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B.5.0 TANK CLOSURE

This section describes the representative tank closure process that has been included in the alternatives to allow an equitable comparison of alternatives. Closure is a term that refers to the final disposition of the tanks and associated piping, any residual waste that remains in the tanks following remediation, equipment that may be left in the tanks, and any soil or groundwater contamination associated with the tank farm operations.

Under the Tri-Party Agreement, both SSTs and DSTs are RCRA hazardous waste management units that will be eventually closed under State Dangerous Waste regulations (WAC 173-303). Three options exist for this closure: 1) clean closure, involving removal of all waste and waste constituents, including tank, debris, contaminated equipment, and contaminated soil and groundwater; 2) modified closure, which involves a variety of closure methods but requires periodic (at least once after 5 years) assessments to determine if modified closure requirements are being met; and 3) closure as a landfill with waste remaining in-place and corrective action taken for contaminated media under post-closure requirements. All three options require the submittal and approval of closure plans by Ecology. There is currently insufficient information available to make a decision on how to close the tanks, so closure is not within the scope of this EIS. However, decisions (such as the percent of waste recovery) on how to treat and dispose of the tank waste may impact the level of closure activities in the future.

To provide information on how closure activities would be affected by remediating the tank waste, a representative approach to tank closure (closure as a landfill) has been included in each of the TWRS alternatives to allow an equitable comparison of the alternatives. This is described in the following text.

Closure would address 149 SSTs, 28 DSTs, and approximately 60 MUSTs and includes the other ancillary equipment associated with waste tank activities. Closure would apply as follows.

• Both the SSTs and DSTs would be stabilized to prevent dome collapse by gravel filling for all ex situ vitrification alternatives and the In Situ Fill and Cap alternative. The gravel-fill process would involve the uniform distribution of sized, crushed rock throughout the tank including the tank dome, using a gravel slinger. This commercially-proven technology is used in filling ship holds and silos with materials such as grain or cement. Tests performed at the Hanford Site have verified the use of this technology with local materials in a tank-like environment.
• Ancillary equipment and MUSTs would be grout filled for stabilization in all treatment alternatives with the exception of the No Action and Long-Term Management alternatives. Ancillary equipment and MUSTs would not be excavated or packaged. Ancillary equipment would include diversion boxes, catch tanks, valve and pump pits, process pits, diverter stations, receiver vaults, condensate tanks, risers, transfer piping, and piping encasements associated with SST operations. Pipelines would include lines between tanks and process facilities, air and steam supply lines, raw water lines, and drains.
• During tank farm closure, ancillary equipment items and MUSTs would be stabilized in place with an appropriate grout material. The physical immobilization of contaminants provided by grout could be augmented by using sequestering agents, such as zeolites, that would be capable of chemical bonding with contaminants. If ancillary equipment was plugged at one or more points, several access ports would have to be installed to ensure complete grout filling.
• For purposes of assessing the environmental impacts associated with dispositioning of ancillary equipment and MUSTs as part of closure, it was assumed that the entire void volume within the ancillary equipment would be filled with grout and that no ancillary equipment or Inactive MUSTs would be excavated, packaged, or disposed of as LAW or mixed waste (WHC 1995i).
• Surface barriers (Hanford Barriers) would be placed over SSTs and DSTs for all alternatives except the No Action and Long-Term Management alternatives . Barriers would also be placed over the LAW vaults described in the Ex Situ Intermediate Separations, Ex Situ No Separations, Ex Situ Extensive Separations, Ex Situ/In Situ Combination 1 and 2, and the Phased Implementation alternatives.