SWSA 6 Composite Analysis

John Tauxe was a contributing author to this work while a researcher at ORNL:

COMPOSITE ANALYSIS FOR
SOLID WASTE STORAGE AREA 6
ORNL-6929

EXECUTIVE SUMMARY

This Composite Analysis (CA) for Solid Waste Storage Area 6 (SWSA 6) on the Oak Ridge Reservation (ORR) has been prepared in response to the Defense Nuclear Facilities Safety Board (DNFSB) Recommendation 94-2 and the commitments by the U.S. Department of Energy (DOE) to the DNFSB in the Implementation Plan, Defense Nuclear Facilities Safety Board Recommendation 94-2 (DOE 1996b). This CA was prepared according to the Guidance for Composite Analysis of the Impact of Interacting Source Terms on the Radiological Protection of the Public from Department of Energy Low-Level Waste Disposal Facilities (DOE 1996a) and presents a complete analysis of other potential contributing sources of radioactive contamination to the potential releases of radioactive contaminants from the low-level waste (LLW) disposal operations at SWSA 6. This document is a companion document to the Performance Assessment for the Continuing and Future Operations at Solid Waste Storage Area 6 (ORNL 1997). Together, the two documents provide the necessary technical basis for the issuance of a Disposal Authorization Statement for disposal operations at SWSA 6, once both documents are reviewed and accepted by DOE. This document has been prepared to facilitate the review of the CA according to the Interim Review Process and Criteria for Composite Analysis (DOE 1996c).

The CA provides an estimate of the potential cumulative impacts to a hypothetical future member of the public from SWSA 6 disposal operations and all of the other sources of radioactive material in the ground on the ORR that may interact with contamination originating in SWSA 6. The projected annual dose to a hypothetical future member of the public from all contributing sources is compared to the primary dose limit of 100 mrem per year and a dose constraint of 30 mrem per year. For projected doses greater than the primary dose limit, an options analysis is required to identify alternatives for reducing the dose to levels below the limit. For projected doses less than the primary dose limit but greater than the dose constraint, an options analysis and ALARA (as low as reasonably achievable) assessment for further dose reduction is required. For projected doses less than the dose constraint, an ALARA assessment may be warranted, but only if it would be cost-effective.

Consistent with the CA guidance, dose estimates for the first 1000 years after disposal are emphasized for comparison with the primary dose limit and dose constraint. However, to provide more complete information on the potential long-term impacts of existing radioactive sources on the ORR, estimates beyond 1000 years also are presented. The point of assessment is specified in the guidance as the DOE land use boundary. For this CA, the current land use boundary, which is the edge of the ORR 20 km (12 miles) downstream from the confluence of White Oak Creek (WOC) with the Clinch River, is used as the point of assessment. The current land use plan for the ORR is being revised, and may include a reduction in the land currently controlled by DOE on the ORR. The possibility of changes in the land use boundary is considered in the CA as part of the sensitivity and uncertainty analysis of the results, the interpretation of results, and the conclusions.

SWSA 6 is located on the north shore of White Oak Lake (WOL), an impoundment of WOC created by White Oak Dam (WOD). Most of the radioactive materials disposed of over the period of operations of ORNL were disposed of in other locations in the WOC watershed upstream of SWSA 6 which is closest to WOD. A complete description of SWSA 6 and the site characteristics is provided in the SWSA 6 Performance Assessment (PA) (ORNL 1997).

The WOC watershed occupies parts of both Melton Valley and Bethel Valley, each with differing geohydrologic characteristics. The identification of potential sources of radioactive materials in these separate areas is being addressed as separate Comprehensive Environmental Response, Compensation, and Liability Act (CERCLA) actions. The contributing sources in Melton Valley are described in detail in the Melton Valley Remedial Investigation (MVRI) (DOE 1997b,c), a primary source of information for this CA. The contributing sources in Bethel Valley include the ORNL main plant area and SWSA 3 and currently are the subject of the Bethel Valley Remedial Investigation and Feasibility Study (BVRI/FS), to be completed in 1998.

The environmental setting for the facilities considered in this CA is typical of the Tennessee Valley. The landscape is characterized by rolling hills with hilltops typically 30 m (100 ft) above the lower portions of the valleys. The watershed includes four geologic groups common to the Valley and Ridge Province, with the Conasauga Group associated with Melton Valley and the Chickamauga Group associated with Bethel Valley. The soils in the area are thin and predominantly derived from weathered residuum of the underlying bedrock. The WOC watershed is interior to the ORR, distant from local population centers, and discharges to the Clinch River, a major tributary of the Tennessee River. The groundwater present in Melton and Bethel valleys has a limited yield and is largely localized between the many ephemeral drainages within WOC watershed. Groundwater is shallow and typically occurs as a water table aquifer. This groundwater is not used as a water supply on the ORR. Groundwater is recharged by precipitation and anthropogenic sources, and subsequently discharges to surface water within the watershed. Surface water discharges and the transport of contaminants within the watershed are heavily influenced by the effects of storms. The average discharge of WOC is 0.42 m³/s (15 cfs), and the average discharge of the Clinch River where WOC discharges to the Clinch River is 140 m³/s (4900 cfs).

The radioactive material from sources in the ground that could interact with the contamination from LLW disposed of at SWSA 6 are subject to remediation under the CERCLA process. While in progress, this process is incomplete for the facilities which could interact with SWSA 6. The feasibility studies (FS) for Melton Valley and Bethel Valley are in preparation, and so a Record of Decision (ROD) has yet to be developed. Because the CERCLA process is not complete, this CA does not consider the impacts that will occur from CERCLA actions that have yet to be defined. Instead, where no remediation has begun the CA considers the existing sources only in their present condition, which is equivalent to assuming that the CERCLA process would result in a selection of the no-action alternative. Future RODs that prescribe actions which might affect the results of this CA would require a revision of this CA as part of the CA Maintenance Program.

The CA considers the potential releases of unclassified radioactive material from SWSA 6, both the portion addressed in the revised SWSA 6 PA and the portion associated with disposals prior to September 26, 1988. A complete description of the disposed inventory in the SWSA 6 facility after September 26, 1988, is provided in the PA (ORNL 1997). The contributing sources to this CA also include the historical disposal facilities at SWSA 5, SWSA 4, and SWSA 3. Other primary contributing sources include the gunite and associated tanks (GAAT), the Surface Impoundments Operable Unit (SIOU), and Pits and Trenches area. These disposal facilities are associated with a variety of secondary sources of contamination present in WOC watershed, which were considered independently, such as the WOC floodplain. The sources of contamination were considered by identifying subbasins within the WOC watershed, using the approach taken in the MVRI. The inventories in each of these subbasins were defined using the data presented in the MVRI. For Bethel Valley sources, estimates were based on the information from the Remedial Action Program, which was developed prior to the initiation of the BVRI/FS.

The data used in this CA were identified using the data quality objectives (DQO) process. The data presented in the MVRI were subjected to a formal DQO process as part of the preparation of the MVRI. The approach taken in preparing this CA was subjected to a DQO process to ensure that the approach was consistent with other programs and that the best data available were utilized in the preparation of the CA. The various programs associated with the facilities addressed in the CA were involved in establishing the scope of the analysis and were involved in the review of the results.

Radioactive source inventories of important radionuclides in the various subbasins in Melton and Bethel valleys were derived from existing disposal records and available sampling data. In the absence of specific data, inventories were estimated by extrapolating known disposed radionuclide distributions at SWSA 6. Potential peak release rates to groundwater and surface water were estimated using a simple first-order leaching model. Transport of radioactive materials in water was the only release pathway considered in the CA because historical data on releases of radioactive material from the ORR and the findings of the PA have shown that releases by other pathways are insignificant compared with releases in water.

The travel time to WOD for each radionuclide was determined from the travel times in the SWSA 6 PA. The travel time for a given radionuclide in the CA is equal to the difference between the average time of the peak mass releases from the source and the peak concentration at WOD resulting from the SWSA 6 PA calculations. Decay of each radionuclide was incorporated into the calculations. The peak mass release rate from the source term for each radionuclide from each subbasin was summed and diluted by the discharge at WOD to calculate the total concentration at WOD. Three different approaches were used to estimate the timing of the peak concentrations at WOD. In the first case, all released radionuclides were assumed to arrive immediately. This case neglected the effect of travel times for the radionuclides considered and is interpreted as an extreme upper bound. For the second case, arrival time bins of 0–100, 100–1000, and greater than 1000 years were used. The placement of peak concentrations of each of the radionuclides in the various time bins was based on the travel time results from the SWSA 6 PA. For the third case, time bins of 0–100, 100–500, 500–1000, 1000–5000, 5000–10,000, and greater than 10,000 years were used. Again, the placement of peak concentrations of each of the radionuclides in the various time bins was based on the results of the SWSA 6 PA. Concentrations in the Clinch River were estimated from concentrations at WOD by applying a dilution factor of 330, based on the relative average discharges of WOC and the Clinch River.

The resulting estimates of the peak radionuclide concentrations for each of the three approaches for evaluating time bins were used to obtain estimates of the annual dose to individuals for the points of assessment at WOD and the Clinch River. The dose was estimated for a scenario of an individual using the contaminated surface water at either WOD or the Clinch River as a water supply. Doses were estimated for the drinking water, milk, meat, fish and swimming pathways for each radionuclide and were summed to give a total dose.

The calculated peak concentrations and doses were examined with respect to land use, remediation activities, source terms, environmental transport, and the overall results from the analysis. Of greatest importance to understanding the calculated results are the sensitivity and uncertainties associated with land use. With the current land use controls and monitoring programs for WOC and the Clinch River, the proper point of assessment is the Clinch River after all discharges from WOC have been fully diluted in the Clinch River. However, changes in land use controls as a result of the current effort to revise the land use plan for the ORR could lead to the release of and public access to land currently controlled by DOE and use of the Clinch River in the mixing zone. This change in land use control could result in exposure of an individual to contaminated discharges from WOC that are not diluted by the Clinch River. For this potential change in land use controls, the doses attributed to the use of water at WOD is more appropriate. The extent of remediation activities prescribed by the CERCLA process could significantly reduce potential doses at WOD. While the improvements in the estimates of the inventories evaluated, source term estimates, or environmental transport would reduce uncertainty in the results, the dose calculated at WOD is expected to exceed the primary dose limit, and the dose calculated in the Clinch River is expected to be below the dose constraint.

The results calculated by the model were interpreted to account for inconsistencies with the more robust analyses presented in the PA for SWSA 6. This interpretive analysis led to a more rational selection of time bins and an extension of the peak doses of some radionuclides to multiple time bins. The interpreted peak radionuclide concentrations were converted to doses using the same methodology applied to the model results for use in the options analysis. The results for the case of the peak concentrations for all radionuclides arriving at WOD at the same time were dismissed because they run counter to the known transport characteristics of radionuclides on the ORR. For the purposes of the CA, only those doses associated with the first 1000 years after disposal are considered. The interpreted results lead to a total peak dose in the Clinch River of 5.0 mrem per year in the time period of 0–100 years. Doses in the Clinch River after full dilution of WOC decline over the remainder of the compliance period. The interpreted results lead to a peak dose at WOD of 540 mrem per year in the time period of 100–500 years. The peak dose at WOD during the time period of 0–100 years was estimated to be 150 mrem per year. During the period of 500–1000 years the peak dose is 93 mrem per year, and after 1000 years the dose declines to 65 mrem per year. The radionuclides associated with the largest contribution to dose are ⁹⁰Sr, ²⁴¹Am, ²⁴⁴Cm, ¹⁴C, and ²³⁹Pu.

An ALARA analysis that considered both sets of results was performed for the CA. For the case in which the point of assessment was the Clinch River, the calculated individual doses were sufficiently low that there was no basis for conducting an ALARA assessment for further dose reduction. For the analysis at WOD, the calculated doses exceed DOE's primary dose limit, in which case alternatives for reducing the dose below the limit must be identified. The specific alternatives will be selected under the CERCLA process. Consequently, the identification and selection of these alternatives as part of the CA is inappropriate. However, the analysis indicates that the continued control of land to prohibit routine access to WOC and the right shore of the Clinch River by members of the public would lead to doses well below the primary dose limits without further efforts to remediate the existing sources of contamination.

The main conclusion obtained from the options analysis is that doses attributable to all contributing sources in the watershed are less than the primary dose limit of 100 mrem per year, and the dose constraint of 30 mrem per year, as long as the current system of land use controls and monitoring programs continues over the period of assessment. Potential mitigating actions for the doses at WOD were identified in the options analysis, which were derived from the current efforts for developing a FS for Melton Valley. One of the remediation levels identified for the Melton Valley FS, as part of the CERCLA process, is to achieve a risk range of 10–4 to 10–6 for residential use of surface water before discharge to the Clinch River. Accomplishing this level of remediation as part of the CERCLA process would remove the need to continue land use control and monitoring programs as they are currently practiced, and would meet the primary dose objective and dose constraint for the CA.

The CA provides an analysis of the doses resulting from disposals and other sources in SWSA 6 and WOC watershed, and concludes that the primary dose limit and dose constraint are met under certain conditions. Significant uncertainties are included in the analysis, with the largest uncertainty associated with the inventory of radioactive materials that could contribute to dose. The conclusion of the CA depends on a continuation of land use controls by DOE to prohibit public use of lands along the shoreline of the Clinch River and continued monitoring of WOC and the Clinch River. The results presented warrant continued maintenance of the CA to ensure that appropriate mitigating measures are continued for protection of the public health and the environment.