Civil Engineering News; Environmental Engineering; June 2003

A frozen pond of radioactive waste will soon be thawed or excavated at the Oak Ridge National Laboratory, in eastern Tennessee, marking the end of an innovative containment demonstration project conducted by the U.S. Department Energy (DOE).  For almost six years the DOE has maintained the barrier of frozen soil around a 9,041 sq ft (840 m2) area contaminated with strontium 90, polychlorinated biphenyls (PCBs) and other toxins.

 

In the late 1950s the contaminated site was known as the HRE pond – an impoundment at the bas of a hill that stored low-level liquid wastes produced by a prototype nuclear reactor in what was called the Homogeneous Reactor Experiment.  The pond, actually more of a hole, had been excavated  to bedrock to depths reaching 25 ft (7.6 m) and then filed with toxins.  It was decommissioned in 1970, backfilled with clay and shale, and capped with an asphalt pavement.

When tracer tests in the 1990s determined that the site was leaching contaminants into a nearby waterway, the DOE needed to act fast to contain the toxins.  Rather than seal the area with a permanent structure such as a slurry wall, the department’s Office of Science and Technology decided to test the effectiveness of frozen soil as a temporary barrier.

Engineers often freeze soil to stabilize a foundation or seal off groundwater.  The technique involves pumping chilled antifreeze through a network of underground pipes.  In order to avoid the risk of leakage, engineers relied on a modified passive heat removal system – a series of 6 in. (150 mm) diameter siphons filled with carbon dioxide and fitted with a heat exchanger at the top.

Engineers from Arctic Foundations, Inc., of Anchorage, Alaska – a firm that has been freezing foundations since the 1950s – placed 50 siphons at roughly 6 ft (1.8 m) intervals along the periphery of the site, and installed them at depths of 30 ft (9.1 m).  When the air at the top of each siphon is cooler than the soil at the bottom, the carbon dioxide condenses and travels to the bottom of the pipe.  There the condensate evaporates and carries heat from the ground to the air above.  The cycle continues until the temperature of the ground is less than the temperature of the air.

In cold climates, such passive ground freezing systems can function without any outside energy source.  Since temperatures in Tennessee are rarely below freezing, the engineers equipped the top of each siphon with a 30 hp (22.4 kW) expansion refrigeration system.  “It’s the same type of system that the butcher down the road from you may have in his blast freezer,” says Ed Yarmak, a chief engineer, for Arctic Foundations.

It took about two months to drill the holes for the siphons.  With the devices placed, crews covered them with quartz sand and then patched the holes in the original asphalt cap.  To prevent heat from penetrating the barrier, crews installed a cap of extruded polystyrene insulation in three 2 in. (50 mm) thick lifts.  The system was activated in September 1997 and by November of that year the cylinders had frozen the soil between them, forming a perimeter barrier about 300 ft (91 m) long, 12 ft (3.7 m) thick, and 30 ft (91 m) deep.

Engineers from Arctic installed eight vertical wells containing temperature sensors and piezometers around the contained pond to monitor its condition.  Four siphons inside the containment also were equipped with temperature and pressure sensors.  The barrier and the monitoring equipment were installed for about $1.2 million, and the cost of running the refrigeration unit over the past six years has averaged about $15 per day.

The barrier’s performance was carefully monitored for 12 months by the U.S.  Environmental Protection Agency with groundwater tracer tests.  An independent study has not been conducted since then, but every other day engineers from Arctic have monitored the temperature of the wall via an online data logger.

The frozen barrier was always viewed a temporary means of confining the contaminants to the HRE pond until they could be extracted and buried in a landfill licensed for radioactive waste.  Bechtel Jacobs, the locally based joint venture in charge of a large-scale cleanup at Oak Ridge, will issue a request for proposals this summer for bids to excavate the material – estimated at about 4,200 cu yd (3,200 m3) – as well as other contaminated sites at the compound.

It took four months from the start of construction for the frozen barrier to take shape, but it could take two years for the soil to thaw completely.  Greg Eidam, a Bechtel Jacobs project manager, says that once a contractor is chosen, that company will decide whether to thaw the ground or excavate it frozen.

by Greg Brouwer