Charles R. Hayes, Jr.
Westinghouse Savannah River Company

Lawrence T. Ling
USDOE

Jeffry L. Newman
Westinghouse Savannah River Company

In July 1997, the Nation's first closure of a high-level radioactive waste (HLW) storage tank, Tank 20, was completed at the Savannah River Site, near Aiken, South Carolina. Under the site's Federal Facility Agreement (FFA), the Department of Energy (DOE) is required to remove from service twenty-four waste tanks that do not meet secondary containment requirements. Success in meeting environmental standards and guidelines, evaluating the residual source term and selecting the appropriate tank system and residual waste material stabilization configuration was achieved by close and frequent cooperation with regulatory staff. South Carolina and the U.S. Environmental Protection Agency (EPA) approved the plan for tank closure in July, 1996. The Nuclear Regulatory Commission (NRC) staff also had no objections to the plans for tank closure. Fate and transport modeling of future human health and soil and groundwater impacts met regulatory performance objectives. The Tank 20 system alone cannot practically be isolated from other operational systems for the purposes of assessing potential remedial actions. The assessment of soils and groundwater around and below the waste tanks will occur upon closure of a geographical grouping of tank systems and their associated support services. DOE's institutional control over the area surrounding the tank farm will continue for the next 100 years, after which the area will be zoned industrial for an indefinite period with deed restrictions on the use of groundwater.

Tank 20, a 1.3 million gallon capacity, single-shelled underground carbon steel vessel, was closed by first removing all waste that was technically and economically feasible after which approximately 1000 gallons of waste remained, and then backfilling with several layers of grout. These included a bottom layer of a specially developed and formulated chemically reducing grout to retard the movement of waste constituents from the closed tank; a thick layer of Controlled Low-Strength Material to provide structural and overbearing support; and a free-flowing, strong grout layer to fill the voids and make potential future access difficult.

The Savannah River Site (SRS) occupies approximately 300 square miles adjacent to the Savannah River, principally in Aiken, Barnwell, and Allendale Counties of South Carolina. The site is owned by the DOE and is operated by the Westinghouse Savannah River Company (WSRC). Environmental restoration is emphasized in the current site mission. However, since the early 1950s, the primary mission of the site was to produce nuclear materials for national defense. The chemical separations processes used to recover uranium and plutonium from production reactor fuel and target assemblies in the chemical separations area at SRS generated liquid high-level radioactive waste. This waste, which now amounts to approximately 34 million gallons, is stored in underground tank systems in the F- and H-Areas near the center of the site. DOE has committed to remove from service those HLW tank systems that do not meet the standards set forth in Appendix B of the SRS FFA. DOE, the EPA, and the South Carolina Department of Health and Environmental Control (SCDHEC) signed the SRS FFA pursuant to Section 120 of the Comprehensive Environmental Response, Compensation, and Liability Act (CERCLA) for the comprehensive environmental remediation of SRS; the agreement became effective in August 1993. After wastes are removed from individual tank systems, the tanks will be closed under and removed from the industrial wastewater permits that regulate their operation.

DOE is closing HLW tank systems, which are permitted by SCDHEC under authority of the South Carolina Pollution Control Act (SCPCA), in accordance with South Carolina Regulation R.61-82, "Proper Close-out of Wastewater Treatment Facilities." This regulation requires that such closures be performed in accordance with site-specific guidelines established by SCDHEC to prevent health hazards and to promote safety in and around the tank systems. To facilitate compliance with this requirement and in recognition of the necessity for consistency with ultimate remedial action of the SRS under the FFA, SRS adopted a strategy for HLW tank system closure that includes evaluation of an appropriate range of closure alternatives with respect to pertinent, substantive environmental requirements and guidance and other appropriate criteria (e.g., technical feasibility, cost). The general strategy for HLW tank system closure is consistent in its substance with comparative analyses performed as part of a Resource Conservation and Recovery Act (RCRA) corrective measures study/CERCLA feasibility study under the FFA.

In February, 1996, SRS began dialogue with EPA and SCDHEC on the topic of HLW tank closure. Between February and July 1996, the three parties met bi-monthly to review/develop plans for closure and to identify the environmental requirements and guidance applicable to the closure of the F- and H-Area HLW tank systems. Compliance with these requirements will ensure that closure of the HLW tank systems will be protective of human health and the environment and consistent with final remedial action for SRS as implemented under the SRS FFA. These requirements along with the SRS general plans for closure were compiled into the "Industrial Wastewater Closure Plan for the F- and H-Area High Level Waste Tank Systems". This "general" plan for tank closure was approved by both EPA and SCDHEC in July, 1996.

A National Environmental Policy Act Environmental Assessment for the closure of all SRS high level waste tanks was also completed in July of 1996. The preferred alternative- to remove the waste from the tanks, to bind up residual waste, fill the tanks with a backfill to prevent collapse and inadvertent intrusion, would lower human health risks and increase safety of employees. A Finding of No Significant Impact was issued by DOE in August of 1996.

Although regulatory approval was obtained for the "general" closure plan, a review of more specific plans for Tank 20 was necessary. Therefore, a tank-specific document entitled "Industrial Wastewater Closure Module for the High Level Waste Tank 20 System" was developed. The Tank 20 module included waste residual characteristics and ultimate stabilization methods specific to Tank 20. SCDHEC approved the Tank 20 closure module in January, 1997. Individual modules will also be developed for other tanks and related systems.

The performance standards for HLW tank system closure are generally numerical standards, such as concentration or dose limits for specific radiological or chemical constituents released to the environment. These numerical standards apply to various environmental media, at different points of compliance, at various periods during or after closure. They are used to develop performance objectives that provide a basis for comparison of different tank system closure configurations. The performance objectives for HLW tank system closure will be the groundwater protection standards applied at the point where groundwater discharges to the surface (seep-line) and the surface-water quality standards applied in the receiving stream. Closure options were evaluated to show conformance with the performance objectives as part of the overall evaluation [similar to compliance with applicable or relevant and appropriate requirements (ARARs) as one of the nine CERCLA criteria].

For the tank closure project, a construct for apportioning performance objectives known as a groundwater transport segment (GTS) was utilized. GTS's represent the approximate flow-path of contaminants from a tank system or group of tanks. For fate and transport modeling, the GTS is a convenient method to identify all potential sources whose contaminant plumes may overlap. The GTS also assists in conceptualizing and reducing the complex hydrogeologic variables of the site into a conservative, simplistic data package for input into the EPA-recognized Multimedia Environmental Pollutant Assessment System (MEPAS) Model.

Further, the GTS was used to apportion the performance objectives among all sources (both tank and non tank) contained within the GTS. The apportionment is based on the relative impact at the point of exposure at the time of greatest impact of the various sources in the GTS. To demonstrate compliance with drinking water standards, a hypothetical receptor is assumed to drink the groundwater at the location of maximum concentration of the GTS at an agreed upon point of exposure (i.e., the seep-line).

Due to the three-dimensional nature of groundwater flow and leakage between the stacked aquifer layers beneath the affected area, each GTS contains three layers. The boundaries of the Water Table Aquifer layer of the GTS, which is the first aquifer layer impacted by a future release from the Tank Farms, were used to define the boundaries for the underlying Barnwell-McBean Aquifer layer of the GTS. In turn, the Barnwell-McBean Aquifer layer of the GTS will control the boundaries of the underlying Congaree Aquifer layer. Therefore, the fate and transport modeling at each tank farm include components for each of the aquifer layers within each GTS.

Since the highest hypothetical exposure is to a worker at the seep-line (where groundwater intersects with the stream), the following Performance Objectives are applicable: (1) compliance with the SCDHEC Primary Drinking Water Standards for radionuclides (i.e., 4 mrem/year beta-gamma dose and 15 pCi/L total alpha concentration) at the seep-line, and (2) compliance with the SCDHEC water quality criteria, criteria to protect aquatic life, or Maximum Contaminant Level, whichever is more restrictive, for non-radiological constituents at the seep-line.

The modeled dose contribution from drinking groundwater at the seep-line from Tank 20 is 0.0055 millirem per year. The modeled dose contribution from all the F-Area tanks will contribute a total of 1.9 millirem per year. This is well within the performance objective of 4 millirem per year.

DOE employed Construction Technology Laboratories, Inc. (CTL) to determine the emplacement, chemical, and mineralogical properties of three reducing grout formulations, each designed to have an oxidation potential (Eh) less than zero and a pH greater than 9.5. CTL formulated the mixes with various parameters and desired properties, including (listed in decreasing priority): reducing conditions, alkalinity, flowability, self-leveling capability, compressive strength, cohesiveness, low water/cement ratio, low permeability, chemical composition, engineering properties (setting time, set strength), and minimal bleeding. The three grouts, identified as mix 1, mix 2, and mix 3, each had the same basic ingredients except for the type of cement used. Each grout contained sand, ground blast boiler slag, silica fume, water, water reducing agents (also known as super-plasticizers), set retarders, and sodium thiosulfate. Mix 1 was a Type I cement-based grout, mix 2 was a Type I/K cement-based grout, and mix 3 was a Type V cement-based grout. The sand, silica fume, and slag amounts were slightly adjusted to account for the different amounts of cements used for each mix.

CTL designed a battery of tests intended to examine each of the properties previously listed. Several of these tests allowed CTL to observe several of these parameters simultaneously. To approximate the application of stabilizing sludge, CTL used a simulated material. CTL added non-radioactive surrogates that are chemically similar to the radioactive elements of concern such as plutonium and technetium. Simulant was placed in three 45 foot long pie-shaped forms. Grout was introduced at various flow rates to determine how the sludge interacted with grout. It was concluded that the grout displaced the sludge with the sludge eventually "floating" on top of the grout layer. This meant that if the grout were poured through the center riser of the tank, most of the residual sludge in the tank would end up on top of the reducing grout within a few feet of the tank wall. Therefore, a technique was devised to take advantage of this lifting quality.

It was successfully shown in pilot testing and final placement that the distribution of sludge could be greatly improved by pouring the grout into the tank in multiple locations and by pouring in two lifts with an addition of a dry grout mixture between lifts. Grout was alternately poured through six opposing locations then center poured, simulating the location of the seven risers in Tank 20. Dry grout was distributed on top of the first lift, with a second lift poured the next day. The simulated sludge was effectively incorporated within the grout layer.

A challenge to operationally closing the first ever HLW tank was the need to develop specific requirements and guidance relative to such closure. DOE believed sufficient guidelines and requirements existed for other activities dealing with both HLW and Low Level Waste which, if properly implemented for operational closure of HLW tank systems, would assure protection of the public, workers, and environment. For example, criteria for "incidental waste" determination associated with waste removed from HLW tanks at Hanford existed. Working with the NRC staff, SRS used these criteria as well as 10CFR61 as a basis for developing specific closure criteria for Tank 20 and the subsequent tank closed, Tank 17.

In July 1997, NRC and SR entered into a Memorandum of Understanding (MOU) and Interagency Agreement (IA) for the NRC to review the methodology employed by SRS for HLW tank closure and the basis for the "incidental waste" determination. This review is consistent with NRC statutory responsibility in accordance with the Energy Reorganization Act of 1974 with respect to waste that may be subject to NRC licensing. The result of this review, expected Spring 1998, should enable SRS to better quantify "incidental waste" determination requirements.

Additionally, relative to Tanks 20 and 17, the environmental impact at the point of compliance associated with the closed tanks was determined to be sufficiently low to enable proceeding with closure activities prior to completing the requested NRC review of SRS methodology. DOE recognized and accepted the minimal risk in proceeding forward without completion of the NRC review. The NRC as well as SCDHEC were informed of SRS intentions to close these tanks prior to the completion of the NRC review, and expressed no objections. Subsequent to closure of Tanks 20 and 17, SR will not conduct further HLW tank closures until completion of the NRC review.

The NRC considers that HLW wastes would be incidental wastes provided they:

• Have been processed (or will be further processed) to remove key radionuclides to the maximum extent that is technically and economically practical
• Will be incorporated in a solid physical form at a concentration that does not exceed the applicable concentration limits for Class C low-level waste or alternative site-specific classification limits as set forth at 10 CFR 61
• Are to be managed, pursuant to the Atomic Energy Act, so that safety requirements comparable to the performance objectives set out in 10 CFR 61 are satisfied

The closure configuration for the Tank 20 System included filling the tank with a "sandwich" of grouts. The first layer consists of approximately 30 inches of chemically reducing grout. The purpose of this grout is to provide long-term chemical durability against leaching of the residual waste by aggressive agents in the environment such as groundwater and acid rain. This reducing grout is a self-leveling back-fill material which will encapsulate not only the residual waste, but also some of the equipment remaining inside the tank. The reducing grout is composed primarily of cement, blast furnace slag, masonry sand, and silica fume. The reducing grout itself was placed in several stages. The first layer was placed in liquid form using multiple riser pour locations. The dense grout lifted the waste sludge, which is less dense, off of the tank bottom and spread it across the tank. The loose waste sludge was then immobilized by blowing in dry powdered grout. The dry particles hydrated, incorporating the water into the grout powder, and formed a hard mass. More liquid grout was poured in to form a domed cap which fully encapsulated the waste within the grout layers. A total of 518 cubic yards of liquid grout was used along with 141,620 lbs. of dry grout material to form this grout/sludge "sandwich".

On top of the reducing grout, and constituting the bulk of the tank's fill material, is approximately 7,500 cubic yards of controlled low strength material (CLSM). CLSM is an inexpensive, self-leveling grout fill material composed of sand and cement formers which is readily available from local concrete suppliers. CLSM provides sufficient strength to support the overbearing weight of the closed tank. The CLSM layer was placed to about 32 feet deep, to within 9-12 inches of the top of the vertical wall of the tank (spring line). The CLSM was pumped into the tank in a manner similar to that used for the reducing grout.

From the spring-line area up to the bottom of and including the tank top riser ports, a strong grout was poured on top of the CLSM. This strong grout is a low viscosity, free-flowing grout with a compressive strength in the normal concrete range of 2,000 to 3,000 pounds per square inch and is beneficial for filling void spaces. In addition, this relatively strong grout will discourage an intruder from inadvertently accessing the waste. A reducing grout was poured into the lower section of each riser; the high flowability of the reducing grout is effective in filling small voids in riser openings. In the upper section of each riser, a 5,000 pounds per square inch grout was used to finish off the top of the riser. The reducing grout will be injected in the remaining equipment (evaporator pot, piping, and etc.) to ensure that such voids are filled to the fullest extent practical. In addition, this relatively strong grout will discourage an intruder from inadvertently accessing the waste. Finally, metal or wooden formwork will be installed around each of the riser ports. Riser fill material caps (grout or concrete) will then be poured at these locations, which will effectively seal both the riser ports and the concrete plugs currently used to seal the riser openings.

The back-fill materials were manufactured by a subcontractor, Throop Inc., at an on-site portable continuous feed batch facility and pumped directly into the tank. The grout plant consisted of two separate continuous feed volumetric mixing units. Raw materials (cement, sand, water, and admixtures) were fed into the plants from adjacent silos and storage bins. Each plant was equipped with a mixing auger which provided continuous stirring. The mixed grout left each of the augers into a common diesel driven reciprocating piston grout feed pump. The pump fed the product into a 5" ID cast iron delivery line. The line ran at a downward slope for approximately 1100 feet directly to the top of Tank 20. Mixer trucks were not used during the tank filling phase of closure. Less than 50 truck loads were needed to finish the strong grout back-fill of the tank risers.

Tank 20 had seven available openings in which to pour grout; six perimeter locations and one center. Each location had a dedicated 5" ID flexible hose in which to pour grout. Each hose ran from a common platform mounted on top of Tank 20 to just above each opening in the tank. A flexible hose piece at the center permitted the grout installer to connect the transfer line to the appropriate pour location. Tremmies were attached to the pour location flexible hose and were placed inside the tank.

The entire tank filling operation was observed using a remotely operated video camera. The closure crew visually verified that all voids in the tank were filled. The grouts and CLSM were shown to be very flowable while in the liquid state and were able to self level and fully surround and enclose tank equipment. After visual inspection, SCDHEC approved the final as-closed condition of Tank 20 on July 31, 1997.