RADIOACTIVE SOIL CONSOLIDATION AT THE SAVANNAH RIVER SITE

Dr. W. Dean Hoffman, J. Mike Griffith
WSRC

ABSTRACT

This paper for Radioactive Soils Consolidation was prepared in order to share evaluation of on-site disposal alternatives for low-level radioactive soils and debris from various Resource Conservation and Recovery Act (RCRA)/Comprehensive Environmental Response, Compensation, and Liability Act (CERCLA) waste units within the Savannah River Site (SRS).

Twenty RCRA/CERCLA waste units have been identified at SRS that contain low-level radioactive soils and debris and will be used as the baseline for disposal capacity evaluation. One remedial management option is to consolidate this material into one disposal facility constructed within the boundaries of SRS. The overall objectives of this paper were to provide decision makers adequate information to compare Soils/Debris Consolidation Facility (SDCF) alternatives, select the preferred SDCF option(s) to other remedial actions, including those involving treatment, that have been preliminarily identified for the individual SRS low-level radioactive RCRA/CERCLA waste units. The SDCF could provide for a reduction of receptor exposure to the contaminant source by decreasing the total radioactively contaminated area (footprint) at SRS requiring institutional controls, minimizing the number of radioactive waste sites requiring long-term monitoring and maintenance, and streamlining by using generic remedies for the number of candidate units.

INTRODUCTION

A large portion of the Environmental Restoration program at the SRS includes remediation of inactive basins that received radioactive effluents from various production processes. At the present time there are 20 of these units in the ER program at SRS representing approximately 562,000 cu-yds. One method currently being considered to remediate these sites is excavation and disposal in a centralized engineered disposal facility. In order to determine the feasibility of the soil consolidation concept at SRS, an alternative study has been developed. The purpose of the alternative study is to evaluate and select a preferred alternative for consolidating soil from these various waste units. These alternatives range from construction of a new facility to the use of existing facilities such as the high-level waste tanks. In addition, the selected soil consolidation alternative has been evaluated against existing technologies which include in-situ treatment, capping, or disposal at an off-site (SRS) facility. This paper summarizes the alternative report and provides the current status of the soil consolidation concept at SRS

Approach and Report Organization

The Soil/Debris Consolidation Facility (SDCF) Alternatives Screening Report was developed using the Feasibility Study (FS) process as detailed in the National Oil and Hazardous Substances Contingency Plan (NCP) of November 20, 1985 (50 FR 47973), the Superfund Amendments and Reauthorization Act (SARA) of October 17, 1986, and the amended NCP of March 5, 1990 (55 FR 8666). The general framework of the report is based on the U.S. Environmental Protection Agency (EPA) document Guidance for Conducting Remedial Investigations and Feasibility Studies Under CERCLA (EPA, 1988a), the U.S. Department of Energy (DOE) document Remedial Investigations/Feasibility Study Process, Elements, and Techniques Guidance (DOE,1993), and the Westinghouse Savannah River Company (WSRC) document RCRA Facility Investigation/Remedial Investigation Program Plan, Revision 1 (WSRC, 1993a). The general FS process was selected for development of this document since the process provides an established method by which different remedial actions can be evaluated and compared. The preferred alternative(s) will be selected based on information contained in this SDCF Alternative Screening Report and based on DOE, EPA, South Carolina Department of Health and Environmental Control (SCDHEC), and community acceptance.

Estimated Volume of Impacted Soils

Based on process knowledge, the estimated volume of contaminated waste at the 20 units is 430,100 m³ (562,500 yd³). Based on the estimated volume and increase in volume due to excavation, the SDCF would require a minimum of an estimated 460,000 m³ (600,000 yd³ ) of storage space. The estimated activity contained in theses waste units is 860 curies.

Facility Design Alternatives

Potential on-site disposal alternatives have been developed to address radioactive soils from the 20 waste units. Institutional controls will be part of all alternatives developed for the SDCF except for the no action alternative. As required by the NCP, the no action alternative is provided as a baseline for comparison.

Disposal methods for radioactive waste are well documented. The initial list of disposal methods was developed through review of literature and by an expert panel made up of low-level radioactive waste experts from academia, the government, and the private sector. Off-site disposal alternatives--such as augered shafts, underground mines, rock cavities, deep seabed, and extraterrestrial space--were considered inappropriate alternatives for conditions at SRS or would not meet the specific goals of the SDCF.

The potential disposal alternatives that address radioactive soil contamination and a brief description of the alternatives are as follows:

• Alternative 1: No Action
  
  Alternative 1 would consist of no action, i.e., there would be no construction of a radioactive soils/debris consolidation facility at the Savannah River Site. The No Action alternative is developed to provide a baseline alternative for comparison purposes.
  
  
• Alternative 2a: Earthen Below-Grade Facility--Cap, No Bottom Liner, or Leachate Collection System
  
  Alternative 2a would consist of excavated, shallow-land, below-grade trenches. Bulk waste soils would be placed within the trenches, and then the trenches would be covered with either a single layer soil cover or an engineered multi-layer capping system. Neither an engineered bottom liner system nor a leachate collection system would be included under Alternative 2a; however, an operational liner would be used during the facility operation to control and manage precipitation within the trenches.
  
  
• Alternative 2b: Earthen Below-Grade Facility--Cap, Bottom Liner, and Leachate Collection System
  
  Alternative 2b would consist of excavated shallow-land below-grade trenches with an engineered bottom liner system. The bottom liner system would consist of polymeric liners such as high density polyethylene (HDPE) and a drainage layer. A leachate collection system would be included in the liner system. Bulk waste soils would be placed within the trenches. Closure would include covering the trenches with either a soil cover or an engineered, multi-layer capping system.
  
  
• Alternative 3a: Earthen Above-Grade Facility--Cap, No Bottom Liner, or Leachate Collection System
  
  Alternative 3a would be engineered as diked area directly on the land surface. Bulk waste soils would be placed within the diked area, and then the area would be covered with either a soil cover or an engineered, multi-layer capping system. No engineered, bottom liner system is included under Alternative 3a; however, an operational liner would be used during the facility operation to control and manage precipitation within the trenches.
  
  
• Alternative 3b: Earthen Above-Grade Facility--Cap, Bottom Liner, and Leachate Collection System
  
  Alternative 3b would be engineered as diked area directly on the land surface with a below-grade, bottom liner system. The liner system would consist primarily of polymeric liners such as HDPE and a drainage layer. A leachate collection system would be included in the liner system. Bulk waste soils would be placed within the diked area. Closure would include covering the area with either a soil cover or an engineered, multi-layer capping system.
  
  
• Alternative 4: Earthen Combined Above and Below Grade Facility--Cap, No Bottom Liner, or Leachate Collection System
  
  Alternative 4 would consist of engineered shallow-land trenches; however, the depth of trenches would be such that bulk soils disposed of within trenches would be piled below- and above-grade. Closure would include covering the area with either a soil cover or an engineered, multi-layer capping system.
  
  
• Alternative 5a: Above-Grade Concrete Vault
  
  Above-grade concrete vault disposal would consist of placing soils within B-12 boxes and placing the boxes within an engineered concrete structure located above-grade. The vault would be completely closed by placement of a concrete roof over each backfilled cell. Additionally, an earthen capping system would be constructed over the vaults for closure purposes.
• Alternative 5b: Below-Grade Concrete Vault
  
  Below-grade concrete vault disposal would consist of placing soils within B-12 boxes and placing the boxes within an engineered concrete structure below-grade. The structure would consist of reinforced concrete floors, walls, and roof. An earthen capping system would be constructed over the vault for closure purposes.
  
  
• Alternative 6a: Existing K-Reactor Retention Basin - 44% (Plus Similar Expansion - 56%)
  
  The K-Reactor Retention Basin is a lined, below-grade facility having a 190,000 m³ (248,000 yd³) capacity. Under Alternative 6a, the basin would be expanded to obtain a capacity of 450,000 m³ (600,000 yd³) which would effectively support the predicted waste volume from the identified waste units. Following placement of the bulk soils within the basin, the basin would be closed using either a soil cover or an engineered, multi-layer capping system.
  
  
• Alternative 6b: Existing High-Level Waste Tanks - 30% (Plus Alternative 2a - 70%)
  
  Fifty-one-million gallon steel tanks exist at SRS for the temporary storage of high-level waste. The current closure plans of the high-level waste tanks, following removal and treatment of the high-level waste, is to inject a grout mixture into the tanks and construct a capping system over the tanks. Under Alternative 6b, soils within the waste units would be processed with grout and injected into the high-level waste tanks. Only 30% of the total waste unit volumes could be injected into the high-level waste tanks; therefore, an additional disposal facility with a capacity of 302,000 m³ (400,000 yd³) would be needed to accommodate the remaining waste volume.

Evaluation of SDCF Alternatives

The alternatives formulated undergo a screening evaluation to reduce the number of feasible disposal options. Disposal alternatives retained after screening in this section are subjected to a detailed evaluation in conjunction with other potential remedial action for waste unit soils/debris. Siting of the disposal facility is discussed in a separate document; however, the conceptual unit model was developed based on a disposal facility siting location near the burial grounds in E-Area.

Six of the ten disposal alternatives were rejected. The no action alternative was preliminarily retained to provide the basis for comparison of other alternatives. Alternatives 2b and 3b (liners) were rejected because given the limited lifetime of the bottom polymeric liner system, the liner system would not provide added value in regards to protecting human health and the environment. Alternatives 5a (Above-Grade Concrete Structure) and Alternative 5b (Below-Grade Concrete Structure) were rejected on the basis of the lack of design capacity flexibility and cost. Alternative 6a (Existing K-Reactor Retention Basin) was rejected due to hydrogeology. Alternative 6b (Existing High-Level Waste Tanks) was rejected because it does not meet the objectives of the SDCF; implementation of the alternative poses significant worker exposure, and other inherent implementability concerns. The retained alternatives are identified as follows:

At this screening level and with the exception of the no action alternative, very little difference is apparent between the retained disposal technologies. Each technology would meet siting criteria, and a preliminary performance assessment indicates that given site conditions and waste and contaminant characteristics, the effectiveness of the disposal technologies are similar. Therefore, for the purposes of the Alternatives Screening Report, the differences between Alternatives 2a, 3a, and Alternative 4 were considered design aspects; and the selection of the appropriate design will be deferred to the stakeholders' decision.

The SCDF alternative evaluated during the detailed analysis will encompass the general aspects common to each the retained disposal alternatives. However, in relation to the capping system design of each alternative, a risk management decision must be made regarding which hypothetical future receptor presented in the conceptual unit model is more appropriate. Based on contaminant leaching modeling conducted, a soil cover having a hydraulic conductivity of 1 x 10^-5 cm/sec (4 x 10^-6 in./sec) would be appropriate for protection of a hypothetical receptor at the closest surface water entity (Upper Three Runs Creek) over a 1,000 year period (via groundwater ingestion). For a hypothetical receptor at the SDCF boundary (via groundwater ingestion), a capping system having a hydraulic conductivity of 1 x 10^-6 cm/sec with certain waste unit volumes requiring pre-disposal treatment would be the only option which would not cause an exceedance of the MCL over a 1,000 year period. Review of the known and predicted activities of iodine-129 and uranium isotopes in the waste units indicates that a conservative estimate of the volume of waste requiring pre-disposal treatment would be 76% of the total [460,000 m³ (600,000 yd³)]or 350,000 m³ (460,000 yd³).

The pre-disposal treatment technology most applicable to attaining protection of human health and the environment at the SDCF boundary is ex-situ stabilization/solidification (grout). Therefore, for purposes of the Alternatives Screening Report, two separate SDCF alternatives will be evaluated within the detailed analysis in consideration of the two different future hypothetical receptors. The SDCF alternatives developed for detailed analysis are the following:

• Dispose of Waste Unit Soils/Debris in an Earthen Facility/Cap (1 x 10^-5 cm/sec)
• Ex-Situ Grout of 76% of Waste Unit Soils/Debris and Dispose of Waste Unit Soils/Debris in an Earthen Facility/Cap (1 x 10^-6 cm/sec)

SCDF Alternatives Comparison to Baseline

The NCP requires that a FS consider a range of alternatives that include treatment to reduce toxicity, mobility, or volume of contaminants. Alternatives were developed that would immobilize contaminants or reduce the volume of contaminants or contaminated media and, to the maximum extent feasible, eliminate or minimize the need for long-term management. Alternatives have also been developed which involve little or no treatment, but which provide protection to human health and the environment by preventing or controlling exposure to the contaminants through engineering or institutional controls. As required by the NCP, the no action alternative is provided as a baseline for comparison.

Potential remedial alternatives have previously been developed to address contaminated soils/debris for three out of the 20 identified RCRA/CERCLA waste units. Each of the three waste units have similar waste composition and contaminant characteristics as the other 17 identified waste units. Therefore, the alternatives developed and evaluated in the CMS/FS reports for the three waste units were utilized to evaluate a common remedy for the entire 20 waste units including two SDCF disposal alternatives. As stated earlier, the overall purpose of Alternatives Screening Report is to provide sufficient information to make an informed decision as to the feasibility of consolidating the soils and debris into one disposal unit versus managing the 20 waste units separately.

The following are the remedial alternatives that address radioactively contaminated soil and debris common to each waste unit.

• Alternative 1: No Action
• Alternative 2: Cap Units
• Alternative 3: In-Situ Grout Contaminated Soil and Debris, Cap Units
• Alternative 4: Excavate Contaminated Soil and Debris, Ex-Situ Grout, Replace Treated Soil and Debris, Cap Units
• Alternative 5: Excavate Contaminated Soil and Debris, Dispose at the SRS E-Area Vaults
• Alternative 6a: Excavate Contaminated Soil and Debris, Dispose at the Radioactive Soil and Debris Consolidation Facility (1 x 10^-5 cm/sec Soil Cover)
• Alternative 6b: Excavate Contaminated Soil and Debris, Ex-Situ Grout 76% of the Waste Soils, Dispose at the Radioactive Soil and Debris Consolidation Facility (1 x 10^-6 cm/sec Capping System)
• Alternative 7: Excavate Contaminated Soil and Debris, Dispose at the Nevada Test Site

The results of the detailed analysis based on NCP criteria are summarized in Table I.

Conclusion and Path Forward

Although the Alternative Screening Report does not provide any conclusions or recommendation to Soil/Debris Consolidation, it identified that locating a group of waste sites in a central location can be implemented. Also, the report identified that capping and treating the waste to protect the groundwater receptor and capping the waste to protect the surface water receptor are available options. A decision cannot be made until regulatory and community acceptance is obtained as well as decisions on cleanup standards, facility design, and costs.

In addition, an assessment of the sites identified in the study was used to determine which sites are likely candidates for soil consolidation. The following screening criteria were used to evaluate the 20 waste sites: 1) Location, 2) Reduction in Industrial Footprint, 3) Risk Reduction, and 4)Contribution to Groundwater Contamination. Using this criteria limits the number of sites was limited to four which may be acceptable candidates for excavation and disposal. These sites are the Savannah River Laboratory Seepage Basins, Ford Building Seepage Basin, TNX Burying Ground, and the Old TNX Seepage Basin. These sites account for approximately 81,700 cu-yds. Applying this strategy significantly reduces the volume and therefore the inventory of radionuclides requiring disposal. Therefore, a reduced disposal capacity would be required which may make other disposal options viable, e.g., E-Area slit trenches, HLW tank cover soil, etc. Additional analysis will be required to determine if these options are technically and economically viable.