WSRC-MS-2001-00092

Optimization of Remedial Design for Remediation of SRSís Radioactive
Seepage Basins by In Situ Stabilization/Solidification

Amit Ganguly
Westinghouse Savannah River Company
Aiken, SC 29808

 

This document was prepared in conjunction with work accomplished under Contract No. DE-AC09-96SR18500 with the U.S. Department of Energy.

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Historical Background

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 nationís 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:

The remedial action objectives were to-

Conceptual Model of Remedy

 

Stabilization/Solidification (S/S) Technique & Process

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:

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 Ė 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.

Lessons Learned

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

Optimization of S/S design

Optimization of S/S design entailed the following steps:

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

Grout mix optimization (pre-designed grout mix):

  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 basinsí 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.

Optimization of Field implementation of S/S

Optimized Acceptance Criteria for S/S

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.