SWSA 6 was evaluated for the hydrologic and atmospheric transport of contamination from each disposal unit. Transport through these media provided the basis for demonstrating compliance for the all-pathways, air pathway, and the water resource performance criteria.
The hydrologic transport of contamination from SWSA 6 considers the release of contamination from the individual disposal units using the SOURCE1 and SOURCE2 computer programs. For the purpose of demonstrating compliance with the performance objectives for off-site releases, the parameter values were selected with the intent of overestimating actual releases from disposal units. These verified codes estimated the release rate of contamination by considering the wetting of the waste by infiltration of water into the waste, and the subsequent leaching and transport of radionuclides from the waste form by diffusion and advection. The complex forms of wastes, disposal units, and concrete barriers were approximated, including the behavior of the concrete pad and drainage systems for the tumulus type disposal units. The process of degradation and cracking of concrete over time were included in the analysis and were used to identify the hydrologic isolation to be credited to the various disposal units in SWSA 6.
Contamination released from the disposal units was analyzed for transport through the environment using the Unified Transport Model (UTM) and the U.S. Geological Survey Method of Characteristics (USGS MOC) computer programs. The transport of contamination in surface water and groundwater took into account precipitation, shallow subsurface transport in soils, infiltration, percolation, recharge, sorption, streamflow, and radioactive decay. A projected closure plan was utilized that was derived from the Accelerated Cleanup Plan developed for the ORR. The computer programs used for analyzing the environmental transport of contamination are documented, verified, and validated to the extent that they provide reasonable representations of site performance. The results of these programs were used to estimate potential doses from existing waste disposal operations and to estimate additional waste loadings to IWMF that would permit SWSA 6 to meet the performance criteria for the all-pathways and water resources performance criteria.
Contamination released from disposal units by atmospheric transport of volatile radionuclides (3H and 14C) was also analyzed to the extent needed to demonstrate compliance with the air pathway performance criterion. This very conservative approach presumed the release of radionuclides by diffusion through the media overlying the disposal units, with all volatile releases being in the chemical forms of 14CO2 or 3H2O. The results of this analysis were used to estimate potential doses from existing waste disposal operations, but no limitations on future waste disposals were identified that were not bounded by other exposure pathways. Non-volatile radionuclides were shown to not contribute to the total dose for the air pathway.
The analysis of potential doses to off-site individuals and inadvertent intruders attributable to disposal facility operation were estimated using several assumptions. For individuals residing outside the SWSA 6 buffer zone, which is either a 100-m buffer zone surrounding the SWSA 6 disposal units, or an expanded buffer zone that includes additional small tracts of land within SWSA 6, annual doses were estimated for direct ingestion of contaminated water, ingestion of milk and meat obtained from dairy and beef cattle that drink contaminated water, external exposure from swimming in contaminated water, and ingestion of fish obtained from contaminated water. In addition, doses to off-site individuals due to atmospheric releases of the volatile radionuclides 3H and 14C were evaluated. Following the 100-year active institutional control period, inadvertent intruders were assumed to be exposed according to one of four different scenarios --the "agriculture," "resident," "post-drilling," and "discovery" scenarios--all involving direct intrusion into disposal units. The discovery scenario was assumed to occur once in an individual's lifetime, whereas the other scenarios were assumed to involve continuous exposure. The agriculture scenario was assumed to occur at 300 years after the disposal for all disposal units constructed with engineered barriers, but at 100 years after disposal for the biological trenches constructed without engineered barriers. The resident, post-drilling, and discovery scenarios were assumed to occur at 100 years after disposal for all disposal units, but were not considered relevant for the biological trenches. The dose analysis for inadvertent intruders includes considerations of mobilization and transport of radionuclides from the various disposal units prior to the time intrusion was assumed to occur, based on the results of the SOURCE1 model. For the purpose of the intruder dose assessment, the parameter values used in the code were selected with the intent of overestimating the quantities of radionuclides remaining in the disposal units over time.
Although the results of the PA indicate that existing disposals at IWMF are in compliance with the performance objectives of DOE O 5820.2A and, thus, that the IWMF can continue to accept waste in the future, the results of this revised PA indicate that all closed disposal units at SWSA 6 and the existing disposals at IWMF are in compliance with all of the performance objectives of DOE O 5820.2A. The results also provide allowable inventory limits per pad for the disposal of LLW on the unused portions of the IWMF. Of the 63 radionuclides considered in detail, one radionuclide is limited by the specific activity of the radionuclide, two are limited by the groundwater performance criterion, and the remaining 60 radionuclides are limited by the criteria for the protection of a hypothetical inadvertent intruder. These limits were determined by using the composite model for SWSA 6 to evaluate the additional loading of each radionuclide on IWMF that would yield the dose prescribed by the performance criteria for SWSA 6 for the groundwater pathway, surface water pathway, air pathway, all-pathways, and inadvertent intruder analysis. The most restrictive limit for each radionuclide determined the allowable inventory of each radionuclide at the IWMF. For the evaluation of existing wastes on the IWMF, which contain multiple radionuclides, the sum-of-fractions rule was applied to each pad on the IWMF to verify the acceptability of the wastes currently disposed of on the IWMF.
The sensitivity and uncertainty of the results are important in interpreting these results and evaluating compliance of SWSA 6 with the performance objectives of DOE O 5820.2A. Sensitivity analyses were used to identify the modeling parameters that had the greatest effect on the results. The identified parameters were used in the uncertainty analysis to determine the confidence to be attributed to the results. The uncertainty analysis incorporated the subjective evaluation of the acceptability of each component of the modeling of site performance as well as the quantitative evaluation of the model component uncertainty. The results of the uncertainty analysis indicate that the greatest source of uncertainty is associated with the waste inventory and that uncertainty in the calculated results increased as the time increased. Furthermore, the uncertainty in the waste inventory was greatest with the waste previously disposed of in the SWSA 6 disposal units.
The question of whether releases of radionuclides from SWSA 6 to the general environment would be as low as reasonably achievable (ALARA) was considered in the revised PA. Based on an assumption that a population the size of the city of Oak Ridge would use the water from the Clinch River near the discharge of White Oak Creek as a drinking water supply, and taking into account that SWSA 6 would comply with the performance objective for protection of surface water at WOD, the projected collective dose over 1000 years was sufficiently low that the expected number of health effects would be zero. Furthermore, the maximum individual dose from the use of water in the Clinch River is expected to be less than 1 mrem per year. Therefore, the projected impacts from waste disposals in SWSA 6 are sufficiently low that further efforts to reduce releases from SWSA 6 based on the ALARA objective are unwarranted.
The results of the PA suggest the need for changes in the current and future operations at the IWMF to address the concerns of the DNFSB and to ensure compliance with the performance objectives of DOE O 5820.2A. A recommended strategy for future IWMF operations has been developed to provide the needed assurance that the performance objectives will be maintained. The strategy has five parts: (1) buffer zone expansion, (2) restricted disposal limits for problematic radionuclides, (3) removal of selected vaults, (4) discontinuation of the most probable activity factor, and (5) development of off-site disposal capacity.
The recommended expanded buffer zone, which was applied to this revised PA, reduces the compliance requirements for groundwater protection within SWSA 6 but provides a consolidated area for waste management and environmental compliance that is within the SWSA 6 facility boundary. Some radionuclides were identified as problematic with respect to compliance with the performance objectives in the initial PA, and the results of this revised PA for these radionuclides, as shown in the uncertainty analysis, become less certain as time increases. These two factors suggest the reduction of the inventory limits for the disposal of these problematic radionuclides to 1% of the limits calculated in the PA. This substantial reduction in the disposal limits for future disposals at IWMF provides additional assurance that the performance objectives will be met, and it significantly reduces the potential uncertainty of compliance. Furthermore, the land use boundary for SWSA 6 may extend well beyond the existing facility boundary once the revised land use plan for the ORR is completed. The existing buffer zone boundary for SWSA 6, that extends slightly beyond the SWSA 6 facility boundary, was used as the point of assessment for this revised PA, which adds an additional margin of safety. These reduced disposal limits are to be applied to the vaults currently disposed of on the IWMF. Vaults that are identified as exceeding these modified disposal limits are to be removed from the IWMF. Waste characterization practices at ORNL have significantly improved since the publication of the initial PA, such that the use of a most probable activity factor, which artificially increases the inventory of radionuclides in each vault, is no longer justified and is to be discontinued. Finally, the use of SWSA 6 as a disposal facility will be discontinued once the IWMF is filled to capacity with waste. Once the IWMF is filled, future disposal of waste generated at ORNL will be performed at an off-site disposal facility. These recommendations are to be adopted as part of the waste disposal operations at the IWMF.