Advanced Containment: Foundation Technique Traps Waste In Soils

Waste Treatment Technology News; April 1998

A foundation-shoring technique has led to a waste containment technology.  Arctic Foundations, Inc. (5621 Arctic Blvd., Anchorage, AK 99518; Tel:  907/562-2741, Fax: 907/562-0153) has pioneered a ground-freezing process it calls a “cryogenic barrier.”  Originally developed to prevent foundation slumping after seasonal freeze/thaw cycles in the Alaskan tundra, the company was approached by the Department of Energy (DOE) to undertake a project freezing soils surrounding a contaminated pond at Oak Ridge National Laboratory (ORNL).  The 316,000-gallon pond, which formerly stored nuclear reactor cooling water, contains large amounts of cesium-137 and strontium-90.

The freezing of the ORNL pond commenced in September, 1997.  Elizabeth Phillips, the DOE’s project manager for the cryogenic barrier demonstration, tells WTTN that the installation phase has been completed and the verification phase is now underway.

Phillips is pleased with the project results so far.  The technology has proved even less expensive than the DOE originally thought.  Initial installation costs were $1 million.  The thermoprobe freezing units, built to last 100 years, are very cost-effective.  The project’s maintenance costs are minimal as the system produces little or no waste and the refrigerant is constantly recycled.  Electricity to the probes runs approximately $15-22 a day.  Phillips adds that electrical costs could be eliminated in future projects by using solar collectors to operate the probes.

As part of an EPA project demonstration at Oak Ridge Lab’s Homogeneous Reactor Experiment, the cryogenic barrier was created by installing “thermoprobes” around the perimeter of the former storage pond.  The thermoprobes were sunk to a depth of 30 feet and spaced 6 feet apart.  Additionally the system contains an electrically powered condenser, an interconnecting piping system, a control system, and a two-phase working fluid.

The thermoprobes and piping were placed around and beneath the waste, and the working fluid circulates through the system.  Operating as thermo-siphons, the fluid-filled probes remove warmth from soils as the fluid cycles from gas to liquid phase.  The selection of working fluid will vary according to specific waste applications.  Carbon dioxide is being used at the Oak Ridge site, but other choices include freon, butane, propane, or ammonia.  Cooler air temperatures allow the probes to operate passively, the cool air pulling refrigerant down into the soil.  Mechanical means are required in warmer weather to transport the refrigerant into soil.

The pond area is prepared with a 6-inch insulating layer of polystyrene and topped with a reflective coating to reduce heat absorption.  As the refrigerant enters the soil, the ground freezes concentrically around each probe.  Eventually these circles overlap to form a solid barrier of frozen earth with temperatures near –40°C.