Organic Polymers for Stabilization/Solidification - Stabilization and Solidification of Hazardous, Radioactive, and Mixed Wastes - page 86

offset by higher waste loading efficiencies for some types of waste. For example,
one study comparing the cost of polyester encapsulation versus conventional cement
process for dry nitrate salt wastes concluded that the polyester was 28% less expen-
sive ($11.50/kg waste for the polyester system compared with $16.09/kg waste for
hydraulic cement process). 9 Thus, cheaper material costs do not guarantee cheaper
processing costs. Actually, total cost should be compared, if possible, including
processing costs, the effects of volume change, storage costs awaiting final disposal,
transport costs, and cost for final disposal. A cost comparison at Oak Ridge National
Laboratory (ORNL) indicated that the processing costs for stabilizing transuranic
(TRU) waste was dwarfed by the cost of storing the canisters of treated TRU waste
on-site, transporting these canisters to the Waste Isolation Pilot Plant (WIPP), and
disposing them in the WIPP. Unlike thermoplastic binders (e.g., bitumen and poly-
ethylene), which can be remelted after solidification, once thermosetting resins
polymerize they can no longer be processed again.
Thermosetting resins including polyester styrene and vinylester styrene have
been used commercially to treat low-level radioactive waste at commercial nuclear
power plants and at some DOE facilities. Due primarily to the relatively high cost
of materials, the technology has not been widely adapted for waste treatment, but
still maintains a niche for the treatment of some types of “problem” wastes, e.g.,
evaporator concentrates.

5.3 THERMOPLASTICS

The organic chains in thermoplastic polymers are not usually cross-linked; thus,
they are solid at ambient temperature but can be heated, melted, and reformed upon
cooling without impacting the material structure or properties. These polymers can
be applied for waste encapsulation by mixing the waste with the molten polymer
and allowing it to cool to a monolithic solid waste form. Since no chemical reaction
is required to form the solid, thermoplastics are not susceptible to the failures
associated with cement grout and thermosetting polymers. Maximum waste loading
is limited by processability and the performance requirements of the final waste
form. For example, above maximum loading limits, the viscosity of the waste-binder
mixture precludes homogeneous mixing. Likewise, with very high waste loadings,
waste particles may not be fully encapsulated, leading to interconnected pores and
higher leachability. 13 Processing temperatures for organic polymers are higher than
100°C, so aqueous wastes cannot usually be processed directly due to product
foaming as moisture evaporates. Some process techniques are available (e.g., kinetic
mixing, vented extrusion, wiped film evaporation) to alleviate or minimize these
effects, but if the waste contains significant concentrations of moisture (e.g., > 5
wt%), pretreatment to remove residual moisture is often the most efficient approach.

5.3.1 B ITUMEN

Bitumen is a high-molecular weight thermoplastic organic polymer consisting of a
mixture of organic solids and organic liquid or oil that is derived from the distillation