Technology Development & Applications Group
Macroencapsulation
Background
With support from the U.S. Department of
Energy-Office of Technology Development (DOE OTD), Brookhaven National
Laboratory (BNL) has developed a low- density polyethylene encapsulation
process for low-level radioactive, hazardous, and mixed wastes. Polyethylene
is an inert, low permeability, thermoplastic material that is highly
resistant to chemical attack, microbial degradation and radiation damage.
These properties combine to provide an extremely durable and stable final
waste form for the disposal of DOE and commercial waste streams. Original
development efforts focused on microencapsulation of wastes, in which the
waste is combined with molten plastic in an extruder to form a homogeneous
mixture and is then cooled to form a monolithic solid final waste form.
Since no chemical reactions are required for solidification, the process is
compatible with a wide range of wastes including aqueous concentrates such
as nitrate, chloride, sulfate and borate salts, off-gas scrubber salts,
incinerator ash, ion exchange resins, and spent molten salt oxidation
residuals.
More recently the process has been applied to macroencapsulation,
in which larger pieces of waste not suitable for extrusion processing (e.g.,
lead, debris) are surrounded by a layer of clean polyethylene to isolate the
contaminants from the environment. The Environmental Protection Agency (EPA)
has identified polymer macroencapsulation as the Best Demonstrated Available
Technology for radioactive lead solids (D008) and mixed waste debris,
defined as materials exceeding 60 mm in particle size. Under current
regulations, macroencapsulated debris does not require performance testing
such as EPA's Toxicity Characteristic Leaching Procedure (TCLP).
Process Description
 The polyethylene macroencapsulation process utilizes a single-screw plastics extruder to melt, convey and pump molten polyethylene through a die and into a waste form container. A schematic diagram of an extruder is shown in the figure to the right. The extruder consists of an auger-type screw enclosed in a barrel to which polyethylene, typically in the form of beads, is gravimetrically fed from a storage hopper resting above the feed throat. The polyethylene is gradually heated and melted as it is conveyed by the screw through independently controlled temperature zones. The heat for melting the polyethylene is provided through the heaters and by frictional heat which is generated by a gradual decrease in the screw channel depth between flights along the screw.
Temperatures are set to achieve a uniform molten output flow. The required processing temperatures are dependent on the type and grade of resin used. Recycled resins may be used in this process, providing a valuable new end-use for industrial and post-consumer solid waste plastics.
As shown in the figure on the left, the molten polyethylene from the extruder die is poured into a waste container in which waste materials have either been suspended or supported. Techniques for generating uniform macroencapsulated waste form products depend on desired waste form geometry, extruder/die configuration, processing conditions, and type of resin used. Figure 2 shows a waste form container that utilizes an interior cage-like basket to hold the waste requiring solidification.
Commercialization
Commercial application of polyethylene macroencapsulation has recently been initiated by Envirocare of Utah with support from DOE and assistance from BNL. Envirocare is a Nuclear Regulatory Commision (NRC) licensed disposal facility for naturally occurring radioactive materials, low-level radioactive wastes and is the only licensed diposal facility in the US for mixed radioactive/RCRA hazardous wastes. The
Envirocare polyethylene macroencapsulation process uses a 4.5 inch single-screw extruder and having received permits from the State of Utah, has successfully begun processing over 400,000 pounds of DOE mixed waste solids such as lead bricks, as well as other DOE and commercial mixed waste debris.
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Last Modified: November 12, 2009
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Linda Satalino
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