Amit Ganguly
Westinghouse Savannah River Company
Aiken, SC 29808

This report was prepared as an account of work sponsored by an agency of the United States Government. Neither the United States Government nor any agency thereof, nor any of their employees, makes any warranty, express or implied, or assumes any legal liability or responsibility for the accuracy, completeness, or usefulness of any information, apparatus, product or process disclosed, or represents that its use would not infringe privately owned rights. Reference herein to any specific commercial product, process or service by trade name, trademark, manufacturer, or otherwise does not necessarily constitute or imply its endorsement, recommendation, or favoring by the United States Government or any agency thereof. The views and opinions of authors expressed herein do not necessarily state or reflect those of the United States Government or any agency thereof.

This report has been reproduced directly from the best available copy.

Available for sale to the public, in paper, from:  U.S. Department of Commerce, National Technical Information Service, 5285 Port Royal Road, Springfield, VA 22161,  phone: (800) 553-6847,  fax: (703) 605-6900,  email:  orders@ntis.fedworld.gov   online ordering:  http://www.ntis.gov/support/ordering.htm

Available electronically at  http://www.osti.gov/bridge/

Available for a processing fee to U.S. Department of Energy and its contractors, in paper, from: U.S. Department of Energy, Office of Scientific and Technical Information, P.O. Box 62, Oak Ridge, TN 37831-0062,  phone: (865 ) 576-8401,  fax: (865) 576-5728,  email:  reports@adonis.osti.gov

The Savannah River Site (SRS) located adjacent to the Savannah River, in Aiken and Barnwell counties of western South Carolina. SRS is owned by the United States Department of Energy (US DOE). SRS has historically produced special nuclear materials for our nation’s defense programs. This effort was discontinued in 1988. Chemical and radioactive wastes are by-products of nuclear material production processes. These wastes have been treated, stored, and in some cases, disposed of at SRS.

The Savannah River Site recently began remediation of several radiologically contaminated basins. These unlined basins contain radiological contaminants, which potentially pose significant risks to human health and the environment.

Waste materials handled at SRS are regulated and managed under Resource Conservation Recovery Act (RCRA)/Comprehensive Environmental Response, Compensation and Liability Act (CERCLA). Selection of the remedy, required to protect human health and the environment, is based on the National Oil and Hazardous Substance Contingency Plan (NCP) selection criteria. The final selected remedy is typically summarized in the Record of Decision (ROD) for the waste unit. The completion of the ROD process requires agreement between US DOE, US EPA, SCDHEC, and the public.

A plug-in approach was used and recorded in a "Plug-In-ROD" to design a common remedy for such radioactively contaminated basins with similarities in history of use, contaminants, risk, and location in current industrial use areas adjacent to existing nuclear facilities. This plug-in approach allows early remediation with considerable cost savings through reduction in documentation.

The selected remedy consists of the following actions:

• In-situ stabilization/solidification of the contaminated wastes (contaminated soils in basins and around pipelines, interior of pipelines, vegetation, and other debris).
• Installation of a low permeability soil cover.
• Institutional Control and maintenance of the cover.

The remedial action objectives were to-

• minimize contaminant migration and treat potential threat source materials (PTSM)
• protect site workers and future residents from direct exposure to radiation
• reduce infiltration, intrusion, and surface erosion

In situ stabilization is a well-known technique for immobilizing contaminants within soil and preventing the spread of contamination into the groundwater. The soil is mixed with cement or other grout that chemically absorbs and immobilizes the contaminant materials.

In situ stabilization/solidification of three basins (considered as lead basins) were completed in between late spring of 2000 and early spring of 2001. Lessons learned from these projects are being used to optimize the remedial design and construction requirements for stabilization/solidification of the plug-in candidate basins.

For the lead basins following activities were performed as an integrated set of tasks that would result in the successful completion of the S/S of wastes in the basin soils and pipelines:

• Laboratory-scale grout mix development
• Demonstration of the selected S/S technology and application of the radiological worker safety procedures in a clean area
• Pilot testing for optimization and field adjustments of the grout mix and the S/S technique followed by final S/S of all basin wastes inside the basin (in-situ)
• Verification of completion of successful S/S

Verification of successful completion of S/S was performed through quality control inspection and testing of S/S waste samples. Considering that in-situ soil stabilization/solidification, for treating radiological constituents of concerns is an evolving technology, a conservative approach was taken in the determination of performance criteria for the stabilized/solidified waste in those basins. The performance requirements for verification of successful S/S of wastes were derived from the results of a treatability study and US EPA guidance documents. 28-day compressive strength and leachability were the primary criteria. Additional secondary criteria (for S/S process control) were hydraulic conductivity, pH, temperature, gas generation, radiation exposure, effects of nitrite/nitrate, sulphite/sulphate, strength of the S/S waste under immersed and irradiated conditions, etc.

Several successful Portland cement based grout mixes were developed through the process of bench-scale development and testing of basin waste samples mixed with treatment reagents in the laboratory. Typically grout mixes were composed of Type I or Type II Portland cement mixed with various proportions of silicate (in blast furnace slag or zeolite form), bentonite, fly ash and super plasticizer (in a few applications to enhance mixing operation). Consistent with SRS geology, soil types ranged from silty-sand to clayey sand to sandy clay. Occasionally stiff clay was encountered at basin bottoms, which required application of super plasticizer.

Shallow soil mixing techniques were selected for the stabilization/solidification treatment. The first two projects started with two different type of S/S technique and equipment – one with the horizontal mixing technique with a shearing injector equipment and the other with vertical column mixing technique with single auger equipment.

The single auger mixing technique encountered less number of constructibility issues when compared with the single auger mixing technique. Both techniques required multiple pilot tests to determine the necessary operational parameters (e.g., grout flow rate, mixing time, number of passes, etc.) for mixing the clayey soils at basin bottom. However, because of its equipment configuration (limited depth of mixing and slow energy during horizontal mixing and cross mixing) the shearing injector technique required mixing in at least two layers. It also encountered relatively more problems related to ensuring uniform mixing at interfaces (either at the joint of two layers or at the required target depth). In addition, number of test samples required for verification testing of primary criteria (acceptance criteria) and secondary test parameters (for process control) were significantly large. Consequently, the process of sample collection, analysis and testing became very involved and expensive.

The lessons learned from the S/S of aforementioned lead basins may be summarized as follows:

• During the mixing operation, grout injector plugging occurred several times.
• Variations in the grout mix injection caused an increase in the weight of the grout mix in the stabilized mass, resulting in a significantly stronger grouted mass. [Note: If the grouted waste in the radioactive basin achieved very high strength and if nonconforming conditions were found in those stronger areas of the grouted waste, reworking of those areas to resolve the nonconformance would require application of extensive safety and protective measures to meet ALARA requirements.]
• High hydration rates resulting in high temperature rise within the grouted mass was observed during the curing process.
• Cracks (extending through the stabilized mass) were observed, the cause of which is suspected to be the high temperature rise during curing.
• Difficulties were encountered in the installation and extraction of some of the sample collection tubes.
• Excavation of the grouted soil in the demonstration area indicated that the depth of mixing was inadequate and non-uniform around the perimeter of the grouted area.
• Variations in physical soil properties were encountered in the OFASB area resulting in non-uniform grouted mass.

Optimization of S/S design entailed the following steps:

• Development and selection of the qualified grout mix for treatment of basin soils by grouting.

• Determination of key parameters for soil-grout mixing to produce reasonably homogeneous grouted waste.

• Resolution of field implementation issues

• Acceptance criteria for the stabilized/solidified wastes and its basis

Results of the optimization of S/S design are as follows:

1. The 28-day unconfined compressive strength values (ranging from 300 psi to 600 psi) of all four mixes far exceeded the 50 psi strength. Overall, strengths of sandy soils and grout mixture were higher than those for clayey soil and grout mixture.
2. All four mixes effectively reduced the lechability of gross alpha and non-volatile beta. The Leachability Index (LI) of the treated sample, per ANS 16.1 leach test, varied from 9.8 to 13, which were greater than 6. Formulations with high mix of silicate were relatively more effective in reducing leaching of gross alpha and non-volatile beta (i.e. produced higher value of LI) compared to other two formulations.
3. All four mixes had 28-day hydraulic conductivity equal to or lower than 1x 10^-6 cm/sec. [Note: The average hydraulic conductivity of the surrounding Fuquay soils (typical for the seepage basins’ soils) is approximately 1x 10^-3 cm/sec. (Rogers 1990). Per the US EPA guidance (US EPA 1989), the hydraulic conductivity of the stabilized/solidified material should be two orders of magnitude below that of the surrounding soil.]
4. Laboratory test results from the study also indicated that all four grout mixes could handle the variability in lithology that may be expected within the basins. However, confirmatory field tests will be required prior to full-scale implementation of the grout mix.
5. Presence of low levels of chloride, nitrite, nitrate, sulfate and sulfide had little to no inhibitory effect on the cementitious reactions, which occurs during the curing of the Portland Cement.
6. Moisture content in the soils tested ranged from 11% to 23%. Variability within this range had little or no effect on the S/S test results. Soils with <10% moisture content will require addition of water to minimize drying and cracking of the grout-treated soil.

• Mixing equipment capable of delivering a high energy and high shear mixing action, which is necessary to thoroughly mix the clay component of the soils with the injected grout.
• The basin soils or the grout mix may require monitoring of moisture content to ensure flowability of the design grout mix.
• The mixing technique will require field adjustments for proper application of the qualified grout mix to ensure thorough mixing of the grout mix with the contaminated soils to the required depth.
• Resolution of injector plugging entailed reconfiguration of the shearing injector or development of a method of clearing the injector consistent with ALARA principles.

• Preconditioning of wastes and increasing the number of passes, without injecting additional grout, for both auger mixing and shearing injector mixing accomplished improvement in the uniformity of the soil-grout mixture.

• Insulating the grouted mass with a soil layer for temperature control during the curing process to reduce the potential for thermal cracks.
• Based on above-mentioned field observations the primary focus during the field implementation phase will be to achieve a uniformly mixed and reasonable homogeneous grouted waste. The unconfined compressive strength of the grouted mass will be tested and monitored to ensure that the quality of the grouted mass is not adversely affected by addition of excessive water during the above-mentioned field adjustments. In addition, a few random samples will be tested for verification of reduction in leachability.

The Acceptance criteria for the stabilized/solidified waste are as follows:

1. Strength > 50 psi (the suggested compressive strength per US EPA guidelines for durability and adequate strength to withstand overburden pressure from the soil cover).
2. Uniformity/homogeneity of the grouted waste, by visual inspection of grouted waste samples (to ensure reasonable uniform mixing of the waste with the design grout mix thereby, accomplishing adequate stabilization/solidification of the contaminated waste in the grout). High energy mixing equipment is required to accomplish uniform mixing.
3. Leachability Index of 6 or higher, per ANS 16.1 for the in-situ stabilized waste.