NOVEMBER 1998 CIVIL ENGINEERING MAGAZINE
|CONTAINING THE COLD WAR’S HOT WASTE
Ernest E. Carter, P.E.
|The U.S. Department of Energy buried more than 3 million m3 of radioactive waste generated during the Cold War in shallow, unlined earthen pits around the country. That waste, which is often mixed, has the potential to contaminate the environment over time. The complex process of safely excavating, processing and reburying the waste in a more suitable vault presents a risk to the life and health of workers.As our environmental awareness developed, we realized that earthen pits did not provide sufficient containment and that digging it up might be even more dangerous than leaving it in the ground. One microscopic particle of radioactive contaminated dust can lodge in the lungs of a worker and cause a cancer many years later. But a new method of creating and verifying bottom barriers under existing waste dumps can help create safer long-term containment for long-buried radioactive material without the hazards of digging it up.
Much of the buried matter consists of mixtures of chemical hazardous waste, transuranic waste, low-level radioactive waste and ordinary industrial debris. Some is undocumented, which means no one knows what’s in it. The cardboard, wood and thin steel containers that the waste was placed in have deteriorated, so large volumes of soil are now contaminated as well.
Waste was buried in earthen pits because that was the accepted method of dealing with it at the time and because it was generally thought that we could dig it back up and transfer it elsewhere if necessary. But the process of excavating the radioactive material safely requires such complex and elaborate facilities that, statistically speaking, ordinary construction accidents can be expected to kill several workers. In spite of stringent safety measures, workers have already been killed in such accidents. By contrast, the old disposal sites, inadequate as they are, have not killed or sickened anyone.
Certain technologies can remove a percentage of the radioactive particles, but no regulator would ever certify the resulting material to be “clean.” So far, the only method for disposing of radioactive material is sealing it off from the rest of the environment.
What if, instead of trying to create some new solution that will last forever, we just try to improve the old solution? Although it is impossible to prove that any man-made structure will last forever, or even 100,000 years, advancements in engineering over the last 50 years will surely allow us to reengineer existing dumps to last 1,000 years. After all, roads and structures built by the Roman Empire 2,000 years ago exist to this day, and ancient Egyptian tombs have preserved dead kings for thousands of years.
By selecting thermodynamically stable grouting agents that are chemically similar to natural materials known to be stable over thousands of years, we can project that our materials will be stable for similar amounts of time.
IN SITU CONTAINMENT
In situ containment, or isolating a waste from the environment without disturbing it, can take two forms. The first is encapsulating the waste into a single monolithic block of durable material. The second is entombing the waste inside a dry vault.
Encapsulation, or solidifying waste into a durable, solid, waterproof chunk, has the potential to contain radioactive material safely for thousands of years. But encapsulating every bit of debris in a landfill is difficult. Varying moisture levels and nonhomogeneous waste types surrounded by voids or impermeable materials complicate the task. The encapsulating material must contain mobile organic liquids, soil, salt and heavy metals, as well as pieces of junk as big as a Buick.
Jet grouting, one option for encapsulation, uses a rotating drill pipe to position ultra-high-pressure (exceeding 34,400 kPa) jets of liquid grout underground. The grout pulverizes and mixes subterranean soils and debris into a solidifying column. Multiple overlapping columns effectively convert nonhomogeneous soil and waste into a monolithic soil-cement rock.
Conventional jet grouting is a messy process that would expel contaminated liquid spoil back to the surface in amounts equivalent to more than 100% of the column volume. The high water content of the cement slurry used in civil construction jet grouting makes a relatively permeable rock that does not adequately stabilize some wastes. Fortunately, high-tech grouts and a containment structure with a false floor can solve both problems.
A new jet grouting technology using patented hardware developed by engineers at the Idaho National Engineering and Environmental Laboratory (INEEL) in Idaho Falls and patented grout materials developed by Carter Technologies Co., Houston, can encapsulate waste without returning spoils to the surface. In 1997 this technology was demonstrated on an actual radioactive contaminated site known as the Acid Pit at INEEL. This is an area where radioactive acids and heavy metals were disposed of by pouring them out on the ground. The project conclusively demonstrated that jet grouting can be safely applied to a radioactive site without releasing any contamination.
The demonstration relied on TECT HG, a hematite analog grout with rheological properties that allow relatively small volumes of the grout to disrupt and thoroughly mix with soil and debris, reducing the amount of grout required and spoils produced. The grout seems to bind contaminated soil, dusts, particulate fumes, debris, oils, salts, mercury and sludge into a low-permeability, ceramiclike product that is expected to have geologic durability similar to natural hematite.
Waxfix, a plastic molten waxy grout, was also demonstrated on a cold test site. The patented grout saturates essentially all of the dust and debris in a landfill, solidifies organic liquids and prevents water from contacting the wax-impregnated waste. The treated waste can be left in place or excavated without generating contaminated dust. Tests by Brookhaven National Laboratory in Upton, N.Y., showed the permeability of both grouts as a grout/soil mixture to be less than 1 x 10ÿ10 cm/s.
Field-testing in nonradioactive landfills has shown that these sites can be jet grouted without significant spoil returns to the surface by using the special grouts. Sites below the water table have essentially no void space and will produce some spoils even with the special grouts.
To contain these contaminated spoils and keep them from returning radioactive material to the surface during jet grouting tests, engineers at INEEL covered the project site with a thrust block, a structural concrete platform with a false floor. The block’s concrete slab, with structural ribs on its underside, provided a large volume to contain whatever spoils came back to the surface. Preformed holes in the thrust block platform defined the drilling locations and the spacing of the grout columns.
To keep workers from being exposed to any contaminated airborne particles, crews mated a negative-pressure shroud on the drilling unit to the thrust block and maintained negative air pressure under the thrust block by pulling the air through a high-efficiency particulate air (HEPA) filter. An accordionlike hose covering the drill pipe was connected to the shroud, which enclosed all surfaces that touched the waste within the negative-pressure system. Air emission monitors and radiation detection surveys around the work area were used to confirm that no radioactive particles or airborne emissions escaped from the work area.
DRY VAULT CONTAINMENT
INEEL engineers estimate that jet grouting encapsulation will cost approximately $2,000 per cubic meter. A less expensive alternative for large sites is to create a dry, airtight and watertight vault around the waste. The watertight feature is especially important because water, the universal solvent, causes leaching and migration of wastes.
The biggest challenge in creating a dry vault around existing waste is installing a watertight bottom barrier. Most environmental proj-ects attempt to use a natural confining layer such as a clay stratum as a bottom, but these materials are not sufficiently impermeable to keep water out of the vault. The vault can stay only as dry as the relative humidity of the soil at the bottom interface.
Jet grouting can be used to form a bottom barrier, but usually the barrier will have many small leaks that are difficult to find in the field.
To surmount the leakage problem, Carter Technologies Co. has patented a more positive method of constructing and verifying barriers under a waste site. The method is really a suite of technologies that apply to different situations and soil types. The process includes a simple method of cutting horizontal or inclined pathways between multiple directionally drilled holes, between two trenches or between a hole and a trench. That pathway is then filled with grout to form a permeability barrier, a permeation pathway or a structural panel.
The cuts are made by a cutter operating in a catenary fashion between the holes or trenches. The edges of the pathway do not have to be parallel and there are no fixed depth limits. The cut may be horizontal, vertical or continuously variable (as in the case of forming a basin-shaped structure).
There are two basic types of cutting hardware. One is primarily an abrasive mechanical cutter capable of going through hard soil and even rock. This system is similar to a diamond wire quarry saw used to cut granite blocks.
The other system is a linear version of jet grouting, which cuts mainly by high-pressure jets of grout. This method works best in soft soil and is quite fast. Paths up to 30.5 m wide between holes may be formed with a single cut. A predecessor of this device was used to form horizontal panels from multiple 3.05 m wide panels at a test site in Oklahoma. Both systems have self-proving characteristics because the hardware physically passes through the length and width of the cut to mechanically verify continuity.
With either system, a damaged or broken cutter can be replaced during cutting. If contractors encounter rock, they can even replace the jet cutter with the abrasive cutter and continue.
Many separate subterranean panels can be joined together to form large, multihectare basins. The geometry of the cutter mechanism is arranged so that the next adjacent cut is inherently joined to the previous cut.
House-sized barriers can be formed under structures using a single cut in many soil types. Such barriers can be made from one of several special plastic grouts, which have ultralow permeability and the ability to compress or stretch. These grouts also self-heal any cracks. Industrial uses may include stopping migration of radon or methane gas into the basement of a building. The cost of these bottom barriers is estimated to be approximately $320 per square meter of barrier.
There are also special lifting grouts that can form a solid barrier up to a meter thick from an initial cut only a centimeter thick. All methods have a means for recovering quickly from breakage or obstructions underground and for preventing subsidence and pinching out of the barrier prior to hardening.
These techniques can be used to form not only a horizontal floor but also an entire basin structure—rectangular or bowl-shaped—under and around a waste burial site. In the case of a radioactive mixed-waste site, the method also includes the means for passively verifying the initial and continued integrity of the barrier. This system makes the entire structure so airtight that virtually all the moisture inside can be removed by a series of vacuum extraction and dry air injection cycles over a long period.
The resulting dry tomb containing the desiccated waste is expected to last for thousands of years. Continued integrity is verified by soil gas humidity sensors under the cap.
Two major types of grout are available. The first, called TECT SG, is a dense but low-viscosity liquid made of cementitious materials with a specific gravity of up to three times the density of water. The high density provides buoyant support for the overburden soil above the cut. The grout remains liquid for several weeks before hardening into a low-permeability material. Additional flow of grout causes a buoyant lift that results in a barrier much thicker than the abrasive cut.
The second grout is a molten polymer wax that contains additives that allow it to permeate even wet clay soils. Called Waxfix, the plastic molten waxy grout permeates the soil above and below the cut to create a barrier much thicker than the original abrasive cut. The polymer can be used to preseal a site where the high-density grout is later used to create a thick solid barrier.
Once an air- and watertight bottom barrier is in place, it will still be necessary to keep the waste dry and detect any leaks that may develop. Carter Technologies’ patented system places an airtight top cap to the bottom barrier without the gas vents standard in most top caps. Controlled ports on the cap can introduce dry air or inert gas into the vault at one end while vacuum extraction removes air and moisture from the other end, effectively drying out the contents.
The integrity of the vault is monitored with internal and external soil gas pressure sensors. A log of the effect of normal barometric pressure variations outside versus changes inside demonstrates the integrity of the vault. Low-cost humidity sensors sealed under the cap can monitor the environment inside the vault.
If leakage occurs in the future, additional drying can remove the moisture before the vault’s containment capability is compromised. Chemical tracer gas placed in the ground can identify the locations of such leaks, allowing repairs by additional grouting or flooding of the semidry vault with a permeating grout.
The total volume of buried radioactive wastes continues to grow, and the cost of safe retrieval, reprocessing, and redisposal is staggering: estimates range from $8,000 to $20,000 per cubic meter. In situ encapsulation will cost only a 10th as much, and in situ dry vaults will cost only a tiny fraction of that. In situ containment, in either form, may turn out to be the faster, safer and cheaper alternative.