Site Description
The Hanford Nuclear Reservation is the largest and most complex of the 16 major defense-related facilities operated by the U.S. Department of Energy (DOE). Hanford occupies 550 square miles bordering on the Columbia River, approximately 50 miles from the TriCities area (population 340,000) in South Central Washington. The primary purpose of Hanford was to provide reactor-produced radioactive isotopes for use in defense-related nuclear weapons production. The 100 Area is located in the northern part of the site along the southern border of the Columbia River. Nine nuclear reactors are located along the Columbia River in the 100 Area.
The N reactor was the first to be constructed as a major production reactor at the Hanford Site. It differs from the other reactors at Hanford in that it was designed for a dual purpose, capable of producing special nuclear materials and steam. The N reactor went into production in December 1963 and shutdown was ordered in October 1991.
Environmental studies conducted at the Hanford Site have concluded that discharges of ground water to the Columbia River at the 100-N Area contribute a significant portion of the total waterborne offsite radiation exposure due to Hanford operations. (DOE-RL 1990a).
Major Environmental Concerns
The 100, 200, 300, and 1100 Areas at Hanford have been included by the U.S. Environmental Protection Agency (EPA) on the National Priorities List. A total of 33 problem units were identified within these Areas. The 100 Area contains a number of large ground water plumes in five operational areas that are contaminated with tritium, nitrates, chromium, and strontium-90.
The 100 Area also has a large volume of contaminated soil that includes mixed waste from near-surface liquid disposal units such as cribs, french drains, retention basins, and miscellaneous spills. These soil contaminants include chromium, cesium, strontium, cobalt, and nitrates. The estimated total radioactivity is 300,000 curies, most of which is presumed to be low-level waste. The elevated water levels resulting from discharge to the1301-N crib and trench were responsible for the formation of numerous springs in the 100-N Area along the Columbia River, and the area came to be known as "100-N Springs").
There are 108 sources of potential or known contamination in the 100-N Area (DOE-RL 1990a). The major contributors to the contamination found in the N-Springs effluent into the Columbia River are the 1301-N Crib and Trench and the 1325-N Crib and Trench. These facilities received low-level liquid radioactive effluent flows containing small quantities of nonradioactive chemical constituents. Radioactive effluents and wastes were generated as a result of 105-N reactor operation during its operating life of approximately 25 years and continue to this day. The 1325-N crib and trench, which used the soil to load radioactive isotopes, has been the major contributor to ground water and soil contamination and suspected releases to the Columbia River.
Other 100 Area problem units include contaminated and non-contaminated piping systems that supported the nuclear reactors; effluent piping from the reactors to numerous retention basins and then to the Columbia River; eight radioactive fuel-storage basins; nine reactor cores that need to be decontaminated and decommissioned; radioactively contaminated surface soils and biota; a 78-acre burial ground with unsegregated wastes and radioactive contamination; nine petroleum underground storage tanks, some of which have leaked; 100 Area reactor buildings, stacks, laboratories, and ancillary structures that need to be decontaminated and decommissioned; and potentially contaminated sanitary and laboratory sewers. (DOE/ID/12585-92 Vol. 2 GJPO-104.)
Proposed Solutions to Major Environmental Concerns at the 100-N Springs
Installation of a CRYOCELL¨ frozen soil cut-off wall can divert groundwater flow to eliminate Strontium-90, Tritium and other discharges into the Columbia River.
The 100-N Springs area is a viable candidate for the use of frozen soil barriers for hydraulic diversion. As Phase I of this proposal, the CRYOCELL(R) barriers would be designed to meet site specific conditions and objectives and would be installed near the edges of contaminated plumes to direct the contaminated groundwater flow towards a treatment zone. Due to the flexibility of a frozen soil barrier, a capture treatment zone can be incorporated into the overall design and referred to as a Permeable Reactive Wall Section. This wall section should be installed in a chosen location and have reactive areas located at desired depths designed as "residences" for replaceable cells of the selected remediation processes. Possible remediation processes for consideration are PNL's Clinoptilolite (Kitty Litter) sorption process, a MAG*SEP process, the LASAGNA(tm) process, or others. The chosen passive remediation processes can be used singularly or in combination. After passive remediation processes are complete, the frozen soil barrier and the passive remediation sections or "residences" can all be removed.
ResonantSonic drilling (also a DOE "Green Book" technology) is an innovative and cost effective approach to environmental drilling applications. This technology is hydraulically driven, and incorporates a sonic drill head. Series of high frequency sinusoidal wave vibrations are generated into a steel drill pipe, creating a cutting action at the bit face. The tremendous forces produced by the resonant head and the external flush nature of the specialized drill pipe force material displaced by the bit into the borehole wall. The result, no drilling spoils. It is also possible to resonate a closed-end steel pipe into the ground, displacing formation media similar to a pile driver. ResonantSonic drilling can produce boreholes of various depths and dimensions, yield continuous, relatively undisturbed cores, and requires no air, mud, water, or other circulatory medium for drilling. The method is extremely fast (up to one foot per second in certain formations), and can drill through virtually any media from horizontal to vertical. Again, this is with no drilling spoils, which provides a direct reduction in risk and overall site cost by reducing the waste volume produced during installation of a CRYOCELL containment barrier.
In Phase II, CRYOCELL(R), providing full enclosure (bottom), is proposed for containment of the 1301-N and 1325-N Crib to halt contamination transport from these sources and isolating them for remediaiton activities.
Distinct features of CRYOCELL(R) frozen soil barrier 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, *Environ-mentally friendly (no excavation for installation, repairs or removal), less exposure to contaminants (this provides tremendous safety advantage to workers during installation, and throughout the operational lifetime of the barrier), *Cost-effective short-term, long-term installation, *Applicable ISO 14,000 Standards will be incorporated into the overall design of the installation, *Efficient and cost effective drilling techniques are available, specific to the 100-N Springs area, for the installation of the required underground systems which provide little or no spoils, *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 these Solutions
Unloaded construction costs for Phase I or the installation of the wall along the 100-N Springs is estimated at $7-$10 million. Operational costs including power for 10 years is estimated at $2-$3 million. The Permeable Reactive Wall Section is estimated to cost between $2 -$4 million. The remediation process chosen will be priced by its respective developer/owner. For Phase II, CRYOCELL containment (full enlcosure) of the 1301-N crib and trench is estimated $5-$7 million and containment (full enclosure) of the 1325-N crib and trench is an additional $5-$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.