U.S. Department of Energy; Office of Environmental Management; Oak Ridge Operations ; Site Technology Coordination Group Newsletter; Summer 1997
This summer a team at Oak Ridge National Laboratory (ORNL) demonstrated soil freezing as a means of radionuclide containment at a Superfund waste site.
The demonstration site is an earthen pond dug in 1955 to receive waste from Homogeneous Reactor Experiment (HRE) conducted at ORNL between 1951 and 1961. The 316,000-gal pond, meant for low-level liquid waste (< 1,000 cpm/mL), also received highly contaminated fission products, principally 137Cs and 90Sr. The pond was backfilled and sealed in 1970.
Freezing began in September. Upon completion, a 12-ft-thick ice wall will extend up from the bedrock, which lies about 20 ft from the surface. Fluorescent dyes have been used to determine groundwater flow around the pond. A second dye test will be run sometime in the fall, after the frozen layer has been established, to verify that contaminants are no longer leaving the pond site.
An array of cooling pipes called “thermoprobes” is being used to freeze the soil at the site and maintain it at temperatures below 0°C. These freezing units are hybrid thermosyphons. If the ambient air is cool enough, the system will operate passively, driven by the natural air-soil temperature differential. In warmer weather, a mechanical refrigeration system will be used to drive the thermoprobes.
The thermoprobes are composed of 6-in. pipes that contain a heat-exchange coil in the upper 3 ft of the unit, which condenses the carbon dioxide working fluid in the thermoprobe. The condensate flows down the interior of the thermoprobe, where it picks up heat from the surrounding ground and evaporates. The evaporation cools the ground, and the vapor rises to recondense in the heat-exchanger. The heat-exchange coils in the thermoprobes are cooled by direct expansion of R-40a, an environmentally friendly refrigerant, to a temperature of –30 to –40°C. The heat is withdrawn from R-40a by a pair of electrically driven, air-cooled refrigeration condensing units. The energy requirements for this system are estimated to be modest. Once activated, the cooling units will consume about 1,000 kW/d of electricity during the freezing step; they will require about 500 kW/d to maintain the barrier.
Arctic Foundations, Inc., of Anchorage, Alaska, was awarded the DOE direct contract to design the system specifically for the HRE site and to install and operate it. It is the company’s first collaboration with DOE. The project is being funded by the Subsurface Contaminants Focus Area of the DOE Office of Science and Technology, the DOE Office of Environmental Protection Agency’s Superfund Innovative Technology Evaluation Program, and Arctic Foundations, Inc.