Site Description
Used to store highly radioactive and chemical wastes, the 149 single-shell underground storage tanks are clustered in "tank farms" located near Hanford's primary chemical processing facility, the Purex Plant. Constructed between 1943 and 1967, these tanks received effluent from processing of irradiated fuels for recovery of plutonium, uranium and other reactor-produced materials. The tanks were constructed of a single layer of carbon steel, reinforced with concrete. The tops of the tanks are concrete domes, covered with six to nine feet of soil.
Operating procedures called for continually adding waste
to a tank while maintaining liquid levels at 70% to 80% and
allowing the material to "self-concentrate" from heat
produced by fission product decay. As the tanks were filled,
the wastes separated into an insoluble sludge layer and a
supernatant (pumpable liquid) layer. In some cases, "if
solution were removed or lost from the tank and the sludge
allowed to dry, temperatures in excess of 10,000 degrees F
could result" (Purex 105-A Waste Storage Liner Instability
and Its Implications on Waste Containment and Control; Beard
and Hatch, ARCO - Oct. 31, 1967).
Major Environmental Concerns
Considered "the greatest long-term risk to the public health and the environment" within the DOE weapons complex, the single-shell tanks contain significant amounts of plutonium, and other highly toxic materials (DOE/ID/12584-92 Vol.2 GJPO-104). Currently, approximately 40.9 million gallons of sludge and saltcake, and 19.3 million gallons of liquid are present in the Hanford tanks (WHC-EP-0182-34). An estimated 300 million Curies of high-level radioactive hazardous and mixed waste are present in the tanks. Of the total, 67 tanks are known or assumed to be leaking and 24 tanks have been classified as containing potentially explosive levels of ferrocyanide (>100g/mole). An additional 23 tanks are considered explosion risks resulting from accumulation of hydrogen, 11 have self-sustaining high heat (>40,000 BTU/hr) concerns, and 8 have potentially flammable mixtures of organic materials.
This highly toxic material has the "potential for release of high level waste due to uncontrolled increases in the temperature or pressure" within the tanks. Concern exists that an event similar to the January 28, 1965 sudden steam release from a single-shell tank will occur in the future. During this event, earth in the immediate vicinity of tank 106-C trembled and a lead cover was dislodged, releasing steam and "a few gallons of liquid." Radiation doses of 400 R/hr were measured one foot from the spill. Upon inspection, it was learned that the bottom of the tank had bulged upward 8.5 feet. This was a result of sludge migration from the tank into the annular space between the steel liner and concrete reinforcement. Increasing sludge temperatures in the presence of water in this confined space caused a near total failure of this single-shell tank. Hanford's high-heat, ferrocyanide, and hydrogen-containing tanks are all considered similarly potentially unstable.
During the 1970's, as much as 1 million gallons of water
was sprayed into single-shell tank 105-C in order to
facilitate cooling of the tank contents. It is likely that
800,000 gallons of this contaminated cooling water has
escaped into the surrounding soil and groundwater
(WHC-EP-0182-34). The process of adding water to maintain
tank temperature of high-heat tanks continues, potentially
adding substantial amounts of radiologically-contaminated
water to the underlying aquifer. Available information
suggests that in the absence of institutional controls
(adding water), the contents of the 106-C tank could climb
to uncontrollable temperatures. This has the potential to
lead to a catastrophic failure of 106-C and contamination of
the surrounding area, including the Columbia River.
Proposed Solutions to Major Environmental Concerns Related to the Single Shell Tanks
Containment of the 149 Single Shell Tanks at Hanford is possible using a CRYOCELL® (frozen soil) barrier providing full enclosure (bottom) of the single shell tank clusters and other contaminated apparatus. CRYOCELL® is 10-15 foot thick barrier formed by circulating refrigerant through dual tube bore holes spaced around the area to be contained. This freezes the soil moisture and reduces permeability, providing complete containment of actual plumes (e.g., groundwater contaminated with tritium) or potential plume sources (e.g., tanks). Cost studies indicate frozen wall technology to be cost competitive with competing tech-nologies. All chemical and high level radioactive waste present at these locations can be stopped from leaching further into surrounding soils and aquifers.
Distinct features of cryogenic containment applications are: *Impervious to leakage, *Can provide full enclosure (bottom), *Can be installed in any type of soil (e.g. arid and coarse desert), *Earthquake resilient because of self-healing characteristics, *Easily repairable in-situ (if necessary), *Monitorable on continuous, real-time basis from remote location, *Removable, cost-effectively, without excavation, *Environmentally friendly (no excavation for installation, repairs or removal), less exposure to contaminants, *All construction activities, including installation and barrier operations equipment, would be performed outside of the secured area of the tank clusters, *Efficient cost effective drilling, as techniques are available, for the installation of the required underground systems which provides little or no spoils, (this provides a tremendous safety advantage to workers during installation, and throughout the operational lifetime of the barrier, *Cost-effective short-term, long-term installation.
A key attraction of CRYOCELL® is its potential compatibility with other remediation technologies, as it will allow contaminated soil to be isolated in an impervious frozen soil containment envelope. The application of a remediation process within an impervious enclosure allows for the application of water, or the use of enhanced liquids, as an efficient upgradient in a recycle process without the risk of releasing the subject fluids into the surrounding environment.
Barrier integrity, monitoring and verification system for
cryogenic systems include constant assessment of
temperature, pressure, and soil moisture characteristics by
installing the necessary components along with underground
freezing assemblies, providing real-time monitoring and
control. In addition, ground penetrating radar and
electropotential devices can provide assurance that a
barrier has formed a hermetic seal, offers a visual
representation of the forming ice barrier, and locates any
unfrozen "windows" in the barrier. These systems combined
with the temperature monitoring systems provide continuous
real-time surveillance of the ice barrier, which can be
remotely monitored. Three-dimensional imaging through
computer animation techniques enhance the monitoring process
throughout the barrier's life-cycle.
Estimated Cost of this Solution
Unloaded construction costs for containment of the "C" tank cluster (16 tanks) at Hanford is estimated at $16 million. Operational costs including power for 20 years of uninterrupted service of the CRYOCELL® barrier is estimated to be $7 million.
Technology Maturity
The CRYOCELL® technology has been field tested and is
ready for field implementation. It has also been documented
in a DOE Innovative Technology Summary Report, (known as a
"Green Book") entitled Frozen Soil Barrier Technology, and
DOE/EM-0296, Subsurface Contaminants Focus Area Technology
Summary, August 1996. It is one of the few containment
barrier technologies listed in the Environmental Protection
Agency's SITE Technology Program, 7th & 8th Editions. It
has also been lab tested for hazardous waste confinement
using Hanford soils. Diffusion tests were conducted for
Chromate and TCE, Radioactive Cesium, Decane and Calcium
Chloride Brine in 1995 for DOE by J.G. Dash, R. Leger and
H.Y. Fu of the Physics Department of the University of
Washington, Seattle, Washington. All tests indicated that
the barrier was impermeable. A copy of this report is
available upon request. Ground freezing has been used in
civil engineering to provide groundwater control and
structural excavation support under the most severe
engineering and design condition for over 100 years