Engineers charged with devising repositories for nuclear waste face no small task.
The repository must be maintenance-free, prevent its contents from escaping either as liquids or noxious gases, resist intrusion by plants, animals and humans, and be acceptable to the general public and a vast army of state and federal regulators. Last, but not least, it must be able to isolate the nuclear waste for at least 1,000 years.
According to Richard Wing and Glendon Gee, writing in a recent issue of Civil Engineering, the knowledge of the ancients is playing a large role in designing the perfect caps for modern dumps to contain materials that could prove dangerous to generations who will not be born for centuries yet to come.
Before beginning the process of building a test cap at the Hanford nuclear waste site in the state of Washington, engineers studied tombs and temples that have survived for millenia. Many of the ancient sites have survived intact in spite of centuries of weathering and erosin as well as human and animal exploration. Of particular interest were the methods used by the engineers of antiquity to prevent water from reaching protected chambers within the sites even in regions that experience annual flooding.
Natural geological analogs of artificial pits were also examined. Some natural pits in the vicinity of the Hanford site have remained essentially unchanged for 13,000 years when the last ice age receded. What both the natural and artificial pits appeared to have in common was a layering of fine soils over coarser materials which, in turn, capped a more or less impermeable layer of clay or asphalt that prevented water from either entering or leaving.
At the Waterford site, a series of 10 layers, of differing depths and composition, were placed over the test pit. The top layer consists of about 3 feet of fine loam that stores water for recycling back to the atmosphere and to support plant life to aid in evaporation and transpiration. Beneath the topsoil lies nearly 12 feet of layers differing in composition from silt to coarse gravel and broken asphalt similar to that removed from roads being resurfaced. These layers are designed to prevent root systems and animal burrows from penetrating below the top layer and to help create a favorable water cycle for the surface vegetation.
The coarse materials may also discourage human explorers from digging deeper into the cap. Next comes a 6-inch layer of asphalt designed to prevent water from percolating into the interior or water and gases from escaping. The coarse gravel just above is expected to serve as a drainage medium to channel percolating water to the edges of the pit. Finally several feet of compacted soil lie beneath the asphalt covering the stored materails.
Tests on the experimental site have been going on for the past six years and early results have the design performing as expected. The vegetated barrier systems will store or return to the atmosphere at least three times the maximum average annual precipitation for the region. Asphalt samples from the site are being compared with artifacts that are between 500 to 3,000 years old in order to determine the aging process in buried environments. Preliminary studies show the asphalt cap should last well over the required 1,000-year period. No gases or contaminated waters have been detected escaping from the site. Because it is impossible to test the system for a millenia of average weather, it is being stressed with snow, water, and temperature variations that would occur in a 100-year time span over short time periods.
The major problem with sites being sealed for periods of a millenia or more has little to do with the physical barriers being devised. As the centuries pass, the reasons for the sites are sure to be lost to succeeding generations who may do archeological digs to see what their ancestors were at such pains to conceal. The meanings of today’s hazard logos, such as that for radioactivity, will also likely be meaningless to our descendents of centuries from now. Study groups are trying to devise non-linguistic warning signs for the sites that could be embedded into the asphalt layer to give future explorers a sense of the dangers that lie within.
Clair Wood is a science instructor at Eastern Maine Technical College and the NEWS science columnist.