THE HANFORD PROBLEM
A basic problem with the tank wastes at the Hanford Reservation is that leakage into the underlying soils has already occurred in the 200E and 200W operating areas. A total of 177 buried tanks have been installed in these two areas to store various radioactive waste streams. These are organized into 12 tank farms with 149 single-shell tanks, and 6 newer tank farms with 28 double-shell tanks. The first single-shell tanks were put in operation in 1944, and as of 1993, leaks were known or are assumed to have occurred in 67 of the 149 single-shell tanks (Hanlon, 1995). Leakage problems have occurred in every one of the single-shell tank farms; each tank farm has at least one and as many as 10 tanks with leaks. It is very difficult to determine precisely which tanks have leaked. Consequently, the actual number of tanks which have leaked or are capable of leaking is not known.
The Hanford Federal Facility Agreement and Consent Order (Triparty Agreement) between Washington State Department of Ecology, U.S. Environmental Protection Agency, and U.S. Department of Energy (1989), states that essentially all wastes must be removed from both single- and double-shell tanks and processed for off-site disposal away from Hanford, unless technically infeasible. This will not, however, eliminate the need for an effective method of protecting and monitoring the soils beneath the single-shell tank farms that are already contaminated. In effect, the soils beneath each of the 12 single-shell tank farms will need to be isolated to prevent contaminant migration, whether the tank contents are removed or not.
A question can therefore be raised; Why cannot an appropriate method of containment-in-place be implemented that isolates the waste in the tanks, as well as the contaminated soil beneath? We emphasize that any containment method that could be used to protect the contaminated soils from further migration beneath the tanks could also enclose the tanks and, therefore, might well provide an effective method of isolating wastes that remain in these tanks.
Furthermore, most of the aqueous phase of the waste has already been removed from almost all of the single-shell tanks (U.S. Department of Energy, 1987), leaving a semi-solid phase that will be difficult to remove. The technology currently being considered for removal of the solid phase is hydraulic sluicing. The addition of large volumes of water to a tank that is expected to leak or is known to have leaked will require prior isolation of the tank using subsurface barriers. If the technology already exists to isolate the tank and prevent migration of waste materials under the extreme conditions of hydraulic sluicing, the same technique should be able to prevent contaminant migration in the absence of hydraulic sluicing. Thus, there is considerable merit to reexamining the tank management strategy.
This reexamination should be conducted on a tank-by-tank basis, as well as a tank farm-by-tank farm basis. If containment technologies are capable of isolating the tanks, then waste removal might only be necessary for those special circumstances where the tanks contain materials that are particularly hazardous-for example, where there is the potential for an explosion, where there are excessive temperatures, or where there are large inventories of long-lived radionuclides.
