Organic Polymers for Stabilization/Solidification - Stabilization and Solidification of Hazardous, Radioactive, and Mixed Wastes - page 87

of petroleum. Its main applications are for asphalt highway paving and roofing, but
it has a number of attractive properties for waste solidification, including:

• Low permeability to water

• Resistance to acids, alkalines, and salts

• Low viscosity at relatively low process temperatures that allow good

mixing with solid waste particles

Bitumen was first introduced for treatment of radioactive waste in Belgium in
the early 1960s and is still in use today. Bitumen is available in several grades based
on the method of production, such as:

• Distilled bitumen; softening point 34 to 65°C

• Oxidized or blown bitumen (air is pumped through petroleum during

manufacturing); softening point 70 to 140°C

• Cracked bitumen (pyrogenic breakdown of heavy molecules); softening

point 77 to 85°C

• Emulsified bitumen (emulsification of bitumen in soapy water) 14

Direct distillation or oxidized bitumens are most commonly used for waste
solidification processes. Composition and physical properties can vary considerably
from batch to batch, but typically bitumen specified for waste encapsulation has a
density of 1.01 g/cm 3 at ambient temperature, a softening point of 90°C, and a flash
point of > 290°C. 5 Bitumen has been used to treat a broad range of low-level
radioactive wastes including sludge, evaporator concentrates, incinerator ash, sol-
vents, and debris. It has been deployed in Belgium, France, England, Poland, Russia
and other former Soviet countries, Korea, and Japan. Several bitumen systems were
installed in the U.S., but due to poor efficiency and safety issues, they are not
commonly used.
Several methods have been applied for bituminization of radioactive waste. One
method uses preheated molten bitumen in a wiped film evaporator, in which injected
liquid wastes are vaporized. The resulting particulate salts and other waste solids
are then mixed with the bitumen and discharged to a drum to cool and solidify. The
steam is condensed and captured, along with any other volatiles and carryover
particulates. A typical wiped film evaporator system installed at the waste treatment
facility at the Korean Atomic Energy Research Institute (KAERI) is shown in Figure
5.3. A second method uses a twin-screw extruder, similar to systems used in the
plastics industry. Solid or liquid waste and molten bitumen are fed into the extruder
and are mixed continuously and conveyed by the action of the intermeshing screws.
The heated barrel keeps the bitumen molten and, in the case of aqueous waste,
vaporizes the liquid, which is then drawn off of the mixture through several vacuum
ports and condensed. The condensate is recycled through the process as long as it
remains radioactive. Like the thin film evaporator, the combined mixture of waste
solids and bitumen is discharged to a drum for cooling and solidification. A typical
extrusion process for bitumen encapsulation is shown in Figure 5.4. 12 A production-
scale twin-screw bitumen processing system was installed at the Palisades Nuclear