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<br />Ou, EN(x\EER1NG/0CT0BER 1994
<br />Ell] I Ell, 3HE 9
<br />N. RICHARD WING
<br />GLENDON W. GEE
<br />Engineers and scientists at the Department of Energy's Hanford site in Washington believe they
<br />have developed a maintenance free waste -site surface barrier, made from natural materials,
<br />that will last for 1,000 years. They are ready to monitor a 5 acre prototype, recently constructed
<br />over a decommissioned wastewater -disposal facility/. If it performs as expected, this cap could
<br />have significant impact on waste -disposal systems nationwide.
<br />xhuming and treating wastes may not always be the most effec-
<br />tive way to remediate a site. In some cases, in-place disposal
<br />with a protective cap offers the best protection for human health
<br />and the environment. This is especially true of Department of En-
<br />ergy (DoE) sites where radioactive wastes may be better left
<br />undisturbed. Yet in-place disposal requires a protective covering
<br />that can guarantee isolation of the wastes for centuries. No
<br />proven long-term barrier currently exists, but a recently con-
<br />structed 5 acre prototype barrier may be just such a cap.
<br />Since 1985, a team of engineers and scientists on the Han-
<br />ford Site Permanent Isolation Surface Barrier Development
<br />Program have been working to create a cover that could isolate
<br />wastes at DoE's Hanford site near Richland, Wash. for 1,000
<br />years. The development team includes engineers and scientists
<br />from Westinghouse Hanford Co. and the Pacific Northwest
<br />Laboratory (PNI), both of Richland. Kaiser Engineers Hanford
<br />Co., also of Richland, provided design support for barrier -relat-
<br />ed projects, and worked with Westinghouse and PNL to develop
<br />the definitive design drawings and construction specifications
<br />for the prototype barrier.
<br />Current cover designs, such as those recommended by EPA,
<br />have a short design life, usually no more than the 30 -year post -
<br />closure period specified under the Resource Conservation and
<br />Recovery Act (RCRA). During this relatively brief time, engi-
<br />neers can monitor and maintain the barrier and correct any
<br />problems. Many wastes, however, must be isolated for much
<br />longer periods, from centuries to millennia. For these waste -
<br />management situations, the relatively short-term designs are
<br />not satisfactory. Many synthetic construction materials that
<br />might be effective for decades cannot be relied on to perform
<br />satisfactorily, or even exist, more than 1,000 years.
<br />At Hanford, a cap is needed to isolate single -shell tank
<br />wastes, transuranic -contaminated soil sites and sites where
<br />transuranic -contaminated solid wastes are buried. Decommis-
<br />sioned facilities, solid -waste sites, low-level waste sites and haz-
<br />ardous -waste sites could also take advantage of this barrier in-
<br />stead of current short-term cover designs.
<br />In developing such a cap, the design team focused on sever-
<br />al objectives. We decided that a long-term cap should:
<br />• Function in a semiarid to subhumid climate.
<br />• Limit the recharge of water through the waste to the water
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<br />table to near -zero amounts. We selected a design objective of
<br />0.05 cm of water per year (1.6 x 10-9 cm/s)—based on prelimi-
<br />nary performance assessments.
<br />• Be maintenance -free.
<br />• Resist plant, animal and human intrusion.
<br />• Limit the exhalation of noxious gases.
<br />• Minimize erosion -related problems.
<br />• Meet or exceed RCRA cover -performance requirements.
<br />• Isolate wastes for a minimum of 1,000 years.
<br />• Be acceptable to regulators and the public.
<br />We organized our work into 15 groups of tasks to resolve
<br />technical concerns, covering project management, biointrusion
<br />control, water infiltration control, erosion and deposition con-
<br />trol, physical stability testing, human interference control, pro- !.
<br />curement of barrier construction materials, prototype barrier
<br />designs and testing, model applications and validation, natural'
<br />analog studies, long-term climate change effects, interface with
<br />regulatory agencies, RCRA equivalency, technology integration
<br />and transfer, and final design.
<br />THE NATURE OF THE PROTOTYPE
<br />We first looked to nature—to the stability and performance of
<br />natural analogs that have existed for thousands of years. We
<br />studied, for example, many of the borrow pits at the Hanford
<br />site. Unchanged for about 13,000 years, they consist of fine
<br />soils overlying coarser materials. We also paid attention to con-
<br />structed mounds used to protect tombs or used as temple plat-
<br />forms hundreds to thousands of years ago. Many of these an-
<br />cient mounds have survived extremely well and are still intact.
<br />The permanent isolation surface barrier we developed and
<br />constructed at the Hanford site consists of a variety of different
<br />natural materials placed in engineered layers directly over a
<br />stabilized waste zone to form an above -grade mound with re-
<br />dundant protective features (see figure). The barrier consists
<br />of a fine -soil layer placed over other layers of coarser materials
<br />such as sands, gravels and basalt riprap. Each layer serves a
<br />distinct purpose.
<br />THE PERMANENT ISOLATION SURFACE BARRIER DEVELOPED AND CON-
<br />STRUCTED AT DOE'S HANFORD SITE CONSISTS OF A VARIETY OF DIFFERENT
<br />NATURAL MATERIALS—FINE SOILS, SANDS, GRAVELS, BASALT RIPRAP AND
<br />ASPHALT—PLACED IN ENGINEERED LAYERS DIRECTLY OVER A STABILIZED
<br />WASTE ZONE. PHOTO BY DENNIS R. MYERS.
<br />0885-7024-/94-0010-0038/$02.00+25¢ per page
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